PHOTOGRAPHY ITS PRINCIPLES AND PRACTICE By C. B. NEBLETTE Member of the Faculty of Texas A. and M. College Fellow of the Royal Photographic Society of Great Britain Editor, The Month in Applied Photography, Photo-Era Magazine SECOND EDITION LONDON CHAPMAN & HALL, LTD. Eleven Henrietta Street, W.C. 2 1931 Copyright, 1927, by D. VAN NOSTRAND COMPANY 1930, by D. VAN NOSTRAND COMPANY, Inc. All Rights Reserved This book, or any parts thereof, may not be reproduced in any form without written permission from the publishers. First Published, January 1927 Second Printing, August 1928 Second Edition, November 1930 Printed in the V.S. A. LANCASTER PRESS, INC. LANCASTER, PA. TO MY PARENTS and A VERY BEAR FRIEND PREFACE TO SECOND EDITION In the three years that have elapsed since the appearance of the first edition, the results of continued investigation of the principles of the photographic process have been such as to render necessary a thorough revision of certain portions of this work. Much of that portion of the chapter on emulsions which deals with the nature of the sensitivity centers has been rewritten to bring this section into line with recent in- vestigations on this subject. Several alterations have been made to the chapter on orthochromatics and the material on the chemical con- stitution of color sensitizing dyes is new. The chapter on the latent image has been largely rewritten ; the older theories, such as the sub-halide, are discussed frankly from the his- torical viewpoint and the emphasis has been placed, where it properly belongs, on the modern theories of latent image formation. In view of the current discussion regarding the usefulness of H. and D. speeds, it has been thought advisable to add to the chapter on sensitometry a section on H. and D. speeds and effective speeds. A new section has been added on -the reciprocity function and the more generally used polarization photometers have taken the place of the now obsolete H. and D. instrument, a description of which was included in the first edition because its principle is more easily understood by the student than the more complex polarization instruments. Other additions include a table of the properties of the more com- mon developing agents, sections on the exhaustion of developing solu- tions through continuous use, development at high temperatures, the newer theories of the fixing process, and a revision of the material on the characteristics of printing papers. It is a pleasure to acknowledge the suggestions and criticisms of my good friend Dr. S. E. Sheppard and the helpful advice of many others, especially Dr. C. W. Burchard, Professor of Organic Chemistry at this institution. C. B. Neblette, F.R.P.S. College Station, Texas January 4, 1930 PREFACE TO FIRST EDITION Manifold as are the applications of photography in all branches of science and industry and great as has been the increase in our knowl- edge of its basic principles in recent years, comprehensive and ade- quate instruction in the subject has been largely neglected by our uni- versities and technical institutions. Despite its daily application to the practice of almost every branch of science and industry, and in- deed in every walk of life, as well as its importance from the stand- point of pure science, there is not, within the knowledge of the writer, a single university or technical institution in this country which offers a thorough and complete course in the science and practice of photog- raphy. The literature of photography is widely scattered among a large number of journals, some of which have long since disappeared, while until comparatively recently no worthy attempt had been made to- wards the abstracting and indexing of photographic information. Excluding papers in the periodical press, photographic literature can for the most part be divided into two classes: (i) works of an ele- mentary nature designed for the beginner and paying but scant at- tention to the fundamental scientific basis of the subject and (2) works of an encyclopedic nature designed principally for reference purposes, such as Dr. J. M. Eder's monumental work in German, the Ausfuhrliches Handbuch der Photographie and Fabre's Traite En- cyclopedique de Photographie in French. Valuable as these works may be, they are not textbooks in the true sense of the word and there is in fact no work dealing both with the science as well as the practice of photography which is especially adapted for use as a text. The present work is an attempt to meet that need. It embraces the features which several years' experience in the teaching of the subject has shown the writer to be desirable in a work designed for college instruction. No attempt has been made to compile a complete treatise on the subject, while at the same time the fact has been kept in mind that a superficial treatment of the subject, one which is concerned with effects rather than causes and with operations rather than scientific principles, is undesirable in a work of collegiate grade. Accordingly vii viii PREFACE TO FIRST EDITION it has been the aim of the writer throughout to present as clearly and as concisely as possible the fundamental principles of the science of photography, omitting nothing of primary importance necessary to an understanding of the subject and paying particular attention to the proper coordination of the facts to one another. The practical side has not been lost sight of, however, and several of the chapters deal with their subject more from the standpoint of technique rather than science. These, it is hoped, will render the work useful not only to the student but to the practical worker as well. An apology, perhaps, is due for the omission of certain subjects and for the brief treatment accorded to others. This was, however, to a certain extent, demanded by the scope of the work which is that of a text rather than a treatise. Accordingly a large number of the un- settled controversies have either been omitted, or but briefly treated, as it was felt inadvisable to consider in a work of this nature subjects which still await satisfactory solution. Footnotes throughout the text will show the extent to which I am indebted to others, while to the following authorities I desire to place on record my appreciation of their invaluable services : Dr. C. E. K. Mees, Dr. S. E. Sheppard, Dr. A. P. H. Trivelli, all of the Eastman Research Laboratory ; Mr. F. F. Renwick ; Dr. Walter Clark of the British Photographic Research Association ; Dr. Hermann Kellner of the Scientific Department, Bausch and Lomb Optical Company ; Carl J. Reich of the Gundlach-Manhattan Optical Company; Mr. George E. Brown, Editor of the British Journal of Photography; Mr. Frank Roy Fraprie and Mr. E. J. Wall of American Photography; Drs. Walters and Davis of the Bureau of Standards; and Miss Bess Spence of this institution who has assisted me in seeing the work, through the press. To all others who have assisted in the preparation of this work in any way, a cordial acknowledgment of appreciation is also due. C. B. Neblette College Station, Texas, 1926 CONTENTS Chapter I. The Development of Photography Introduction. The Development of the Camera. Jean Baptiste Porta. The Camera Obscura with Lens. Early Records of the Photochemical Action of Light. The Forerunners — Davy and Wedgwood. Life and Work of Joseph Nicephore Niepce. Life and Work of Jacques Mande Daguerre. The Daguerreotype Proc- ess. Later History of the Daguerreotype. The Positive Process of Bayard. Life and Work of Henry Fox- Talbot. The Calofype Process. Miscellaneous Paper Processes. Introduction of Glass. Scott-Archer and the Introduction of Collodion. The Collodion Process. Inconveniences of the Collodion Process. Modifications of the Collodion Process. Introduction of Collodion Emulsion. In- troduction of Gelatino-Bromide Emulsion. Improvements in the Gelatino-Bromide Process. Development of Printing Processes with Silver Salts. Platinum Printing Processes. Printing Processes with Bichromated Colloids. Chapter II. The Camera and Darkroom The Box Camera. The Miniature Camera. Folding Hand Cam- eras. The Professional Camera. The Reflex Camera. The Prin- cipal Adjustments of Cameras. The Swing Back. The Reversible Back. Other Movements. Shutters. Tripods. The Darkroom. Ventilation. Size. Arrangement. Water Supply. The Illumina- tion of the Darkroom. The Safelight. The Efficiency of Dark- room Safelights Trays, Tanks, Commercial Tanks and Gradu- ates. Miscellaneous Features. Chapter III. Photographic Optics. Introduction. Refraction of Light. Dispersion. Lenses and Image Formation. Image Formation according to the Gauss Theory. The Position of the Nodes. The Principal Focus of a Lens — Focal Length. Focal Length and Size of Image. Angle of View. Con- jugate Focal Distances. Extra Focal Distances. Theory of Depth of Focus. Factors Controlling Depth of Focus. The Intensity of the Image. Speed of Lenses — Systems of Diaphragm Notation. Effective Aperture. Loss of Light in Lenses Due to Absorption and Reflection. Variation in Relative Aperture with Distance of Subject. Chapter IV. The Aberrations of the Photographic Objective. . Introduction. Chromatic Aberration. Spherical Aberration. Coma. Curvative of Field. Distortion. Astigmatism. Flare and Flare Flare Spot. Unequal Illumination. ix X CONTENTS Chapter V. The Photographic Objective 105 Part I. The Astigmats : The Single Collecting Lens. The Single Achromat. Semi-Achromatic or Soft-Focus Lenses. The Aplanat or Rapid Rectilinear. The Petzval Portrait Lens. Part II. The Anastigmats : Introduction. Cemented Symmetri- cal Anastigmats. Alternate Form of the Double Anastigmat. The Four Glass Element — the Protars. The Five Glass Element. Sym- metrical Lenses with Air-Spaces. The Gauss Construction. Stein- heil's Unofocal. Graf Variable and Anastigmat. Beck's Neostig- mat and Isostigmat. The Plasmat. Dallmeyer's Stigmatic. Ru- dolph's Early Protars. The Unar. The Tessar. Combination of Air Space and Cemented Surface — Later Developments. Serrac. X-Press. Radiar. The Cooke Triplet. Development of the Cooke Triplet after H. D. Taylor. The Aviar. Aldis. Heliar, Dynar. Pentac. Ernostar. Part III. The Teleobjective : Principle. The Compound Tele- objective. Early Fixed-Magnification Teleobjectives. Anastigmatic Fixed-Magnification Teleobjectives. Dallmeyer's Adon. Chapter VI. The Photographic Emulsion 149 Introduction. The Two Classes of Emulsions. General Outline of Operations in Emulsion Preparation. Gelatine. Light Sensitive- ness of Silver Salts. The Preparation of Emulsions. Emulsifica- tion. Gelatino-Bromo-Iodide Emulsions. Digestion of the Emul- sion. Fog. Theory of Digestion. Eliminating the Soluble Salts. The Silver Bromide Grain of Photographic Emulsions. The Sensi- tivity of the Silver Halide Grain. Grain Size and Distribution and Its Relation to the Photographic Properties of Emulsions. Chapter VII. Orthochromatics 168 Light and Color — the Spectrum. Visual and Photo-Chemical Lumi- nosity. History of Dye Sensitizing. Known Facts Regarding Color Sensitizing. Constitution of Color Sensitizers. Color Sensitizing Dyes. Color Sensitizing by Bathing. Hypersensitizing. The The- ory of Light Filters. Orthochromatic Filters. Contrast Filters. Orthochromatic Methods in Landscape Photography. Orthochro- matic Methods in Portrait Work. Photographing Color Contrasts. Chapter VIII. The Latent Photographic Image 194 Photo-Physical and Photo-Chemical Change. The Latent Image. Hydrogen Peroxide. Intensification with Hydrogen Peroxide. Sodium Arsenite. Reversal by Chemical Reagents vs. Reversal by Light. Photo-Regression. Physical Development of the Latent Image after Fixation. The Photosalts. Action of Oxidizing and Halogenizing Agents on the Latent Image. Visible Darkening of the Ag Halides. The Latent Image. Modern Conceptions of the Mechanism of Exposure. CONTENTS xi Chapter IX. Sensitometry... 209 What is Sensitometry? Resume of Sensitometric Investigation. Standard Light Sources. Sensitometers. Relation of Time and In- tensity. Densitometers. Opacity — Transparency — Density. Ex- posure and Development of Sensitive Materials for Speed Deter- mination. Relation of Exposure and Growth of Density. The Characteristic Curve. The Significance of the Characteristic Curve. Inertia as an Inverse Measure of Speed. Variation of the Inertia. Effective and H. and D. Speeds. Wedge Methods of Sensitometry. The Perfect Negative. Density-Exposure Relation and Correct Reproduction. Latitude of Sensitive Materials. De- velopment and the Reproduction of Contrasts. Constant Density Ratios. An Important Difference. Development and Contrast. Gamma as a Measure of Contrast. Gamma and the Characteristic Curve. Calculation of Gamma. Gamma Infinity. Chapter X. The Exposure of the Sensitive Material 240 The Problem. Light Intensity and Exposure. The Subject. Speed of Plate. Speed of Lens. Determination of the Time of Exposure. Exposure Meters. Corrections for Special Subjects. Visual Meters, Types, Principles and Use. Chapter XI. The Theory of Development 253 Introduction. The Invasion Phase. The Chemical Reaction within the Cell — The Reduction Phase. The Precipitation Phase. De- velopment as a Reversible Reaction. The Action of Sulphites, Solu- ble Bromides and Alkali in Organic Developing Solutions. The Physical Chemistry of the Developing Process. The Induction Period. The Velocity of Development. The Velocity Constant. Calculation of the Time of Development for a Given Gamma. Ef- fect of Temperature on Development. Calculating the Temperature Coefficient. Time of Development at Various Temperatures. The Action of Soluble Bromides in Development. The Relative Reduc- ing Energy of Developing Agents. Chapter XII. Organic Developing Agents 276 Developing Power. Classification of Developing Agents. Organic Developing Agents. The Significance of Group Relations. De- veloping Characteristics of Organic Developers. Formulae for Principal Developing Agents. Formula for Combined Developers. Chapter XIII. The Technique of Development 293 Introduction. The Sulphites in Development. The Alkalis in De- velopment. The Value of Desensitizers. The Development of De- sensitizing Agents. Desensitizing in Practice. Development by In- spection. The Watkins System of Factorial Development. What xii CONTENTS Determines the Factor. Accuracy of the Factorial System. Thermo Development. The Watkins System of Thermo Development. De- veloping Speeds of Commercial Plates. Developers. Instructions. Thermo-Development with Glycin. The Efficiency of Time De- velopment. Development at High Temperatures. Chapter XIV. The Laws of Fixation and Washing 314 Action of Sodium Thiosulphate on the Silver Halides. The Mecha- nism of Fixing. Influence of the Concentration of Thiosulphate and Temperature on Time of Fixation. Influence of Ammonium Chlo- ride on the Rapidity of Fixation. When are Plates Fixed? Ex- haustion of the Fixing Bath. The Fixation of Prints. Plain Fixing Baths. Acid Fixing Baths. Acid Fixing and Hardening Baths. Troubles with the Acid Fixing and Hardening Bath. Extra Hard- ening Baths. The Mechanism of Washing. The Efficiency of Washing Devices. The Washing of Prints. Methods of Deter- mining the Presence of Thiosulphate. Hypo Eliminators. Chapter XV. Defects in Negatives 333 The Why of Defects. Thin Negatives. Dense Negatives. Fog on Negatives. Local Fog. General Fog due to Light. Chemical Fog. Dichloric Fog. Developer Stains. Silver Stains. Miscellaneous Stains. Transparent Spots. Opaque or Semi-Opaque Spots. Mis- cellaneous Troubles. Chapter XVI. Intensification and Reduction 345 Part I. Reduction. The Three Classes of Reducers. Farmer's Reducer. Mercury and Cyanide. Iodine Cyanide. Belitiski's. Per- manganate. Bichromate. Proportional Reducers. Super-propor- tional Reducers. Theories of Super-proportional Action. Practice of Persulphate Reduction. Intensification, Definition, Methods and Characteristics. Mercury Intensifiers. Monkhoven's. Mercuric Iodide. Silver Intensifiers. Chromium Intensifiers. Uranium. Sulphide. Lead. Copper. Sensitometry of Intensification. Local Reduction and Intensification. Chapter XVII. Printing Processes with Silver Salts 364 Part I. Developing Papers : Characteristics of Printing Papers. Adapting the Paper to the Negative. Developing Papers. Expo- sure. Developers. The Safelight. Development. Factorial De- velopment. The Proper Factor. The Short Stop. Fixing. Wash- ing. Drying. Alteration of Contrast Reduction and Intensifica- tion of Prints. The Glazing of Prints. Part II. Gelatine P-O-P: Toning. Instantaneous Toning. Black Tones with P-O-P. Fixing. CONTENTS xiii Chapter XVIII. Projection "Printing. 390 Introduction. Fixed Focus Enlarging Cameras. Apparatus for Projection Printing with Daylight. Apparatus for Projection Print- ing with Artificial Light. Self-focusing Apparatus for Projection Printing. Illuminants for Projection Printing. The Mercury- Vapor Lamp. The Electric Arc. Incandescent Lamps. Securing Even Illumination without Condensers. The Condenser in Projec- tion. Condensing Lenses with Diffusing Media. The Projection Lens. The Projection Easel. The Negative for Projection Print- ing. The Technique of Projection Printing. Focusing. Determin- ing Exposures in Projection Printing. Relative Exposure, Scale and Aperture in Enlarging or Reduction. Enlarging from Small Negatives. Introducing Clouds in Enlargements. Enlarged Nega- tives.— Sensitive Materials. — Exposure. Development. Chapter XIX. Lantern Slides and Transparencies 419 The Negative. Lantern Plates. Printing Frame for Contact Print- ing Exposing. Printing by Projection. Developers. Develop- ment. Fixing, Washing and Drying. Masking. Spotting. Bind- ing. Advertising Slides. Toning of Lantern Slides by Restrained Development. Physical Development. Colors on Development with Thiocarbamide. Toning of Lantern Slides. Reduction and In- tensification of Slides. Chapter XX. The Toning of Developed Silver Images 431 Introduction. The Sulphur Toning Processes — the Print. The Hypo-alum Process. Zanoff's Controlled Hypo-alum Method. Sul- phur Toning with Acid Hypo. Toning with the Polysulphides. Single Solution Sulphur Toning Processes (Shaw's Process). The Indirect Process of Sulphide Toning. Rebleaching of Sulphide Toned Prints. Indirect Sulphide Toning with Intermediate Develop- ment. Mercury Sulphide Toning (Bennett's Process). Toning with Copper. Toning with Uranium. Iron Toning Processes. Toning with Vanadium. Minor Toning Processes. Chapter XXI. Platinotype and Iron Printing Processes 452 Introduction. Theory of the Process. Commercial Papers and their Treatment. Exposure. Development. Variations in Contrast. Variations in Color. Silver Platinum Papers. The Kallitype Proc- ess. Blue Printing. Chapter XXII. Printing Processes Employing Bichromated Colloids. I. (Carbon and Carbro) .... 462 The Chemistry of Pigment Printing with Bichromated Colloids. The Carbon Process. Carbon Tissues. Double and Single Trans- CONTENTS fer. Sensitizing the Tissue.- Exposure. Development. Double Transfer. Transferring to Rough Papers. The Carbro Process. The Bromide Print. Sensitizing the Tissue. Transfer. Redevel- opment of the Bromide Print! Development of the Carbro. Car- bon on Bromide. Multiple Printing. Chapter XXIII. Printing Processes Employing Bichromated Colloids. II. (Gum-Bichromate and Allied Processes) 481 Introduction. Materials. The Negative. Formulas. Effect of Varying Proportions of the Coating Mixture. Coating. Drying. Exposure. Development. Registration. Gum-Bromide and Gum- Platinum. The Powder Processes. Formula of E. J. Wall. Res- inopigmentype. Chapter XXIV. Printing Processes Employing Bichromated Colloids. III. (Oil, Bromoil and Trans- fer 494 Introduction. Materials for the Oil Process. Papers for the Oil Process. Brushes.. Pigments. Sensitizing. Exposing. Pigment- ing. Incorrect Exposure. Drying and Mounting. Duvivier's Proc- ess. The Bromoil Process. The Choice of the Paper for the Bro- mide Print. The Production of the Bromide Print. Bleaching of the Bromide Print. Chemical Theory of the Bleaching Operation. Fixing. Producing the Relief. Pigmenting. Natnias Method of Pigmenting. Defatting the Finished Bromoil. Bromoil Transfer. The Bromide Print. Preparation of the Bromoil. The Transfer Paper. The Transfer Press. Transferring the Pigment. Zaeper- nick's Chemical Transfer Method. Multiple Transfer. Chapter XXV. Copying 520 Introduction. Apparatus for Copying. Methods of Illuminating the Print. Copying Cameras. The Objective for Copying. Focus- ing. Copying to Scale. Exposures in Copying. The Copying of Subjects in Pure Black and White. Development of Process Plates. Copying Photographs or Like Subjects in Monochrome. The Pho- tography of Colored Objects. Photography of Small Objects in the Studio. Chapter XXVI. Natural Color Photography 537 Introduction. Processes of Direct Color Photography — The Bleach Out Process. Processes of Direct Color Photography — Processes of Light Interference. Natural Color Photography by Trichromatic Methods. Making the Three Color-Sensation Negatives. Additive and Subtractive Three-Color Photography. Subtractive Printing- Processes. Multi-Color Screen Plates. The Autochrome Plate. CONTENTS xv The Compensating Filter. Handling of the Autochrome Plate. Exposure. Development. Reversal of the Image. Varnishing. After-Treatment of Autochromes. The Agfa Color Plate. Duplir eating Processes of Screen-Plate Color Photography. The Duplex Method. Appendix. A List of the More Important Reference Works on Photography 559 References to Technical Journals 561 Index ILLUSTRATIONS Fig. 1. The camera obscura from an old print. (Courtesy of the Smithsonian Institution) 2 2. Johann Heinrich Schulze. (From Eder's biography) 6 3. Thomas Wedgwood. (Chalk drawing, author unknown) 7 4. Joseph Nicephore Niepce (Courtesy of the Societe Francaise de Photo- graphic) 9 4a. Birthplace of Niepce 10 4&. Statue to Niepce 10 5. A heliographic print by Niepce, 1824 11 6. Louis Jacques Mande Daguerre 12 7. Tomb of Daguerre. (Courtesy of the Societe Francaise de Photo- graphie) 14 8. Fuming cabinet for the Daguerreotype process 15 9. Developing box for the Daguerreotype process 16 10. An early Daguerreotype portrait. Often stated to be the first portrait by photography 17 11. William Henry Fox-Talbot. 19 12. Frederick Scott Archer. Drawing from an old print reproduced in J. Werge — The Evolution of Photography 22 13. The wet collodion process in the field. (From an old manual) 24 t 14. Portable laboratory for field use with wet collodion 25 15. Dr. Richard Leach Maddox 27 16. Typical miniature camera for plates and for roll film 39 17. Hand cameras for plates and for roll film 41 18. Professional view camera 43 19. Principle of the reflex camera 44 20. Principle of the swing back. Use of the swing back for securing greater depth of focus 46 21. Ventilation of the darkroom. (Courtesy of Eastman Kodak Company) 53 22. Floor plan of darkroom for amateur use 54 23. Eastman indirect darkroom lamp 57 24. Design for indirect darkroom lamp. (Krug, American Annual of Pho- tography, 1922) 57 25. Eastman developing lamp 58 26. Wratten darkroom safelight lamp 58 27. Drying cabinet for plates and films 64 28. The principles of refraction 66 29. Refraction in a medium with parallel sides 67 30. Refraction in a prism 67 31. Dispersion in a prism 68 32. Principal forms of simple lenses 68 33. Image formation with positive or converging lenses 69 xvii xviii ILLUSTRATIONS 34. Course of light pencils through a negative lens '. 69 35. Image formation according to the Gauss theory 70 36. Image formation according to the Gauss theory 70 37. Position of the nodes in common forms of simple lenses 71 38. Table for the calculation of angle of view 73 39. Graphic illustration of the principle of depth of focus. (Von Rohr) . . 79 40. Intensity of the optical image. (Brown) 81 41. Chromatic under correction 90 42. Chromatic over correction 90 43. Irrationality of dispersion 91 44. Spherical aberration 93 45- Coma. (Kellner) 94 46. Two forms of coma. (Piper) 95 47. Curvature of field. (Under-correction) 96 48. Curvature of field. (Over-correction) 96 49. Distortion 97 50. Under and over correction for distortion 98 51. Astigmatic deformation. (Kellner) 100 52. Optical flare 101 53. Constriction of aperture for the marginal pencils. (Brown) 102 54. Greater focal length of marginal pencils resulting in lower intensity. (Brown) 103 55. The angling of the oblique ray. (Brown) 103 56. Relation between the angle of view and the diminution of the optical intensity of the image. (Zschokke) '. . . . 104 57. Wollaston's meniscus 105 58. The Chevalier or French landscape lens 106 59. Grubb's landscape lens 106 60. Dallmeyer's rapid landscape lens 106 61. Goddard's landscape lens — Dallmeyer's rectilinear landscape lens 107 62. Sutton's triplet 109 63. Dallmeyer's triple achromatic 109 64. Harrison and Schnitzer's Globe lens 109 65. The Aplanat or R. R. no 66. Portrait of Petzval in 67. Petzval's portrait objective 112 68. Modifications of the Petzval portrait objective. ( (a) Dallmeyer, (6) Voigtlander, (c) Zinc-Sommer) 112 69. Cooke F/1.5 modification of the Petzval construction 113 70. The Goerz Dagor 115 71. Watson's Holostigmat : 116 72. The Collinear and Orthostigmat of Voigtlander and Steinheil 116 73. Rudolph's Protar Series Vila 117 74. The T-R anastigmat 118 75. The Celor and Syntor of Goerz 119 76. The Dogmar of Goerz 120 77. The Gaussian objective 120 ILLUSTRATIONS xix 78. Rudolph's Planar. 121 79. Kollmorgen's Aristostigmat 122 80. Lee's T. T. H. Opic , . . 122 81. iThe Leiss Biotar F/1.4.... 123 82. Rudolph's Plasmat 123 83. Steinheil Unofocal 125 84. Graf Anastigmat 126 85. Beck's Isostigmar 127 86. Beck's Neostigmar 128 87. Dallmeyer Stigmatic 128 88. Rudolph's unsymmetrical Protar 129 89. Rudolph's Unar 130 90. Rudolph's Tessar 13 1 91. Dallmeyer Serrac 132 92. Ross X-press 133 93v Gundlach Radiar 133 94. Cooke triplet ' 134 95. Action of the central diverging lens J35 96. Cooke Aviar 136 97. Aldis Series lies 137 98. Aldis Series II and III 137 ^ 99. Harting's Heliar 137 " ,t 100. Harting's Dynar 138 , 101. Dallmeyer's Pentac 138 102. The Ernostar 139 103. Dallmeyer's compound teleobjective 141 104. Petzval's orthoskop \ . . 142 105. Martin's Bis-Telar 143 106. Zeiss Magnar 143 107. Ross Telecentric 143 108. Dallmeyer large Adon 143 109. Booth teleobjective 144 110. Radiar teleobjective 145 in. Cooke Telekinic F/3.5 H5 112. Zeiss Teletessar 146 113- Ross Teleros 146 114. Lee's T. T. H. Telephoto 146 115. Voigtlander's Tele-Dynar 146 116. Dallmeyer's Adon 147 117. Emulsion press 161 118. Photographic emulsion under microscope 162 119. Size frequency distribution of silver halide grains in a portrait film and lantern slide emulsion 166 120. Three-color print of the spectrum 121. Visual luminosity of the spectrum after Abney 1 169 122. Spectral sensitiveness of the silver halides 170 xx ILLUSTRATIONS 123. Spectral Sensitiveness of a Gelatino-bromo-iodide Emulsion Color- Sensitized with Various Dyes 178 124. Portrait on ordinary and panchromatic emulsions 188 125a. Photograph of manuscript in blue with red corrections using a green filter 190 1256. Photograph of manuscript in blue with red corrections showing use of red filter 190 126. Wood sections on ordinary and panchromatic plates. (Courtesy of II ford Ltd.) 192 127. Chapman Jones plate speed tester 212 128. H. and D. sector wheel and exposing apparatus 212 129. Densitometers. (1) Martens; (2) Filmograph; (3) Sanger-Shepherd Density Meter 215 130. Illustrating the relation of opacity, transparency and density 218 131. The characteristic curve 222 132. Step chart illustrating the theory of the characteristic curve 223 133. Characteristic curve secured by crossed wedges 227 134. Latitude and the characteristic curve 231 135. Development and constant density ratios 231 136. The geometry of gamma. (Brown) 236 137. Sea and sky 242 138. Sea view and shipping ., 243 139. Open landscape 243 140. Average landscape 244 141. Outdoor portrait 245 142. The Invasion phase of development. (Mees) 254 143. Growth of density with time of development. (Neitz).... 260 144. Curve showing growth of density with time of development. (Neitz) . . 260 145. H. and D. method for calculating the time of development for given gamma 264 146. Watkins method of calculating the T. C 270 147. Stokes time development chart 271 148. Effect of a soluble bromide in the developing solution on the plate curve, 272 r49. Density depression with a soluble bromide. (Neitz) 273 150. Effect of soluble bromide on the densities. (Watkins) 273 151. Influence of temperature on time of fixation 316 152. Influence of concentration of hypo on time of fixation 317 153. Influence of ammonium chloride on the time of fixation 319 154. Windoe's washing apparatus 328 155. Trox washer for roll film 329 156. Sensitometric action of different reducers. (Neitz and Huse) 346 157. Action of a proportional reducer on the plate curve. (Neitz and Huse), 35° 158. Sensitometry of photographic intensification. (Neitz and Huse) 361 159. Bench for local reduction. (British Journal of Photography) 363 1600. 1606, 160c. Adapting the scale of the printing paper to the negative.. 161. Printing machines for amateur and for professional use 373 162. Effect of time of development upon the characteristic curve of paper . emulsions 376 ILLUSTRATIONS xxi 163. Electrically operated print washer. (Pako) 379 164. Centrifugal water pressure type of print washer. (Halldorsen) 380 165. Rotary belt dryer. (Sickle) 381 166. Principle of projection printing 390 167. Box enlarging camera , 391 168. Daylight enlarging camera 392 169. Projection printing apparatus for use with daylight 393 170. Enlarging lantern for artificial light 394 171. Proper position of the carbons of an arc light for use with alternating current 396 172. Parallax reflector for use with incandescent electric sources 398 173. Securing even illumination with five incandescent electric sources 398 174. Forms for the lighthouse using reflected light. (Wall) 399 175. The function of the condenser 400 176. Conjugate foci in enlarging 401 177. Adjustment of the light source with condensers 402 178. Loss of light between condensers due to the use of a long focus lens for projection. (Candy) 404 179. Loss of covering power owing to the use of short focus projecting lens with condensers. (Candy) 404 180. Ingento enlarging easel for use with printing frame 406 181. Westminister enlarging easel 406 182. Scatter of light by negatives. (Callier) 408 183. Graduating focusing scale for enlarging. (Lockett) 412 184. F and S Lantern slide printing frame 420 185. Century lantern slide camera for reduction 421 186. Slide making by reduction using daylight 422 187. Device for holding lantern plate in position when using enlarger for lantern slide making by reduction. (Charles) 423 188. Actinometers for carbon printing 468 189. Squeegee board for carbro. (Farmer) 476 190. Curves showing the influence on contrast of variations in proportions of gum, pigment and sensitizer in the gum-bichromate process. (Anderson) 484 191. Owens' frame for multiple printing 487 192. Zerbe's method of registration 489 193. Proper position of the brush in pigmenting. (Mortimer and Coult- hurst, The Oil and Bromoil Processes). 500 194. Results in pigmenting. (Partington) 510 195. Transfer presses 515 196. Prett's transfer press 516 197. Copying stand 520 198. Book holder for copying 520 199. Illumination of the copy using daylight 522 200. Copying apparatus for artificial light. (Rose, The Commercial Pho- tographer) 522 xxii ILLUSTRATIONS 201. Method of securing white or black backgrounds. (Photo, courtesy of D. J. Pratt) : 536 202. Sanger Shepherd three-color camera 541 203. Butler's one-exposure, three-color camera. . 541 204. Autochrome screen. X 125 .. 547 205. Duplex screen 554 1 a CHAPTER I THE DEVELOPMENT OF PHOTOGRAPHY Introduction. — Photography is the science of obtaining images of objects by the action of light on sensitive substances. The word photography is due probably to Sir John Herschel and is derived from two Greek words ( = light, ypaia = write) meaning to draw by light. The two sciences of optics and chemistry form the basis of pho- tography, the former being concerned with the production of the image in the camera, the latter with the composition and treatment of the sensitive surface which reproduces the image cast upon it by the lens. The Development of the Camera. — The invention and development of the camera forms a most important phase in the history of pho- tography. Camera is a Latin word originally meaning ait enclosure with a vaulted, or arched, cover, but in course of time came to mean a room. Thus the camera obscura means a dark room, except for the illumination which comes through the small opening which serves as the lens. (Fig. i.) We do not know by whom, or at what date, the principle of the camera obscura was first discovered. There was, properly speaking, no invention of the camera obscura, for the prin- ciple is a natural phenomenon which must certainly have been ob- served many times by man without having excited any particular interest until someone more enterprising than the rest set about to find the cause of the phenomenon and its possible applications. At whatever date this may have taken place, we have at least a reference to the principle of the camera obscura as early as the time of Aris- totle. This learned Greek in his Problemata published about 350 B.C. refers to the fact that the image of the sun formed by the rays of light passing through a square aperture appears circular. He also noted the amplification of the image as the distance from the aperture is increased. Even a man with his intellect, however, appears to have been unimpressed, so that the camera obscura, properly speaking, es- caped him and he has really no place in its history. 1 2 PHOTOGRAPHY From the time of Aristotle there is no mention of the camera obscura for many hundred years. Alhazen in his Thesaurus Optica written in the eleventh century, although not published until 1572, seems to have been more or less familiar with its principle although he does not mention it specifically. Roger Bacon in his Perspectiva, published in 1267, has a passage which many have taken as the first description of the camera obscura, but it is so indefinite that it may (Courtesy of the Smithsonian Institution) Fig. i. The Camera Obscura from an Old Print be equally as well regarded as applying to the projection of images. There are other passages in his Opus Majus which seem to indicate a knowledge of the principles of the camera obscura but these likewise are couched in such vague terms that we are hardly justified in crediting him with the discovery of the camera obscura. The first precise and complete account of the camera obscura is to be found in one of the unpublished manuscripts of Leonardo da Vinci quoted by Venturi in his Essai sur les Ouvrages physico- mathematiqucs dc Leonardo da Vinci which was published at Paris in 1797. The following is Venturi's translation of the passage in the works of Leonardo: The following experiment shows how objects send their images to intersect on the albiginous humor inside the eye. When the images of illuminated objects enter into a very dark chamber by a small round aperture, if you receive these images in the interior of the room on a piece of white paper placed at some dis- THE DEVELOPMENT OF PHOTOGRAPHY 3 tance from the aperture, you will notice on the paper all the objects in their proper forms and colors : they will be lessened in size and will be reversed, and that in virtue of the intersection already noticed. If the images come from a place lit by the sun, they will appear as if painted on the paper, which should be very thin and looked at from behind. The aperture should be made in a very thin piece of sheet iron. Leonardo then goes on to give a diagram showing the arrange- ment of the aperture and screen and the course of the light rays. The manuscript is undated, but as Leonardo died in 15 19 it probably dates from several years previously. It is noteworthy that he does not refer to it in any way as an invention, which would lead one to be- lieve that he was not sure of having been the first to describe the same. In Csesariano's translation of Vitruvius' Treatise on Architecture, published at Como in 1521, there is a passage referring to the camera obscura as having been discovered by a Benedictine monk, Don Pap- nuito. Libri in his Historie des sciences mathematiques en Italie (Paris, 1841) says that although this is the first published description of the camera obscura, the observations of Leonardo must certainly have been made at an earlier date. Maurolycus, an eminent mathematician of Messina, in his Photismi de Lumine et Umbra ad Perspectivam et radiorum incedentiam fa\- cientia, published in 161 1, but finished in 1521, treats the subject mathematically and gives several theorems relating to the passage of light through small apertures and was apparently well acquainted with the formation of images in this way. The next references to the camera obscura are found in Germany, where we find Erasmus Reinhold and his pupils Gemma Frisius and others using the same to observe eclipses of the sun without danger to the eyes. Reinhold probably used the camera obscura in this way as early as 1540. Jean Baptiste Porta. — The connection of Jean Baptiste Porta with the discovery of the camera obscura arises from a passage in his Magia Naturalis, a remarkable work published in Naples, 1553, when he was fifteen years of age. Although there are at least five accurate and precise descriptions of the camera obscura prior to the time of Porta, still he is popularly credited with its discovery. It is quite likely that this misconception arose from the fact that Arago, the eminent secretary of the French Academy of Sciences, in his address before that body on the occasion of the presentation of the details of 4 PHOTOGRAPHY the Daguerreotype process took the opportunity to make a few re- marks on the historical phases of the subject in which he referred to the work of Porta with the camera obscura. There is nothing to show that Arago had investigated the subject thoroughly and, al- though he did not distinctly credit Porta with the discovery of the camera obscura, the prominence given him by one of Arago's emi- nence established him in the popular mind as the inventor of the camera obscura. As a matter of fact, Libri, a colleague of Arago, in a work on the history of the mathematical sciences in Italy, already referred to, called attention to the work of Leonardo and several others who anticipated Porta in the discovery of the camera obscura. This work, however, apparently never reached the photographic fraternity, with the result that year after year the writers of text- books in referring to Porta as the inventor of the camera obscura, firmly established the myth in the popular mind and it was not until the appearance of Dr. Eder's Geschichte der Photographie, and the work of Waterhouse, that the work of Porta's predecessors was properly brought before the photographic world.1 The Camera Obscura with Lens. — The first definite description of a camera obscura with a lens is found in a work on perspective, La Pratica della Prospectiva, by a Venetian nobleman, Daniello Barbaro, which was published at Naples in 1568. In 1585, seventeen years after the appearance of this work and four years prior to the ap- pearance of the second edition of Porta's Magia Naturalis, iri which the description of the camera obscura with lens occurs, another Venetian nobleman, Giovanni Battista Benedetti, in a book of mathe- matical and physical observations published at Turin, again refers to the camera obscura with lens. The lens used by Barbaro and Benedetti was planoconvex in form. Kepler, who took up the study of the camera about the beginning of the seventeenth century and investigated it thoroughly both theo- retically and practically, was the first to perceive the advantage of a compound objective composed of concave and convex lenses. In his Dioptrice published in 161 1, he deals with the principles of refraction, of image formation, and the properties of various forms of lenses and their combinations. He speaks of the disadvantages of the plano- convex form as regards the small field and the advantages of the use 1 For a biography of Porta and an account of his predecessors see Bull. Soc. franc. Phot., 1923, 10, 52. THE DEVELOPMENT OF PHOTOGRAPHY 5 of a concave lens with the convex. Kepler really did a great deal towards placing the optics of the camera on a firm foundation, a branch of his work which has not received the attention which it de- serves. Early Records of the Photochemical Action of Light. — The tan- ning of the skin by light is one of the many common evidences of the action of light which could hardly escape the attention of man, even in the savage state, but this action is so slow that it is not sufficiently striking to excite more than casual interest. More than 300 years before Christ, Aristotle observed that the green color of plants was due to light, for plants which had become bleached in the dark turned green again on exposure to light. The first observation on record of the action of the atmosphere on silver is by Pliny about 100 A.D., but his observations are probably only a record of the action of the at- mosphere on metallic silver. In the eighth century, however, Jabir Ibn Hayyam, often called Geber, observed the darkening of silver nitrate on exposure to atmospheric action. In 1556 Georgius Fabricus recorded the fact that crude silver chloride, or horn silver, an ore frequently found in the mines of Frieburg, darkens on exposure, and Boyle, an early English chemist, writing about 1686 speaks of the sensitiveness of gold. In 1725 a Russian field marshal prepared a remedy in which ferric chloride is used, using the action of light to reduce it to the ferrous state. That the darkening of the silver salts is due to light and not to the action of various vapors of the atmosphere was first definitely re- corded by Johann Henrich Schulze in 1727. (Fig. 2.) While ex- perimenting at an open window with a solution of chalk and aqua regia, which accidentally contained a trace of silver, he was surprised to find that the surface of the solution which was exposed to light had changed to a dark purple color, while the body of the solution re- moved from the light had not changed. Following up his observa- tions, he made a fresh solution of chalk and aqua regia, which he exposed to light under precisely the same conditions as the first so- lution. As this mixture was unaffected, he rightly concluded that the sensitiveness of the first solution had been due to the trace of silver. Cutting a stencil in opaque paper he placed the same around a bottle containing some of the mixture and exposed it to light. In this way the words or sentences were accurately and distinctly re- 6 PHOTOGRAPHY produced on the chalk sediment and the result was looked upon as a marvel by ignorant people. Fig. 2. Johann Heinrich Schulze. (From Eder's biography) While Schulze was undoubtedly the first to secure an image by the agency of light, his work, although far in advance of his time, is hardly sufficient to earn for him the title "Discoverer of Photogra- phy " such as has been given him by Eder and others. The work of Schulze was a great advance, and we are certain a great stimulus to later work along similar lines, but as he made no attempt to use the camera obscura, nor to " fix " the image which he obtained, it seems hardly fair that he should be termed " Discoverer of Photography." 2 In 1763, Dr. William Lewis published an account of his investi- gations on the cause of the discoloration of bone, ivory, wood, etc., when treated with silver nitrate and exposed to light, and in his His- tory of Discoveries Relating to Light, Vision and Color, Dr. Joseph Priestley refers to the previous work of Schulze and Lewis. The principal interest, however, which we have in this work of Priestley 2 For an interesting biography of Schulze see Eder, Johann Heinrich Schulze — Der Lebenslauf des Erfinders des Ersten Photographischen Verfahrens und des Bcgrunders der Geschichte der Medisen. Wien, 1920. A full translation of Schulze's paper describing his researches with silver salts appeared in the Photographic Journal for 1898, 38, 53. THE DEVELOPMENT OF PHOTOGRAPHY 7 is its connection with a later experimenter, Thomas Wedgwood, who without doubt was led to the subject through its pages. In 1777, Carl William Scheele noted the influence of various colors on the rate of darkening and found that blue and violet light were much more active in darkening silver nitrate than red or orange. Scheele also investigated the chemical changes involved in the darken- ing of silver chloride and discovered that the effect of light on this substance is to cause the evolution of chlorine. Plerschel in 1800 discovered the heat rays beyond the visible red, and the following year Ritter discovered, by photographic means, the existence of the very active ultra-violet beyond the visible violet. FlG, 3. Thomas Wedgwood. (Chalk drawing, author unknown) The Forerunners — Wedgwood and Davy. — Neglecting the work of Boulton in 1777, Charles in 1780, and Lord Brougham in 1795, whose claims to the previous discovery of photography are too vague to be seriously considered, we arrive at the important work of Wedg- 8 PHOTOGRAPHY wood, who made the first definite step towards the discovery of pho- tography. Thomas Wedgwood, fourth son of the great potter Josiah Wedg- wood, was born the fourteenth of May 1771. (Fig. 3.) On account of his delicate health, most of his education was conducted at home and he had as tutor a Mr. Alexander Chisolm, who had formerly acted as secretary to Dr. William Lewis and from whom Wedgwood was no doubt able to learn of the work of Schulze and the others who had preceded him. Wedgwood, together with Humphrey Davy, then a rising young chemist, repeated the work of Schulze with silver nitrate and were able to make prints of leaves, and similar objects, but were unable to prepare a paper sufficiently rapid to permit of its use in the camera obscura. Davy made some important additions to the work of Schulze. He found that silver chloride was more sensitive than the nitrate, and using the concentrated light of a solar microscope, he was able to secure images of small stationary objects on his silver chloride paper. But neither Wedgwood nor Davy were able to find a means of " fixing " the image, or dissolving the unacted-upon silver salt so as to render the image permanent. The poor health of Wedg- wood was no doubt partly responsible for this, and the^whole subject was abruptly terminated by his death at the early age of thirty-one years. After his death, a joint paper, written probably by Davy, was brought before the Royal Institution and appeared in the Journal for 1802 under the following title: An Account of a Method of Copying Paintings upon Glass, and of Making Profiles by the Agency of Light on Nitrate of Silver, by T. Wedgwood with observations by H. Davy. Davy does not appear to have paid any attention to the subject after the death of Wedgwood and no further work on photography was done in England until the researches of Talbot beginning about 1835. While the work of Talbot logically follows that of Wedgwood on account of the close similarity of the methods adopted by the two experimenters, we must first consider the important work of Niepce in France, whose researches began about 1812. The Life and Work of Joseph Nicephore Niepce. — Joseph Nice- phore Niepce (Fig. 4), the first man to obtain a permanent photo- graph, was born at Chalons-sur-Saone on March 7, 1765. His father was a man of means and Nicephore and his brother, Claude, had a tutor in language and science in which both early showed especial THE DEVELOPMENT OF PHOTOGRAPHY 9 interest. Designed for the Church by his parents, the Revolution upset his plans, and Niccphore joined the army in >792 arid served two years in Italy, when ill health compelled him to resign his commission (Courtesy of the Sociili Fran(aise de Photographic) Fig. 4. Joseph Nicephorc Nicpce and return to his country estate, where, having married, he spent the remainder of his long life in scientific pursuits, of which photography was by no means the least. His brother Claude, to whom he was devotedly attached, resided with him until 181 1, when, in order to further his scientific work, he left for Paris and finally to Kew in England. Unfortunately Niepce left no written account of his work and our only source of information is from his correspondence to his brother Claude. In 1827 Nicephore visited his brother Claude in England and brought with him some prints and a paper which he hoped to bring before the Royal Society, but having refused to make public the methods employed for making the prints, the rules of the Society compelled them to refuse the communication. The same year he met Daguerre in Paris and after overtures lasting PHOTOGRAPHY Fig. 4b. Statue to Niepce THE DEVELOPMENT OF PHOTOGRAPHY 11 two years the two investigators signed articles of partnership to con- tinue for ten years, during which time the two would work to their joint advantage. Niepce made no further advance on his process after this date and on his death in 1833, in his sixty-eighth year, his son Isidore succeeded him in the partnership. Fig. 5. A Heliographic Print by Niepce, 1824 The basis of the process worked out by Niepce was the discovery that bitumen of Judea or " Jews' pitch " becomes insoluble upon ex- posure to light. Niepce dissolved bitumen of Judea in oil of lav- ender and spread a thin layer on stone or metal plates. The sensi- tized plate was then exposed for several hours under the transparent drawing to be reproduced, after which the plate was immersed in oil of lavender which dissolved the parts unaltered by light, leaving the plate bare in these places and accurately reproducing the outlines of the drawing. By treating the metal plate with acid, an image in relief was produced from which prints could be secured in an ordi- nary printing press. One of these early prints, dating from 1826, is the Cardinal plate, illustrated in Fig-. 5. A letter of Niepce recently discovered by G. Cromer 3 shows that Niepce was successful in the use of the camera obscura as early as 1826. The letter describes what is perhaps the first permanent re- 3 Brill. Soc. franc. Phot., 1922, g, 69. 12 PHOTOGRAPHY production of a natural object by photographic means. The time of exposure required was from six to eight hours, so that, while Niepce may be said to have discovered photography, his process was of little practical value. Nevertheless we must not lose sight of the fact that, although far from perfect, his process was the first by which a permanent reproduction might be secured by photographic means and, furthermore, he was the first to successfully make use of the camera obscura, so that to Niepce belongs a large share in the discovery of photography. Fig. 6. Louis Jacques Mande Daguerre The Life and Work of Jacques Mande Daguerre. — Jacques Mande Daguerre (Fig. 6), who invented the Daguerreotype, the first practi- cal process of photography, was born at Cormeilles, a small village THE DEVELOPMENT OF PHOTOGRAPHY 13 about ten miles from Paris, on November 18, 1787. His father was court crier of the village and his mother from one of the village fam- ilies. During the Revolution his father lost his position and moved to Orleans, where the young Daguerre grew up. He was educated in the public schools of France and early showed especial aptitude for drawing ; producing, it is said, creditable portraits of his parents and friends at the early age of thirteen. At the age of sixteen, he left Orleans to begin life in Paris. There he finally secured employment with Degotti, a flourishing scene painter, and at this the young Daguerre made rapid progress, soon equalling, and finally excelling his master, so that his services were much sought after by the lead- ing theaters. During the years 1816 to 1821 he assisted Pierre Pre- vost with his panoramic paintings of the cities of Europe and during this time it is probable that he first became acquainted with the camera obscura. In the production of the large paintings required for the diorama, which he opened in Paris in either 1822 or 1823, Daguerre frequently made use of the camera obscura and it was the remarkable beauty and perfection of the image produced by this instrument that led him to attempt to find a way by which the image might be made perma- nent. His investigations seem to have begun about 1824. /Two years later he received word, probably from Chevalier, an optician from whom he had been in the habit of purchasing the apparatus necessary for his experiments, that the subject was also occupying the attention of a man in the Provinces, Joseph Nicephore Niepce. Daguerre im- mediately wrote to Niepce suggesting an exchange of secrets, but let- ters to Niepce received but curt replies until 1827 when Niepce was called to England on account of the serious illness of his brother Claude. Stopping in Paris he met Daguerre and cordial relations were established between the two investigators. On December 5, 1829, the two workers signed an agreement of partnership to continue for ten years, during which time each would work to their mutual advantage. After the death of Niepce in 1833, if not before, Daguerre discarded the method of the elder investigator and started out on different lines. In 1835 he informed Isidore Niepce, who had succeeded to his father's interest in the partnership, that he had reached a certain amount of success, and after two years more of perfecting details, a company was formed to buy out the process foi the sum of 200,000 francs. This, however, was a failure and in their extremity the two were forced to appeal to the Government. THE DEVELOPMENT OF PHOTOGRAPHY 15 Daguerre showed specimens, and placed a written account of his process in the hands of Arago, the eminent physicist and astronomer, in January 1839. Arago was impressed with the possibilities of the process and brought the matter to the attention of the Home Minister, to whom Arago's endorsement was sufficient, and on his recommenda- tion the Government awarded Daguerre a life pension of 6,000 francs yearly and to Isidore Niepce one of 4,000, on the condition that the in- vention be published without patent, this money being paid by France " for the glory of endowing the world of science and of art with one of the most surprising discoveries that honor their native land," to quote the official document. The stipulations were agreed to and in August of the same year (1839) the details of the process were made public before the Academie des Sciences. Interest in the process spread rapidly and the inventor made a small fortune in the sale of apparatus for the process. Daguerre died at ' Petit-sur-Marne in 185 1 at the age of sixty-three, having lived to see the science take a large and important place in the affairs of the world. (Fig. 7.) The Daguerreotype Process. — The Daguerreotype process occu- pies such an important place in the history of photography that an extended description of the same will not be out of place. A silver plate, or copper plate covered with silver, is rubbed with tripoli and olive oil and polished with rouge and cotton wool to ob- tain a highly polished, perfectly smooth, faultless surface. This polished plate is placed with its polished side down in a fuming Fig. 8. Fuming Cabinet for the Daguerreotype Process cabinet (illustrated in Fig. 8) on the bottom of which is a thin layer of iodine crystals. As the iodine evaporates the vapors come in con- tact with the silver and form silver iodide. After passing through 16 PHOTOGRAPHY several successive changes of color, the surface of the silver plate becomes blue and when this stage is reached the plate is removed, placed in the holder, and exposed. Exposure with such a plate for three to four hours produces only a faint impression of the silver iodide and if it had not been for the accidental discovery of the latent image, and the possibility of developing the image by chemical means, Daguerre would have fared no better that his predecessors. For- tunately, however, Daguerre discovered that mercury had the power of bringing out the visible image, so that the exposure necessary was shortened to three or four minutes. The discovery of the latent image, and the possibility of developing the same, was the greatest step towards the realization of practical photography made by Daguerre and one which must forever entitle him to a high place among those who have contributed to the advancement of the science. Development is conducted in a developing box (illustrated in Fig. Fig. 9. Developing Box for the Daguerreotype Process 9). The heat of the spirit lamp under the dish of mercury causes the mercury to condense on those parts of the image affected by ex- posure to light and the image gradually develops as more and more mercury is deposited until a complete reproduction of the original is obtained. For fixing, Daguerre at first used common salt, but soon after the publication of the process Herschel called attention to the use of " hypo " which was immediately adopted.4 4 The chemical reactions involved in the Daguerreotype process cannot be said to be fully understood even at the present time. For a very complete discus- sion of the same see Waterhouse, " Lessons from the Daguerreotype," Photo. /., 1899, 39, 60, and 1898, 38, 45. THE DEVELOPMENT OF PHOTOGRAPHY 17 Later History of the Daguerreotype. — Daguerre's early plates re- quired an exposure of three to four minutes but the following year (1840) a London science lecturer, Mr. Goddard, discovered the fact that a combination of bromine and iodine was much more sensitive than either alone and a great increase in rapidity was secured, while the time of exposure was reduced by the introduction of the really wonderful Petzval portrait lens by Voigtlander the following year. The first attempts at portraiture appear to have been made in America. The claims of Dr. J. W. Draper and Robert Cornelius of Philadelphia to bave made the first human portrait by photography have been reviewed by L. T. W. in the British Journal of Photog- raphy. It appears that the portrait of his sister was made by Dr. Draper on March 31, 1840, while Robert Cornelius opened a studio for the Daguerreotype process in Philadelphia on February 18, 1840. As Cornelius must certainly have made experiments before opening a studio professionally it appears that he, and not Draper, was the first to make a portrait by the Daguerreotype process. Draper's por- Fig. 10. An Early Daguerreotype Portrait Often stated to be the first portrait by photography trait, reproduced in Fig. 10, is the first Daguerreotype portrait which is now in existence, none of the results of Cornelius being in existence, so far as known.5 6 Brit. J. Phot., 1920, 67, p. 420. 18 PHOTOGRAPHY The Daguerreotype process lasted only about ten years or until the discovery of the wet collodion process by Scott-Archer in 1851. It was almost entirely a portrait process and was not used for landscape and other exterior work to any extent. The Positive Process of Bayard. — Bayard, one of the founders of the Societe Francaise de Photographie, demands a few lines for the positive process which he worked out prior to the announcement of the Daguerreotype. On June 24, 1839, two months before the details of the Daguerreotype process were made public, Bayard exhibited a collection of silver prints made by a method entirely different from that employed by any of the early workers. His process produced a positive print direct and without development. Paper was soaked in 'a solution of ammonium chloride, dried, floated on silver nitrate and after drying in the dark it was exposed to daylight until com- pletely darkened. Before exposure, it was placed in a solution of potassium iodide and exposed while wet in the camera. The action of light bleached the paper, producing a positive result which was washed and fixed in potassium bromide. The prints were permanent and some are said to be in existence at the present day. The Life and Early Work of William Henry Fox-Talbot.— Wil- liam Henry Fox-Talbot (Fig. 11) was born February 11, 1800, at the ancestral home of the Talbots, Lacock Abbey, in Wiltshire. He was of an old and well-established family, the Talbots ranking among the oldest families in England, while his mother was a daughter of the Earl of Ilchester. He was educated at Harrow and Cambridge, leaving the University in 1821 with highest honors, and for two years was a member of parliament, but politics did not interest him and he retired in 1835 to devote the remainder of his life to science. Talbot was a versatile experimentalist; his earlier years were devoted to photography, but in later years he wrote on a wide range of subjects, as spectrum analysis, inscriptions in Egypt, the optical phenomena of crystals and integral calculus, while apparatus in the memorial collection at the Royal Photographic Society speak of his interest in electrical and physical science. Talbot relates in his Pencil of Nature, published in London 1844, that in 1833 he was sketching on the shores of Lake Como in Italy with the camera obscura, but without much success owing to his lack of knowledge of drawing. On his return to England in January the year following he determined to follow up the work of Schulze and THE DEVELOPMENT OF PHOTOGRAPHY 19 Wedgwood on the action of light on silver salts. His first experi- ments with silver nitrate and silver chloride were unsuccessful, as the paper was not sufficiently sensitive to light. As a result of many trials, Talhot found that a far greater degree of sensitiveness was obtained with silver chloride if a weak solution of salt was employed, Fig. II. William Henry Fox-Talbot producing what he termed an " imperfect " chloride, which was very much more sensitive to light. Using paper prepared in this manner he was able to readily obtain prints of tracings, leaves, etc., as had Wedgwood and Davy before him and with whose work he was fa- miliar. Talbot, however, was successful where earlier investigators had failed ; he found that a solution of common salt would dissolve the unacted-upon salts and render the image permanent. In 1835 ne found that the sensitiveness of his paper was greatly increased by giving it successive washings in salt and silver and exposing it while still wet. With paper so prepared he made a picture of his home, Lacock Abbey, using the camera obscura during this same year (1835). 20 PHOTOGRAPHY The details of Talbot's process, which the inventor styled " Photo- genic Drawing," were first made public in a communication to the Royal Institution by Faraday on January 25, 1839, and a week later (January 31, 1839) Talbot himself read a paper on the subject before the Royal Society, of which he was a member. This was Talbot's first paper, and it will be observed that it was published almost eight months before the details of the Daguerreotype process were made public. The Calotype Process. — The principal work of Talbot, however, was the Calotype process invented by him in 1840. In this process silver iodide was used, the paper being impregnated with silver iodide and immediately before exposure was washed over with a mixture of gallic acid and silver nitrate. After an exposure of about a minute, the image was developed in gallic acid and silver nitrate. Talbot had at last grasped the idea of a developer for bringing out the latent image. After fixing and drying, the paper negative was placed over a similar sheet of sensitized paper and exposed to obtain the positive proof. The process was fully described by Talbot before the Royal Society on June 10, 1841. It is practically certain that in the use of gallic acid to increase the sensitiveness of his paper, and also as a developer, Talbot had been anticipated by an English clergyman, Rev. J. B. Reade, but as the latter did not publish an account of his work, Talbot's discovery was independent and original with him. Miscellaneous Paper Processes. — The work of Talbot was soon estimated at its true value by a French investigator, Blanquart-Evard. A process devised by him was practically identical with that of Talbot except for the employment of silver chloride instead of iodide. Nu- merous other processes were invented by various workers, none of which are of more than historical interest, and these we will merely mention, referring those who may desire further information to Robert Hunt's Treatise on Photography published in London 1854. Ainphitype — invented by Sir John Herschel. Anthotype — invented by Sir John Herschel. Catalysotype — invented by a Dr. Wood. Chromatype — process using chromic acid. Chrysotype — invented by Sir John Herschel. Energiatype — invented by Robert Hunt. Fluorotype — so called from use of salts of fluoric acid invented by Robert Hunt. THE DEVELOPMENT OF PHOTOGRAPHY 21 The Introduction of Glass. — Paper is not an ideal medium for negatives owing to its relatively coarse grain, which destroys fine de- tail, and its opacity, which makes printing slow. To overcome these difficulties Sir John Herschel early attempted to substitute glass, but his process was unsuccessful because he did not recognize that images of sufficient opacity can be obtained only in the presence of albumen, gelatine, or some similar substance, which is capable of attracting and combining with the silver salts. The need of such a substance was recognized by Niepce de Saint Victor, a nephew of Nicephore, who was responsible in 1847 tor a method in which albumen was used. The white of an egg was beaten up with potassium iodide and com- mon salt and the clear liquid poured over a glass plate and allowed to dry. In this state the plates could be kept for some time. Im- mediately before exposure the plate was dipped in a bath of silver nitrate, which caused the formation of a sensitive silver-chloro-iodide within the pores of the albumen. The plate was exposed either wet or dry and developed in gallic acid. Although no gain in rapidity was made by the albumen process, the results were much clearer and the negatives printed more rapidly, so that the process was immensely popular .until the introduction of collodion. Scott-Archer and the Introduction of Collodion. — In 1847 Seh6n- bein and Bottcher discovered gun cotton and the following year Maynard of Boston showed that the same might be dissolved in a mixture of alcohol and ether to produce a substance of a viscid na- ture which is termed collodion. In 1849 Le Gray, a French investi- gator, suggested the use of collodion in photography and in a book published in 1850 Robert Bingham, assistant to Faraday, suggests the use of collodion in place of albumen, but the credit for the invention and publication of a workable process employing collodion is due to Frederick Scott-Archer. (Fig. 12.) The inventor of the collodion process was born at Stortford in 1813 and in early life became a sculptor. He took up the Calotype process in 1847, it is said, for the purpose of making, records of his work. We do not know just when he began experimenting with col- lodion but in 1850 his collodion process was so far advanced that he described it to a few friends, from whom he received some assistance, and the following year the details of the process were published in The Chemist for March 1851. Archer appears not to have recog- 22 PHOTOGRAPHY nized the value of the process, for he did not patent it, but so com- plete and perfect was his process that it at once took the place of all other processes, remaining supreme in the field for almost thirty years, and is even to-day unsurpassed for certain branches of work. Fro. 12. Frederick Scott-Archer. Drawing from an old print reproduced in J. Werge — The Evolution of Photography Archer was a fertile inventor and made several minor additions to photographic processes which we do not have the space to record, but was a poor business man, and upon his death in May 1857 m practically a state of poverty, the sum of 747 pounds was raised by subscription among friends, and shortly afterwards, Mrs. Archer having passed away, the Government granted the children a pension of fifty pounds a year as "their father was the discoverer of a sci- entific process of great value to the nation from which he had reaped little or no benefit." THE DEVELOPMENT OF PHOTOGRAPHY 23 The Collodion Process. — The following is an outline of the col- lodion process : 6 1. Prepare pyroxyline by immersing cotton wool in equal parts of nitric and sulphuric acids for fifteen seconds, after which wash thoroughly in water. * 2. Dissolve the pyroxyline in a mixture of equal parts of sulphuric ether and absolute alcohol to obtain collodion. 3. Add some soluble iodide, and also a little potassiunjj.bromide. 4. Pour on a clean glass plate and allow to set. 5. Take the coated plate into the darkroom and immerse in a bath of silver nitrate (thirty grains to the ounce of water) for a minute. Here a chemical change takes place resulting in the formation of a sensitive silver-bromo-iodide in the pores of the collodion. 6. Place plate in holder and expose. 7. Take plate back to darkroom and develop by pouring over it a solution of water, acetic acid, and pyrogallic acid. 8. Fix by immersion in a bath of sodium thio-sulphate ("hypo"). After the introduction of collodion, photography for the first time became really popular. Out of this newborn interest in the subject arose several institutions which were to stand until the present time and to exercise a favorable influence on the further developments of the science. The Royal Photographic Society was founded in 1853 as the Photographic Society of London, and the following year the Societe Francaise de Photographie was organized at Paris. In 1854 the well-known British Journal of Photography was established as a monthly, becoming a weekly in 1859, while the year previous had wit- nessed the birth of the Photographic News. Inconveniences of the Collodion Process. — While a notable ad- vance upon all previous processes, the collodion process was subject to several grave objections. It is absolutely necessary that the plates be exposed and developed as quickly as possible after their prepara- tion before the surface has had time to dry. For this reason the wet plate process, while well adapted to the studio, is not so suitable for landscape work, or for general amateur use. A heavy equipment had to be carried in the form of a tent, sensitizing bath, developing trays, fixing and developing solutions, and a plentiful supply of pure 6 For a full description and formula see : Wet Collodion Photography, by C. W. Gamble; The Wet Collodion Process, Arthur Payne. 24 PHOTOGRAPHY water. Some idea of the inconveniences of outdoor photography with the collodion process may be had from Fig. 13 and Fig. 14; the former shows the photographer " en route " with his outfit and the latter the outfit in use in the field. Sometimes the outfit was arranged to be car- ried on a cart drawn by a donkey, an example of which is in the museum of the Royal Photographic Society of Great Britain. Fur- thermore if the exposure was a long one, as might easily be the case with interiors where exposures run to several hours, the surface of the plate dried and the picture was spoiled. Lastly in cold weather the sensitizing bath, solutions, water supply and plates would freeze, so that photography in winter, or in cold climates, was well nigh im- possible. Despite these obvious drawbacks, some of the work of this period ranks with the best that photography has produced. The work of Rejlander, the portraits of Solomon, Mrs. Cameron, and much of the famous work of H. P. Robinson were all done with collodion, and will ever remain as notable tributes to the enthusiasm and energy of these untiring workers. Fig. 13. The Wet Collodion Process in the Field (From an old manual) THE DEVELOPMENT OF PHOTOGRAPHY 25 Modifications of the Collodion Process. — To overcome the defects of the collodion process John Spiller and William Crookes in 1854 7 proposed the use of a deliquescent salt, such as magnesium nitrate, to keep the collodion moist and allow it to be kept several hours be- fore use. The same year George Shadbolt and Maxwell-Lyte advised the use of honey and grape sugar to prevent evaporation. The most successful method, however, was the collodio-albumen process devised Fig. 14. Portable Laboratory for Field Use with Wet Collodion by Taupenot in 1855. 8 In this process the plate after having been coated with iodized collodion in the usual manner was then flowed with albumen and allowed to dry, when it was immersed in a bath of silver nitrate, washed and dried. The plates so prepared were very slow, about six times slower than ordinary collodion, but would keep well and were rather extensively used by landscape workers. In 1855 Dr. Hill Norris of Birmingham described a process 9 in which the plates were first washed in water and then immersed in pyrogallic acid, after which they were dried and kept until wanted. The following year he took out a patent for a collodio-gelatine process, the sensitive collodion plates being covered with a solution of gelatine in order to prevent its condensation on drying and to keep in a sensi- tive state. Dry plates so prepared were placed on the market and large numbers were sold between 1855-1866. Among other processes having as their object the production of dry 7 Philosophical Magazine. 8 La Lumiere, Sept. 8, 1855. sJour. Phot. Soc. of London (R. P. S.), May 1855. 26 PHOTOGRAPHY plates we may mention the tannin process of Major Russel, intro- duced in 1861 ; the albumen-beer process of Capt. Abney, 1874; the resin process of the Abbe Desprats and the oxymel process of Llewelyn. The Introduction of Collodion Emulsion. — The term emulsion is properly applied to a liquid holding another immersable liquid in sus- pension in an extremely fine state of division. In photography the term is a misnomer for a mechanical suspension of a solid sensitive salt of silver in a finely divided state in a substance such as collodion, albumen or gelatine. While Gaudin, and Dixon and Fry 10 had met with some success in the preparation of a workable collodion emulsion, it remained for Sayce and Bolton to work out the first satisfactory method for the preparation of a suitable collodion emulsion for photo-' graphic purposes in 1864.11 These workers added nitrate of silver to a bromized collodion thus producing a sensitive bromo-silver collodion. Plates coated with this emulsion were flowed over with tannin and dried. Later improvements consisted in increasing the amount of silver and the addition of tannin directly to the emulsion. Many others added suggestions of importance, among whom may be men- tioned Carey Lea, Col. Stuart Wortley, George Dawson, Thos. Sutton and W. J. Stillman. For several years after the introduction of collodion emulsion the excess silver salts were removed by washing the plates after coating. In 1871 Sutton suggested the use of a " corrected " emulsion in which the proportions of bromide and silver were so adjusted as to leave neither in excess, but because of the practical difficulties in determin- ing the proper proportions the method is not satisfactory. In 1874, Bolton showed that the emulsion might be washed before the plates were coated 12 and the following year Rev. Canon Beechey described 13 a similar method using pyrogallic acid as a preservative. This method was perhaps the most reliable and uniform method of preparing collodion dry plates, and plates so prepared became an article of commerce. While not so rapid as ordinary wet collodion, the Beechey plates were sufficiently rapid for exterior work, requiring an exposure of from 30 to 60 seconds with a diaphragm equivalent to F/16. 10 La Lumiere, Aug. 1853; Photo. News, 1861, p. 193. 11 Brit. J. Phot., September 9, 1864. 12 Brit. J. Phot., Jan. 16, 1874. 18 Brit. J. Phot., Oct. 1, 1875. Harrison, History and Handbook of Photog- raphy, Appendix. THE DEVELOPMENT OF PHOTOGRAPHY 27 The Introduction of Gelatino-Bromide Emulsion. — The first record of the application of gelatine to photography was the unsuccessful at- tempt of Niepce de Saint Victor in 1847 as a vehicle for holding silver iodide on glass plates. In 1853, Gaudin gave a formula for what we would now term gelatino-iodide emulsion but his method was not practical. His exper- iments, however, led him to recognize the fact that bromide of silver is more sensitive in combination with gelatine than iodide of silver. The use of gelatine as a preservative of wet collodion by Norris in 1856 we have already noticed under collodion emulsion. Fig. 15. Dr. Richard Leach Maddo In 1868, W. H. Harrison 14 published the results of his experiments on the emulsification of silver bromide in gelatine but his method was of no practical value, the principal significance of his work being the use of an alkaline developer. While Le Gray, Smith, Harrison and Sutton had either suggested uBHt. J. Phot., Jan. 17, 1868. PHOTOGRAPHY the use of, or had experimented with gelatine, it is to Dr. R. L. Mad- dox (Fig. 15), an English amateur, that we owe the first really work- able method of preparing gelatino-bromide emulsions. His method was fully described in an article in the British Journal of Photography for September 8, 1871, under the following title: "An Experiment with Gelatino-Bromide." The introduction of gelatine pointed the way to plates of a higher degree of sensitiveness than had been pos- sible with collodion, so that while the process of Dr. Maddox was not revolutionary and complete in itself as was that of Scott-Archer, it marks an epoch in the development of photography. In the method described by Dr. Maddox, silver bromide was formed in the presence of gelatine, the emulsion containing an excess of silver and a small amount of aqua regia. Without further treatment the emulsion was coated on glass plates, dried, and exposed. Develop- ment was conducted with pyrogallic acid and intensification with pyro and silver nitrate followed. With our present knowledge it is not hard to see why Dr. Maddox did not meet with complete success. He does not seem to have real- ized the necessity for washing the emulsion so as to remove the excess silver salts, although this was regularly done with collodion emulsion processes. Consequently the presence of the excess salts of silver and the nitric acid from the aqua regia, acted as a restrainer and made the plates very slow. It is noteworthy that Maddox had some idea of the ripening processes which have meant so much to the development of the gelatino-bromide process, as he tried to increase the sensitiveness of his plates by fuming with ammonia (a method that had been pre- viously applied to albumen paper) but without success. Very little attention was paid to the work of Maddox at the time, but two years later Burgess advertised a gelatino-bromide emulsion in the English photographic journals.15 The method employed by Mr. Burgess in the preparation of his emulsion was never published and did not prove to be a commercial success, but he was the first to show that excellent results could be obtained on gelatine, and that gelatino-bromide emulsion could be produced which was equal in sensitiveness to wet collodion. Improvements in the Gelatino-Bromide Process. — The same year that Burgess introduced his emulsion commercially, King and John- son described independently of each other two methods for removing 15 July 18, 1873. THE DEVELOPMENT OF PHOTOGRAPHY 29 the excess of silver salts from the emulsion. King's method con- sisted in placing the emulsion in a container of vegetable parchment or bladder ; the whole being immersed in a large vessel of water, un- der which circumstances the soluble salts pass outwards through the parchment into the water. Johnson's process, described in the same issue of the British Journal of Photography,1* advised the use of an excess of soluble bromide — a point of great importance — and plain washing of the shredded emulsion in running water to eliminate the excess salts. On account of its simplicity and effectiveness this method has been generally adopted. In November of the same year Richard Kennett, an amateur re- siding in London, took out a patent 17 for a method which he discov- ered of preserving the emulsion and the following month announced his " sensitive pellicle " which was nothing more than a dried, sensi- tive gelatino-bromide emulsion. The pellicle was quite successful and remained on the market for about ten years. In 1874 Bolton suggested that only a small part of the gelatine be used for preparing the emulsion, the rest being added afterwards — a procedure which later proved of great value when the effect of heat on the emulsion was discovered. The same year Stas observed that several forms of silver bromide are possible and that heating formed the most sensitive compound.18 The same year (1874) gelatine plates first appeared on the market, manufactured by the Liverpool Dry Plate Co. Bromide paper appeared at the same time. In 1878 Bennett showed that the sensitiveness of gelatino-bromide emulsion might be greatly increased by keeping the emulsion at a temperature of 90 degrees Fahr. for five to seven days.19 This added a great impetus to the development of gelatino-bromide emulsion and another firm of dry plate makers took the field — Messrs. Wratten and Wainwright. The prolonged stewing of the emulsion at 90 degrees, however, was not only troublesome but led to trouble owing to the partial decom- position of the gelatine, so Mansfield announced in 1879 20 that this long and troublesome process could be avoided by forming the bro- mide of silver in a weak solution of gelatine which was then boiled 16 November 14, 1873. 17 B. P. No. 3782 of November 20, 1873. 18 Annates de Chimie, Fifth Series, vol. Ill, p. 289. 19 Brit. J. Phot., March 29, 1879. 20 Brit. J. Phot., August 22, 1879. 30 PHOTOGRAPHY for ten to fifteen minutes, the remainder of the gelatine being added when the solution had cooled. Emulsification in a portion of the gelatine, the remainder being added after digestion, was a repetition of the advice given by Bolton in 1874. In May 1879 Captain Abney showed that a good emulsion might be formed by precipitating silver bromide in glycerine, the precipitate after two or three washings with water being mixed with the proper amount of gelatine to form the emulsion. The object of this method was to save the trouble of washing the emulsion in order to eliminate the excess silver salts as required in the usual process. In 1877 Johnson described the use of an aqueous solution of am- monia for the ripening of gelatino-bromide emulsion.21 Not much at- tention seems to have been paid to this communication, however, and it was not until Monkhoven in 1879 22 suggested that the increased sensitivity of the emulsion produced by prolonged heating might be due to a change in the molecular state of the silver bromide along the lines of the work of Stas and showed that silver bromide might be changed from the ordinary to the most sensitive green state by treat- ment with ammonia that much interest was taken in the subject. The following year Eder investigated the matter very thoroughly and per- fected a process using ammoniacal silver oxide and later discovered the advantageous influence of ammonia and ammonium carbonate on the ripening of gelatino-bromide emulsion in the cold.23 The follow- ing year Abney showed the advantage to be gained from the use of a small amount of iodide in gelatine emulsions. The addition of iodide at first reduced the speed of the emulsion to a certain extent, but gave clearer negatives having greater density. A small percentage of iodide is used in nearly all modern plates. In the meantime, the spread of gelatino-bromide emulsion had been exceedingly rapid and by 1882 gelatine emulsion had almost com- pletely displaced collodion, except for some few specialized purposes. The Introduction of Film. — The introduction of the dry plate made unnecessary the dark tent, the nitrate of silver bath, and other inconveniences of the wet-collodion process, and resulted in a large in- crease in the number of amateurs. The next step in the simplification and consequently the popularization of photography was the introduc- tion in 1884 of the stripping film of Eastman and Walker of Rochester, 21 Brit. J. Phot. Almanac, 1877, p. 95. 22 Brit. J. Phot., October 17, 1879. 23 Sitsungsber. Akad. Wiss. Wien, 1880, 81, 679. THE DEVELOPMENT OF PHOTOGRAPHY 31 N. Y. This was a gelatino-bromide emulsion coated on paper in such a way that after development and fixing, the image could be stripped from the paper support and dried on glass. This film was supplied in rolls and a roll holder was provided to adapt it to the plate cameras then in common use. From the standpoint of the general public, this was a great advance but the process was still intricate and difficult for the amateur on account of the delicate handling demanded in trans- ferring the image from paper to the final glass support. Four years later the stripping film with its paper base gave way to a transparent base of nitrocellulose. The discovery of a new base of the sensitive emulsion equal in every way to glass and with the added advantages of being flexible, light and unbreakable combined with the introduction of the daylight loading roll film camera (1891) brought photography within the reach of all. Development of Printing Processes with Silver Salts. — The de- velopment of positive printing processes begins with Fox-Talbot's Calotype process in 1841, although it was not until after the inven- tion of the collodion process that much progress was made in this line. For printing from his early negatives, Fox-Talbot employed what we to-day term the salted paper process. Paper of suitable surface and texture is immersed in a weak solution of salt, after which* it is dried, and in this state it may be kept indefinitely. Just before use, it is sensitized in silver nitrate and dried, after which it is exposed to daylight under the negative to be reproduced. When the image is sufficiently dark, the print is removed and toned in a solution of gold chloride which is followed by fixing in a bath of " hypo." Talbot's early prints, however, were not toned, as gold toning does not seem to date back further than 1849. We will have more to say regarding plain salted paper later on as it is to some extent in use at the present time. Le Gray appears to have been the first to suggest coating the paper with albumen before sensitizing in order to obtain a higher gloss, al- though Fox-Talbot is often credited with the same. Albumen paper was quite popular and practically the only paper used from 1860- 1885. A method of using collodion in place of albumen was described by G. Wharton Simpson in 1864 24 and collodio-chloride of silver papers 24 Photographic Yearbook, 1865, p. 63. 32 PHOTOGRAPHY were introduced commercially by Obernetter of Munich two years later, but the process did not attract much attention until the introduc- tion of a much improved product by Liesegang of Diisseldorf in 1886. Gelatino-chloride papers appear to have been first employed by Palmer and Smith as early as 1866, but no details were published. Full details for the preparation and use of gelatino-chloride papers were published by Abney 25 in 1882 and, papers of this type were in- troduced commercially two years later by Obernetter of Munich and a few years later by Liesegang of Diisseldorf and Uford of London. The above papers are all members of the class known as printing- out-papers; that is, they produce a visible image upon exposure and there is no after development. The now popular developing papers appear to have had their prototype in a process used by Blanquart- Evrard in 1851, but it was not until 1874, after paper coated with gelatino-bromide emulsion had been introduced by the Liverpool Dry Plate Company, that developing papers made any headway. In 1880 Morgan and Kidd established a factory at Richmond (England) and in 1885 Eastman of Rochester introduced the machine coating of paper in the roll with gelatino-bromide emulsion after which gelatino-bromide paper began to assume importance as a printing process. Directions for the preparation and use of emulsions of silver chloride for positive printing were published by Eder and Pizzighelli in i88i.2e The first papers of this type were introduced commercially by Dr. E. Just of Vienna in 1883 and shortly afterward in England by Edwards and Warnerke but " Velox " introduced in American by the Nepera- Chemical Company from the formula of Dr. Leo Baekeland was the first to achieve wide popularity. Its successful introduction paved the way for a large number of similar papers in all important countries. The use of gelatino-chloro-bromide emulsions for positive printing was introduced also by Eder and Pizzighelli in 1883.27 Chloro-bromide emulsions are especially adapted to the production of warm-tone images and papers of this class are widely employed for portrait work at the present time. Many lantern slide plates and positive cinematograph films are also coated with emulsions of this type. Platinum Printing Processes. — In 1832 Herschel discovered that 25 Phot. News, 1882, 24, 300. > 26 Die Photographie mit Chlorsilbergelatine mit Chemischer Entivicklung , Vienna, 1881. 27 Phot. News, 1883, 25, 98. THE DEVELOPMENT OF PHOTOGRAPHY 33 light had a reducing action on platinum compounds, especially in the presence of an organic salt such as ferrous oxalate. Hunt in 1854 tried to turn this to account by coating paper with ferric oxalate and platinic chloride, but he failed to realize an essential point, the two salts must be in solution before the reaction can take place. It is to William Willis Jr. that we owe the platinum process in its present form. He took out his first patent in 1873, a second in 1878 and the last in 1880. Under the last patent, paper is coated with a mixture of potassium chloroplatinite and ferric oxalate. This is exposed under the negative until the image is sufficiently printed when it is removed and placed in a solution of potassium oxalate, in which the reduced iron salt is soluble, and as it is dissolved by the oxalate it attacks the platinum compound and reduces it to the metallic state. After im- mersion in several baths of hydrochloric acid to remove the iron salts* which remain, the print is washed and dried. Sensitiveness of Chromic Compounds and Bichromated Colloids.— The first to observe the sensitiveness of chromium compounds to light appears to have been Mungo Ponton, an Englishman, who in 1839 discovered that paper soaked in a bichromate and dried was sensitive to light. The following year Becquerel discovered that when the paper was sized with starch it became more sensitive and he decided that the sensitiveness of the chromium compound was due to the presence of organic substances used for sizing the paper. In 1852 Fox-Talbot found that when bichromate is mixed with gelatine and exposed to light the gelatine is rendered insoluble. In 1855 Alphonse Poitevin discovered that if a colored substance be added to gelatine, sensitized with potassium bichromate, and exposed to light under a negative, the unaffected parts might be washed away leaving an image formed by the colored substance held in the insoluble gela- tine formed as a result of the action of light. This was the founda- tion of the carbon and gum-bichromate processes. Poitevin also dis- covered that a bichromated gelatine film when exposed to light and allowed to swell in water would take a greasy ink on the exposed portions but not on the unexposed portions. From this he developed a process of photomechanical printing known as collotype and at a later date the processes of oil and bromoil, based on the same prin- ciple, were brought out by others. Poitevin may thus be termed the father of printing processes employing bichromated colloids. The Development of the Carbon and Gum-bichromate Process. — 34 PHOTOGRAPHY In 1858 John Pouncy of Dorchester, England, was granted a patent for a carbon process based upon the same principles as that of Poitevin. His method consisted in brushing over paper a mixture of bichromatized gelatine and carbon : the paper after drying being ex- posed under the negative and developed in water. His results were far from satisfactory, however, because the half tones were lacking. The same year the Abbe Laborde showed the reason for this saying : " In the sensitive film, however thin it may be, two distinct surfaces must be recognized, an outer and an inner which is in contact with the paper. The action of the light commences on the outer surface. In the washing, therefore, the half tones loose their hold on the paper and are washed away." The same year J. C. Burnett, Blair and Schouwaloff to overcome * this defect suggested the expedient of exposing from the back of the paper, but in i860 Fargier in France showed that the best way was to coat the exposed film with collodion, then transfer it to glass and then wash away the soluble gelatine from the back. This method, however, was too complicated for general use. In 1864 J. W. Swan patented carbon tissue, which is simply paper coated with gelatine and pigment, which, after sensitizing in bichro- mate, is exposed under the negative and transferred before develop- ment to another support. The tissue backing is then stripped off leaving the pigmented gelatine on the new support. Development is effected by washing away the soluble gelatine in water. This was the first really practical process of pigment printing in which the pig- ment is incorporated with the bichromated colloid before exposure. The gum-bichromate process, now so popular among pictorialists of a certain class, is nothing more than Pouncy's carbon process which he described bef ore a meeting of the Photographic Society of London in 1858. It was brought to the front about 1895, largely as the re- sult of the work of Robert Demachy, Ch. Puyo and other French pictorialists. Abroad, it passed out upon the advent of the oil and bromoil processes but in America it has held its ground and is still popular in many quarters. In 1873 Marion 28 found that a sheet of paper immersed in bichro- mate and exposed to light so as to produce a faint image will transfer its image to a sheet of pigmented carbon tissue when the two are placed in contact with one another. This is due to the fact that there is left in the image some chromate of chromium, the salt formed as 28 Brit. J. Phot, 1873, p. 342. THE DEVELOPMENT OF PHOTOGRAPHY 35 a result of the action of light on the bichromate, and this diffuses into the pigmented tissue and renders it insoluble just as if it had been exposed to light. Manly in 1899 introduced 29 a process of pigment printing based upon this principle under the name of ozotype. This failed to catch on, however, and in 1905 the same worker introduced 30 his ozobrome process. In this a sheet of ordinary carbon tissue was soaked in a solution containing potassium bichromate, potassium fer- ricyanide and potassium bromide. It was then placed in contact with an ordinary bromide print which had been previously soaked in water to become limp. After being kept under pressure for several minutes the carbon tissue was stripped from the bromide print, squeezed to its final sup- port and developed as usual. The bromide print thus takes the place of the negative, so that an enlarged negative is not required when prints larger than the original negative are desired, nor is daylight necessary at any stage. Ozobrome for a while was quite popular but finally fell into disuse. It was revived, however, in an improved form by H. F. Farmer in 1919 under the name carbro and in its improved form has become so popular that it is again bringing carbon printing back to the attention of amateurs and professionals and may eventually supersede carbon, except where critical definition is required. The Development of the Oil and Bromoil and Powder Processes.— A second process was worked out and patented by Poitevin in which a bichromated gelatine film without pigment was exposed under a negative. This gelatine film upon exposure to light under the nega- tive became more or less insoluble in various portions according to the gradations of the negative. When immersed in water, the soluble gelatine absorbs water and becomes so charged with water that it will repel a greasy ink, while the shadows, being insoluble, do not ab- sorb water and will accept the ink. Accordingly when a roller charged with greasy ink is passed over the paint, an image is formed in greasy ink which adheres to the shadows but not to the highlights of the print. This process was the forerunner of a number of photo- mechanical processes, which are beyond the scope of this work, and the oil, bromoil and powder processes. Poitevin's patent of 1855 is not very explicit, but in 1858 Asser was granted a patent for a process based upon the same principle and in which very precise directions were given. 29 British Patent No. 10,026/1899. 30 British Patent 17,007/1905. 36 PHOTOGRAPHY Two years previously the Due de Luynes through the Societe Fran- coise de Photographie had offered a prize of 10,000 francs to the per- son discovering a process by which absolutely permanent prints might be produced. The President of the Societe, M. Regnault, the fa- mous chemist, in announcing the offer called attention to the perma- nency of carbon and suggested that experiments be conducted with a view to obtaining prints in carbon. Two Frenchmen, Garnier and Salmon, starting from Poitevin's patent of 1895 worked out a process in which the bichromated gelatine was exposed to light under the negative, then soaked in water and pure finely divided carbon dusted over it. The carbon adheres only to the unexposed parts and in this way an image is secured. This was the beginning of the so-called powder processes. Rawlings' process of oil printing (1904) is actually little more than a modification of the process covered by Poitevin's patent of 1855. Rawlings advised the use of brushes rather than a roller for applying the ink, thus making possible the control of the various tones of the print by varying the amount of ink deposited. This feature served to attract various workers who wished to have a ready means of alter- ing the tone values of their prints and the process rapidly gained in popularity among pictorialists. In 1889 Howard Farmer found that when a gelatine film contain- ing finely divided silver, as in a negative or positive, is immersed in a bichromate, the gelatine in contact with the metallic silver is rendered insoluble exactly as though it had been exposed to light in these por- tions. Upon this property of a bichromated colloid is based the bromoil process The first suggestion of the rationale of this process is due to E. J. Wall.31 It was taken up and worked out practically by C. Welborne Piper.32 Conclusion. — With this our history of the development of photog- raphy must be brought to a close. Many are the names and processes which we have been compelled to barely mention and not a few have been omitted altogether, while all have been treated in outline only, so that only a general idea of their essentials has been gained. It is hoped, however, that this short account has been of sufficient interest to encourage the student to follow up the subject by outside reading in larger and more comprehensive works and to assist in this worthy end a short list of the leading historical works on photography is appended. 31 Phot. News, 1907, 51, 299. 32 Phot. News, Aug. 16, 1907. THE DEVELOPMENT OF PHOTOGRAPHY General Reference Works Brothers — A Manual of Photography. London, 1899. Brown — Who Discovered Photography? Photo-Miniature, No. 60. 1903. Colsen — Memoires des Createurs des Photographic Paris. Eder — Geschichte der Photographic Halle a/S. Eder — Johann Heinrich Schulze. 1917. Fouque — Sur l'lnvention de la Photographic Paris, 1867. Harrison — The History of Photography. London and New York, 1885. Litchfield — Tom Wedgwood, The First Photographer. London. Potonniee — Histoire de la Decouverte de la Photographie. Paris, 1925. Schiendel — Geschichte der Photographie. Shepperley — A History of Photography. London, 1929. Tissandier — History and Handbook of Photography. London, 1874. Werge — The Evolution of Photography. CHAPTER II THE CAMERA AND DARKROOM I. The Camera The Box Camera. — The simplest and the cheapest possible camera which you can purchase is one of the box form. Such cameras are cheap because they, are made in large quantities by machinery and because they do not have the capabilities and adjustments of more expensive models. Naturally they are more limited in their scope and cannot be used for a wide range of work. However, since they are simple and easily operated they form an excellent camera for the beginner, who has not yet become familiar with the various ad- justments which render the more expensive instrument capable of handling a wide range of subjects more efficiently. Box cameras are supplied in several sizes from 1^5x23^ to 3/4 x 5/^ inches and in both roll film, pack and plate models, although the last named have practically disappeared from the com- mercial market. On account of the greater bulk of the camera the smaller sizes are most popular and since the roll film or film pack models are lighter and more convenient to use they have practically superseded plate cameras of this type. Box cameras are generally fitted with cheap single achromatic lenses which cannot be used at a larger aperture than F/16. In bright light, from 8 to 3 o'clock, snap- shots may be made if the subject is open and not in shadow. Under other conditions the camera must be placed on a firm support and a time exposure given. Since the positions of both lens and film are fixed it is impossible to focus and the lens is so placed that all objects from infinity to within 10 to 15 feet of the camera are defined with satisfactory, if not critical sharpness. This avoids one of the dif- ficulties of the beginner and hence such cameras, under the proper conditions of light, give good results with the minimum of trouble and skill on the part of the user. The Miniature Camera. — The miniature camera, or V.P. camera as sometimes termed, ranges in size from a^/i x 6 cm. (1.77x2.36 inches) to 6^x9 cm. (2.56x3.5 inches), is more expensive, and 38 THE CAMERA AND DARKROOM 39 is designed to be fitted for a wide range of serious work with the minimum of inconvenience to the owner when not in use. The par- ticular feature of these cameras is their portability. They are small and light, so that they may be carried in an ordinary pocket without annoyance and brought into use quickly and with the minimum of effort when desired for use. At the same time such cameras are capable of really serious work, when they are handled with skill, since when fitted with good lenses the small negatives enlarge readily to medium sizes. A typical example of an instrument of this type is shown in Fig. 16. When purchasing a camera of this kind it is well to remember that although they are rather expensive it is well to get the best and espe- cially to secure a good lens since good sharp definition will be required for subsequent enlargement, while a large aperture will enable snap- shots to be made when otherwise impossible. Another important thing to examine is the soundness of construction. While a certain Fig. 16. Typical Miniature Camera for Plates and for Roll Film sacrifice in stability is necessary in order to prevent undue weight and size it is desirable that the instrument be sufficiently strong to with- stand long-continued usage. Many cameras of this type are rather overloaded with adjustments and movements, which are useful at certain times but are more often simply a hindrance to fast work. In the opinion of the writer the following are the most important features of a miniature camera: 1. Body of aluminum or better Duraluminum. 2. A platform so that the lens is covered when the camera is folded. 40 PHOTOGRAPHY 3. When opened the front should lock with the lens in focus for objects at a medium distance, say 15 to 30 feet. 4. Further focussing should be provided for with either a lever or pinion, conveniently located. 5. The focussing scale, shutter speed and diaphragm scales should all be visible from the top of the camera so that any adjustments may be made while the subject is being followed in the finder. In the case of cameras designed for use with direct view finders the shutter and diaphragm scales should be visible from the viewpoint of the eye when following the subject in the finder. 6. The finder should be placed as close as possible to the lens in order that the correspondence between the two may be as perfect as possible. 7. The lens should be a high-grade anastigmat, with a large aper- ture, as F/4.5 or larger, in a shutter with a wide range of speeds from one second to 1/200. The advantages as regards convenience certainly lie with the min- iature camera using roll film but many of the disadvantages of plates are removed when small sizes are used. Thus weight becomes neg- ligible, and the only remaining difficulties are those of loading and unloading plate-holders, while the advantages of focussing and selec- tion of particular plates for different purposes are valuable to the serious worker. At the same time when facilities are lacking for the use of plates, film packs may be used in an adapter which may be loaded or unloaded in daylight and f ocussing done just as with plates. Film packs thus offer the same advantages as both films and plates. In the last few years a new class of miniature camera using standard 35 mm. motion picture film has become prominent. The picture area in the case of these cameras ranges from the standard motion picture size, 24 x 1 inch (18x24 mm.) to double this size, or 1x1^2 inches (24x36 mm.). When constructed with a high degree of precision and fitted with the better grades of anastigmat lenses, these cameras become very capable instruments. The short focus objectives, with which they are fitted, have great depth of focus and thus render practi- cal the use of much larger apertures than on larger cameras which must be fitted with lenses of tonger focal length. With good optical defi- nition, the fine grain of commercial motion picture film allows of con- siderable enlargement without loss of definition. Folding Hand Cameras. — Folding hand cameras are made in sizes from 2^4 x 3*4 to 4 x 5 inches (or from 6.5 x 9 to 9 x 12 cm.) in both THE CAMERA AND DARKROOM 41 film and plate models. There is without doubt a greater demand for this class of instrument than any other — a fact which is evident from the wide range of models provided by the various manufacturers. For one thing, the contact prints are sufficiently large to satisfy the require- ments of the average amateur, while the instrument itself, although less portable than the miniature camera, is easily slipped into the coat pocket or slung over the shoulder by means of a leather strap. Roll film models of this class call for the barest mention since they are the cameras in general use by the larger body of amateurs. They are ideally fitted to the needs of most amateurs for whom the camera is only a method of keeping a record of their happy experiences on trips and during vacation. For serious photographic work of a gen- Fig. 17.- Hand Cameras for Plates and for Roll Film eral nature they are not so well adapted since they are not provided with means of focussing on the ground-glass, and consequently can- not be used for copying and work of a similar nature. For this rea- son many prefer to purchase one of the light plale cameras and use film-pack whenever the advantages of lightness and daylight loading are important. The more expensive plate cameras of this class are exceedingly ver- satile instruments and are capable of doing almost anything that the average photographer is likely to demand. They are fitted with re- versible backs so that pictures may be made either vertically or hori- zontally without turning the camera on its side and many of them have a long bellows which enables them to be used for copying and 42 PHOTOGRAPHY photographing small objects. In addition, the long extension oermits the use of long-focus lenses, which for portraiture and certain kinds of landscape work are very desirable. Some of these cameras are also fitted with a swing back which en- ables the plate to be kept in a vertical position while the bed of the camera is tilted upward in order to include the whole of a tall object on the plate. Various other features are supplied on the different in- struments such as rising and falling front, wide angle bed for using wide angle lenses, and sometimes sliding fronts are fitted. Since these cameras are always fitted with a finder and focussing scale they may be either held in the hand or placed upon a tripod. They are thus suitable for both the most exacting work and at the same time may be used as a hand camera whenever desired. There is little ques- tion that this is the most efficient instrument for really serious photo- graphic work which the amateur can buy. Typical examples of hand cameras for both plates and film are shown in Fig. 17. The Professional Camera. — The view and studio cameras in gen- eral use by the professional photographer do not differ greatly from the folding plate camera which we have just described. They are usually more substantial and consequently more bulky, while they are of larger size — the ordinary sizes being 5x7, 7x11, 8x10 and 11 x 14 inches. In addition to greater stability, the movements fitted to professional cameras have a wider range of adjustment than those of the more compact hand camera, which is not intended for such a broad range of work. Not only are the adjustments fitted to the pro- fessional camera of greater latitude, but there are, in the better ones, adjustments which are not found on the hand camera. These provide the extreme range of movement necessary in technical photography. The lens board is also larger so that long focus lenses of large aperture may be accommodated. In addition there is provision for focussing from either the back or the front — a valuable feature when wide angle lenses are in use, since in this case focussing may be done from behind and there is no danger of a part of the camera bed appearing in the picture. Some cameras of this type known as banquet cameras, made for such work as their name indicates, have an arrangement by which the lens may be tilted downward while keeping the plate vertical, so that large groups may be photographed from above with the minimum of distortion (Fig. 18). The studio camera is in general similar to the view camera except that it is much heavier and larger, the rising front is dispensed with THE CAMERA AND DARKROOM 43 as is also front focussing. The lens board is larger so that the large bulky portrait or anastigmat lenses of long focus and large aperture may be readily accommodated. The Reflex Camera. — The principle of the reflex camera requires a word of explanation since it is radically different from any of the cameras which we have already described. Fig. 19 shows a typical reflex in cross section. The rays of light from the object pass through Fig. 18. Professional View Camera the lens and are reflected by the mirror to the focussing screen at the top of the camera, where the image is of course in its normal unreversed position, i.e. right side up. Behind the mirror is the focal plane shutter and behind this the sensitive plate. The focal plane shutter consists of a long opaque curtain with apertures of varying lengths, any one of which may be made to pass across the front of the plate at a high speed. When the image has been focussed on the ground-glass and the exposure lever is depressed the mirror swings up out of the way and forms a light-tight joint with the focussing screen. As soon as the mirror reaches this position it automatically operates the shutter. Thus two distinct operations are performed in the interval between the action of the exposure lever and the actual exposure. First the mirror is released and swings up, and then an aperture in the curtain of the focal plane shutter passes over the plate and makes the exposure. However, in a well-made reflex the mirror and shutter are so well coordinated that the time interval is not more than 1/10 to 1/5 of a second. The reflex camera offers several distinct advantages possessed by 3 44 PHOTOGRAPHY no other camera, which renders it well worth its cost, which is neces- sarily rather high owing to the care needed in manufacturing and properly adjusting the intricate mechanism. The image can be seen in full size, right side up on the focussing screen until just before the exposure is made. Thus the reflex is superior to the folding film camera in that it is possible to focus accurately on the ground-glass and not have to depend upon focussing scales. It is superior to the Fig. 19. Principle of the Reflex Camera ground-glass focussing plate camera in that the image is right side up so that composition and placement of the subject is simpler and also in the fact that the exposure can be made immediately without the operations of closing the lens, inserting the plate-holder, withdrawing the slide, etc. Furthermore, very rapid exposures are possible since the ordinary focal plane shutter works up to a maximum speed of 1/1000 second. THE CAMERA AND DARKROOM 45 Aside from its expense, the principal objection to the reflex is its bulk and weight. There is no doubt that where portability is an im- portant factor the average reflex is rather out of question. The x 4/4 instrument weighs from four to five pounds and occupies a space of approximately 5x5x6 inches, while the 4x5 size is cor- respondingly larger. The first mentioned size is the more popular of the two. For those who demand portability and yet desire reflex ad- vantages the 2y2 x 3^ size may be recommended, while the 5 x 7 size is practically obsolete except among professional workers. To overcome the bulk of the box form reflex many manufacturers, especially in foreign countries, have placed folding models on the market. These are much more costly, and are neither as substantial in construction nor do they possess the usual bellow extension, extent of rising and falling front, etc., so that at present the box form type is, perhaps, still the best. While especially suitable for photographing objects in rapid motion, the reflex is by no means limited to work of this class. Indeed for all ordinary work it is the most certain and con- venient instrument to use. It is of course not suited to architectural work when a swing-back is required and in most cases cannot be well used for copying, but for all general work in the field or at home the reflex is ideally adapted. The Principal Adjustments of Cameras. — The principal adjust- ments of cameras are the rising and falling front, the vertical swing or swing-back as it is commonly termed, the horizontal or side swing, and the reversible back. The rising and falling front is an arrangement for raising or lowering the position of the lens in order to increase or decrease the amount of foreground included. While at times necessary in all kinds of work, it is particularly valuable in architectural work where it is necessary to include the whole of a tall building. The amount which the lens may be raised is usually expressed as a fraction of the greatest length of the plate. Thus if the rising front on a 4x5 camera allows the lens to be raised one inch above its normal central position the degree of rise is said to be 1/5. The amount which the lens can be raised varies in different makes of cameras but is always greater in view cameras than in the more compact hand and stand cameras. Wide limits of rising front are sometimes required in exacting cases and at any rate it is well to secure a camera allowing the maxi- 46 PHOTOGRAPHY mum rise and fall for a camera in its class as the reserve rise will at times help one out of difficulty. To secure the full advantages of a camera having extreme rise, a well-corrected lens with a reserve covering power is required, since when the lens is raised above its normal position it is the margins of the field rather than the center that are used and consequently the greater the demand for good cor- rection, since the definition of a lens is never so good near the margins as at the center. Reserve covering power is needed in order that the plate may be completely covered when the lens is fully raised. While many film cameras are provided with a rising and falling front its utility is in this case somewhat doubtful, since the finder cannot be relied upon to show just what is included when the lens is not in its normal position. Several makes of cameras, however, are fitted with self-adjusting finders which more or less accurately indi- cate the exact limits of the picture when the lens is raised above its normal position. The Swing-Back. — The swing-back is an adjustment for swinging the back of the camera at an angle to the bed so that the plate may be kept in a vertical position, when the camera is pointed upwards in A f Fig. 20. Principle of the Swing Back Use of the swing back for securing greater depth of focus order to include a lofty subject on the plate. In a of Fig. 20 the camera is supposed to be absolutely level so that the parallel lines of the subject A and B are represented by parallel lines A' and B' in the image formed by the lens L. In this case there is no distortion. However, when the camera is tilted upwards as in b of Fig. 20 the THE CAMERA AND DARKROOM 47 sensitive plate is no longer parallel with the subject AB and conse- quently the parallel lines A and B of the subject are represented as converging lines in the image. However, if the camera is fitted with a swing-back, the plate can be brought to a vertical position by prop- erly adjusting the back and distortion will be avoided although the bed of the camera be tilted upwards. However, as will be observed from c of Fig. 20, the axis of the lens is no longer at right angles to the sensitive plate, but crosses it obliquely, so that the use of a small diaphragm is necessary to obtain sharp focus over the entire area. The size of the diaphragm required depends upon circumstances and can only be determined by examination of the ground-glass image. The other use of the swing-back is in focussing different planes at varying distances from the camera sharply, without using small dia- phragms. Suppose we are required to photograph the side of a hill on which A and B represent objects of interest which it is necessary to focus sharply without the use of small diaphragms. With the plate in the horizontal position CD it is evident that the distance BD is greater than AC and that only one of these objects can be sharply focussed unless a small diaphragm is used. However, if the swing-back is ad- justed so that the sensitive plate occupies the position DE, the distance of the plate from A is increased while BD remains the same. By watching the focussing screen while making these adjustments it is easy to bring about some compromise which will allow a larger stop to be used than would otherwise be possible. In every case in which the axis of the lens is not at right angles to the plate a certain amount of distortion results, so that this movement cannot be used for certain kinds of work, but on landscapes, portraits, etc., the small amount of distortion may pass unnoticed. Indeed in some cases it is a positive advantage since it emphasizes the nearer objects. At any rate the worker must determine for each particular case, by- examination of the image on the ground-glass, whether the distortion is objectional or not. Swing Front. — The swing front is an adjustment which is very useful at times and which unfortunately is included on very few cameras, even those built exclusively for professional use. When the front is hinged at the base, as in some reflex and stand cameras, the chief function of the swing lens is to provide an increased amount of rising front when employed with the swing back. When the camera is tilted upward and the back adjusted vertically so as to preserve the 48 PHOTOGRAPHY parallelism of upright lines in the picture, the shift in the image is proportional to the tilt given the camera, but if the camera is provided with a swing front, the lens may be made parallel to the back thus we include still more of the sky and less of the foreground so that we secure, in effect, an extreme rise of front which is frequently very necessary, as in photographing a modern skyscraper. Furthermore, the lens is parallel to the plate, so that less stopping down will be re- quired for sharp definition. Even of greater service is a swing front which swings from the lens rather than the base of the camera front. A swing of this description, when used in conjunction with the rising front, may effectively take the place of the swing back. If, when the lens is thus raised extremely high, the lower corners of the plate are badly illuminated or cut off, the lens can be tilted slightly upward and the illumination on the plate will be restored without material shift of the image. Another advan- tage in the use of the swing front is that by pointing the lens slightly upward or downward it is possible to obtain increased depth of focus without the distortion which is produced when the swing back is used for the same purpose. In this way objects near the camera can be brought into sharp focus at the same time that the distance is sharp and with a comparatively large stop. The use of the swing lens in this manner, however, presupposes the use of a lens which has a sharp field larger than the plate. The Reversible Back. — The reversible back allows the back of the camera to be reversed so that the picture can be made either hori- zontally or vertically without turning the camera on its side. This is a very convenient feature and is found on all of the more expensive plate cameras except those which must be made extremely compact. It cannot be had on any roll film camera, all of which must be re- versed when a horizontal picture is required. Some cameras are fitted with what is termed revolving backs instead of reversible backs. These serve the same purpose but, as their name indicates, they are revolved from one position to the other without being detached from the camera. Other Movements. — Some cameras are also fitted with side swings which allow the plate to be adjusted with reference to horizontal ob- jects. While at times valuable, the horizontal or side swing is not nearly so important as the vertical swing or swing-back, and for that reason it too is found only on the professional view camera and the more expensive hand and stand cameras. THE CAMERA AND DARKROOM 49 The sliding front is an adjustment fitted to only a few cameras and these are generally view cameras for professional use. When two pictures are made on the same plate, the lens may be moved to each side in order that the center of the field may be used. There are many other adjustments fitted to various makes of cameras which are not sufficiently universal in application to require attention. Shutters. — Photographic shutters may be conveniently divided into four classes according to whether they are placed before, between or behind the lens or in the position of the focal plane. Of the first class little need now be said for shutters of this class have almost com- pletely disappeared. The few types of behind-the-lens shutters re- maining are used almost wholly for studio portraiture. This leaves us then the two classes of shutters in general use, namely, the between- the-lens, or diaphragm shutter as it is sometimes termed from its close proximity to the diaphragm, and the focal plane shutter. The between-the-lens shutter consists of from three to five blades of thin steel or hard rubber, which open and close from the center, the time during which the leaves are open being controlled either by an air-brake or a chain of gears. Most of the later shutters, such as the Ilex, the Wollensak, and the Compur, to mention only a few well- known makes, belong to the last named class. The shutters controlled by a chain of gears are generally more accurate in their timing than those controlled by pneumatic means although this is not always the case. The accuracy of timing in most shutters, even of the best types, leaves much to be desired. The speeds marked on most shutters are only approximate and the actual speeds not infrequently show considerable departure from the marked values. The difference between marked speed and actual speed is generally most evident in the shorter exposures. The ex- posure marked 1/250 is seldom more than 1/200 and frequently much less. The lower speeds, on the whole, show less deviation, although it is not altogether uncommon to find a shutter which shows but little variation in actual speed between the indicated speeds of 1/25 and 1/100 of a second. Shutters, the speeds of which are approximately correct when new show considerable variation after a season or two, although the modern gear-controlled shutters are less likely to " go off " than the others. The focal plane shutter is thus termed because it operates close to the focal plane and immediately in front of the sensitive material. It 50 PHOTOGRAPHY consists of a light-tight curtain pierced with slits of various width. This curtain is carried on two rollers, one of which is under tension. When the exposure lever is released, the curtain rolls off of one roller on to the other, making the exposure as the open slit passes in front of the plate or film. A wide range of exposures may be had by vary- ing the size of the slit in the curtain and by increasing or decreasing the tension, which controls the speed at which the slit is drawn past the plate or film. With a focal plane shutter the actual speed values correspond more closely to the marked values than with the between-the-lens shutter, ex- cept when old and the tension weakened. The efficiency of the focal plane shutter is higher than that of the between-the-lens shutter, effi- ciency being taken as the actual amount of light which reaches the plate in a given time of exposure. No matter what the exposure, the leaves of a diaphragm shutter take some time to open and close. Thus the lens is working at full aperture for only a portion of the exposure. With the focal plane shutter, however (provided the shutter is placed close to the surface of the sensitive material), the full aperture of the lens is utilized, so that considerably more light reaches the plate or film in a given exposure time than with a between-the-lens shutter. The efficiency of the best diaphragm shutters varies between 50-60 per cent ; that of the best focal plane shutters between 80-90 per cent, so that there is a distinct advantage in the use of focal plane shutters at the higher speeds. Tripods. — Metal tripods have largely replaced the wooden variety except for use with the larger professional cameras. A great many of the light and compact metal tripods are lacking in stability and are suitable only for the lightest cameras. Nothing is more annoying than a wobbly tripod and except where weight and size are the deter- mining factors, the lighter metal tripods should be avoided. While a trifle less compact, those tripods in which the members are hinged so that the legs open up in the manner of a folder have greater strength and are more easily erected, as the sliding type with time tends to give trouble from bending. Few metal tripods have a head which allows the camera to be tilted except by unequally spreading the legs. Tilting and revolving tripod heads, however, may be bought quite reasonably and are almost in- dispensable. Special stands have been introduced for home portrait and general interior work by a number of firms. The leg braces and THE CAMERA AND DARKROOM 51 central elevating post of these make their use preferable to the usual form of tripod for interiors or wherever there is danger of slipping. Lens-Hoods. — Lens-hoods are necessary when the lens is pointed towards the source of light and advisable at all times, especially with compactly mounted, large-aperture, anastigmat lenses. These, unlike the older lenses, generally have no projecting hood, while their short barrel length, large aperture and more complex construction all favor the formation of internal reflections which reduce the brilliancy of the image. The general disregard of lens-hoods is simply another instance in which the craze for compactness and simplicity has got the best of what is the better practice so far as optical performance is concerned. So little is the value of the lens-hood appreciated at the present time that it is 'almost impossible to find a really efficient lens-hood for a small camera. Any black lined tube, however, which projects as far as possible in front of the lens without cutting into the picture, serves the purpose, whatever it may lack in convenience. Finders. — Finders are of two general types : ( 1 ) reflecting and (2) direct. The reflecting type is that usually fitted to hand cameras and consists essentially of a small camera obscura with a slip of mirror at an angle of 45 degrees to the lens, so as to reflect the image to the top. Most of the finders fitted to hand cameras are too small to be used to the best advantage. Furthermore, they do not show the image as it will be recorded by the lens. The customary finder shows only approximately what will be included in the picture. The separation between lens and finder is not important for distant objects but is for very close objects, say within 5 to 6 feet. Consequently, a finder which indicates correctly the field included with a distant object may fail to do so with a close one. When the lens is raised or lowered the image in the finder is not altered proportionately. This limits the use of the rising front on roll film cameras. The " Sybil " finder of Messrs. Newman and Guardia, however, provides for this by a set of number marks and the " Idento " finder of Messrs. Adams has an automatic masking arrangement which insures correspondence when lens is raised or lowered. The direct finder may be a rectangular lens on which the image is viewed from a sight rod, or it may be simply a wire frame the di- mensions of the picture area, attached to the lens or shutter mount having a sighting rod on the side of the camera. Either form is pre- ferable to the reflecting finder: (1) it allows the use of the camera at eye-level, which is usually preferable to waist-level, (2) it is much 52 PHOTOGRAPHY easier to follow moving objects and (3) the size of the image is larger. Direct finders may also be made to compensate for the raising or lowering of the lens although in most cameras this is not done. II. The Darkroom The Size of the Darkroom. — The important part played by the darkroom in the quality and volume of the work produced is not as fully realized as it should be. As a consequence, we have many dark- rooms which are mere makeshifts, which are ill arranged and result in serious loss of valuable time and materials, and in some cases in- jurious to the health of the photographer and to the sensitive ma- terials he uses. It is well worth while to pay particular attention to making the darkroom an orderly, well-arranged place, which is both healthful and pleasing, a place which one will not object to living in. The size of the darkroom is in most cases determined by the cir- cumstances attending to its location. The proper size is determined by the character as well as the volume of work carried on. For the average amateur there is no particular advantage in a room larger than ten by twelve feet, while about six by six feet may be regarded as the minimum. The advantages of a workroom about eight by ten or ten by twelve feet are : the greater ease in heating and ventilating, and less danger from stray light from openings in the walls or from around the door or window. A room with these dimensions allows an enlarging lantern to be installed and provides room for separate benches and sinks for different operations, as plate changing, plate developing, printing, washing, etc. For commercial work no definite size can be stated, as this will de- pend upon the class of work and upon the volume of business. In any case, room should be provided to allow sufficient space for each op- eration so that there may be a separate and distinct place for each operation and for the materials and equipment required for this par- ticular operation. When this is done things are not so often mis- placed, broken, overlooked or destroyed. For a large business it is an advantage to divide the workroom into several smaller workrooms each of which is equipped and used for one particular purpose and no other. Thus we may have one moderate-sized room for plate changing and development, another for printing, and still another for the storage of chemicals and for preparing solutions. Of these three rooms in most cases the printing room requires to be the largest, and THE CAMERA AND DARKROOM 53 the room for developing next in size, while the chemical storage room may be comparatively small since it is not in constant use. If only a small amount of enlarging is done the apparatus may be installed in the printing room, but if enlarging forms an important part of the business it is well to provide a separate place for this purpose. Sxcareo location Or /niet L cxhmmt. r Should Be on Oura/oc Wall. -I -a- M JNU/AAI N£0 Fig. 2i. (Courtesy of Eastman Kodak Company) Ventilation of the Darkroom Ventilation. — A matter of particular importance, to which little or no attention is generally paid, is proper ventilation. In the opinion of the writer it is undesirable to have as a darkroom one which is permanently dark. If conditions will permit, it is far better to have a room which includes at least one window, or more if the room is large, which is provided with a tight-fitting, light-proof blind which may be quickly opened to admit air and light and as quickly closed for work. While this will be of considerable service in ventilating the darkroom, and may be sufficient for the amateur who only works for a short period of time, something more is required in large establish- ments where the room is used throughout the day. Here it is neces- sary to provide light-proof air vents in order to allow the entry of fresh air and if the exit vents are fitted with suction fans so much the better. We illustrate in Fig. 21 a plan for the ventilation of the 54 PHOTOGRAPHY workroom which will be found quite satisfactory. A ten-inch pro- peller fan will handle about 300 cubic feet of air per minute and is large enough for a room containing about 4000 cubic feet of air. While this may seem elaborate and unnecessary expense, it can be proved that the gain in general efficiency more than compensates for the initial cost while the good will of the employee cannot be estimated in dollars and cents. Arrangement. — The arrangement of the darkroom is a matter de- serving particular attention. In laying out the floor space the aim should be to allow plenty of space to enable an operation to be car- ried on quickly and efficiently without hindrance to other work which may be going on at the same time. To do> this there should be a separate place for the materials and apparatus for each operation and this should be convenient to the place where the work is carried on so that articles required may be readily accessible. Forethought along these lines and careful planning of the workroom with a view to the requirements will save much time and labor later. Two very suitable arrangements for the amateur are shown in Fig. 22, one for a room and the other for a closet which is to be con- verted into a workroom. It will be observed that in both cases the space for loading plates and developing is placed behind the door where there is less danger from stray light. The space for loading plate holders or development is marked A in both floor plans, while B represents the sink in which the fixing bath and washing tanks are kept. The enlarging lantern may be conveniently placed in either at C and a separate bench for the printing machine and cabinets for papers and plates may be placed along one of the free sides of the room. Fig. 22. Floor Plan of Darkroom for Amateur Use THE CAMERA AND DARKROOM 55 The Water Supply (Sinks). — As a large amount of water is re- quired for most photographic operations, the water supply is an im- portant item in the location of the darkroom, which should be lo- cated, if possible, so that water may be easily installed. For while running water is not an essential, at least for the amateur who works at intervals, though it may be regarded as an absolute necessity for the professional, it is a decided convenience and adds much to the pleasure of the work. Owing to the location of the darkroom it may be im- possible to install running water either because the mains are not available or the cost is prohibitive. In such cases the amateur will find a very good substitute in a large water cooler of about five gallon capacity. This should be fitted over the sink in such a position that the tap is conveniently located for the drawing of water for the dilu- tion of solutions and rinsing of plates after development. A similar container may be placed below the sink. Operations which require a large amount of water, as the washing of plates or prints, will then be carried out in another room where running water is available. Opinions vary regarding the proper size and the construction of the sink. In the opinion of the writer it is a mistake to have a sink smaller than 18 by 36 inches. A sink of this size is just sufficient to carry a cold and a hot water tap together with the negative fixing tank, which should always be kept in the sink in order that there may be no danger from hypo infection. Larger sinks offer advantages in that they may contain in addition the negative washing box and the print washer or other similar apparatus. On the other hand they quite frequently occupy valuable room and are otherwise objectionable because they keep the room damp and unpleasant and cause metal goods, as the enlarging lantern, to rust. In arranging for the sink the worker must be guided by his own requirements and by the space available. The sink itself may be of wood coated with a waterproof paint, such as Probus ; of cement, of enameled steel, or of lead. Steel enameled sinks are perhaps the most satisfactory and are really the cheapest in small sizes but are obtainable only on special order in very large sizes and are also very expensive. Large sinks are there- fore generally made of either cement or wood coated with a water- proof paint. Taking into consideration the labor involved in con- structing the same, the wood sink is the cheaper and is perfectly sat- isfactory, provided it is kept well coated with a water-, acid- and alkali-proof paint. In the laboratories of the Division of Photog- 56 PHOTOGRAPHY raphy at The Pennsylvania State College, the writer for several years had two wooden sinks, covered with an alkali- and acid-proof paint, in almost constant use and they have proved perfectly satisfactory. The only precaution to be taken is to renew the coating of paint once or twice a year. The majority of sinks, however, are made of con- crete, which is resistant to all acids and alkalies of such strength as are used in ordinary photographic practice. The following directions for the manufacture of a large concrete sink were given at a meeting of the Photographers' Association of America some twelve or thir- teen years ago. A framework of half-inch boards is first built on the support where the sink is to be placed, and on this a thick layer of cement and sand in the proportion of cement two parts and sand three parts is laid, about an inch thick. While this is setting, an in- ner framework of half-inch boards, about two inches shorter than the outer one and without any bottom, is prepared and when the bot- tom layer of cement is set, this inner framework is rested upon it, and the tops of the inner and outer framework are kept steady at a distance of about an inch apart by two strips of wood attached at distances at the top. This forms a mould between the two frame- works and the bottom layer of cement, and into this mould more cement mixture is poured and allowed to set. The waste pipes should be put in before the cement sets and placed a little below the surface to allow for the shrinkage which occurs upon drying. To strengthen the sink large nails, or pieces of iron or steel, may be im- bedded in the cement and if thoroughly covered they will not rust. When the cement has become thoroughly hard the forms may be re- moved and work begun immediately. If the somewhat rough surface is objected to for any reason the cement may be coated with any of the compounds used for finishing cement surfaces and will then be perfectly smooth and resistant. In fitting taps over the sink care should be taken that they are, not so low that large graduates or containers cannot be placed under them. Placing the taps too high is also to be avoided owing to the trouble from splashing. A height of about fifteen inches from the bottom of the sink is a fair distance. The Illumination of the Darkroom. — There are two systems of darkroom illumination, direct and indirect. Indirect light, while hav- ing been adopted by some large commercial houses, has been neglected by the amateur and even by the average professional. The advantages THE CAMERA AND DARKROOM 57 of indirect light are many, as will soon be observed by one who installs it, and the amount of light which may be present in the darkroom with- out danger of fog on even the most sensitive of modern plates, when handled with reasonable precautions, will astonish one. As a matter of fact most darkrooms arc too dark and in the end not so safe as sup- posed, since it is necessary to work quite close to the light — not one of which is really safe. An overhead light will give an even, diffused Fig. 23. Eastman Indirect Darkroom Lamp light all over the room and there is no difficulty in finding articles which may be required. Lamps for indirect lighting are supplied by the Eastman Kodak Company (Fig. 23) but there is nothing to prevent the worker from Fig. 24. Design for Indirect Darkroom Lamp. (Krug, American Annual of Photography, 1922) making his own if he so desires, as the construction is quite simple. The light-box itself (see Fig. 24) is made of thin sheet iron and is 8 x 10x6^ inches in size. There are two ventilators, one in one side 58 PHOTOGRAPHY and one in the bottom of the box, which must be made so as to prevent any white light from passing. The interior of the box is painted with a matte white or aluminum paint and the electric socket is placed so as to bring the filaments of the bulb nearly in the center of the box. The top is hinged so as to permit of changing the safelight to suit emulsions of different character. The box is suspended from the ceiling by four chains or wires attached to each corner and the electric light cable is brought clown from the wall tap which should be close by. To secure the full advantage of the light the ceiling should be a white matte; however, if this is not the case a sheet of Beaver board about four feet square and painted white may be fixed to the wall directly above the light. Under these conditions the illumination will cover an area about sixteen feet square so that additional light will be needed for a large room. A 25- or 40-watt bulb will supply sufficient light ; very large bulbs will melt gelatine safelight screens. The safelights used will, of course, be those which are adapted for the type of sensitive material in use. This matter we will consider shortly. A very convenient lamp for the development of prints or enlarge- Fig. 25. Eastman Developing Fig. 26. Wratten Darkroom Safelight Lamp Lamp ments or for either time or factorial development, where there is no occasion for examining the negative by transmitted light, is the pendant light (Fig. 25). As the safelights are interchangeable the same light may be used for developing either plates or prints, the safelights being changed as the occasion demands. THE CAMERA AND DARKROOM 59 Among the many excellent types of lamps on the market for direct illumination the Wratten (Fig. 26) may be mentioned for the facility with which the safelights may be changed, for perfect ventila- tion, and for the uniform and safe illumination by using only reflected light. All the lamps which we have mentioned are for electric light only and certainly no one who has access to electric current will use any- thing else. Where gas or oil must be used the best plan is to place the light outside of the darkroom itself and arrange a holder for the safelight in the wall of the room. Where this cannot be done it is necessary to purchase one of the gas or oil lamps obtainable from dealers. In purchasing examine first the size of the lamp (5 by 7 inches should be regarded as the minimum size for the safelight) ; second the arrangement for changing safelights for different plates ; and third see that the ventilation is well taken care of. Most com- mercial oil lamps are deficient on all of these points but more par- ticularly the second and third. Do not make the mistake of buying a cheap lamp, but examine carefully the various models and do not hesitate to pay a fair price for a well-built and efficient lamp which will fulfill your requirements. The Safelight. — The term safelight is in a sense a misnomer as there is really no such thing as a " safe " light, for any light that is bright enough for the eye to observe will affect a plate if it is ex- posed to it sufficiently long. It is entirely a matter of time. It is well to remember that, regardless of the screen used for development, the plate should be exposed to the light as little as possible, only at the beginning of development to see if the plate is evenly covered and then towards the end of development to determine when the plate should be removed, and the tray should be kept covered at all other times. Under these conditions it is possible to use a fairly bright light for development with comparative freedom from fog. In the earlier days of photography when plates were not nearly so rapid or as color sensitive as they are at the present time, the com- mon materials used were ruby glass and canary fabric. While both of these materials pass a considerable amount of active light, they were satisfactory with the plates then in use and indeed are still widely used, but with the advent of the modern highly color-sen- sitive plate the ruby glass screen is giving way to the modern safe- light, composed of certain dyes, which are selected so that the com- 60 PHOTOGRAPHY bined transmission is that to which the plate for which it is designed is least sensitive. These screens are made by several manufacturers, in different series, for plates of different color sensitiveness, or they may be made at home, but as there are many excellent screens at very reasonable prices on the market it is a mistake and false economy to make one's own. The Efficiency of Darkroom Safelights. — With any darkroom screen we naturally wish to secure the maximum safety with the greatest visual intensity, or in other words the brightest light that is safe for the plate. Thus, if a plate is sensitive only to the ultra-violet, violet, and blue, and is insensitive to green and red, either a green or a red safelight might be used with safety. The most efficient of the two screens, and therefore the best choice for practical work, would depend upon the relation between the sensitiveness of the plate and the sensitiveness of the eye for any particular color. The visual in- tensity is higher for the green than the red, but the latter has less ac- tion on the plate, so that with ordinary plates where a fair volume of light may be used, the red screen is used in preference to the green, but in the special case of the panchromatic plate, which is sensitive to practically the entire visible spectrum, so that only a very small volume of light may be used, the green is chosen because of its greater visual intensity. The standard of safety adopted for the products of at least one manufacturer of screens is that no effect should be produced on the plate when it is exposed to the safelight at a distance of one meter for thirty seconds. Most screens will produce fog if rapid or highly color-sensitive plates are exposed to their action for much over a half minute. A good working test of safety is to place a plate facing the safelight and about two feet away and expose part of it for thirty seconds, leaving part of the plate unexposed by covering it with sev- eral coins, or a piece of black paper. If the plate shows signs of fog, after development in total darkness for the usual time, the light is un- safe for the plate and either a weaker light source should be used, the plate handled at a greater distance from the lamp, or the safelight should be changed to one which more nearly transmits light to which the plate is insensitive. Before leaving the subject of safelights we would again caution the worker about exposing plates to the safelight more than is absolutely necessary. This is particularly important when loading holders and THE CAMERA AND DARKROOM 61 before the plate is placed in the developing solution, as the sensitive- ness decreases as development proceeds, so that a light which may be safe for examining the plate when partly developed may be relatively unsafe for the same plate in the dry state. It is not a difficult matter to learn to load plates in the dark and it is far more satisfactory to do so. With time development there is no necessity for observing the plate at all unless greater or less contrast than the normal is desired, and then it is not necessary to examine the plate until development is nearly complete. The same applies to development by inspection. Particular care must be taken when the factorial method of develop- ment is used, since in this case the plate must be held rather close to the light at the beginning of development in order to observe the time of appearance of the image. This difficulty may be overcome by the use of a desensitizer. Trays, Tanks and Graduates. — Trays having the same dimensions as the plate used are really convenient only for special operations such as reducing, intensifying, etc., and it is far better to purchase de- veloping trays which will accommodate at least four plates at a time. There is very little to choose between hard-rubber, porcelain, or steel enamel trays. All withstand all ordinary photographic solutions. While the first are expensive and easily broken, the second are rather expensive and also somewhat clumsy on account of their weight, and the last has the disadvantage that the enamel becomes chipped in time and exposes the metal to the action of the solutions. Composition trays are as satisfactory as any and if broken can be replaced with less outlay than any of the others. Now that tank development is so largely employed by both the amateur and the professional it may be well to say a word regarding the tanks for the development of plates and also those adapted to the use of roll film. Practically all of the tanks on the American market which are designed for the use of plates are made of nickeled steel and fitted with a waterproof cover so that the tank may be turned over on its end in order to agitate the developer and prevent uneven density. In several of the tanks on "the market provision is made for pouring in the developer through a light-proof trap after the plates have been loaded into the tank. Then, when development is complete, the developer may be poured out through the same tap and replaced by pure water for rinsing and finally by the fixing bath. Thus all operations except the loading of the tank with plates may be done 62 PHOTOGRAPHY in full daylight, while it is quite an easy matter to load the tank with plates without a darkroom by using a changing bag. Other tanks have very ingenious loading devices which enable the tank to be loaded quickly without the danger of touching the sensitive surface or exposing the plate to the light very long. The Kodak film tank is an example of the all-by-daylight method of developing roll film whose use is too well known to require more than a brief mention. Only three sizes of graduates are required for all ordinary purposes, although others may be useful at times. The sizes most generally re- quired are a one-ounce minim graduate, one of about 8 ounce capacity and a larger one of 16 or 32 ounce capacity. For the larger sizes the cheap tumbler graduates having pressed instead of engraved lines are sufficiently accurate. By paying somewhat more one can secure graduates having opaque graduations which can readily be seen in the darkroom, so that there is no uncertainty in preparing solutions while developing. Two special forms of graduates also require at- tention, viz. : the graduated beakers used by chemists which are prac- tically unbreakable, and the combined graduate and mortar and pestle which is extremely useful for powdering chemicals or crushing tablet developers, as the Tabloid or Scaloid products. It is essential that all trays, tanks, and graduates be kept thoroughly clean. Many of the stains and other defectg which perplex the amateur and trouble the professional at times are due to unclean trays or other similar equipment. One of the most effective means of rapidly removing any ordinary chemical impurity from a vessel is by the use of a solution of potassium bichromate and sulphuric acid. The proportions are not so very important and the following will be found about right: Water to make 32 ounces (1,000 c.c.) Potassium bichromate 4 ounces (125 gms.) Sulphuric acid (commercial) 4 ounces (125 c.c.) This is poured into the vessel and will act in a minute or so, when after a rinse or two and some swabbing with a tuft of absorbent cot- ton the tray or graduate will be chemically clean and available for any photographic operation. The solution should be kept in a glass-stop- pered bottle, as it will destroy a cork. Commercial Tanks. — In commercial establishments where a larger volume of work is done, the use of open tanks is general. The sensi- tive material, whether plates, cut film or roll film is carried on suitable THE CAMERA AND DARKROOM 63 hangers through the operations of developing, fixing and washing. The tanks for use with plates and cut films hold from I to 3>£ gallons of developer and accommodate from I to 2 dozen plates or cut film at a time. The larger tanks used for the development of roll film hold from 10-50 gallons of developer and take from 10-50 rolls at a time. With such installations, it is not the usual practice to discard the developer after each batch of plates or film but to keep the solution up to normal strength by the addition of fresh solution. Lately, there have been introduced for the development of roll film automatic developing machines which carry the loaded film hangers through the developing, rinsing, fixing and washing tanks without at- tention on the part of the operator. These machines are capable of developing up to 500 rolls of film per hour and require the services of but two men ; one to attach the rolls of film to the conveyor and one to remove the films in the drying room. They have proven entirely suc- cessful and are used in a number of the larger photo finishing plants. Some Miscellaneous Workroom Features. — It is always advisable to keep the major part of the stock of sensitive materials on hand outside the workroom but provision ought to be made to keep those which are constantly in use where needed in the workrooms and in such quantities as may be required. To this end it is well to con- struct tight wooden cabinets to contain all plates, films and papers in general use not only in order that such materials may be kept in an orderly condition but to prevent injury by the moisture always present in the workroom. Such cabinets may also be provided for containing loaded and unloaded plate holders and for the weighing and mixing of chemicals. The drying of plates and films is a matter of some moment, espe- cially in large commercial establishments where it is necessary to dry batches of plates or films at a uniform rate day in and day out re- gardless of the weather conditions outside. We illustrate in Fig. 27 a small cabinet designed for plates or cut film and intended more par- ticularly for the serious amateur or small professional. For a larger establishment and where provision must be made for drying roll film more elaborate equipment is necessary. The size of the cabinet will be governed largely by the number of films handled in a batch and the number per day. It should be sufficiently large, however, to contain all the films it is likely to be called upon to handle at one time with- out undue crowding and ample space should be allowed for the cir- 64 PHOTOGRAPHY culation of air on all four sides and through the center. Either slid- ing or swing doors may be provided, while if desired one may place doors on two opposite sides, the films direct from the washing tank being inserted from one side and removed from the other after dry- ing. To secure a thorough and even circulation of air it is advisable to provide both the bottom and top of the cabinet with an air cabinet. Both the upper and lower walls of the bottom air cabinet should be made perforated, using ^2 inch holes spaced two inches apart each way. The holes of the two walls should not coincide, however, or Fig. 27. Drying Cabinet for Plates and Films the purpose of the air chamber will be defeated. The bottom of the upper air chamber should also be perforated and provision made at the top for the installation of a suction fan to create a current of air through the cabinet. Provision for suspending the films will natu- rally depend upon the type of hangers employed and can be worked out by the individual to meet his own particular case. Heat may be used for drying if a thorough even circulation of air is assured but not otherwise. The best results are obtained at 95 degrees Fahr. and a thermometer should be kept within the cabinet and in plain view to see that this temperature is not exceeded. Drying cabinets for roll film are an article of commerce and may be obtained from a number of dealers should the worker prefer not to build his own. Such cabinets may also be used for drying prints on squeegee plates. Commercial apparatus for this purpose is also available. CHAPTER III PHOTOGRAPHIC OPTICS Introduction. — According to the theory now generally accepted, light is a wave motion in an elastic medium known as ether. The vibratory motion of the molecules of a body are communicated to the ether and a transverse wave spreads out in all directions with a velocity of approximately 300,000,000 meters per second. When this wave motion strikes the eye it produces the sensation which we term light. We distinguish between luminous and illuminated bodies: the former radiate light of themselves, the latter are simply bodies on which the rays from a luminous source fall. Among the former are included the sun, all artificial lights as oil, gas, electric and magnesium, and every body heated above a certain point. Illuminated objects com- prise all bodies in the unobstructed path of rays from a luminous source. The intensity of illumination on an illuminated body depends upon the strength of light at the source, the distance of the illuminated body from the light source and the density of the intervening medium. While light waves spread out in all directions from the source, the vibrations which reach any point travel a straight line joining that point and the source. This is known as the rectilinear propagation of light. A cone of rays from a luminous point is known as a light-pencil, and is said to be homocentric to all other pencils which proceed from the same luminous point. The central line of this light-pencil is termed the axis. If we follow the rays from the source they are said to be divergent; in the opposite direction, convergent. Refraction of Light. — In a homogeneous medium the path of a ray of light is always straight, but the passage of the ray from one medium to another, in general, alters both its velocity and its direction. The alteration in the direction of a ray upon passing from one medium to another is known as refraction. The angle which the incident ray makes with the normal to the surface at the point of separation of the two mediums is known as the angle of incidence, while the angle of the refracted ray to the normal is known as the angle of refraction. A ray (see Fig. 28) A moving in a medium 1 meets at B the surface 65 66 PHOTOGRAPHY CD separating the medium i from a denser medium 2. Upon striking the surface CD, the ray is divided into two rays BE and BF, the former the reflected and the latter the refracted ray. The angle of the reflected ray is equal to the angle of the incident ray, so that the angle of incidence equals the angle of reflection. When the ray passes from one medium to another of greater density, as from air to glass, the re- L Fig. 28. The Principles of Refraction fracted ray BF is bent towards the normal, but when passing from a dense medium to one of lesser density the path of the refracted ray lies away from the normal. The angle of deviation of a ray upon passing from one medium to another is known as the index of refraction. Since refraction is due to a change in the velocity of light the refractive index of a medium is equal to the velocity of light in air or a vacuum the velocity of light in the given medium The amount which the refracted ray deviates from the direction of the incident ray, or the index of refraction, can be determined from the size of the angles ABL and BFL'. Thus the angle BF = ABL — BFL' . Or, where i is the angle of incidence, r the angle of refrac- tion and D the deviation, the value of D is obtained by the following equation : D= (i- r). Thus far we have only considered refraction at one surface. When we have a plate of a dense medium, as glass, a ray of light is refracted both upon entrance and also upon emergence. Let ABCD (Fig. 29) be a block of glass with sides AB and CD parallel. The ray of light PHOTOGRAPHIC OPTICS 67 a strikes the surface AB at /. This ray is refracted from the normal on entering the denser medium and proceeds to c. Upon passing from the surface CD into a medium of lesser density, refraction again occurs Fig. 29. Refraction in a Medium with Parallel Sides but this time towards the normal. Since AB and CD are parallel, the normals are parallel and the emergent ray is parallel to the incident ray. There is a lateral displacement but not a change in direction. However, if the sides are not parallel the course of the ray is altered as shown in Fig. 30, in which the course of the light ray is traced A Fig. 30. Refraction in a Prism through a prism. The ray proceeding from a meets at I the side of the glass prism ABC. It is refracted towards the normal, n — n, and follows the course indicated by I — c reaching at c the surface of the glass prism ABC. On passing out of the prism into air refraction occurs again, but this time away from the normal along the line cd. The total angle of refraction is thus equal to the angle at o. D = (* -[»') - (r - r'). Dispersion. — So far we have considered only monochromatic light or light having one definite color, but in practice we do not deal with monochromatic light but with daylight, or at least a light source hav- ing a wider range of emission than one narrow band of the spectrum. 68 PHOTOGRAPHY When white light is passed through a prism the different rays are not all refracted to the same degree but according to their ref rangibility ; those of short wave-length, as blue and violet, being refracted to a greater degree than those of longer wave-length, as orange and red (Fig. 31). The angular separation between the constituents of a ray Fig. 31. Dispersion in a Prism of composite light produced by refraction of the ray in passing through another medium is known as dispersion. Dispersion was formerly thought to be dependent upon refractive power so that substances having a high refractive power necessarily had a high dispersive power. This is now known to be incorrect and it has become possible to prepare glasses with high refractive power and low dispersion and vice versa. Dispersion results in what is termed chromatic aberration, the correction of which is of considerable importance in photographic objectives and will be fully treated in the following chapter on the aberrations of the objective. Lenses and Image Formation. — There is a very close similarity be- tween a lens and a combination of prisms and in fact we may consider mm Fig. 32. Principal Forms of Simple Lenses a lens as a prism having an indefinite, or infinite, number of sides. Single lenses are divided into two classes according to whether they are diverging or converging ; the former are known as negative lenses, the latter as positive. The principal forms of converging and diverg- ing lenses are shown in Fig. 32. It will be observed that the former PHOTOGRAPHIC OPTICS 69 are thicker at the center than at the edges, while the latter are thicker at the edges than at the center. The position and character of the image formed by a positive lens will be seen from Fig. 33. In this case AB represents an object Fig. 33. Image Formation with Positive or Converging Lenses placed at a distance from the lens which is much greater than the focal length of the lens. For purposes of illustration the course of only three rays proceeding from the object points will be shown. The three rays from A pass through the lens and are refracted so that they intersect one another at A', while the rays from B intersect at B' in a similar manner. Likewise the rays from any point on the line AB on passing through the lens will be refracted and form a corresponding point on the line A'B'.1 The image in this case is real and inverted. The point at which the rays begin is termed the object point and the point at which they intersect after having passed through the lens is known as the image point or the focal point, while the distance from a point in the lens known as the nodal point to the focal point is known as the focal length. From Fig. 34 it will be observed that a negative, or dispersing, lens forms no real image but only a virtual one since the focal point lies between the lens and the object. Image Formation according to the Gauss Theory. — We owe to Gauss the conception of the nodes and nodal planes of a lens, also called principal or Gauss points after their discoverer. If the positions of these are known, the image can be constructed from the object 1 We are assuming, of course, a fully corrected lens. Fig. 34. Course of Light Pencils through a Negative Lens 70 PHOTOGRAPHY without knowing anything about the actual course traversed by the rays in their passage through the various glasses. In Fig. 35 are shown the nodes N2N1 in a single lens. A ray of Fig. 35. Image Formation According to the Gauss Theory- light entering at an angle so that it reaches the node at N2 (node of admission) acts as if it was carried parallel to the axis to Nt (node of emergence) after which it continues on in a straight line. If we imagine the planes P2P2 andP1P1 passing through the nodes N2 and N1 parallel to the axis, we will see that a ray of light, as ab, parallel to the axis on reaching the lens passes undeviated to the plane of emer- gence and is there bent so that it passes through the focal point. If the lens is reversed the node of admission becomes that of emergence and vice versa. Thus every lens may be said to have a front focus and a back focus, the former measured from the node of admission, the latter from the node of emergence, to the focal point. In Fig. 36 Nt and N2 are the nodes of a lens, P1P1 and P2P2 the Fig. 36. Image Formation According to the Gauss Theory corresponding nodal planes, F± and F2 are the front and rear foci re- spectively, so that F1N1 is equal to F2N2. Let ab represent an object having its lowest point on- the lens axis bb%. The position of point a in the image may be found by drawing two rays AP1 parallel to the axis and thence through the rear focus Fv the other ray through the front focus F2 to the plane of admission P2 and thence parallel to the PHOTOGRAPHIC OPTICS 71 axis. The meeting point of these two rays (oj is the image point of a. Any other point may be found in like manner, and hence if the position of the nodal planes is known the rays can be traced through the lens whether the curves or glasses of the objective are known or not. The Position of the Nodes. — The positions of the nodes and nodal planes in a single lens vary with the type of lens. In Fig. 37 the posi- Fig. 37. Position of the Nodes in Common Forms of Simple Lenses tions of the nodal planes are shown for a number of the common types of single lenses. The light is assumed to be passing from left to right in the direction indicated by the arrow which also indicates the axis of the lenses. Na and Ne represent the nodes of admission and emer- gence respectively. When the curves are equal the nodes are centrally placed ; when the curves are unequal the nodes lie nearer the side hav- ing the greatest curvature. When a lens has a plane surface one node lies at the vertex of the convex surface, while in the case of a meniscus lens one node lies outside of the lens, or " free." In combinations of lenses the positions of the nodes vary considerably. They may lie within the lens itself near the diaphragm, in front of, or behind the lens. When the nodes are situated a considerable distance in front of the lens we secure a great focal length with a short distance between the rear of the objective and the ground-glass. This is the principle upon which the teleobjective is based.2 The Principal Focus of a Lens — Focal Length. — When the object is at an infinite distance from the lens so that the incident pencils of light are parallel on entering the lens the focus of the emergent pencil constitutes the principal focus of the lens. Since either side of the lens may face the subject, there are two principal foci, both of which are equidistant from the node of emergence but on op- posite sides of the lens. 2 For a more complete discussion of the nodes and their action see Piper, The First Book of the Lens, pp. 31-32 and 45-49. 72 PHOTOGRAPHY The distance between the back surface of the lens and the sensitive plate constitutes what is called the " back focus." It bears no definite relation to the focal length and its only value is to indicate the bellows extension necessary. The term is now seldom used except in connec- tion with telephoto lenses. Focal Length and Size of Image. — Assuming a concrete distance between lens and subject, the greater the focal length of the ob- jective the larger the size of the image. Except with very near ob- jects, the size of the image varies directly as the focal length of the objective. Thus with a given distance between lens and subject the image produced by a twelve-inch lens is twice as large as that pro- duced by a lens with a focal length of only six inches. Stated dif- ferently, to obtain a given size of image the distance between the lens and the object decreases as the focal length. While the perspective produced by a lens is always scientifically ac- curate, regardless of the distance between the lens and the subject, it does not necessarily accord with our established idea of perspective. Owing to the fact that the eye is a long focus instrument and accus- tomed to viewing objects at a relatively great distance, photographs made with the lens very close to the subject appear to possess violent and unnatural perspective to the eye, so that the result is unsatis- factory to our aesthetic sense even though it may be scientifically ac- curate. Perspective is determined entirely by the distance between the lens and the subject and is independent of the focal length of the lens, but since in practice when using a lens having a short focal length the tendency is to get close to the object in order to secure an image of satisfactory size, short focus lenses have gained the reputation of producing violent and disagreeable perspective. There is, however, no actual foundation for this, for if the distance between the lens and the subject is the same the perspective of identical sections of the subject will be the same. Thus if two plates of the same size are exposed, using two lenses of different focal lengths at the same distance from the subject, one plate will include very much more of the subject than the other. However, if this latter is trimmed so as to include the same portion of the subject as the other the two prints will be identical, except in size. Consequently a lens of long focal length gives superior perspective only because the distance between PHOTOGRAPHIC OPTICS 73 the lens and the subject must be greater for an image of a given size. The choice of a suitable focal length is governed by the size of the plate and the requirements of the subject. In certain cases short focus objectives must be used in order to include all of the subject from a given viewpoint. In such cases violent and unnatural perspective cannot be avoided and must be accepted. Generally, however, there are no such limitations and the focal length is, within certain limits, a Focal Length in Inches Fig. 38. Table for the Calculation of Angle of View matter of convenience. For all general work it is well to choose a lens the focal length of which is equal to, or slightly greater than, the diagonal of the plate. For pictorial work or portraiture, how- ever, longer focus lenses are desirable and for such work lenses hav- 74 PHOTOGRAPHY ing a focal length two, or even three, times the diagonal of the plate are used.3 Angle of View. — The angle of view of any objective is the angle subtended by two lines drawn from the node of emergence to the corners of the plate in use. The shorter the focal length of the lens in relation to the size of plate the greater the angle of view and the more of the subject included from a given viewpoint. It is often of advantage to be able to calculate the maximum angle of view for a given lens on a certain sized plate and to render this a simple matter we have reproduced a chart by which this information can be simply and rapidly secured. The maximum angle of view possible with any lens depends upon the relation between its focal length and the size of the plate which it will cover with satisfactory definition. For a given size of plate, the shorter the focal length of the lens the greater is the angle of view, while on the other hand, the larger the plate which a lens of given focal length will cover with sufficiently critical definition the greater is its angle of view. Thus a lens of six inches focal length will include a much greater angle if used on a 5x7 plate than the 4x5 for which originally designed. Most lenses, however, particu- larly if of high speed, will not cover satisfactorily a plate much larger than that for which they are designed unless considerably stopped down. Some lenses, such as those built on the " Tessar " construc- tion, do not increase in covering power to any extent when stopped down and naturally their angle of view is limited by the plate for which they are designed to cover. A lens made to cover with sat- isfactory definition a field large in proportion to its focal length is termed a wide angle lens. While no definite rules exist for classifying lenses according to angle, lenses including an angle of less than 45 degrees may be considered as narrow angle; those including angles from 45 to 75 degrees as medium angle, and any lens including an angle above 75 degrees as a wide-angle objective. Conjugate Focal Distances. — When the distance of the subject is less than infinity, any object point and its corresponding image point are termed conjugate foci, and the distance from the object point to the nodal plane of admission and that from the image point to the 3 For a complete discussion of the fundamentals of perspective see paper by J. C. Dollman in the Photographic Journal, 1923, 63, 315. The same paper will also be found in the British Journal of Photography, 1923, 70, 411. PHOTOGRAPHIC OPTICS 75 nodal plane of emergence form what are termed the conjugate focal distances. These distances are interdependent and connected by a certain formula. Let u represent the distance of the object, v that of the image, and / the focal length of the lens. Then I _ I _|_ i / u v If the values of any two of the above terms are known it becomes a simple matter to find the other from the following: f _ vu v + u fi> v-f From the formula u-f 1=1 + 1 f 11 V it will be evident that when the distance of the object and that of the image are equal (i.e. u equals v) both distances are equal to 2f. The image is thus the same size as the object and the term symmetric foci, applied by Prof. Thompson, is apt. The linear ratio of the object and the image is expressed as Distance of image (v) Distance of object (w) but for purposes of calculation it is much simpler to express the ratio in terms of u and f, or v and f (f representing the focal length of the lens). From Fig. 36, representing the principles of image formation according to the Gauss theory, it will be seen that triangles abF2 and F2N2P2 are similar, whence jV2P2 _ F2N2 ab bF2 By construction, N2P2 is equal to and F2N2 is equal to / and bF2 equals v ■ — f. 4 76 PHOTOGRAPHY Therefore, Image _ R = / _ Object u — f From triangles a1b1F1 and P1N1F1 Image _ R _ v — f Object / Extra Focal Distances. — From the last two formulae it is evident that calculation is made much simpler when the distance of the ob- ject or the image is reckoned from the front or rear focal point re- spectively. These distances which are therefore u ■ — / and v — f are known as the extra focal distances. The extra focal distance of the object is denoted by Ev and that of the image by Eu. Therefore, Ev=v-f = fR, Eu=u-i=fR- In other words, the extra focal image distance is equal to the focal length of the lens multiplied by the ratio of image to object, while the extra focal object distance is equal to the focal length divided by the ratio of image to object. Calculations of the conjugate distances can be made quite simply by considering the distance between the image and the object to be divided into five separate distances. These are: 1. The extra focal distance — focal length multiplied by the number of times of reduction or enlargement. 2. One focal length. 3. The nodal space — generally negligible. 4. One focal length. 5. The extra focal distance — focal length divided by the number of times of reduction or enlargement. In the case of reduction (1) and (2) are on the object side of the lens, but when enlarging (5) and (4) are on the object side of the lens and (1) and (2) on the image side. When copying same size (1) and (5) are alike and are equal to one focal length, so that the whole distance from object to image is equal to four focal lengths, if the nodal space be neglected. PHOTOGRAPHIC OPTICS 77 The above are the fundamental formulae covering the relation of the conjugate foci when enlarging or reducing and almost any re- quired calculation relating to the sizes and distances of object and image may be worked out from the formulae above.4 Theory of Depth of Focus. — In explaining the theory of depth of focus, more properly termed depth of field, use will be made of the illustration developed by Moritz von Rohr.5 In Fig. 39 let OaO.^ represent an object, parts of which are at different distances from the lens L. For the sake of simplicity the latter is represented without the nodes necessary for a complete rep- Fig. 39. Graphic Illustration of the Principle of Depth of Focus. (Von Rohr) resentation of its action. Suppose the lens to be focused on 01( a point image Ix will then be formed on the focussing screen or sensi- tive plate. With the focussing screen, or the sensitive plate, at this point it is clear that the image of 02 is formed at I2 behind the screen and that of 0? at /3 in front of the screen. In other words the posi- tion of the image point varies with the distance of the object point and the lens cannot produce a point for point image upon a plane subject unless the subject itself is a plane surface. When this is not the case, rays proceeding from object points nearer to, or farther from, the lens than the plane focussed for do not produce a point 4 Numerous formulae for the calculation of the various problems pertaining to scale in optical reproduction were given by Mr. George E. Brown in the British Journal of Photography for 1921, 68, pp. 702-705. 5 Zur Geschichte und Theorie des Photographischen Teleobjectiv. 1897. Also Eder's Jahrbuch for 1906. 78 PHOTOGRAPHY image on the focussing screen but instead a circular disk. However, owing to the fact that the eye has its limits of definition and cannot appreciate critical sharpness, these disks may appear as point images to the eye if their diameter is less than a certain size which is termed the circle of confusion. The circle of confusion thus represents a circle of the largest diameter possible without producing perceptible unsharpness to the eye. The largest circle which will appear as a point to the eye is a variable dimension depending upon the distance from which the photograph is viewed. It is found that at a distance of 12 inches, which is a normal viewing distance for prints up to ap- proximately 8x 10, a circle with a diameter of not more than i/ioo of an inch will appear as a point to the normal eye. Consequently this has to a certain extent been adopted as a standard for satisfactory definition in photographic work. It is to be noticed that this value applies to the finished print but not necessarily to the original negative. For instance when a negative is enlarged the standard requires to be reduced in proportion to the degree of enlargement in order that the circles of the enlarged print may not exceed the maximum permissible diameter. Where negatives are to be subsequently enlarged the circle of confusion should not exceed 1/250 of an inch in diameter and in cinematography still less departure from critical sharpness is allow- able : in this latter case the diameter of the circle of confusion allow- able is about 1/600 of an inch. Hence, the better plan of expressing the circle of confusion is in terms of the viewing distance, as adopted by Continental writers, rather than the adoption of an arbitrary stand- ard. According to the latter view, the permissible circle of confusion is not a definite value, but a variable, which depends upon the distance from which the print is viewed. It is found that any object, regard- less of size, will appear as a point to the eye when viewed at a dis- tance equal to, or greater than, 3,400 times its diameter. This is equivalent to the very small angle of 1 minute (or a 5,400 part of a right angle). In order to be sharply perceived the angle must be at least 5 minutes, hence objects subtending angles of 1-4' are per- ceived more or less distinctly and appear practically sharp to the eye. This view of the variable standard of the admitted disk of confusion has much to commend it, and in view of the widespread practice of enlarging from small negatives it would probably be better if it were PHOTOGRAPHIC OPTICS 79 universally adopted and the older method of an arbitrary standard abandoned.6 Factors Controlling Depth of Focus. — The factors which control depth of focus are: 1. The focal length of the objective. 2. The aperture of the lens. 3. The distance of the plane in sharp focus. 4. The permissible diameter of the circle of confusion. 5. The presence of spherical aberration. The extent of field in sharp focus, other conditions remaining con- stant, is greater the shorter the focal length of the lens. The depth of field varies inversely as the square of the focal length, if near ob- jects be excepted. It is for this reason that fixed-focus cameras and cameras focussed by scales are generally fitted with lenses of the short- est focal length consistent with the size of image required for the plate in use. Increasing the rapidity of the lens reduces quite rapidly the depth of focus. Thus with a lens of six inches focal length focussed on infin- ity, all objects up to 20 feet of the camera will be sharp if the lens is used at F/16. If the lens is opened up, however, say to F/4, then the depth of focus will only extend from infinity to within 75 feet of the camera. The difference in the extent of field in sharp focus at different apertures is even more noticeable when a plane comparatively near the camera is focussed for. From what has already been said it will be evident that depth of focus in any given case will depend considerably upon the standard of definition adopted as satisfactory. Thus the depth of focus will be much greater if a circle of confusion equal to 1/100 of an inch is accepted as satisfactory in place of 1/250 inch or less. For contact prints up to 8 x 10 the former standard is satisfactory but when smaller negatives are enlarged up to that size the latter standard, or an even higher value, must be adopted if the resultant enlargement is to appear critically sharp on inspection at the normal viewing distance of 12 inches. The presence of spherical aberration increases the apparent depth of focus since it destroys critical definition and as a result there is no 6 For a full discussion of the two methods and the factors on which the same are based see " Theory and Practice of Depth of Focus " by George E. Brown in the British Journal of Photography, 1922, p. 476 et seq. 80 PHOTOGRAPHY clear dividing line between that portion of the subject which is sharp and that which is unsharp. An excellent example of this is seen in the modern soft-focus objectives which have greater depth of focus than other objectives of the same focal length and aperture because the difference between the portions in focus and those slightly distant from the plane f ocussed on is not so readily noticeable, and also because a circle of confusion greater than i/ioo of an inch is ac- cepted as permissible. If the depth of such an objective be considered from the latter viewpoint, it is quite possible that it might have no depth whatsoever since the circles of confusion would nowhere be less than i/ioo of an inch in any part of the field. The Intensity of the Image. — The intensity of the image formed by the lens is naturally a matter of considerable importance since on it depends the time of exposure required to impress the image on the sensitive film or plate. It is evident that where other conditions are constant the times of exposure with any two lenses will be in inverse proportion to the intensity, or brilliancy, of the image which they project on the sensitive material. Neglecting for the moment losses in the incident light due to absorption or reflection by the glasses of which the lens is composed, the intensity of the image on the axis is the volume of light admitted divided by the area over which it is spread. The intensity of the image at points removed from the axis is naturally dependent to a certain extent upon the optical perform- ance of the lens, but the intensity of the image on the axis is governed solely by the volume of light passing through the lens divided by the area over which it is spread in order to form the image. The volume of light admitted is of course dependent upon the area of the diaphragm aperture. Since the areas of circles are to one an- other as their diameters squared the volume of light transmitted by any two lenses is directly proportional to their diameters squared. Thus the volumes of light passed by any two lenses having an aperture of d1 and d2 are to one another as d2 : dt2. In case the light from d1 and d2 is spread over the same area, in other words the two lenses form an image of the same size, the in- tensity of the image will be directly proportional to dx2 and d„2. Thus the intensity of the image, is proportional to d2. Suppose, however, a lens with an aperture of diameter dy (Fig. 40) has a focal length of f±. If O be a luminous point in an object at such a distance from the lens that the incident rays are practically PHOTOGRAPHIC OPTICS 81 parallel, then the image will be formed at the principal focus of the lens ft and may be represented by FtFx. Now suppose this lens is replaced by another having the same aperture but with a focal < a x »- L_=5s Fig. 40. Intensity of the Optical Image. (Brown) length twice as great as fx so that f2 will be twice as large as that of FXFV = 2/,. Then the image F2F2 Hence (DiameteA Ft Ft ) (Diameter\ FtFt ) (0 fi h But the areas of the images F1F1 and F2F2 are to one another as the squares of their diameters, hence we may write (1) as follows: (W (F2F2y fi (2) Thus the areas of F1F1 and F2F2 are proportional to the correspond- ing focal lengths squared. As the aperture, d, is constant, the inten- sity of the image is thus inversely proportional to the focal length squared, or f2. Hence we have volume of light proportional to d2 # area of image proportional to f ' the intensity of the image, then, is represented by the expression dVf. (3) If this is written (d/f)2, the d/f expresses the ratio between the aper- ture and the focal length and is termed the aperture ratio. Speed of Lenses — Systems of Diaphragm Notation. — The aperture ratio d/f affords a convenient means of expressing the speed of lenses. It is evident that the intensities of two lenses are to one another as the squares of their aperture ratios. Thus if the aperture ratios of two 82 PHOTOGRAPHY lenses are represented as d1/f1 and d2/f2 respectively, the relative in- tensities are as Hence if the intensities of the image are expressed by the aperture ratios d/f, the relative intensities may be found by squaring both ratios and finding the relation between the squares. It is not customary, however, to mark lenses according to the in- tensity of the image, but according to the exposure required, which is of course in inverse proportion to the intensity, for as the brilliancy of the image is increased the time of exposure is proportionately de- creased. Accordingly the diaphragm numbers on lenses represent not the intensities but rather the reverse of these or the " slowness " of the lens. In 1881 a committee of the Royal Photographic Society of Great Britain advised that an aperture ratio of 1 : 4 be adopted as standard and a series of aperture ratios chosen so that each successive aperture would have an area one half that of the preceding, in order that the exposure required for each successive aperture be double that for the one immediately preceding and avoiding altogether the necessity of squaring the aperture ratios in order to find the relative exposures for the various apertures. Since a circle of one half the area has a diameter equal to 1/V2 the series of ratios becomes d _ 1 1 1 1 1 1 J_ J_ J_ / 4 5.6 8 1 1. 3 16 22.6 32 45 64 or in terms of f/d 4 5.6 8 1 1.3 16 22.6 32 45 64 Relative exposure required 1 2 4 8 16 32 64 128 256 It was further advised that in the case of lenses having a maximum aperture ratio intermediate in value between any of the above the F/number, obtained by dividing the aperture by the focal length, be marked upon the mount. Accordingly we have lenses bearing F/num- bers of F/4.5, F/4.7, F/6.3, F/6.8, etc. The relative exposure re- quired for any of these F/numbers may be found by squaring it and the nearest F/number in the series and dividing one by the other. PHOTOGRAPHIC OPTICS 83 Effective Aperture. — In the above sections the area of the aperture has been assumed to be the same as the actual area of the diaphragm, d. This is correct where the diaphragm is in front of the lens as in such cases the diameter of the pencil of light which can pass through the lens is obviously equal to the diameter of the diaphragm. In the case of photographic lenses having the diaphragm between the com- ponents, the diameter of the pencil of light which passes through the lens may be much greater than the actual diameter of the aperture, owing to the fact that the front lens acts as a condenser and converges the incident rays so that a pencil larger than the actual aperture may pass through. The diameter of the pencil which is converged so as to pass through the diaphragm constitutes what is termed the effective aperture. A practical illustration of the difference which may occur between the effective and the actual aperture due to the converging action of the front component was given some years ago by Dr. Zschokke for the Goerz Dagor, a double, symmetrical anastigmat the halves of which are identical. When the front component is used alone in its normal position in front of the diaphragm the effective aperture due to the "coning" action of the front lens is F/11.3. When the rear com- bination is used behind the diaphragm the effective aperture coincides with that of the diaphragm and is reduced to F/13.6. The brightness of the image is therefore nearly one and a half times greater in the first case than in the second, a difference due solely to the converging action of the front component. The difference in the effective and the actual aperture may be even greater with lenses of different design. Thus in the Aldis F/6 lens the actual diameter is only about .87 of the effective aperture. It is evident upon further consideration of the matter that the effec- tive aperture will vary according to which end of the lens faces the subject unless both components are identical. In the case of the Aldis F/6 the components are dissimilar and the aperture when the lens is reversed, with regard to the subject, is only F/6.9 or for all practical purposes F/f. With a lens having the diaphragm in front, the effective aperture varies to a certain extent with the distance of the subject, owing to the fact that the incident pencil of light is divergent rather than parallel. This effect, known as inconstancy of aperture, is so small in most cases that for practical purposes it may be disregarded. It is evident that it is the effective, rather than the diaphragm, aper- 84 PHOTOGRAPHY ture which must be considered in calculations regarding the intensity of the image. Hence it is more accurate to say that the aperture ratio d/f is the effective aperture focal length of lens than simply the aperture divided by the focal length as before. Visual methods of determining the effective aperture have been de- scribed by C. Welborne Piper and by Jobling and Salt,7 but perhaps the method most generally useful consists in focussing the lens on a very distant object and replacing the ground-glass by a card, or other thin opaque material, pierced with a pinhole on the optical axis. A lighted candle, or electric bulb, is then placed next to the pinhole. It is evident that since the latter is at the focus of parallel rays, light from a source in this position will emerge from the lens as a parallel pencil of a diameter equal to the effective aperture. The circle correspond- ing with the effective aperture may be obtained by pressing a scrap of bromide paper against the lens and exposing, or by outlining the same on a sheet of thin paper. Loss of Light in Lenses Due to Absorption and Reflection. — The statement is often made that all lenses require identical exposures when used at the same relative aperture, but this is not strictly true as owing to the differences among lenses in the amount of light lost from ab- sorption and reflection by the glasses of which the objective is com- posed, the actual intensity of the images may vary considerably, not- withstanding the fact that all are marked with the same F/number. The amount of light lost by absorption and reflection in photographic objectives is greater than might be at first supposed. The question has been several times investigated 8 and it has been shown conclusively that the loss varies from 10 per cent in the simplest single lens to nearly 50 per cent in the case of many of the more complex lenses and that few modern anastigmats transmit more than two thirds of the incident light. The losses from absorption do not amount to as much as those due 7 Jobling and Salt, Brit. J. Phot., 1922, 69, 108. Piper, Brit. J. Phot., 1917, 64, 272. 8 Cheshire, Brit. J. Phot., 1912, 59, 597, 645. Zschokke, Brit. J. Phot., 1912, 59, 823. Odencrants, S. I, P., 1925, 5, 87. Forch & Lehrmann, Kinotechnik, 1928, 10, 3. Moffett, Phot. J. Amer., 1920, 59, 44- Nutting, Phot. J., 1914, 54, 187. Goldberg, Der Aufbau des Photographischen Bildes; Ensyk. der Photographic, Hft. 99, pp. 23-43. (W. Knapp, Halle A/S. 1924.) PHOTOGRAPHIC OPTICS 85 to reflection. For this reason it is desirable to keep the number of reflecting surfaces in a lens to the minimum. Dr. Zschokke showed that of two lenses with an aperture of F/6.8, the one a symmetrical doublet having four reflecting surfaces (Dagor), the other composed of four single lenses having 8 glass-to-air surfaces (Syntor), the light transmitted by the former amounted to 88.77 per cent to 65.16 per cent for the latter. Consequently the first lens (Dagor) at an aper- ture of F/7.5 is actually as rapid as the second (Syntor) at its full aperture of F/6.8. The percentage of light transmitted by representative types of modern anastigmats has been measured by Odencrants and more lately Forch and Lehrmann. The following table summarizes the result of their investigations : Loss of Light in Photographic Objectives Objective Marked f/value Transmission Effective F/value Ernostar F/2 46.6% FI3.1 Ruo Pa 58.1% F/3-26 Kinoplasmat F/1.5 38-7% F/2.82 m-5 52.2% F/§.65 Tessar 60.0% F/4.64 F/4.5 63-5% F/5-94 F/4-5 52.7% F/6.39 F/4-5 49-7% F/6.65 W-3 53-4% 778.67 From what has been said, it is evident that the F/number is by no means an accurate indication of the speed of the lens, as it does not take into account the losses due -to absorption and reflection and these, as we have seen, are sufficient to cause considerable differences in the intensity of the image. Perhaps as time goes on we will find a more efficient method based, perhaps, upon the time required to produce a definite amount of photochemical action. At any rate, regardless of the angle from which the matter is attacked, a method of expressing speed which takes into consideration the loss of light due to reflection and absorption would certainly be a step in the right direction. Variation in the Relative Aperture with Distance of Subject. — We have seen before that the size of the image varies with the distance of the object from the lens. As the size of the image increases as the object is brought nearer the lens, so does the conjugate focal distance, v, increase. When the distance of the object is sufficiently great, so that the rays of the pencil of light entering the lens are practically 86 PHOTOGRAPHY parallel, the conjugate focal distance, v, is equal to the focal length, f. The intensity of the image is then expressed as d2/f2. However, if the object point is brought closer to the lens, the conjugate focal distance, v, is no longer equal to f, but becomes progressively greater as the object nears the lens. It is evident then that as long as the aperture remains constant, the intensity of the image will be repre- sented by d2/v2 rather than d2/f2. As the conjugate distance varies with the distance of the object, it is apparent that the intensity of the image, and therefore the time of exposure, will vary with the distance of the object point. In the vast majority of cases where the object is relatively distant from the lens, a condition applying to practically all exterior work and embracing the larger number of subjects, the variation in the aperture ratio is so small as to be practically insignificant in practical work, but in photographing very small objects, copying, enlarging, lantern slide reduction and similar work the variation becomes a mat- ter of considerable importance and must be taken into consideration in calculating the time of exposure. There are two ways in which this may be done. The distance from the rear nodal plane to the sensitive plate or film may be measured and the F/value calculated from d2/v2, but as this method requires a knowledge of the position of the rear nodal plane and the effective aperture it is not as convenient as the second method. This consists in basing the exposure on the nominal F/value as marked upon the lens and increasing it according to the camera extension, v, by multiplying by v2/f2. However, as these calculations are usually required only in copying and similar work, it becomes more convenient to draw up a table tak- ing as a standard the exposure required for copying full size (4 times that required with a subject 24 times the distance of the focal length) and entering the result of the calculation for relative exposures for various extensions by the scale of reduction produced. The follow- ing table of the relative exposure for varying proportions of the image to the original was calculated by Mr. W. E. Debenham several years ago and will be found very convenient in indicating the correction in exposure to be made when copying on various scales. Other methods of making these calculations are given in the chapter on lantern slides and copying. PHOTOGRAPHIC OPTICS 87 Exposures Proportioned Proportion of Image to Proportionate to that Required when Original (Linear) Exposures Copying Same Size l/30 1.07 .27 1/20 1. 10 .28 i/lO I.2I .3 1/8 1.27 .31 l/6 1.36 .34 l/4 1-56 .39 l/2 2.25 .56 3/4 306 .76 1 (Same Size) 4 1 2 9 2.25 3 16 4 4 25 6.25 5 36 9 6 49 12.25 7 64 16 8 81 20.25 9 100 25. 10 121 30.25 11 144 36 12 169 42.25 13 196 49 14 225 56.25 15 256 64 16 289 72 24 17 324 81 18 361 90.25 19 400 100 20 441 1 10.25 General Reference Works Beck and Andrews — A Simple Treatise on Photographic Lenses. Bolas and Brown — The Lens. Cole — Photographic Optics. Czapski — Theorie der Optischen Instrumente. Eder — Die Photographischen Objectiv. igio. Fabre — Traite Encyclopedique de Photographic Vol. I and Supplements. Gleichen — Theorie der Modernen Optischen Instrumente. Harting — Optics for Photographers. Lummer — Contributions to Photographic Optics. Miethe— Photographischen Optik. Moessard — L'Objectif Photographique. Nutting — Outlines of Applied Optics. Piper — A First Book of the Lens. Schmidt — Vortrage iiber photographische Optik. 88 PHOTOGRAPHY Schroeder— Die Elemente der Photographischen Optik. Steinheil and Voit — Handbook of Applied Optics. Southall— Geometrical Optics. Southall — Mirrors, Lenses and Prisms. Taylor — System of Applied Optics. Taylor, J. Traill — Optics of Photography and Photographic Lenses. Von Rohr — Theorie und Geschichte der Photographischen Objectiv. Wallon— Traite Elementaire de l'Objectif Photographique. Wallon — Choix et Usage des Objectifs Photographiques. CHAPTER IV ABERRATIONS OF THE PHOTOGRAPHIC OBJECTIVE Introduction. — In a perfect lens image every point in the object will be represented by a corresponding point on the flat surface receiving the image. It is impossible to realize this ideal and still preserve the speed required for photographic purposes because of the defects or aberrations to which lenses are subject. The more important of these aberrations are : Chromatic aberration, Spherical aberration, Coma, Curvature of field, Distortion, Unequal illumination, Astigmatism. We will consider at some length the causes, effect on the image and manner of correction of each of these. Chromatic Aberration. — Chromatic aberration is a defect caused by the dispersing properties of glass which prevents a lens from trans- mitting white light from a point of the object to a similar point of white light in the image. In other words the ray of light on passing through the lens is broken up into its component colors, the foci of which do not coincide but are situated at varying distances along the axis. The violet rays come to a focus at Fv (in Fig. 41), the red at Fr and the blue, green, yellow, and orange rays to intermediate points be- tween Fv and Fr. The image formed at Fv is that produced by the violet rays, that at Fr by the red rays but in each case there will be superimposed on this image the images produced by the other colors, so that no matter where the sensitive plate is placed between Fv and Fr we cannot secure a sharp image because of the intermingling of the other images, which are not brought to the same focus. In the early days of photography when emulsions were practically 89 90 PHOTOGRAPHY insensitive to any but the violet and blue rays, it was the common prac- tice to rack the lens towards the plate a distance sufficient to place the plate at the focus of the violet image. As the plate had little or no sensitiveness for the less refrangible rays, the definition was not seri- ously affected. With modern color-sensitive materials, this is no longer the case. IB Fig. 41. Chromatic Under Correction With a converging or positive lens, chromatic aberration takes the form shown in Fig. 41, where v is the focus of the violet and y of the yellow rays respectively. This is known as chromatic undercorrection. With a diverging or negative lens, we have a different case, which is represented in Fig. 42 and is termed chromatic overcorrection. ^Hf"^ Axis. Fig. 42. Chromatic Over Correction The method employed in correcting chromatic aberration will now be evident. When a positive and a negative lens of equal power are combined the combination is rendered achromatic. That is to say, it transmits white light as such, but on the other hand the two lenses being of equal but opposite power neutralize one another so that the light rays are not refracted and consequently no image can be formed. However, if the positive lens has a slightly higher refractive index than the negative lens and the latter a higher dispersion than the former, the dispersions may be neutralized without destroying the refracting power so that the lens may be at once convergent and achromatic. For the sake of simplicity we have been considering only two colors, violet and yellow, assuming that if these are brought to the same com- mon focus the other colors will be brought to the same point. How- ever, this is not the case and with two pieces of glass it is only pos- sible to bring two colors to a common focus. This difficulty is due to the fact that the relative dispersions of glass are not the same ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 91 throughout the spectrum. Thus the total dispersion of one glass may be twice that of another and lenses from the two glasses may bring any pair of colors together but will not bring the other colors to the same focal point unless the dispersion of one glass is double that of the other in every part of the spectrum. In most glasses, however, the relative dispersion is not constant and the degree of dispersion AC DBF G H A' C' D' E' F' G' H' Fig. 43. Irrationality of Dispersion varies with the wave-length of the light as shown in Fig. 43. This is known as the " irrationality " of dispersion. The pair of colors chosen for exact coincidence of focus depends upon the purposes for which the lens is designed. Thus in instru- ments for visual use, as the microscope, the green and orange are generally chosen because they, together with the yellow which lies be- tween them, are the brightest colors to the eye. However, since the plate is more sensitive to the ultra-violet, violet and blue than to light of longer wave-length, the colors chosen for photographic purposes are the violet, which lies midway between the very active ultra-violet and the slightly less active blue, and yellow, which is used for focus- sing on account of its luminosity. For all ordinary purposes lenses corrected for only two colors are satisfactory, since the other colors are either brought very close to the common focus or are so much less active that they do not affect the sensitive plate sufficiently to destroy the definition. A lens corrected for two colors is termed an achromat, or is said to be achromatic. The colors which are not brought to an exact focus form what is termed the secondary spectrum. In three-color process work it is necessary that three colors, instead of two, be brought to the same focus in order that the three negatives may be equally sharp and of the same size. By the introduction of other glasses with the proper calculations it has become possible to 92 PHOTOGRAPHY produce lenses in which three colors are brought to the same focal point. These are referred to as apochromats, or are said to be apochromatic. These are generally much slower than other lenses and are not used to any considerable extent for work other than that for which they are designed, since achromatic correction is sufficient for all ordinary photographic purposes. Spherical Aberration. — Spherical aberration results from the use of lenses, the surfaces of which are spherical. In a lens with spherical surfaces the rays of light near the margin of the lens are refracted dif- ferently from those nearer the axis so that the two do not come to a focus at the same point. If it is a converging lens, the marginal rays are brought to a focus closer to the lens than those nearer the axis, while in a dispersing lens the virtual focus is greater for the marginal rays than for the rays nearer the axis. The converging lens is said to be spherically undercorrected ; the dispersing lens overcorrected. Fig. 44 represents the condition of spherical under correction. The parallel rays hlt h2, h3, ht on passing through the lens form image points at different distances from the lens, those passing through the margin of the lens having a shorter focus than those passing through the lens at a point nearer the axis. By taking the distances of the rays from the axis as ordinates and the distances from the image point for the axial zone as abscissa? we may construct a curve showing the degree of under correction present. Bearing in mind that a negative lens forms only a virtual image, parallel rays entering from the right will form virtual image points at different distances to the right of the lens as shown in the dotted lines of Fig. 44. By using hx, h2, hs, h± as ordinates and the image distances as abscissae, in the same manner as before, a curve can be constructed which shows the degree of spherical overcorrection. The method of correction consists in compensating the undercor- rection of a positive lens by combining it with a negative lens whose overcorrection is sufficient to cause the marginal pencils to come to a focus at the same point as the axial pencils. It will be remembered that chromatic correction was corrected in a similar manner by balanc- ing two lenses of opposite errors. To correct for chromatic aberra- tion it is necessary that the lenses possess certain definite focal propor- tions. Now it is possible to make a lens with a definite focal length in rection cannot be made absolutely alike. Fig. 44 shows a lens which have the relative focal lengths necessary for chromatic correction and at the same time make them of such shapes as will correct spherical I ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 93 aberration. Thus since chromatic aberration may be said to depend on the focal length of the lenses and spherical aberration on their shapes it is possible to correct both of these errors in the same com- bination simultaneously. In practice it is virtually impossible to completely correct an objec- tive for spherical aberration since the curves of over- and undercor- rection cannot be made absolutely alike. Fig. 44 shows a lens which has the spherical aberration corrected for the center and a zone near the margin. The rays h2 and ks do not come to the same focus so that by taking their distances and the distance of the rays from the axis, according to the methods used above, it is possible to construct a curve showing the amount of spherical aberration remaining in the combina- 94 PHOTOGRAPHY tion. Perfect correction would be indicated by a straight line but a certain amount of uncorrected spherical aberration remains in even the very highest grade lenses. Coma. — Coma is the name applied to the spherical aberration of the oblique rays of light on passing through the lens. From Fig. 45 it will be seen that the course of oblique rays on pass- ing through a lens is completely unsymmetrical. The rays below the Fig. 45. Coma. (Kellner) axis of the lens are bent more sharply than those above the axis and thus do not meet in a common point but in a series of points. Assum- ing that the sensitive plate is placed at any one of these points of inter- section it is evident that we will not secure an exact image point be- cause all of the rays from the corresponding point in the object are not refracted to the same point in the image. In practice instead of a sharp, well-defined point we secure a small pear-shaped spot which seriously affects critical definition. In Fig. 46, A represents the condition known as outward coma, the points of the pear-shaped blur facing away from the axis of the lens. If the position of the lens is reversed we have the reverse effect be- cause the upper ray is refracted the most and the effect of inward coma is produced. In this case, as the name indicates, the points face the axis. The amount of coma present in any objective may be shown graphi- cally by a curve obtained in much the same manner as that which we have previously used as an expression for spherical aberration (Fig. 45). This curve shows the distances of the different points of inter- ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 95 section measured along the axis of the oblique pencil from a plane laid through the ideal image point. Perfect correction is indicated by a straight line, but as coma is one of the most difficult aberrations to re- move, the line is in practice always slightly curved, for while great strides have been made in overcoming coma, most modern lenses still show measurable amounts. Coma is corrected in two ways : by the use of a diaphragm and by compensation. From the illustration it will be seen that a diaphragm placed in front of the lens will remove the majority of the oblique pencils of light and thus reduce the amount of coma. The principal Fig. 46. Two Forms of Coma. (Piper) method, however, is by neutralizing the inward coma of one lens with an equal amount of outward coma in another lens. If the two lenses in Fig. 46 are combined the outward coma of one is neutralized by the inward coma of the other and if we assume that the amounts of coma present are equal but opposite powers, it is evident that complete neutralization will take place and that the pair as a whole will be free from coma. Further, since it can be proved that opposite forms of coma are given by simply reversing the curvatures of the lens, it is possible to find an intermediate form of lens which is practically free from coma — a discovery utilized by Mr. H. Dennis Taylor in the well- known Cooke triplet objective. Curvature of Field. — As the surfaces of the sensitive materials em- ployed in general photography are always plane, it is essential that the image formed by the lens likewise be plane in order that sharp defini- tion be secured over the entire plate. This means that the focus of 96 PHOTOGRAPHY the oblique pencils of light must lie in the same plane as that of the axial pencils. With all single lenses, however, the focal points of the oblique and axial pencils do not lie on the same common plane but on a curve. With a positive lens (Fig. 47) this curve is concave to the lens since the axial pencils come to a focus at a while the focus of an Fig. 47. Curvature of Field, (Under Correction) oblique pencil is at b rather than at c. When the image curve is con- cave to the lens the condition is known as positive curvature of field. It may also be referred to as under correction for curvature of field. With a negative lens (Fig. 48) the image field is again curved but this time the curve is convex to the lens and the condition is then I ~~7 Fig. 48. Curvature of Field. (Over Correction) known as negative curvature of field or sometimes as over correction for curvature of field. The actual curvature of the image with an uncorrected lens varies with the radii, glass, thicknesses of glass, separation of the component lenses and with the position of the diaphragm and the distance of the object. The curvature of field of a positive lens may be removed by the introduction of a negative lens if the latter is sufficiently powerful ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 97 and placed at the proper distance. A perfectly flat field is not to be expected in any lens, however ; least of all in one of the older construc- tions, such as the Petzval portrait lens, or the aplanat, where a com- promise must be made between curvature of field and astigmatism. In the anastigmats the curvature of field is less pronounced, but even here all objectives show a slight departure from absolute flatness, but the degree of positive or negative curvature is, in the majority of cases, not sufficient to cause serious trouble. Fig. 49. Distortion Distortion. — There are several kinds of distortion but the only one which we intend to discuss in this place is that due to the inability of the objective to reproduce a straight line as such. It is a very objec- tionable fault in a number of branches of work such as copying, archi- tectural photography and the majority of all scientific work. The following diagram will help in explaining without the aid of mathematics the general manner in which distortion is produced. Let Nt and N2 (Fig. 49) be the nodal planes of admission and emergence respectively of the lens L, and let 5C be a diaphragm placed at some distance in front of the lens. The solid lines represent parallel rays of light from a distant object passing through the diaphragm, BC, to the lens, L, and from thence to a focus at /. Let xN and xN' be parallel lines drawn through tne nodal planes of incidence and emer- gence. Let d be the point on the image plane where the line N'd intersects it. d is therefore the true position for the rays, but owing to the fact that a simple lens bends the marginal rays more than the central ones, the image point lies not at d, its true position, but at /, a point nearer the center. When the diaphragm is placed before the lens we have what is termed barrel distortion, a state of affairs represented in Fig. 50. When the diaphragm is placed behind the lens the form of distortion is reversed and is in this case known as pincushion distortion. It is 98 PHOTOGRAPHY preferable, however, to call the first negative and the second positive distortion. The method employed in correcting distortion is to combine two equal but opposite errors. It has been pointed out that with the Fig. 50. Upper : Negative or " barrel-shaped " distortion. Center : Positive or " pincushion " distortion. Lower : Correction of distortion by compound lens in which the negative distortion of one lens is balanced by the positive distortion of the other. diaphragm before the lens we have negative distortion, while when the diaphragm is placed behind the lens we have positive distortion. Then if we use two separate combinations, placing the diaphragm at the proper point between the two, the positive error of one will be neutral- ized by the negative error of the other and a rectilinear or non-distorted image will be produced. Astigmatism. — Astigmatism is one of the most serious of the aber- rations and is at the same time one of the most difficult to correct. While it is not strictly accurate to say that an astigmatically corrected objective was possible only after the introduction of the newer varieties of glass following the investigations of Abbe and Schott, since Martin as well as Beck 1 have shown that anastigmatic objectives may be con-, structed without these glasses, the products of the Jena glass works have played an important part in the development of the anastigmatic 1 Martin in the Omnar produced by Emil Busch, Beck in the Neostigmar Series. ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 99 objective, the series of barium crowns being particularly notable for having contributed largely to the rapid development in objectives of this type. The anastigmatic objective may be said to date from the introduction of the Protar by Rudolph in 1890. 2 Astigmatism is an error which affects only those light pencils which pass through the lens obliquely. It is due to the lens converging the oblique pencils of light to two separate focal lines rather than a point. Astigmatism differs .from spherical aberration in that the lafter affects the central as well as the marginal definition, while pure astigmatism is an error found only on points removed from the axis. Spherical aberration of the oblique pencils may also exist and has already been discussed under Coma. When a pencil of light falls obliquely on the surfaces of the refrac- tive medium the course of the rays in the different planes becomes dis- similar and we must distinguish between two special planes. One of these is the plane which passes through the principal ray of the oblique pencil and is represented as the plane of the drawing. This is termed the meridional plane. Perpendicular to this is the equatorial plane. The condition of astigmatic deformation is shown in Fig. 51.3 The pencil of the light from the window bars at the center of the field passes along the axis and hence the image at the focal point is an exact point for point image, chromatic aberration being assumed to be absent. The pencil from the window bars on the margin of the field, however, passes through the lens obliquely and in so doing the two planes be- come unequally refracted and come to a focus at different points. Consequently we do not secure a perfect image of those points which lie removed from the axis but instead we have a series of image points. Thus the vertical bar comes to a focus (t) before the horizontal bar and when the former is sharp the latter is not. If the position of the ground glass is altered in the direction of s the horizontal bar is brought to a focus while the image of the vertical bar becomes less and less sharp. Hence it is impossible to secure a sharp image of both at the same time no matter in what position the ground-glass is placed. The distance between the focus of the rays in the meridional and those in the equatorial plane forms what is termed the astigmatic difference. From the sectional illustration representing the appearance of the cross bar when the ground-glass occupies various positions between 2 D. R. P. 56.109— April 1890. 3 Courtesy of Dr. Hermann Kellner and the Society of Motion Picture Engi- neers. 100 PHOTOGRAPHY the astigmatic image points t and s, it will be seen that there is a point where both lines are equally sharp although neither is critically sharp. This point represents what is termed the circle of least confusion. If the image point lies near the axis, or the outstanding error is small, the diameter of the circle of least confusion may be so small as to be for all practical purposes a point image. The lens is then considered to be astigmatically corrected and is termed an awastigmat. Unfortunately, however, the difficulties of the designer do not end here. Generally when the astigmatic zones have been removed and astigmatic correction secured the image points lie on a curve and not Fig. Si. Astigmatic Deformation. (Kellner) on a plane perpendicular to the axis of the objective. Further calcula- tion is then necessary in order to bring all of the astigmatic image points as close to a plane surface as possible. When this is accom- plished we have what is termed an anastigmatically flattened field. Flare and Flare Spot. — Both flare and flare spot t can hardly be termed aberrations as they are not concerned with the formation of the primary image, but as they are properties of lenses which affect the character of its image it seems well to treat them at this point. There are two kinds of flare, one caused by reflection of light from a bright object within the lens mount and generally termed flare spot and that due to reflection of light from the surfaces of the lenses themselves. The former may be termed mechanical flare and the lat- ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 101 ter optical flare. The first can be avoided and there is little danger of flare from this source with a lens of a reputable manufacturer, unless old or damaged. Second-hand lenses should be carefully ex- amined for unblackened spots on the mount before purchasing, while the same cause may be looked for when an old lens suddenly begins to give flat foggy images. Optical flare cannot be avoided completely in any lens, and a lens may be excellently corrected otherwise but still useless on account of strong flare. Fig. 52 will illustrate the manner in which optical flare is produced. Let a and a' represent two parallel rays of light passing through the diaphragm and then through the lens and coming to a focus at b. There is a certain amount of reflection at each surface and in the case of the second surface the light is reflected back to the first surface, where it is again reflected back and reaches the plate at c and c'. If the focus of the secondary reflected image is near the same plane as the lens image, a definite spot is formed which destroys critical definition and gives a hazy, foggy effect. Any increase in the number of glasses in the objective increases the number of reflecting surfaces and hence the greater danger from flare in the more complex forms of modern lenses than in the old single achromat. In addition the deeper the curves of the individual glasses the greater the per- centage of light reflected and consequently the greater the danger of flare. The presence of air spaces increases the number of reflecting surfaces so that of two lenses of the same glasses and of the same design the amount of flare will be greater theoretically in the lens in which the components are separated by air spaces. Some forms of modern anastigmats are more subject to flare than others and with all it is advisable to take all possible precautions to remove all sources of the same. Much can be done by the habitual use of an efficient lens hood which really ought to be regarded as an in- tegral part of every ultra rapid objective. Fig. 52. Optical Flare 102 PHOTOGRAPHY Unequal Illumination.- — With every collecting lens, regardless of construction, the center of the field is more strongly illuminated than the marginal portions. This falling off in intensity towards the mar- gins of the field is known as unequal illumination, or diminution of intensity, and is due to two general causes, one of which is regular and common to every lens and the other of which varies with the lens and is dependent upon the construction of the mount. The regular diminution in intensity is due to three distinct causes: 1. The constriction of the aperture for oblique rays. 2. The greater focal length of the marginal rays. 3. The angling of the marginal rays to the focal plane. The manner in which the constriction of the aperture occurs is indicated in Fig. 53, where aa and bb represent the limiting rays of a direct pencil which can pass through the diaphragm ab. cc and dd represent an oblique pencil of light having the same diameter as aa bb. Fig. 53. Constriction of Aperture for the Marginal Pencils. (Brown) It is easily seen that the whole of this oblique pencil cc dd cannot pass through the diaphragm ab because it meets the same at an angle and a portion of the pencil of light is cut off by the diaphragm as indicated by the shaded portions and by the sections of the aperture A and B. Therefore the effective area of the diaphragm is less for an oblique pencil than for a direct pencil and consequently the intensity of the oblique pencil after passing through the lens is less than the intensity of the direct pencil. The second point to be considered is the fact that the focus of the oblique pencils is at a greater distance from the lens than the central pencil. This is shown in Fig. 54, where ab represents the distance of the focus for a central light pencil and ac that for an oblique pencil. It is evident that ac is greater than ab, or in other words the oblique ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 103 pencils have further to travel before coming to a focus than the central pencils, which again means that their effective value is less. Another cause of unequal illumination lies in the angling of the ob- lique pencil. The oblique pencil does not strike the plate perpendicu- Fic. 54. Greater Focal Length of Marginal Pencils Resulting in Lower Intensity. (Brown) larly, as do the central pencils, but at an angle. Thus in Fig. 55 the surface on which it would fall perpendicularly is RS, which is at the angle cSR to the sensitive plate cb. The area of each image point, represented by ES, becomes cS and the intensity from this cause is therefore less than that of the central pencils as SE : cS. The amount of the reduction in the intensity of the image at any point removed from the axis due to the above causes may be calculated Fig. 55. The Angling of the Oblique Ray. (Brown) mathematically provided there is no obstruction of the oblique rays by the lens mount to be considered. Formulae for calculating the diminu- tion in the intensity of the lens image at various distances from the axis were given by R. H. Bow as early as 1866. 4 This relation is rep- resented in Fig. 56 ; 5 the angles of view subtended by the diagonal of the plate are marked along the top of the graph, while the numbers iSfit J. Phot,, 1866, 13, 160. 5 Zschokke, Brit. J. Phot., IQ17, 64, 203. 104 PHOTOGRAPHY below are the ratios of the diagonal of the plate to the focal length of lens corresponding to the angle of view marked on the top line above the graph. The vertical line is marked in exposures, or intensity units, starting with a unit intensity of one on the base. The other cause of unequal illumination lies in the obstruction of the oblique pencils by projecting lens mounts. This occurs whenever the aperture of the lens is very large in proportion to the length of the mount itself. Most modern lenses, particularly anastigmats, are very compactly built with their components close to the diaphragm and the illumination is consequently more uniform than in the case of older tt* IV SV 55* Mf TO* TT* tf *0* <)S' BO* mS* TW* ^* ■I (- 33 o UJ 1 1 ? / / / > y ■1 .4 .» U VL M tt i» M U l-H 1.6 1* Diagonal divii«cl bj Jocal Uigth. Fig. 56. Relation between the Angle of View and the Diminution of the Optical Intensity of the Image. (Zschokke) lenses, such as the Petzval portrait objective, which have a much larger distance between the components in proportion to the relative aperture than do modern anastigmats. General Reference Works The various aberrations of photographic objectives are considered in practi- cally all of the reference works given in the bibliography following chapter III. The following may be recommended as being especially suitable. Hasting — Optics for Photographers. (English translation by Fraprie.) 1912. Lummer — Photographic Optics. (English translation by Thompson.) 1903. Von Rohr — Theorie und Geschichte des Photographischen Objektivs. 1899. CHAPTER V THE PHOTOGRAPHIC OBJECTIVE Introduction. — This chapter is a brief survey of the common types of lenses and the principles employed in their construction. The num- ber of lenses which differ but slightly from a few well-established con- structions is almost without number and owing to the limitations of space it is impossible to treat all of these. Accordingly the chapter will be devoted to the more important principles of construction which have been widely copied on account of their admitted excellence. Part I. The Astigmats Single Lenses. — It is impossible to correct a single lens in any way, hence it is unable to give sharp definition except with a very small diaphragm with the consequent sacrifice of speed. The spherical aberration is at the minimum when the lens is double convex and the radii of the surfaces are in the proportion of 1 : 6. Such a lens, how- ever, is useless photographically because it fails to cover a flat field satisfactorily. Even with small stops the image is sharp only in the center of the field and rapidly falls off towards the margins. In 1812 Wollaston showed that a much better image could be obtained with a concavo-convex, or meniscus, lens than with the usual bi-convex. Wollaston's meniscus (Fig. 57) with the concave side towards the sub- Fig. 57. Wollaston's Meniscus ject gives an image of satisfactory sharpness over a limited field when medium-sized stops are used. Since chromatic aberration cannot be corrected in a lens composed of a single piece of glass, the visual and chemical foci do not coincide and a correction must be made after 1 or, 106 PHOTOGRAPHY focussing. The amount of this correction is equal to the focal length of the lens divided by the v constant of the glass of which it is com- posed. With the glass generally used for lenses of this type the differ- ence of the two foci, or the chromatic difference, is equal to about 2 per cent of the focal length. The principal use of lenses of this type now is for diffused focus, impressionistic photography. Single Achromatic Lenses. — In the preceding chapter it was shown Fig. 58. The Chevalier or French Fig. 59. Grubb's Landscape Lens how it is possible to correct both chromatic and spherical aberration at the same time by cementing to a single collecting lens a dispersing lens of the proper power. A lens so constructed is termed an achromat, or is said to be achromatic, i.e. chromatically corrected. Such lenses may be comparatively well corrected spherically and are able to give sharp definition over a field of medium extent when used at a maxi- mum opening of about F/14 to F/16. Many lenses of this type were constructed in the early days of photography. In 1857 Grubb patented an achromatic lens composed of two concavo-convex meniscus lenses cemented together, the glass nearer the diaphragm being of crown and the other of flint (Fig. 59). In 1864 Dallmeyer introduced his " Rapid Landscape Lens " which is similar to the above (Fig. 60) but differs in the introduction of a third Landscape Lens Fig. 60. Dallmeyer's Rapid Landscape Lens THE PHOTOGRAPHIC OBJECTIVE 107 meniscus of crown glass for the purpose of securing better correction. In 1869 Goddard described a lens having the form shown in a of Fig. 61. For the purpose of securing better correction a convexo- concave meniscus of flint was added to the usual combination of crown and flint, an air space separating the meniscus from the double-con- cave negative lens. In the Dallmeyer " Rectilinear Landscape Lens " patented by Dallmeyer in 1888 the same principle is followed, the two cemented lenses being of crown glass and the separated meniscus of flint. This construction represents, perhaps, the highest attainment of Fig. 61 (a). Goddard's Landscape Lens (b) Dallmeyer's Rectilinear Land- a lens of this class. It has a relative aperture of F/14 and produces an image of satisfactory sharpness over a comparatively large field. The author has used one of these lenses for certain kinds of work for years and has found it perfectly satisfactory. Semi-achromatic Objectives or the Anachromats. — The use of partially corrected lenses at a large aperture so as to secure diffused images may be said to date from the introduction of such lenses by the French pictorialist, M. Puyo, and the construction of the Dall- meyer-Bergheim for Mr. Bergheim, a painter, in 1896. All of the lenses within this class are only partially corrected for chromatic or for spherical aberration and to this they owe the peculiar diffusion or " enveloping image " expressed so admirably by the French word " fiou." The Struss Pictorial Lens and the Kalosat are concavo- convex meniscus lenses of the type represented by Wollaston's me- niscus. The Smith Semi-achromatic, and Synthetic, the Spencer Port- land, the Gundlach Single Achromatic, Bausch and Lomb Plastigmat, Degen Objectif Anachromatique , Hermagis Eidoscope, Koristka Ars and Dallmeyer Soft-Focus are all lenses resembling the single achro- matic lenses previously mentioned in construction, but differing from > scape Lens 5 108 PHOTOGRAPHY them in the use of a much larger aperture and less thorough correction spherically. The majority of these lenses are chromatically corrected although the chromatic aberration of some is only partially carried out. The Gundlach Hyperion, Wollensak' Verito and Smith Visual-Qual- ity are double lenses formed of two components which alone are es- sentially single achromatic lenses. In the Hyperion three foci are available as both components may be used separately. In the Verito two foci only are available as only the back component may be used alone while the components of the Smith Visual-Quality cannot be used separately. The Dallmeyer-Bergheim is a semi-achromatic lense con- structed on the telephoto principle. It consists of a single positive and a negative lens, the space between the two being adjustable by rack and pinion. Varying the distance between the two elements alters the focal length so that the single objective is equal to a battery of lenses having a fixed focal length. The lens is not chromatically corrected and a correction must be made after focussing. The maximum aper- ture is F/6.5. Many of the longer foci anastigmats, which are used principally for portrait work, and also some of the Petzval type portrait lenses are fitted with diffusing devices which introduce a certain amount of aber- ration and soften the critical sharpness of the fully corrected image. The Double Achromat-Aplanat or Rapid Rectilinear. — The single objectives all possess, in addition to astigmatic deformation, coma, curvature of field and incomplete spherical correction, the very serious defect known as distortion. That is, straight lines cannot be repro- duced as such but are rendered as parts of curves and as the diaphragm must be placed before the lens with a single objective, the distortion is barrel shaped. By constructing a symmetrical objective consisting of two achromats the diaphragm may be inserted between the two so that the barrel distortion of one element is balanced by the pincushion dis- tortion of the other and the fault entirely corrected. In addition, owing to the superior correction which may be effected with the sym- metrical construction, a larger working aperture, and consequently greater speed, is secured. Thomas Ross appears to have been the first to make symmetrical ob- jectives. Ross, however, does not seem to have realized the advan- tages of the symmetrical construction in eliminating distortion and for reducing the amount of aberration so that a larger aperture may be obtained. In 1858, Thomas Sutton described a symmetrical triplet objective THE PHOTOGRAPHIC OBJECTIVE 109 composed of a pair of single, collecting lenses with a double-concave negative lens between (Fig. 62) and in i860 Dallmeyer introduced a lens of similar construction (Fig. 63) in which all three elements were cemented achromats. The same year Harrison and Schnitzer brought out the "globe lens" (B. P. 2496/1860). This consisted of two Fig. 62. Sutton's Triplet Fig. 63. Dallmcycr's Triple Achromatic cemented achromatic elements and was termed the globe lens because the outer surfaces formed portions of spheres having a common center (Fig. 64). It was free from distortion but as it was not well cor- rected spherically it had to be used with a comparatively small stop. It was therefore soon replaced by the Aplanat and rapid rectilinear of Steinheil and Dallmeyer respectively. A great advance was made in 1866 when Steinheil of Munchen in- -1 t Fig. 64. Harrison and Schnitzer's Globe Lens troduced a symmetrical objective which he termed the Aplanat (Fig. 65). About the same time, or shortly thereafter, J. H. Dallmeyer of London independently discovered the same construction which he patented under British Patent 1641 and 2502 of 1866. Steinheil evi- dently reached the conclusion that astigmatism would be lessened if the refractive indices of the two glasses employed in constructing the single achromats of a symmetrical objective were more nearly equal. Therefore instead of employing flint and crown glasses, as had his predecessors, he used instead two flint glasses. Dallmeyer also used two flints. 110 PHOTOGRAPHY The first of Steinheifs Aplanats had a relative aperture of F/8; while an even more rapid objective designed for portrait work was patented at a later date. The Aplanat is made in several series having various speeds according to the requirements of the work for which they are intended. Roughly these series may be termed the universal Aplanat, the group Aplanat, the portrait Aplanat, and the wide-angle Aplanat. It is the universal Aplanat with which the public is most familiar. This has an aperture of about F/8 and in many cases allows separate use of the components at F/14 to F/16. At these apertures the defini- tion is satisfactory for all but the most critical work or where the re- sults must be subsequently enlarged. The correction for chromatic Fig. 65. The Aplanat or R. R. aberration and distortion is very good but there is a small amount of spherical aberration, curvature of field and astigmatism remaining. However, where high speed is not required and the lens may be safely stopped down to a small opening, the rapid rectilinear is perfectly satisfactory and is an objective which does not involve a large outlay. The various makes are all made according to practically the same formula and there is no advantage in our discussing the various lenses individually. Practically all wide-angle lenses which are not anastigmats are of the Aplanat construction. For this purpose curvature of field requires to be kept as low as possible while coma and spherical aberration must be highly corrected: therefore the average wide-angle aplanat has a small maximum opening, generally about F/16 to F/18. The Petzval Portrait Lens. — The construction of the Petzval por- trait lens by Voigtlander in 1840 from calculations by Joseph Petzval, a mathematician of Vienna, may be said to mark the beginning of the THE PHOTOGRAPHIC OBJECTIVE 111 serious designing of photographic objectives, as well as the beginning of portrait photography. Before this time the photographer had at his disposal only the single lens with a small aperture and with the in- sensitive materials then available the exposures were so long that por- traiture was practically out of question, except under the most favor- able conditions. The Petzval portrait lens however changed all this and with its aperture of F/6, which was almost immediately increased to jF/4 by Andrew Ross, portraiture for the first time became really Fig. 66. Portrait of Petzval practicable. The development of photography owes much to Petzval's achievement, for coming directly after the discovery of the daguerre- otype it made that process of practical value and thus immensely in- creased the importance of the subject to the general public. The Petzval lens consists of four lenses in two combinations (Fig. 67). The front combination consists of a positive lens of crown glass cemented to a negative lens made of flint glass, while the rear combina- tion consists of two separate lenses, the negative lens being convexo- 112 PHOTOGRAPHY concave and of flint, while the positive lens is double-convex and made of crown. It is completely unsymmetrical and, because of its good correction for spherical aberration and coma, the central sharpness is excellent. It is free from distortion and is chromatically corrected. In opposition to these valuable features it has faults which limit its Fig. 67. Petzval's Portrait Objective usefulness and have caused it to be practically replaced by the later anastigmats, except for studio portraiture. It is not astigmatically corrected, it covers a very small field, has a decided curvature of field, and owing to its length there is a considerable amount of vignetting, or a diminution of intensity towards the margins, which renders it un- suitable for landscape or other work requiring sharp definition from / 3» r 1 \ L 1 1 3SS> n 1 V J 1 4j— ► Fig. 68. Modifications of the Petzval Portrait Objective. Upper, Dallmeyer; Center, Voigtlander; Lower, Zincke-Sommer THE PHOTOGRAPHIC OBJECTIVE 113 corner to corner of the plate. Nevertheless, appearing as it did before any serious attention had been paid to the subject of photographic lens designing, it marks a most brilliant achievement — the greatest single achievement in the annals of the photographic objective. Petzval's original construction has been several times modified by later opticians in order to improve its performance. The most im- portant changes are those of Dallmeyer, Voigtlander and Zincke-Som- mer. In 1866 J. H. Dallmeyer modified the original Petzval design so as to obtain better spherical correction. The change consisted in revers- ing the glasses of the rear combination, placing the flint glass in the rear, the lens in other respects remaining practically unaltered (Fig. 68). In 1879 (D. R. P. 5761/1879) Voigtlander changed the back com- bination to a plano-convex collecting lens of crown to which he ce- mented a concavo-convex lens of flint, the air space being removed completely (Fig. 68). In 1870 H. Zincke-Sommer further modified the original design in order to obtain an increase in the relative aperture. The change con- sisted in placing the positive lens before the negative and leaving an air space between as shown in Fig. 68. The relative aperture was by this means increased to F/2.3. Practically all of the modern portrait lenses, which are not anastig- matic, are constructed according to the Petzval formula, or on calcu- lations based upon the same, and it is beyond the scope of this work to discuss the various lenses of this class now on the market and little of practical value would be gained by so doing. Fig. 69. Cooke F/1.5 Modification of the Petzval Construction Lately patents have been taken out for developments of the Petzval construction for use in cinematography. One of these (B. P. 258,092 of 1925) by Warmisham is for an objective of the type illustrated in Fig. 69, and having a relative aperture of F/1.5. The other patent (B. P. 299,983 of 1927) by H. W. Lee is for a 114 PHOTOGRAPHY combination in which the front doublet of the Petzval lens is replaced by a triplet anastigmat. The back doublet may be of the original Petzval form but better correction is secured by interchanging the two back components ; i.e., placing the Petzval doublet between the dis- persive element and the rear collective element of the triplet. This results in a lens of aperture F/i.8 with an anastigmatically flattened field of about 35 degrees. PART II. THE ANASTIGMATS ' Introduction. — Regardless of how well they may be corrected other- wise all the lenses which we have hitherto investigated contain a seri- ous amount of astigmatism. The character, cause, and correction of astigmatism were discussed in the preceding chapter and if the mat- ter is not clearly in mind this section of the chapter should be re- viewed before proceeding. Steinheil made one step towards the correction of astigmatism in the construction of the Aplanat but complete correction was possible only after the production of the newer varieties of glass by the Jena Glass Works. In order to correct spherical aberration in a cemented system it must have a surface convex to the medium of higher re- fraction, while in order to correct astigmatism the surface must be convex to the medium of lower refractive index. With the varieties of glass known before the introduction of Jena glass the refracting power increased in the same ratio as the dispersing power and it was impossible to .make a spherically corrected achromat that would also be anastigmatic. However after the introduction of Jena glass it became possible to make the collecting lens of glass having higher refraction and lower dispersion than the dispersing lens and thus secure astigmatic correc- tion. The old achromat made of ordinary crown and flint could be corrected spherically but not astigmatically ; the new achromat, how- ever, while corrected astigmatically cannot be spherically corrected. However, by combining the ordinary crown and flint of the old achro- mat with the barium crown glass of the new achromat it became pos- sible to correct both astigmatism and spherical aberration in the same lens. By combining two such achromats both of which are individ- ually corrected to form a symmetrical objective a larger aperture be- comes possible together with correction for curvature of field, coma, and distortion. This is the principle followed in the construction of THE PHOTOGRAPHIC OBJECTIVE 115 most of the symmetrical lenses as the Goerz Double Anastigmat or Dagor, the Voigtlander Collinear, Turner-Reich Anastigmat, etc. The other method is to use two dissimilar combinations, one of which is spherically corrected while the other is astigmatically corrected, the two when placed on opposite sides of the diaphragm forming an unsymmetrical objective which is completely corrected as a whole but not individually. This is the principle followed in the construction of most of the high-speed anastigmats such as the Tessar, Heliar, etc. Cemented Symmetrical Objectives. — The Double Anastigmat. — The Goerz Double Anastigmat, better known in this country as the Dagor, • is a symmetrical objective consisting of two similar combinations each of which is composed of three lenses (Fig. 70). The indices of re- Fig. 70. The Goerz Dagor fraction of the three glasses increase as we pass from one glass to another in the direction of the incident light, i.e. from the diaphragm, as we are considering the single element. Hence the first cemented surface is convex to the medium of lower refraction satisfying the requirements for spherical correction. The second cemented surface is convex to the medium of higher refraction and thus satisfies the requirement for astigmatic correction. Consequently the single ele- ment of three cemented lenses is corrected both spherically and astig- matically in a very simple manner. The Dagor is excellently corrected. Its single elements do not equal the best corrected single lenses, but at an aperture of F/13 give a degree of definition that is satisfactory for most purposes. The relative aperture of the symmetrical objective is F/6.8 for the shorter foci and somewhat less for the longer. Particularly noticeable is the wide field which is critically defined at large apertures and also the extension of this field by the use of smaller stops. In this respect the Dagor is hardly to be surpassed by any other objective. Among other lenses which are constructed according to practically the same principles may be mentioned the following: 116 PHOTOGRAPHY Meyer Double Aristostigmat, F/6.8, F/5.4, F/4.2, Plaubel Triple Orthar, F/6.8 and F/5.4, Koristka Meridan, F/6.8, Hermagis Aplanastigmat, F/6.8, Zeiss Amatar, F/6.8, Degen Double Anastigmat " Normos," F/6.3, F/7.4, Wray Universal Anastigmat, F/6.8. Alternate Form of the Double Anastigmat. — The principle em- bodied in the design of the Double Anastigmat just described is sub- ject to several alterations. A second arrangement consists in the use Fig. 71. Watson's Holostigmat of two negative lenses, one having a lower, the other a higher index of refraction than the interposed positive lens (Fig. 71). Here the arrangement of cemented surfaces is reversed, the indices of re- fraction of the three glasses decreasing as we pass away from the diaphragm. This form of the double anastigmat has been made for several years by Watson of London as the Holostigmat in two series, one with a maximum aperture of F/4.6 and the other F/6.1. The Steinheil Satz-anastigmat also has this design. Practically there is e f u Fig. 72. The Collincar and Orthostigmat of Voigtlander and Steinheil little difference in this form and the one previously described and the corrections of both may be equally well carried out. A further modification (Fig. 72), which was adopted in the manu- facture of both the Collinear and the Steinheil Orthostigmat, consists of a meniscus of low refraction interposed between a double-convex positive lens of high refraction and a double-concave negative lens THE PHOTOGRAPHIC OBJECTIVE 117 of medium refraction. If the central meniscus having low refraction is made quite thick the correction can be carried out very well so that the single elements may be used separately if stopped down. The relative aperture of the Collinear of Voigtlander is F/5.6 while the Steinheil Orthostigmat is made in series up to a maximum aperture of F/6.8. The Four Glass Element — the Protars. — Rudolph's earlier Protars were unsymmetrical anastigmats consisting of two dissimilar com- binations, one being an old achromat spherically corrected and the other a new achromat astigmatically corrected. With the exception of a series for wide-angle work these are no longer made. The series Vila Protar with which we are most familiar is a symmetrical anastig- mat' containing two like combinations each of which consists of an -fit Fig. 73. Rudolph's Protar, Series Vila old and a new achromat (Fig. 73). The single element thus consists of four cemented lenses, the two nearest the diaphragm comprising the new astigmatically corrected achromat, while the other pair are similar to the old Steinheil aplanat and form the spherically corrected old achromat. This single element is the basis of the convertible Protar Series Vila. The speed of the single elements is F/12.5 and because of the increase in the number of glasses to four the lens is excellently cor- rected, its nearest competitors as regards definition being the single elements of the three glass type previously described. When a single element is used separately it should always be placed behind the diaphragm, as it is corrected for rays incident in that direction. When the complete double objective is used the element having the longer focal length should be placed in front, as the correction of the double 118 PHOTOGRAPHY objective depends on having the course of the light rays between the two elements parallel. In comparison with objectives formed of three-lens elements, the single element of the four-lens element is superior but the same does not necessarily hold for the double ob- jective. The design of the Ross Combinable is identical but by changes in the glasses used it has been possible to increase the aperture of the single element to F/li and the double objective to F/5.5 without sac- rificing in any way the corrections of the objective. The principal change consists in the use for the double-concave negative lens, of fluor-crown glass having very low refraction and dispersion for the boro-silicate crown glass usually used. The Five Glass Element. — Before the introduction of the anastig- mat a form of the aplanat was made consisting of three instead of two lenses for the purpose of securing better correction. When the in- troduction of the newer glasses made possible the construction of the new astigmatically corrected achromat Turner and Reich added the ■new achromat to their former three glass aplanat to form the Turner- Reich anastigmat (Fig. 74). This has an aperture of F/6.8 and is Fig. 74. The T-R Anastigmat distinguished by the excellent correction of its single elements. Espe- cially noticeable is the great covering power of the lens at full aper- ture, which is greatly increased if a smaller stop is used, and the lens may then be used as a wide-angle lens on a plate much larger than that for which it was originally designed. In this respect the Turner- Reich anastigmat is unsurpassed. Symmetrical Lenses with Air Spaces — Celor and Syntor of Goerz. — In a cemented system of three lenses containing a bi-concave and a bi-convex lens with an interposed collecting meniscus of low refrac- tion, such as the Steinheil Orthostigmat, anastigmatic flatness is se- cured and spherical aberration corrected by the use of two spherical ce- THE PHOTOGRAPHIC' OBJECTIVE 119 merited surfaces, one of which acts as a collecting and the other as a dispersing lens. In the Celor and Syntor of Von Hoegh (Fig. 75) the interposed collecting meniscus is replaced by an air space, so that it consists essentially of a double-convex collecting lens separated from a double-concave negative lens by an air space having the form of a positive meniscus. Corresponding to the three-lens system this two- lens system contains two contact surfaces, one of which is collecting and the other dispersive, but the contact surfaces are between air and not glass of low refraction. The two-lens system may thus be re- garded as a system derived from the three-lens element by decreasing Fig. 75. The Celor and Syntor of Goerz the necessary power of refraction of the enclosed meniscus until it becomes equal to the refractive index of air, or unity. Then it be- comes possible to eliminate the central meniscus and replace it with an air lens, simplifying the construction and making the lens easier to manufacture. Lenses made according to this design were introduced by Goerz about 1900 in two series, the Celor having a relative aperture of -F/4.5 and the Syntor with a relative aperture of F/6.8. The spherical and astigmatic errors of this design are small and can be practically eliminated or at least to the same degree as in the three glass element, but complete removal of coma is impossible. While coma may be entirely absent in a cemented element of three glasses, complete comatic correction is impossible for an element of two separate lenses without the loss of anastigmatic flatness of field, hence objectives of this class have a certain amount of coma which increases as the aper- ture is enlarged. Lately calculation has shown that better comatic cor- rection may be secured by departing slightly from absolute symmetry in design and an objective of this type but not completely symmetrical 120 PHOTOGRAPHY has been introduced by Goerz as the Dogmar (Fig. 76). The single elements of the two-lens system can be used separately only with very small stops, as its corrections are not nearly so complete as the ce- mented three glass element. A symmetrically constructed objective, each half of which is com- posed of four glasses arranged in pairs with a meniscus shaped air- Fxg. 76. The Dogmar of Goerz space between them, was patented by Warmisham in 1925 (B. P. 260,801). The objective has an aperture of F/4.5 and gives good definition over a field of Jo° . This is supplied by Taylor, Taylor and Hobson especially for aerial mapping. The Gauss Lens. — The Gauss objective is derived from the Gauss telescope and in its simplest form consists of two menisci separated from each other by an air space and having their concave sides fac- ing the incident light (Fig. 77). This type of construction is par- Fig. 77. The' Gaussian Objective ticularly favorable for reducing spherical aberration and may also be well corrected chromatically and astigmatically. In 1896 Paul Rudolph calculated for Carl Zeiss of Jena an ob- jective along the lines of the Gauss construction, which was placed upon the market as the Planar (Fig. 78). The Planar differs from the essential form of the Gauss construction as shown in Fig. 77 by the replacement of the inner menisci by two cemented lenses. These two lenses are made of glasses having identical refractive index but different dispersions. Therefore the inner cemented lenses act as a single lens so far as the refractions of any single color are concerned, THE PHOTOGRAPHIC OBJECTIVE 121 while owing to the difference in the dispersive values of the two glasses the amount of chromatic aberration may be altered simply by changing the curves of the two cemented surfaces which separate the two mediums of different dispersion. In this way the chromatic aberration of the two cemented lenses may be made to equalize the chromatic aberration of the two outer lenses so that satisfactory color correction may be obtained at the same time that the astigmatic and spherical errors are corrected. The relative aperture of the Planar is F/3.5, but owing to the presence of considerable coma the defini- tion at this aperture is not critical and stopping down is necessary for critically sharp definition. The Planar is no longer made, having been replaced by the unsymmetrical anastigmats which have approxi- mately equal speed and superior correction. H. Kollmorgen was the first to show that the Gaussian objective might be chromatically corrected without altering its form or affecting its astigmatic correction. Kollmorgen's method was to make each combination of an anomalous glass pair ; i.e. the coefficient of refrac- tion of the collecting lens with low dispersion must be as large or larger than the dispersing lens. A construction from calculations by Kollmorgen was placed upon the market by Hugo Meyer of Goerlitz in Germany as the Aristostigmat. This objective (Fig. 79) is made in- several series from F/4.5 to F/6.8. The strict symmetrical con- struction is departed from in the larger aperture series in order to ob- tain better comatic correction. Especially notable is the large flat field of this objective which is a characteristic of the Gaussian construction. There is a considerable amount of coma, however, which limits the effective aperture when critical definition is required. Identical in construction with the Meyer Aristostigmat is the Ross Homo centric which is made in four series: F/5.6, F/6.3, F/6.8 and F/8. The single elements of the last three series may be used alone. Fig. 78. Rudolph's Planar 122 PHOTOGRAPHY The Plaubel Double Orthar appears to be of similar construction. It is made in two series with relative apertures of F/6.3 and F/6.8. The Omnar of Emil Bush is along the same lines but the corrections were worked out in a different manner. In the Omnar the two glasses have different indices of refraction, the negative lens having the higher refracting power than the positive, while in the previously de- scribed construction both lenses have practically identical indices of refraction but varying dispersions. The Omnar can be made without the use of any of the newer varieties of Jena glass and is completely anastigmatic. The chromatic and spherical aberrations are well cor- rected and the objective is notable for its large flat field. Coma, how- ever, is present as in all lenses of this construction. In 1920 Taylor, Taylor and Hobson and H. W. Lee patented an improved form of the Gaussian construction which was later intro- duced as the Cooke Series O, Opic F/2. This objective (Fig. 80) consists of six components symmetrically arranged but not identical, two of which are simple meniscus collecting lenses of dense barium Fig. 79. Kollmorgen's Aristostigmat 3S» Fig. 80. Lee's T. T. H. Opic THE PHOTOGRAPHIC OBJECTIVE 123 crown nD 1.6. Between these lenses are two compound dispersive components each consisting of a plano-convex collecting lens of light flint cemented to a plano-concave dispersing lens of barium crown. The difference in the refractive indices of the two cemented lenses must be at least 0.03. The lens has an aperture of F/2 with an angle of view approximately equal to lenses of similar focal lengths working at F/4.5. Since its introduction it has been extensively employed in Fig. 81. Zeiss Biotar F/1.4 press photography, for theater snapshots, and other work requiring short exposures in poor light. The Zeiss Biotar F/1.4 is a Gaussian type of objective which is shown in Fig. 81. The corrections compare favorably with those of the Tessar series F/4.5 and the lens is suitable for color work. It is made in four focal lengths: 25 and 35 millimeters for 16 millimeter cinematograph cameras and 45 and 75 millimeters for standard cine- matograph cameras (B. P. 297,823 of 1927). The Plasmat of Dr. Paul Rudolph. — In 19 18 Dr. Paul Rudolph, the Fig. 82. Rudolph's Plasmat designer of the Planar, Unar, Tessar, Protar and numerous other objectives, calculated a symmetrical objective known as the Plasmat which was placed upon the market by Hugo Meyer of Goerlitz and Suter of Basel, Switzerland. The Plasmat (Fig. 82) consists of two 124 PHOTOGRAPHY similar combinations, each of which is composed of a plano-convex or convexo-concave collecting lens cemented to a double-concave dis- persing lens and a thin meniscus separated from the cemented pair by an air space. The greatest curvatures are all concave to the dia- phragm. The relative aperture is F/4 for the double lens composed of two F/8 elements of equal focal power and F/5.5 for a double objective composed of two elements of unequal focal lengths. The original Plasmats were followed seven years later by two ad- ditional series intended especially for cinematography. These have relative apertures of F/2 and F/1.5 and are known as the Kino- Plasmats (U. S. P. 1,565,205). The construction is fundamentally the same as in the earlier series, the pair of interior dispersing me- niscuses, however, are turned the opposite way from those in the earlier series. The separate components of these lenses are not de- signed for use separately. In the designing of the various anastigmats which have been de- scribed, it has been the aim of the constructors to obtain the most per- fect image of an object plane upon an image plane; i.e., the conditions as exist in copying. The planes before and behind this object plane vary in sharpness with their distance from the plane of sharp focus and with the aperture and focal length of the lens. The depth of field as calculated mathematically is based upon a theoretical lens free from all defects whatsoever. As no objective is perfect, the depth of focus may be less than that indicated by mathematical formulae. The more thoroughly corrected a lens, the more nearly will its representation of depth approach that of the perfect lens. In the ordinary anastigmat the spherical correction is not equally perfect for each color. Rudolph claims to have accomplished this in the Plasmat. Owing to this superior correction of the spherical aber- rations for the different colors, the Plasmat has a greater power of definition; the range of sharp reproduction covers a greater depth of image. Rudolph's claim that the Plasmat shows a greater depth of focus than other lenses of the same optical constants, has aroused a great deal of ' discussion in optical circles. That small differences in correction can influence depth is denied by many. Rudolph claims that his Plasmat has shown this to be true ; others are yet unconvinced. Dr. Zschokke,1 for instance, claims that the greater depth and plasticity in the rendering of the planes is due to the presence of a small amount of uncorrected chromatic aberration. 1 Phot. Ltd., 1921, p. 257. THE PHOTOGRAPHIC OBJECTIVE 125 Two lenses which outwardly resemble the Plasmat have been pat- ented by Hasselkus and Richmond for Ross Ltd. of London (B. P. 295,519 of 1927) and by Carl Zeiss (B. P. 278,338 of 1926). The purpose of both of these is to provide a convertible lens of large aper- ture and possessing a large corrected field. The Ross Airo constructed from the above patent has an aperture of F/4. and covers an angular field of 70 degrees and is especially recommended for aerial map mak- ing. The Zeiss Orthometar designed for the same purpose has an aperture of F/4.5 and a field of 65 degrees. Steinheil's Unofocal. — The usual method of correction for spheri- cal, astigmatic and other aberrations is, as we have seen, the opposi- tion of powerful lenses of different powers so as to secure compensa- tion of the opposing kinds of aberration. In the Unofocal calculated by Steinheil the conditions necessary for securing an anastigmatically Fig. 83. Steinheil Unofocal flat field are met with lenses of very low power free from excessive curvatures and glasses of great thicknesss, both of which react favor- ably on the performance of the objective. In the Unofocal (Fig. 83) there are two double-concave dispersing lenses interposed between two exterior collecting lenses, the general appearance of the objective closely resembling the Celor and Syntor of Goerz. The construction of the Unofocal, however, is based upon an entirely different principle and should not be considered to be in any way allied to the objectives just named. In the first place all four lenses are made of glass having practically the same refractive index and are all of the same focal power. If the lenses were placed in contact this would result in complete neutralization, so that the 126 PHOTOGRAPHY system would no longer have a positive focus, but by slightly separat- ing the elements a converging system having a positive focus is se- cured. Spherical aberration is corrected by two refractions in the same direction assisted by the relations of the lenses and the facing surfaces. Flatness of field is made possible by the use of glass having nearly the same refractive index, while achromatism is secured by suitably balancing the dispersions of the glasses. This design makes possible the construction of an anastigmat which owing to its shallow curves and simple construction is easily manu- factured. The performance of the lens is excellent. The Unofocal is made by Steinheil in several series, the maximum having a relative aperture of F/4.5. There are several other lenses on the market which are based upon the Steinheil principle. Among those which depart but little from the original design the Gundlach anastigmat may be mentioned. This has a relative aperture of F/6.3. The Graf Anastigmat and Variable.— The Steinheil Unofocal is a symmetrical lens, the two glass pairs on either side of the diaphragm being identical. In 191 1 an American, Christopher Graf, designed an unsymmetrical anastigmat based upon the Steinheil design, which was introduced several years later as the Graf Anastigmat. In this objective (Fig. 84) the two inner dispersive lenses are identical with respect to each other and are symmetrically placed. The exterior collecting lenses, however, while similar are not identi- cal either in form or position. An especially notable feature of this construction is the most ex- cellent quality of diffusion secured by lessening the separation be- tween the dispersing lenses and the front collecting lens. The ob- jective is mounted in an adjustable mount in order that the amount of Fig. 84. Graf Anastigmat THE PHOTOGRAPHIC OBJECTIVE 127 diffusion may be regulated by the worker to suit the demands of the subject. The displacement of the collecting lens lengthens the focus of the system, and since the actual aperture remains unaltered, the speed consequently becomes less. The relative aperture of the ob- jective when adjusted for full correction is F/3.8; when set for the maximum usable degree of diffusion, F/4.5. The Beck Isostigmar and Neostigmar. — These lenses were intro- duced by Beck Limited of London, the former in 1906 and the latter in 1910. They differ from the lenses which we have previously de- scribed in that they do not obey the so-called " Petzval condition." According to the Petzval condition in order to secure an anastigmatic objective with a flat field the sum of the focal powers of the individual surfaces, when divided by the product of the refractive indices on either side of the surface, should be zero. These lenses were worked out on certain lines which do not take the Petzvel condition into con- sideration, so that these lenses do not obey the same. The Isostigmar is a five-lens system (Fig. 85), all of the lenses being of low power and uncemented. The two exterior are collective; the three interior dispersive. By careful calculation of the curves and the separation of the components an objective can be calculated which is very well corrected for spherical and astigmatic errors together with coma. It is made in several series with a maximum aperture of F/3.5 in the shorter foci and F/4.5 in the longer. The Neostigmar is a later introduction and is simpler in construc- tion. The single combinations are better and can be used at a larger aperture. It is a four-lens objective of unsymmetrical construction Fig. 8s. Beck's Isostigmar 128 PHOTOGRAPHY (Fig. 86). The two exterior lenses are collecting; the interior dis- persing. Convertibility is secured by removing the third or fourth lens and using the remainder of the system. The Neostigmar is made in several series with apertures of F/6 and F/7.7. One of these series is especially notable for the large field covered with satisfactory definition at F/6. 3 and may be used as a wide-angle lens. The Dallmeyer Stigmatic. — The opposition of a spherically cor- rected glass pair and an astigmatically corrected glass pair, a method adopted by Rudolph in his early Protars, suffers from the disadvantage Fig. 86. Beck's Neostigmar Fig. 87. Dallmeyer Stigmatic that simultaneous correction for astigmatism and spherical aberration is impossible at a large aperture. The conditions necessary to the re- moval of spherical aberration ipso facto increase the amount of astig- matism; hence the objective cannot be well corrected for large aper- tures. In the Stigmatic, calculated for J. H. Dallmeyer by H. L. Aldis, this difficulty is overcome in a novel way. The complete objective (Fig. 87) consists of two new glass pairs to one of which has been added a strongly converging meniscus lens separated by an air space. The two cemented surfaces of the two elements formed of a new glass pair enable astigmatic correction to be obtained, while the addition of the thin, strongly converging meniscus lens enables the opposite spherical effect to be compensated for without affecting the astigmatic correc- tion. The Stigmatic could therefore be made to work at larger aper- tures than the Rudolph objective. It was formerly made in several series, the largest with an aperture of F/4, but is now made in only one series with an aperture of F/6. Rudolph's Early Protar Lens. — Earlier in this chapter we referred to the different methods adopted by Emil Von Hoegh and Paul Rudolph for the correction of astigmatism in photographic objectives. In the pages immediately preceding we traced the development of the symmetrical objective from the double objective formed by the union THE PHOTOGRAPHIC OBJECTIVE 129 of two of the three glass elements as patented by Von Hoegh and Goerz in 1893. In the following pages we propose to trace the de- velopment of the unsymmetrically constructed objective from Ru- dolph's early Protar lens. Paul Rudolph of the Carl Zeiss Werkstatte at Jena was one of the first to take advantage of the newer glasses introduced by the Jena Glass Works in the construction of photographic lenses. The first of Rudolph's anastigmatic objectives with which we are concerned was introduced in 1890 under the name Protar. This objective (Fig. 88) consisted of two glass pairs, one being the old normal glass pair made from the old glasses, the other the abnormal glass pair made from the newer varieties of Jena glass. One of the combinations has a positive astigmatic difference, i.e. the focal length of the rays in the primary section is greater than in the secondary section ; while the other com- bination has a negative astigmatic difference, the focal length of the rays in the primary section being less than in the secondary section. By proper construction, the two opposite effects may be so balanced as to compensate one another so that there is no sensible astigmatic differ- ence when the complete objective is said to be an anastigmat. In the normal glass pair the refractive index of the positive lens is lower than, the adjacent negative lens, while in the abnormal glass pair the positive lens is of higher refractive index than the negative. Both components are separately achromatized, but are not necessarily of the same focal power, as one component may be strongly positive while the other may act simply as a corrective system for the whole. This construction because of its relatively small aperture is no longer made, except for wide-angle objectives where its extensive field makes it very suitable. The wide-angle Protar has this form. The Introduction of Air Spaces — the Unar. — Some nine years after, Rudolph realized the advantages to be gained from replacing the cemented surfaces by air spaces having opposite power. This princi- Fio. 88. Rudolph's Unsymmetrical Protar 130 PHOTOGRAPHY pie was applied to the construction of the Unar introduced by Carl Zeiss in 1899. Several forms of objectives constructed on this princi- ple were described by Rudolph, from which that illustrated in Fig. 89 was adopted for the commercial product. It will be observed that this objective consists of two pairs of glass surfaces, each pair consisting of two surfaces which face one another, i.e. which belong to two consecutive lenses and are separated by an air space but not by the diaphragm. The powers of both pairs of facing surfaces are of opposite sign, the first having a positive astigmatic difference, the second a negative astigmatic difference. The effect of the two pairs of facing surfaces of opposite power is similar to the result obtained in the early Protar of 1890, by the difference in the refractive indices of the crown and flint lenses in the cemented components of the doublet. Just as the two cemented surfaces, one convex to the medium of higher refraction, the other to that of lower refraction, produce opposite effects of astigmatism, so do the pairs of facing surfaces of opposite sign produce the opposite effects of astig- matism which may be completely compensated by making the two op- posite effects of equal power. The introduction of the air space en- ables a greater degree of correction to be obtained, as the number of elements of correction are considerably increased. The aperture of the Unar was F/6. It is no longer made, having been replaced by the Tessar introduced by Rudolph in 1902. Combination of Air Space and Cemented Surface — the Tessar. — In 1902 Rudolph patented the objective issued by Carl Zeiss as the Tessar. The Tessar may be described as a combination of the early Protar and the Unar. It consists (Fig. 90) of four lenses divided into two groups which are separated by the diaphragm. The first group contains a collecting and a dispersing lens separated by an air space having a negative effect. The second group consists of a ce- Fig. 89. Rudolph's Unar THE PHOTOGRAPHIC OBJECTIVE 131 mented negative and a positive lens, the cemented surface having a positive effect. In the early unsymmetrical Protar lens the opposite effects by which astigmatic correction is secured are derived solely from the action of the cemented surfaces. In the Unar the said correction is based on the opposite powers of the two pairs of facing surfaces. In the Tessar these opposite astigmatic effects are obtained by giving to the power of the cemented surface the opposite sign to that presented by the facing surfaces of the other group of lenses. The series of F/4.5 Tessars was followed by one having a relative Fig. 90. Rudolph's Tessar aperture of F/3.5 and built to substantially the same formula (B. P. 273,274 of 1926) and later a series for cinematograph cameras and small cameras working at F/2.7 (B. P. 256,586 of 1925). With the object of improving the comatic correction of the original Tessar, a modified construction was calculated by Bielicke in 1925 (U. S. P. 1,558,073) ; this probably forms the basis of the B. and L. Tessar Series Ic, F/4.5. In 1917 as a result of calculations by Merte of the Technical staff of Carl Zeiss the spherical corrections of the original Tessar were im- proved and the Tessars now produced by Zeiss are probably built ac- cording to this later formula which differs from the original, however, only in the types of glass and the curvatures of the individual glasses (U. S. P. 1,476,195)- On account of its excellent corrections which place it among the best of photographic objectives, and its relatively simple construc- tion which makes it more easily manufactured than other lenses of more complex construction, the Tessar construction has been widely copied and since the patents have expired in practically all countries 132 PHOTOGRAPHY there are numerous lenses by various firms which are essentially identi- cal with the Tessar. The following lenses are of the Tessar type : Aldis Series I : F/4.5 Ernemann Ernon F/4.5 Kodak Anastigmat F/4.5 Koristka Sideran F/4.5 Krauss Trianar F/4.5 Laack Dialytar F/4.5 Plaubel Anticoma F/4.5 Riidersdorf Acomar F/4.5 Salmoirach Phoebus F/4.5 Schneider Xenar F/4-5 Tiranty Transpar F/4.5 Wollensak Velostigmat Ser. II F/4.5 Combination of Air Space and Cemented Surfaces — Further De- velopments.—In 1912 Dallmeyer Limited patented (B. P. 27,518 of r? LS Fig. 91. Dallmeyer Serrac 1912) an objective on the principle of the Tessar which was introduced commercially as the Serrac. This objective (Fig. 91) is identical in design with the Tessar but the glasses are changed and necessarily the radii. In the Tessar the first and fourth glasses are the same, having high dispersion, while the second is of glass having a lower refractive index but higher than the third lens, which is made of glass with a medium refractive index. In the Serrac, as in the Tessar, the first and fourth lenses are of glass with a high refractive index but, unlike the Tessar, the second and third lenses are composed of identical glasses having a medium refractive index. The use of the same glasses THE PHOTOGRAPHIC OBJECTIVE 133 for both of the dispersing lenses makes it possible to secure complete astigmatic correction with shallower radii than is possible in similar systems in which one of the lenses is composed of glass with a high refractive index and low dispersion and the other of low refraction and high dispersion and this reacts favorably on the other corrections. The Dalmac F/3.5 issued by Dallmeyer Limited is a recalculation of the Serrac, only such changes having been made as were necessary to secure satisfactory performance at the larger aperture. In 1913 Ross Limited patented the X-press (B. P. 29,637 of 1913), a lens based upon the same principle as the Zeiss Tessar but differing from it in the use of a rear element consisting of three glasses instead of two (Fig. 92). The first lens of the rear component is made of glass with low refraction, the second with medium refraction and the third with high refraction. The gradual increase in the indices of refraction together with the two cemented surfaces available enables the complete objective to be so corrected that the usual errors are at a minimum for this type of construction. Both cemented surfaces in the rear component are collective in effect. In 1921 the Gundlach Manhattan Optical Company of Rochester, N. Y», introduced the Radiar (Fig. 93), a lens resembling the Ross X-press in the use of a rear element of three glasses but differing from it in that one of the cemented surfaces is collective while the other is dispersive. The three glass element was adopted because it allows a different selection of glasses and affords more latitude in balancing the powers of the component parts of the system. The Uncemented Triplet — the Cooke Lens. — The triplet design was applied to the construction of photographic objectives at a very early date. As early as 1841, Andrew Ross made for Fox-Talbot a Fig. 92. Ross X-press Fig. 93. Gundlach Radiar 134 PHOTOGRAPHY triplet which consisted of a concave dispersing lens of flint, interposed between two equi-convex collecting lenses of crown, and some years later Sutton worked out a similar construction, while in i860 Dall- meyer issued his " Triple achromatic lens " in which each of the three components consists of a cemented collecting and dispersing lens sepa- rately corrected for chromatic aberration. The Cooke lens patented by Mr. H. Dennis Taylor, however, cannot properly be considered as a development of the earlier triplets. Al- though both are triplets, the construction of the Cooke is based on a radically different principle, and the two can be said to be similar only in outward appearance. In none of the earlier triplets is the power of the negative lens more than a small fraction of the sum of the collect- ing lenses, and the idea of utilizing the dispersing lens for flattening the image, and correcting marginal astigmatism, apparently never oc- curred to these men. Fig. 94. Cooke Triplet The Cooke lens is a simple triplet (Fig. 94) consisting of a double- concave dispersing lens interposed between two double-convex collect- ing lenses. The two collecting lenses are identical and are designed to be practically free from coma ; this result being secured by the use of an intermediate form of lens in which the inward coma of one surface is neutralized by the outward coma of the opposite surface. The burden of correcting the entire system is thrown upon the central dispersing lens, which fulfills a threefold office: (1) It flattens the final image and corrects marginal astigmatism, producing a flat astig- matic field; (2) it corrects the color aberrations of the convergent lenses and makes the complete system achromatic; (3) it corrects the residual spherical aberration of the two collecting lenses and renders the final image aplanatic. The action of the negative lens in securing a flat field, free from astigmatism, may be explained with the aid of Fig. 95. L and N are respectively a positive and negative lens of equal focus and made of the same materials. With primary sections of the oblique pencils, the image formed by the positive lens, L, is THE PHOTOGRAPHIC OBJECTIVE 135 curved spherically, p-P, but the interposition of the negative lens, N, throws back the image to the plane Q-Q' which is flat because the curvature errors of the positive lens, L, are exactly neutralized by the opposite curvature of the negative lens, AT. Owing to the fact that this arrangement would result in consider- able distortion, and to the fact that its corrections to a certain extent depend upon the distance of the subject, the single collecting lens is divided into two which are alike in power and shape but turned in opposite directions. The positive focal power of the two converging Fig. 95. Action of the Central Diverging Lens lenses is approximately equal to the negative focal power of the central dispersing lens. Considering the simplicity of construction and the limited number of elements which the designer possessed to correct the usual aberra- tions, the Cooke lens is well corrected and at the time of its introduc- tion (1895) surpassed the anastigmats then known in sharpness of definition over the usable angle of view. Especially notable is the al- most complete absence of coma. The Cooke triplet construction is followed by Taylor, Taylor and Hobson of Leicester, England, in the construction of several series of lenses ranging in speed from F/3.1 to F/8. The more rapid series intended for portrait work in the studio naturally are corrected for a much smaller field than those of smaller relative aperture. Owing to the simplicity of the Cooke triplet construction, its performance and to the fact that the patents have now expired, numerous manufacturers issue under various trade names objectives based upon the same princi- ple. We mention a few: Aldis Series O F/3 Goerz Hypar F/3.5 Riidersdorf (Portrait Anastigmat) Rodenstock Eurynar (Older models) F/4.5 136 PHOTOGRAPHY Rietschel Tular F/6.3 Salmoirach Orion F/4.5 Staebel Kaloplast, Steinheil Cassar Zeiss Triotar '. F/3-5 Development of the Triple Objective after H. D. Taylor — the Cooke Aviar. — In 1918 Arthur Warmisham patented a modified form of the Cooke lens which consists of four simple spaced lenses, two of which are collective and two dispersing. It is essentially a Cooke construction, the idea of a split divergent lens having occurred to Mr. H. D. Taylor in 1898, who was granted a patent for a modification of the triplet in which the central dispersing lens was developed into two f] V7V7 Fig. 96. Cooke Aviar similar lenses of lower individual power (B. P. 12,859 °f 1898). This objective, however, had no advantages over the earlier triplet and was abandoned. By making a special study of coma, Warmisham was able to develop an objective of this type which has a larger flat field than the triplet. The original Aviar as used extensively by the Royal Air Force in the latter years of the World War called for the use of a highly re- fractive baryta crown glass for the front collecting lens. This glass, however, is not very stable and discolors on prolonged exposure to the atmosphere. Accordingly in 1928 Warmisham recalculated the formula to permit the use of a more stable baryta crown without affect- ing the degree of correction (B. P. 312,536 of 1928). Development of the Triplet Objective after H. D. Taylor — the Aldis Lens. — Three objectives of this firm demand attention. Series I la F/6.3 (Fig. 97) may be regarded as a development of the triplet construction of H. Dennis Taylor as described in British Patent No. 1699 °f i899- The central dispersing lens is separated from the front collecting lens by a small air space having a positive effect. The rear collective lens is separated from the front element by the diaphragm. THE PHOTOGRAPHIC OBJECTIVE 137 In the original Cooke triplet the collecting lenses are approximately equal in focal power, here the front collecting lens is much more powerful than the rear. Fig. 97. Aldis Series Ila Series II and Series III may also be considered as having been evolved from the Cooke triplet, although they fall into entirely differ- ent classes. In both of these (Fig. 98) the front collecting lens is cemented to the central dispersing lens while the rear collecting lens of Fig. 98. Aldis Series II and III low power is placed some distance from the cemented element. Series II has a relative aperture of F/6; Series III of F/j.J. Triplets with a Pair of Collecting Cemented Surfaces — the Heliar. — In 1900 Hans Harting calculated for Voigtlander an objective 7 V 7 \ > m Fig. I 99. L Hartir \ g's He liar 1 which is essentially a development of the simple triplet of H. Dennis Taylor, which was introduced commercially as the Heliar. In the Heliar (Fig. 99) the triplet construction is plainly evident from the general similarity of the design. The development made 138 PHOTOGRAPHY by Harting consists in replacing the single collecting lens of the original triplet of H. Dennis Taylor by a cemented element of two glasses. By increasing the number of elements of construction and the introduction of cemented surfaces the initial errors of construction are lessened and excessive curvatures avoided so that the corrections are more easily and fully carried out, resulting in better field covering at a large aperture. The aperture of the Heliar is F/4.5 in all sizes and the area of the field covered with satisfactory sharpness is greater than that of the simple triplet. The Dynar. — Two years later Harting calculated for Voigtlander a similar construction but with the glasses of the cemented com- ponents reversed in position so that all three dispersing lenses are placed together. This construction was introduced as the Dynar (Fig. 100) ; and was made principally for hand cameras and has a maximum aperture of F/6. The Pentac— While the Pentac issued by J. H. Dallmeyer from cal- culations by Lionel Barton Booth is described in British Patent Speci- fication No. 151,506 as a development of the Tessar, the single col- lecting lens of the former being replaced by a cemented component consisting of a collecting and dispersing lens with collecting cemented surface, examination shows that the Pentac has more in common with the Heliar and particularly the Dynar than with the Tessar (Fig. Fig. 100. Harting's Dynar Fig. ioi. Dallmeyer's Pentac THE PHOTOGRAPHIC OBJECTIVE 139 101). Both the Dynar and the Pentac are five-lens systems con- sisting of a doubie-concave negative lens interposed between two col- lecting elements formed of a cemented collecting and dispersing lens and both the exterior lens and the cemented surface are collective in effect. By rigid calculation an objective has been calculated which has an unusually large aperture of F/2.9 and can be well corrected up to a focal length of 12 inches. The Ernostar. — This objective of Ernemann of Dresden is based upon the Cooke triplet. The front collecting (Fig. 102) lens of the Fig. 102. The Ernostar latter, however, has been replaced in the new objective by two elements, each composed of two cemented lenses, the front lens of each pair being collective and the rear lens dispersive in effect. The two ele- ments are separated from one another by an air space having a disper- sive effect. The action of this complex front component is to secure greater convergence of the incident rays so that the path of the ray after passing through the central negative component may be either converging or parallel and not diverging as in the original triplet of H. Dennis Taylor. The division of the front member into two sepa- rate components increases the number of elements at the disposal of the calculator, since there are four glasses and an air space, and this has enabled the corrections to be carried to a high degree of perfec- tion notwithstanding its large relative aperture of F/2. The spherical aberrations, astigmatism and zonal errors are almost completely re- moved and the field is flat and free from distortion. So completely has the chromatic correction been carried out that the objective may be considered to be sphero-achromatized and apochromatic.2 A more simple construction for a triplet with a relative aperture of F/1.8 has been patented by Warmisham (B. P. 280,392 of 1926). 2 British Patents 186,917/1921, 191,702/1922, 193,376/1922, 232,531/1924. Klug- hardt, Phot. Ind., 1924, p. 1008. 6 140 PHOTOGRAPHY The first collecting member and the central diverging member are both cemented doublets, while the rear collecting member is a simple collect- ing lens. Thus, there is secured a 6 glass-to-air surface lens with an aperture of F/i.8. Part III. The Teleobjective Principle of the Teleobjective. — If we take two lenses made of the same glass and having equal but opposite powers, one being negative and the other positive, it is evident that if they are placed in contact with one another the converging power of the positive lens will be exactly balanced by the dispersing power of the negative lens and there will be no alteration in the direction of the incident ray. The combination thus neutralized has no real focus, or it may be said to have infinite focal length. However, if we separate the positive and negative lenses the focal length will gradually shorten until finally we reach the zero position where the focal distance is equal to the focal length of the positive lens, the negative lens then being without effect on the focus. The negative lens may thus be said to take a portion of the image produced by the positive lens and magnify it. The amount of the magnification depends upon the focal length of the objective which in the teleobjective is determined by the distance between the positive and the negative lenses. This distance between the lenses, or A, increases as the focal length decreases and vice versa. The separation of the two lenses also brings about another change. When the two are in contact the principal point coincides with the common lens vertex, but as the components are separated the prin- cipal point moves away from the lens in the direction of the subject. Since the focal length is the distance from the principal point to the point of intersection of the convergent rays, the distance from the focal plane, or the ground-glass, to the lens is less than the equiva- lent focal length. Hence we are able to make use of a long focus objective without a bellows extension of corresponding length. This is the principal point of value of the teleobjective. The Compound Telephoto Objective. — The earliest use of a negative lens in the above described manner is found in the Galilean telescope. Its first use for photographic purposes is credited by Harting to J. Porro in 1851.3 The matter, however, remained unnoticed by the 3 Optics for Photographers, English translation, p. 185. THE PHOTOGRAPHIC OBJECTIVE 141 optical world at large until the latter part of the nineteenth century when it was independently invented by several opticians and is now made by nearly all manufacturers of photographic lenses. As a typical example of the compound teleobjective we may men- tion the design patented by Dallmeyer in his English patent No. 21,933 of 1891. The positive component of this system is the well-known Petzval portrait lens ; the posterior negative element is a symmetrical double combination as illustrated in Fig. 103 and is chromatically Fig. 103. Dallmeyer's Compound Teleobjective and spherically corrected. The two elements are mounted in a tube fitted with an adjustable screw by which the separation of the posi- tive and negative components may be altered to secure any desired degree of magnification. Most manufacturers are in a position to fit, to such of their ob- jectives as may be suitable, a negative combination similar in general construction to the above. When ordering the negative lens the ob- jective to be used as the telepositive should be sent to the manu- facturer in order that the two may be properly adjusted. The advantages of variable focal length and size of image to- gether with short bellows extension are important features and if it was not for the serious disadvantage of lack of speed, which con- siderably limits its usefulness, the compound teleobjective would be widely used. It is not difficult to understand the reason for the lack of speed when we consider the principle upon which the teleobjective is based. The image formed by the positive lens is enlarged (spread over a larger area) by the negative lens ; therefore the intensity of the image is less and a longer exposure is required. The higher the degree of magnification the greater the exposure required. Mathe- matically the aperture of a teleobjective may be expressed as D/ft JIT where D is the aperture of the positive objective, 142 PHOTOGRAPHY /i the focal length of the positive lens, f2 the focal length of the negative lens, and A the separation of the positive and negative lenses. Furthermore some stopping down of the positive lens is nearly al- ways necessary in order that the aberrations of the negative lens (which cannot be completely corrected because the distance between the two elements is subject to considerable variation according to the requirements of the subject) do not unduly interfere with the central definition. This still further lengthens the time of exposure so that hand camera work and the photography of moving objects be- comes possible only in very exceptional cases. For this reason the compound teleobjective is of value only for a limited type of work and has been almost completely replaced by the modern fixed-mag- nification, high-speed, anastigmatic teleobjective. Early Fixed-Magnification Teleobjectives. — While it is impossible to secure a very large aperture with the compound teleobjective, if we fix once for all the separation of the positive and negative ele- ments so that we secure a fixed degree of magnification we are en- abled to considerably increase the working speed of the combination and without any loss of definition. Strictly speaking, the first fixed- 1 I 1 -> - / Fig. 104. Petzval's Orthoskop magnification teleobjective was the Orthoscopic lens worked out by Petzval and introduced commercially by Dietzler in 1856. This ob- jective (Fig. 104) consists of a front positive combination similar to that of the regular Petzval portrait lens and a back negative com- bination with a bi-concave and concavo-convex lens, the two being chromatically corrected so that the objective consists essentially of THE PHOTOGRAPHIC OBJECTIVE 143 two achromats, one of which is collecting and the other dispersing. This rear component magnifies the image in just the same way as in the compound teleobjective but as the usual corrections have to be made for only one degree of magnification, and not an entire series as in the other case, it becomes possible to give to the whole objective an aperture considerably in excess of that which is possible with the compound teleobjective. The possibilities of the Orthoscopic construction were not realized at that time, however, and with the advent of the aplanat it ceased to be made. It was not until 1905 that the first of the modern fixed- focus teleobjectives was introduced, the Bis-Telar of Emil Busch. This was designed by K. Martin and was composed of two cemented doublets (Fig. 105) and had a relative aperture of F/g and a mag- nification-ratio of \2/z- Soon afterwards Zeiss brought out the Mag- Fig. 105. Martin's Bis-Telar Fig. 106. Zeiss Magnar nar (Fig. 106). This was calculated by Rudolph and Wandersleb and had a magnification-ratio of 3 times with an aperture of F/10. The positive component was a doublet and the rear a triplet. The Anastigmatic, Fixed-Focus Teleobjective. — Designers then began to turn their attention towards the more complete astigmatic correction of the teleobjective. In 1912 Ross Limited issued from 0 S — ft- Fig. 107. Ross Telecentric Fig. 108. Dallmeyer Large Adon the calculations of Stuart and Hasselkus the Telecentric/'a. fixed-focus teleobjective, the positive component of which was a cemented triplet and the rear component a cemented doublet (Fig. 107). 'This was 144 PHOTOGRAPHY issued in two series, one working at F/5.4 and the other at F/6.8. Two years later Lan-Davis patented (B. P. 1185 of 1914) an anastig- matic teleobjective which was introduced by J. H. Dallmeyer Limited as the Large Adon. This objective (Fig. 108) consists of a positive component containing a cemented collecting and dispersing lens form- ing an achromatic pair but with considerable remaining spherical aber- ration. The rear dispersing component consists of either two or three cemented lenses which form an achromatic combination and are so corrected spherically as to compensate for the spherical aberration of the front member. In this way a comparatively well-corrected objec- tive with an aperture of F/4.5 was obtained. The same year Lionel Barton Booth calculated and patented (B. P. 3096 of 1914) a four-lens construction in which the members of the positive element were separated by an air space. This had a relative aperture of F/5.8 and was a notable improvement over earlier objec- tives of this class as regards definition and was made by Taylor, Taylor and Hobson in considerable quantities for the use of the British Air Force during the World War. From the standpoint of the manufacturing optician it was desirable to eliminate, if possible, the air space between the two members of the positive element. This problem was solved by Booth, who in 1920 Fig. 109. Booth Teleobjective took out two patents (B. P. 139,719 and 151,507) for fixed-focus, anastigmatic teleobjectives, each element of which consists of a ce- mented doublet (Fig. 109). This construction was introduced by J. H. Dallmeyer Limited in several series as the Ballon. Series VI, XVI and XVIII have a magnification of 2 times and relative apertures of F/5.6, F/7.7 and F/6.5 respectively. Series XVII has a relative aperture of F/6.8 and a magnification of 2^ times. A similar construction was patented by H. W. Lee (B. P. 198,958) and introduced by Taylor, Taylor and Hobson as the Cooke Telic. THE PHOTOGRAPHIC OBJECTIVE 145 This has a relative aperture of F/5.5 and a magnification-ratio of 2 times. The Radiar telephoto anastigmat (Fig. no) introduced by the Gundlach-Manhattan Optical Company is of similar construction. The positive member consists of a cemented doublet with a front col- lecting lens of barium crown and a dispersing lens of heavy flint, while the rear member consists of an inner dispersive lens of barium crown and an outer collecting lens of light flint. The telephoto lenses already described show a small amount of dis- tortion which, for general purposes, is not of serious importance. A non-distorting telephoto as constructed by Taylor, Taylor and Hobson, has the form shown in Fig. in (B. P. 222,709 of 1924). The In the Tele-tessar of Zeiss the rear component is composed of two cemented meniscii the positive member is placed nearest the diaphragm and not on the exterior as in the case of the Dallmeyer Dallons and the Cooke Telic (Fig. 112). The Tele-tessar has a relative aperture of F/S-5 and a magnification-ratio of 2 times. Fig. 1 10. Radiar Teleobjective " barrel-shaped " distortion produced by separating the two elements of the rear dispersive member is neutralized by the opposite type of distortion introduced in the front member by the addition of a collect- ing meniscus, thus resulting in orthoscopy. The Cooke Telekinic F/3.3 has this form. 146 PHOTOGRAPHY In order to construct a fixed-magnification teleobjective with a mag- nification above two and maintain an anastigmatically flat field at the same time it becomes necessary to increase the number of elements. In the Teleros introduced by Ross Limited from calculations by Stuart r 1 ^ 1 I Fig. ii2. Zeiss Tele-tessaar and Hasselkus the rear component is a cemented triplet in which two negative lenses enclose a positive member of glass having lower re- fraction and higher dispersion (Fig. 113). The objective has a re- Fig. 113. Ross Teleros lative aperture of F/5.5 and a magnification-ratio of slightly over two times. H. W. Lee has also patented a fixed-focus teleobjective (Fig. 114) in which the rear component is a triplet with a positive lens of f n r 1 I ■\ 1 > V Ij -1 1 Fig. 114. Lee's T. T. H. Telephoto Fig. 115. Voigtlander's Tele-Dynar low refraction between two negative members of high dispersion. This is manufactured by Taylor, Taylor and Hobson and has a relative aperture of F/5.5 an^ a magnification-ratio of 3. THE PHOTOGRAPHIC OBJECTIVE 147 The Tele-Dynar of Voigtlander also has a rear clement of three members, two of which are cemented and the other separated by an air space (Fig. 115). Several other manufacturers have departed from the simpler constructions already described, but as they are for the most part unknown in this country we do not propose to discuss them further. The Adon. — Before leaving the subject of the teleobjective mention should be made of a construction invented by Dallmeyer and utilized in the construction of the Adon. If the positive and negative elements of a tele-compound are sepa- rated by a difference equal to the difference of their focal lengths, 1 t 1 — — f Equivalent fooml /«ngth l*ns forms virtual im^g* of real iney* formta by postlivo iirti &1 f( Fig. 116. Dallmeyer's Adon incident parallel rays emerge parallel and an ordinary objective fo- cussed for parallel rays when applied to the rear of this combination will' form an image at the focal plane of the ordinary objective, the magnification of the image depending upon the ratio of the focal lengths of the positive and negative lenses (Fig. 116). To maintain the actual F value of the photographic objective for any degree of 148 PHOTOGRAPHY magnification the parallel pencil emerging from the magnifying sys- tem must be as large as the aperture of the objective to which it is ap- plied, therefore the exterior positive element of the enlarging system must be as many times greater in diameter as the lineal degree of magnification desired. Thus we work without loss of speed, the effective aperture being the same as that of the objective alone. This principle, however, can only be used with objectives of moderate diameter and for low degrees of magnification. General Reference Works Eder — Die Photographischen Objectiv. Fabre — Encyclopedique de Photographic Gleichen — Lehrbuch der Geometrischen Optik. Gleichen — Theorie der Modernen Optischen Instrumente. (The English translation by McElwain and Swan contains a table of modern objec- tives which is not found in the original German edition.) Harting — Optics for Photographers. (English translation by Fraprie.) Lummer — Contributions to Photographic Optics. (The English translation by Thompson contains two chapters on British objectives which are not found in the original.) Puyo and Pulligny- — Les Objectifs Anachromatiques. Turriere — L'Optique Industrielle. 1920. (The most complete work on the later anastigmats.) Von Rohr — Theorie und Geschichte der Photographischen Objektiv. CHAPTER VI THE PHOTOGRAPHIC EMULSION Introduction. — Properly speaking, the use of emulsions in photog- raphy dates from the publication of the first practical method of pre- paring collodio-bromide emulsion by Sayce and Bolton in September 1864 but it is in connection with gelatine that the term emulsion is generally associated. The gelatine emulsion which has played such an important part in the development of photography dates from the investigations of an English amateur, Dr. Richard Leach Maddox, whose paper describing the preparation of a sensitive gelatine emul- sion was published in the British Journal of Photography for Sep- tember 8, 1871. His method, however, was not a practical one and gelatine emulsion on a basis similar to that now in use did not appear until several years later. Although a gelatine emulsion had been placed upon the market as early as 1873 by Richard Kennett, gelatino- bromide emulsion of practical utility may be said to have first ap- peared in 1878 after the discovery of the great increase in sensitive- ness to be secured by the application of heat to the finished emulsion. In the meantime three very important points had been cleared up. King and Johnson had shown the necessity for the removal of the soluble salts from the emulsion and indicated means of effecting this; the last named worker had also shown the importance of using an excess of soluble bromide rather than an excess of silver salt; while Bolton had suggested that the emulsion be formed in a small amount of gelatine and the remainder added at a later stage — a method which became very valuable after the discovery of digestion processes with heat. As is fairly well known, the gelatine emulsion which forms the sensitive coating of our plates and films consists primarily of a highly sensitive form of silver bromide and gelatine. If silver bromide is formed in aqueous solution by the double decomposition of a soluble bromide, as potassium bromide, and silver nitrate and the nitrate allowed to stand a short while, the silver halide will begin to pre- cipitate upon the sides and bottom of the vessel. However, if the 149 150 PHOTOGRAPHY silver bromide is formed in the presence of an aqueous solution of gelatine instead of water the solution is at first clear and slightly opalescent and on standing becomes milky or creamy. On standing the silver halide does not precipitate out of solution, as in the case of water, but remains in a homogeneous state. This mixture of finely divided silver halide and gelatine is termed gelatino-bromide emulsion. It is not really an emulsion, however, in the sense in which that term is used in colloid chemistry, but a solution of gelatine carrying in sus- pension minute crystals of solid silver halide. In its simplest form an emulsion may consist purely of silver bromide and gelatine, but at times a small percentage of another halide, chiefly the iodide, but sometimes the chloride, may be added. The available evidence at the present time indicates that in such cases the silver iodide, or chloride as the case may be, is held atomically dispersed within the silver bro- mide and neither combine schematically with the latter nor exists sepa- rately as individual crystals. The processes of emulsion makjng are therefore concerned with the formation of a uniform, homogeneous suspension of a sensitive form of silver bromide in a solution of gelatine. Gelatine. — Gelatine belongs to that class of substances known as col- loids from the Greek «oAAa meaning glue. The substances of this class were termed colloids by a Glasgow chemist, Graham, who found that certain substances in solution such as albumen, glue and gelatine do not pass through an animal membrane, while solutions of crystal- line substances such as common salt do. To the former class of sub- stances Graham applied the term colloids; to the latter class crystal- loids. In colloidal solutions the subdivision of the particles is not so high as in the case of the crystalloids and it is for this reason that they do not pass through filter materials and membranes. Two other terms, sol and gel, were also introduced by Graham. To the liquid solution of a colloid he applied the term sol; to the jelly the term gel. The value of gelatine for photographic emulsions is due to its unique physical properties as well as its chemical composition. The easy reversibility of the transition from the sol to the gel and vice versa, or Hydrosol s=a Hydrogel, is of paramount importance for photographic purposes and it is in this respect that gelatine is distinctly superior to any other colloid. THE PHOTOGRAPHIC EMULSION 151 Gelatine swells in cold water but does not dissolve. Hot water dis- solves it, but on cooling it again forms a jelly even if the concentra- tion of the solution is as low as i per cent. The formation of the jelly from the sol is termed setting and the reverse reaction melting and the temperatures at which the change of state takes place as setting points and melting points. Technical gelatines are broadly classified as hard, medium and soft. A hard gelatine solidifies quickly and becomes quite hard, offering considerable resistance to reswelling. A soft gelatine is exactly opposite in character, solidifying slowly and reswelling quite easily. For emulsions a hard gelatine is easier to work, especially in summer or in hot climates, as the emulsion ad- heres to the support better and does not soften unduly in development. Hard gelatine, however, develops slowly, owing to the fact that the penetration of the film by the developing solution is more difficult. Accordingly, in practice the emulsion maker uses a medium gelatine, combining hard and soft gelatines in the proportions which his experi- ence has taught him to be the best for general purposes. Aside from acting as an emulsifying medium, gelatine acts as a protective colloid. If silver bromide is formed by the combination of solutions of silver nitrate and potassium bromide, using a slight excess of the latter, and the precipitated silver bromide washed to remove all traces of extraneous salts it will be found that on the application of a developer the silver bromide will be immediately re- duced whether exposed to light or not. Sheppard and Mees attrib- ute the protective action of gelatine to the insulation of the nuclei of the silver bromide grain with which effect is associated a delay in the aggregation of the silver amicrons to form larger nuclei.1 Authorities have largely been at a loss to account satisfactorily for the fact that emulsions of much higher speed may be prepared with gelatine than with any other colloid. The earlier conception of the value of gelatine being due to its functioning as a photochemical sensitizer by absorption of halogen has largely been abandoned. Until only a few years ago it was a disputed point as to whether gelatine should be regarded as being directly responsible for the high sensi- tiveness of our modern sensitive materials or whether it acted merely as a passive medium facilitating the growth of the most sensitive form of the silver halide grain. Another disturbing factor was the 1 For an interesting discussion of this subject see " Note on the Function of Gelatine in Development," by Dr. T. Slater Price. Phot. J., 1925, 65, 94. 152 PHOTOGRAPHY action of various gelatines on emulsion sensitiveness. From the earliest days of gelatino-bromide emulsion it had been known that emulsions prepared in precisely the same manner but with different samples of gelatine might vary greatly in light sensitiveness. After methods of determining the size-frequency distribution of grains of silver halide in emulsions had been evolved it was possible to show that emulsions having the same physical characteristics as regards size of grain and size-distribution of grains might vary considerably in light sensitiveness. A long series of investigations in the Eastman Research Laboratory brought to light the existence of what is termed Gelatine-X, the presence of which in ordinary gelatine is largely re- sponsible for photographic sensitiveness. This Gelatine-X has been found to be analogous to allyl mustard oil and to be an allyl isothio- cyanate (C3H5 • NCS) which reacts readily with ammonia to produce a thiocarbamide, e.g. /NHR R-N:C:S NH3 -> C— S \NH2 Experiments show that the group N — — C = S — ■ n is of fundamental importance for photographic sensitizing. Photo- graphically2 active gelatine contains only from i part per 1,000,000 to 1 per 300,000 of the sensitizing substance and the presence of such an exceedingly small amount in a complex, many-sided substance like gelatine accounts for the fact that after fifty years we are just dis- covering the reason for reactions which have been observed since the earliest days of gelatine emulsions. The sensitizing action of thiocarbamide increases with the concen- tration up to a certain point, which varies with the particular emulsion, and then falls off. The fog increases steadily with increase in the thiocarbamide, and rapidly after the maximum speed is passed. With a particular emulsion examined by Sheppard, an increase in speed from 19 H. and D. to 3000 H. and D. was recorded, this maximum being obtained with a concentration of 0.0146 g. of thiocarbamide to each 100 g. of silver halide.3 2 Sheppard, Phot. J., 1925, 65, 380. 3 Sheppard, Phot. J., 1926, 66, 399. THE PHOTOGRAPHIC EMULSION 153 Many other substances have since been found to act as sensitizing agents. Among these are : codein, diethyldiamine,4 iminazole deriva- tives (B. P. 271,475 of 1926, I. G. Farbenindustrie), sodium trithion- ate, tetrathionate (B. P. 255,846 of 1926 — I. G. Farbenindustrie) and thiazole and cystine compounds (B. P. 246,800 and 258,237 of 1925 — I. G. Farbenindustrie). Increasing the sulphur content of gelatine increases the sensitizing properties and the treatment of gelatine with sodium sulphide or carbon disulphide for such a purpose has been pat- ented by the I. G. Farbenindustrie, B. P. 283,222 of 1927. Theourea, mustard oil, and most of the above sensitizers are in- effective when the emulsion is kept in a neutral state and digested by boiling rather than ammonia. The I. G. Farbenindustrie, however, find that the gelatine employed in emulsions of this type may be sensi- tized by the organic disulphides (B. P. 283,223 of 1927). The Two Classes of Emulsion. — Sensitive emulsions may be divided into two classes: (a) those in which the silver halide is formed in the presence of an excess of silver nitrate and (b) those in which the silver halide is formed in the presence of an excess of the soluble halide. Aside from wet collodion, the first class consists principally of emulsions for positive printing out processes such as collodio- chloride and gelatine P. O. P. or similar silver printing papers which produce a visible image upon exposure. The function of the excess silver salt is to act as an absorber for halogen. The second class includes both negative and positive emulsions for development and may be further divided into two classes: (a) those which are used without further treatment after emulsification and (b) those which are submitted to a process of digestion, known technically as ripening, for increasing the sensitiveness and the density-giving powers. This process of ripening consists either of treating the emulsion at rela- tively high temperatures or in the use of ammonia, and will be dis- cussed in greater detail elsewhere. It is sufficient to say for the present that in the preparation of emulsions for positive printing, where a high degree of sensitiveness is unnecessary, ripening plays little or no part, the silver bromide, or silver chloride, being emulsi- fied in such a way as to obtain a very fine grain. In the preparation of highly sensitive emulsions for negative processes, however, ripen- ing plays a very important part. General Outline of Operations in Emulsion Preparation. — The gen- 4 Lumiere and Seyewetz, Rev. franc. Phot., 1925, 6, 291. 154 PHOTOGRAPHY eral outline of the processes involved in the preparation of gelatine emulsion is as follows: 1. The gelatine is allowed to swell in cold water and finally dis- solved by the application of heat. 2. The soluble halide, or halides, are dissolved in water. 3. The required amount of silver nitrate is dissolved in water. 4. The solution of soluble halide is next added to the colloid medium. 5. The solution of silver nitrate is added to the colloid medium. 6. The silver salt and soluble halide unite by double decomposition to form a silver halide. Thus in the case of silver nitrate and potas- sium bromide the reaction is represented by the following equation : AgN03 + KBr -> AgBr + KN03. Silver Potassium Silver Potassium nitrate bromide bromide nitrate 7. Washing, combined with shredding, to completely remove the last traces of soluble salts. 8. The process of digestion ; by standing from 10-20 hours at ordi- nary temperatures or by heating sometimes up to boiling temperature, or treatment with ammonia, in the case of gelatine emulsions. Usually, in the case of gelatine emulsions, the silver bromide is formed in only a portion of the gelatine, the remainder being added immediately after digestion. In this way the danger of destroying the setting power of the gelatine by heat is avoided. Light Sensitiveness of the Silver Halides. — The three silver hal- ides used for photographic emulsions are, in order of their sensitive- ness to light, the bromide, chloride and iodide. Of the three the bromide is much the more important; the chloride and iodide are never used alone but only in combination with the bromide. Slow emulsions for positive printing in which after processes of ripening play a minor part are usually composed of a combination of the bro- mide and chloride while high speed negative emulsions contain gen- erally in addition to the bromide a small percentage of iodide. This does not exist separately in the emulsion, however, but enters into the structure of the silver halide grain. As regards ease of reduction by reducing agents such as photographic developers, the order is chloride, bromide and iodide. Silver bromide precipitated in the absence of colloid media such as gelatine is immediately reduced by developers and is therefore mechanically and chemically unsuited for photographic THE PHOTOGRAPHIC EMULSION 155 purposes. It was formerly believed that there are several modifica- tions of silver bromide but later work 5 has shown that all the various forms of silver bromide from the colloidal suspensions of ultra-micro- scopic particles to the microscopically visible particles of i to 4 microns in diameter are crystals of the same family and represent simply dif- ferences in dispersity or particle size. The Preparation of Emulsions. — The student will have perceived by this time that emulsion making is a very exacting and complex process which demands, not only a thorough training in the chemistry of col- loids and in physical chemistry, but also a large amount of applied knowledge with respect to the operations of emulsion making which can only be gained from actual experience. Our knowledge of the fundamental principles involved in the processes of emulsion making are still unsatisfactory and the subject is in fact more of an art than a science. What we know about the preparation of emulsions and the influence of various factors on the properties of the finished emulsion has been gained entirely by empirical experimentation extending over a long range of years. While constant experimenting has shown the emulsion maker the conditions essential to the preparation of emul- sions of high sensitiveness, we know but little of the fundamental causes involved, the ultimate differences which we find from one emulsion to another, and between different particles of the same emulsion. Work on some of these problems is being conducted at the present time and it is probable that some of these points may be cleared up in the near future. Owing to their commercial value it is difficult to say to what extent these matters will become common knowledge. While in discussing the preparation of gelatine emulsions it will be necessary to divide the subject into three heads — emulsification, ripening or digestion, and washing — it should not be assumed that these operations are entirely separate and distinct and independent of each other, but on the contrary that they are closely related and mutually interdependent upon one another. The sensitiveness of the silver halide grain is influenced by practically every feature of its en- vironment from the time of its emulsification to coating. The con- centration and proportions of the various substances, the temperature at which the various operations are conducted, the character of the 5 Trivelli and Sheppard, The Silver Bromide Grain of Photographic Emulsions, D. Van Nostrand, New York, 1921. Wilsey, Phil. Mag., 1921, 42, 262; 1923, 46, 487. 156 PHOTOGRAPHY gelatine used, the alkalinity or acidity of the emulsion during digestion and the time occupied in the various operations, all influence the sen- sitiveness and character of the emulsion to a marked degree. Thus if emulsification has not been conducted under conditions which are favorable to the formation of relatively large grains of silver halide as well as the proper proportion of the various sized grains, no man- ner of digestion will produce a highly sensitive emulsion. In other words, it is not possible to convert a low speed emulsion into one of high speed simply by digestion ; if an emulsion of high sensitiveness is determined upon, it must be borne in mind from the beginning and conditions provided which are favorable to the formation of the most sensitive grains of silver halide. Hence while for purposes of dis- cussion the various operations will be treated separately, it is to be understood that in reality they are closely related to one another and not separate and distinct as the manner of treatment might indicate. Emulsification. — When silver nitrate and potassium bromide are mixed in the presence of gelatine it is usual to use an excess of the latter salt. In the presence of gelatine, free silver nitrate is easily decomposed during the process of digestion and the emulsion fogs on development. In theory, it should be possible to use equivalent amounts of silver salt and soluble bromide so that neither would be in excess, but it is not possible in practice and therefore it is usual to use an excess of soluble bromide. The proper proportion between the two is a matter of dispute. Dr. J. M. Eder, the celebrated Austrian authority who took a very active part in the development of gelatine emulsions, favored a proportion of 5-4. Sir William Abney, the eminent English investigator, favored a ratio of 1 5-1 1 ; while Ben- nett and Wilson advised 11-7 when using ammonium bromide, and W. K. Burton 42-25. The presence of excess soluble bromide either during precipitation or subsequently during digestion is an essential condition of that part of the ripening process which consists in growth in the size of the silver halide grain. This is because silver bromide is more soluble in solutions containing a soluble bromide and the greater solubility re- sults in the formation of larger crystals. An excess of soluble bro- mide also has the effect of reducing the danger of fog in the prepa- ration of highly sensitive emulsions. In making rapid gelatine emulsions a concentrated solution of silver nitrate is added to a solution of soluble bromide of similar concen- THE PHOTOGRAPHIC EMULSION 157 tration in the presence of gelatine and an excess of soluble bromide. The slight clouding which appears might lead one to assume that the two do not react immediately to form silver bromide but exist sepa- rately for some time. This is not so, for it has been found that silver nitrate and a soluble bromide react at once, even in the presence of gelatine, according to the equation AgN03 + KBr -» AgBr + KN03. If the emulsion is examined with a microscope from time to time as additional silver solution is added, a gradual growth in the number of silver halide grains is observed. At the same time, it will also be observed that the grains already formed are increased in size, show- ing that not all of the silver solution added goes to form new grains of silver halide but that some is added to those already existing. Con- sequently, the grains of silver halide grow, not only in number, but also in size, with the addition of the solution of silver nitrate. The size of the silver halide grain formed depends upon the amount of free potassium bromide present, the temperature and the concentra- tion and rate of addition of the silver solution. The freshly precipitated emulsion, especially if emulsified at a low temperature and under conditions favoring the formation of very small particles of silver halide, is very fine grained and relatively transparent but is only slightly sensitive. When digested by heat or ammonia its speed increases from 100 to iooo times.6 Gelatino-Bromo-Iodide Emulsions. — The addition of a small per- centage of iodide to gelatino-bromide emulsions was advised by Penny in 1878 and was studied in detail by Abney 7 and by Eder.8 The use of iodide makes possible a higher degree of digestion with- out danger of excessive fog and leads to emulsions of higher sensitive- ness. Emulsions containing iodide work cleaner than those of pure silver bromide and give brighter negatives with greater contrast and density. The addition of iodide also changes the spectral sensitivity of the emulsion. As the percentage of iodide is increased there is a definite and progressive shift in spectral sensitiveness towards the longer wave-lengths,9 until the emulsion contains as much as 30 per cent iodide, after which the characteristic spectral sensitivity of silver iodide alone is obtained. 6 See Eder, Handbuch der Photographie, I, 24 (Ed. 1902). 7 Abney, Phot. News, 1880, 174, 196. 8 Eder, Ausfuhrliches Handbuch der Photographie. 9 Huse and Meulendyke, Phot. J., 1926, 66, 306. 158 PHOTOGRAPHY It was formerly thought that the addition of iodide resulted in the formation of a complex of bromo-iodide of silver,10 but it is now known that the silver iodide enters into the crystal structure of the silver bromide crystal. X-ray diffraction studies have shown that the effect of silver iodide is to produce an enlargement of the crystal lattice (Wilsey). The increased sensitiveness of silver bromide when mixed with silver iodide is regarded* as being due to the state of strain set up by the presence of the silver iodide incorporated in the silver bromide crystal structure.11 The larger crystals have been found to contain a higher percentage of silver iodide than the smaller.12 Also in a given emulsion, the large grains are relatively more sensitive than the smaller. While the difference in the percentage of silver iodide present may be the cause of the higher sensitivity, evidence has been obtained which shows that the higher sensitiveness is not due to silver iodide alone. Digestion. — When first mixed, the emulsion is very slow, no matter what formula has been employed, and quite unsuited for use for any- thing but contact printing paper. If the emulsion after mixing is al- lowed to stand at ordinary temperatures for several days the sensitive- ness to light increases. Digestion, or " ripening," of the emulsion at ordinary temperatures is known as cold ripening. The degree of sensitiveness thus obtained, however, is still too low for general use. Rapid negative emulsions may be broadly divided into two classes : (i) acid or neutral emulsion digested by heat, and (2) alkaline emul- sions digested with ammonia, or the alkaline carbonates, either alone or in combination with heat. As a rule, the first class are employed only for the slower emulsions and the more rapid emulsions are pre- pared with ammonia. The addition of ammonia to the emulsion was advised by Johnson in 1877 but it was not until after the investigations of Eder in 1880 that it was used with much success.13 Eder also found that the alka- line carbonates, such as ammonium and sodium carbonate could be used in place of ammonia, but neither has been found to be as effective. The ammonia may be added shortly after emulsification and digestion carried out at normal temperature, or by heat, or the emulsion may be 10 Eder, Photographie mit Bromsilbergelatine, 1903, p. 1 17-122. 11 Trivelli, Recueil des Travaux Chimiques des Pays-Bas, 1923, 42, 714. 12 Renwick, Phot. J., 1924, 64, 360. Sheppard and Trivelli, Journal of Franklin Institute, 1927, 827. 13 Sitsungsber, Akad. Wiss. Wien., 1880, 81, II, 687. THE PHOTOGRAPHIC EMULSION 159 partly digested by heat and the ammonia added after the emulsion has cooled down to about 30 degrees centigrade. Only a very small per- centage of ammonia is required. More than a few per cent increases the grain size unfavorably and produces emulsions which fog easily in development. The exact amount of ammonia which may be em- ployed depends largely upon the type of gelatine used, the conditions under which the silver halides are precipitated and the method of diges- tion. The function of ammonia in ripening is apparently closely con- nected with the formation of the sensitizing substances such as allyl- thiocarbamide described by Sheppard and Punnett. Under given conditions of digestion the sensitiveness varies with the hydrogen-ion concentration. With a given emulsion the sensitive- ness increases with increasing time of digestion to a maximum point which varies with the pH (or hydrogen-ion concentration).14 While it might be concluded that the change in sensitiveness with hydrogen-ion concentration is due to the amount of silver sulphide de- rived from the silver bromide-thiocarbamide reaction, this does not seem to be entirely the case, for it has been demonstrated that the sensitiveness of an emulsion, which is lowered upon the addition of acid, is immediately restored when the hydrogen-ion concentration is brought back to its original value.15 Fog. — Over digestion produces a coarse granular emulsion, the particles of which are visible to the eye in some cases, while the plate fogs in the developer, blackening whether exposed to light or not. All methods of obtaining an extremely sensitive emulsion lead to plates which fog in development. When digested by heat, fog appears earlier in neutral emulsions than in those which are slightly acid and for this reason a trace of acid is sometimes added to make sure that the emulsion is in a slightly acid state. Too much acid, however, is harmful as it delays digestion and affects the gelatine. Alkaline emul- sions are completely digested at low temperatures and tend to produce fog if digestion for a high degree of sensitiveness is attempted. The addition of iodide to emulsions tends to prevent fog, as does the pres- ence of an excess of soluble halide, while, as already mentioned, the addition of a trace of acid to emulsions which are digested by means of heat materially reduces the danger of fog. The danger of excessive 14 Rawling and Glassett, Phot. J., 1926, 66, 495. 15 Rawling, Phot. J., 1927, 67, 42. Sheppard and Wightman, Phot. J., 1929, 69, 22. 160 PHOTOGRAPHY fog is of course much higher with high speed emulsions than with low : in fact a certain amount of fog is inseparable from an extremely sensi- tive emulsion, but the amount is so small, under favorable conditions of manufacture, as to be of little consequence. Theory of Digestion. — There is a progressive growth in the size of the crystals of silver halide during the process of digestion. The smaller grains disappear, combining with the larger grains until the largest crystals reach a diameter at times equal to 8 microns. It has long been supposed that the increase in sensitiveness in digestion is connected with, if not directly due to, the growth in the size of the crystals of silver halide, for it is well established that in general the larger crystals are more sensitive than the smaller. On the other hand, it is possible to prepare low speed emulsions having grains which are appreciably coarser than those of most ultra-rapid emulsions. The relation of size of grain to sensitivity may be expressed by saying that the increased size of grain is a necessary but not a sufficient condition for high sensitivity. The size of grain may be regarded as the ca- pacity factor in the production of the sensitizing specks' of silver sul- phide, or metallic silver, or possibly both combined. Many theories of the process of digestion have been advanced; the earlier ones postulated the partial reduction of silver halide to sub- halide or the formation of a gelatino-silver halide complex, the later ones the formation of colloid silver.16 The recent work of Sheppard on the sensitivity promoting substance in gelatine, while it does not exclude the possibility of colloid silver formation as a factor in the process of digestion, makes it clear that an important, if not the primary factor, in the process is the formation of sulphiding sensitizers and their interaction with silver halide to form highly sensitive nuclei of silver sulphite on the silver halide grain. Eliminating the Soluble Salts. — After the extra gelatine has been added to the digested emulsion, it is well shaken up and then poured out into a porcelain tray and allowed to set. The time required for setting will vary according to the type of gelatine used, the temperature and also the humidity of the surrounding air. Two hours is generally sufficient and often very much less is required. When the emulsion has set it is ready for washing to remove the soluble salts. ieEder, Ausfuhrliches Handbuch der Photographie , III, 37 (1890). Luther, Die Chemischen Vorgange in der Photographie , 1899. Luppo-Cramer, Phot. Korr., 1904, 41, 164. Luppo-Cramer, Kolloidchemie und Photographie, 1920. Renwick, /. Soc. Chem. Ind., 1920, 156T, 39; Brit. J. Phot., 1920, 67, 447, 463. THE PHOTOGRAPHIC EMULSION 161 In dealing with small quantities the emulsion is gathered up in a canvas bag which is placed under the surface of clean cold water and by gentle pressure the emulsion is forced through the interstices of the canvas. For this purpose the canvas should be as coarse as pos- sible. A mesh of about 8 lines to the inch is sufficiently fine. This divides the emulsion up into fine shreds and enables the soluble salts to quickly pass out in running water. Generally the operation is re- peated once or twice and the shreds left in running water for one or two hours. In manufacturing emulsions commercially, presses of the type shown in Fig. 117 for power or hand operation are used. The bottom of the Fig. 117. Emulsion Press and Washer cylinder, which may hold from 15 to 150 liters of emulsion, consists of a nickel-wire screen which cuts the emulsion up into fine shreds. Washing is clone in cylinders of about the same capacity through which water is constantly flowing. In some washing equipment, mechanically operated stirrers are provided to keep the shredded emulsion in motion and thus reduce the time required for washing. The emulsion may now be regarded as complete, but it is customary to add a small amount of chrome alum in order to harden the gelatine 162 PHOTOGRAPHY slightly so that it will adhere to the plate in coating and also remain firm during development, fixing, etc. Since our purpose in this chap- ter is to discuss the subject of emulsions from a theoretical standpoint and not with the idea of enabling the student to prepare his own plates, the operations of coating, drying and packing will be omitted. For information on these points reference should be made to larger and more comprehensive works on the subject. The Silver Bromide Grain of Photographic Emulsions. — When ex- amined under a high power microscope, the photographic emulsion is seen to consist of numerous semi-transparent and practically opaque grains of silver halide imbedded in gelatine. These grains of silver halide are definitely crystalline (Fig. 118) 17 and of various forms and Fig. ii8. The Photographic Emulsion under a Microscope sizes ; the most constantly recurring forms being triangles and hexa- gons, some of which are irregular, while all have rounded corners, but occasionally a long rod-shaped crystal is observed.. The grains also vary in transparency, some being almost completely transparent while others are nearly opaque. Since the opaque grains behave in exactly the same way as the transparent grains, there is no justification for assuming that they are different substances. In addition to these there are ultra-microscopic grains which are beyond the limit of visibility with the highest power of the microscope. Recent investigation has 17 The emulsion shown in Fig. 118 has been greatly diluted in order that the individual crystals may be more clearly seen. Ordinary emulsions as used for coating of plates and film consist of many layers of such grains and contain from 10 to 25 billion of such crystals to the square inch. THE PHOTOGRAPHIC EMULSION 163 shown that these are also crystalline and have substantially the same structure as those of larger dimensions.18 There is no evidence for the existence of non-crystalline silver bromide in photographic emul- sions. The size of the silver halide grains in commercial emulsions ranges from the ultra-microscopic particles of less than one micron to grains as large as 3 or 4 microns. In positive emulsions the larger number of grains are either ultra-microscopic or very small, while in the case of highly sensitive negative emulsions, although a large number of ultra-microscopic grains are present, the majority of. the grains are of microscopic size, while all are of course definitely crystalline. The Sensitivity of the Silver Halide Grain. — Microscopical investi- gation has shown that in spite of the enormous number of grains of silver halide and their close proximity to one another, each individual grain affected by light acts as a single unit and there is no trans- ference of development from one grain to another, unless the two are grouped together in absolute contact; a state of affairs characteristic of some emulsions.21 It has also been found that a grain is either made developable by a certain amount of light or it is not developable. Thus, we do not get partial development for a certain exposure fol- lowed by more for a longer exposure but up to a certain amount of light action the grain is undevelopable and after that amount is reached it is rendered completely developable. The amount of light required to make a grain developable represents what is termed the sensitivity of the grain. Investigation of the number of grains made developable by a given exposure shows that all the grains are not equally sensitive ; because under such conditions all the grains would become developable as soon as the exposure reached a certain value. Microscopical examination at high powers shows that the grains of silver halide differ widely in size and on counting the number of grains made developable in given size-classes, it is found that in one and the same emulsion the sensi- tivity increases with the size of the grain. This does not necessarily mean that all large grains are more sensitive than smaller ones, for with different emulsions the reverse is often true,22 but if we keep to the same emulsion the larger grains are on the average more sensitive than the smaller. There are, however, differences in sensitivity among grains of the same size and shape in the same emulsion. Sensitivity is, 18 Wilsey, Phil. Mag. (1922), 42, 262. 164 PHOTOGRAPHY therefore, not wholly a function of the size of grain, but is due to an inherent difference in the grain itself. Long ago Abegg and Eder suggested that in the process of digestion a trace of the silver halide is reduced to either a sub-halide, or to silver. These traces they referred to as " ripening centers." These served as centers for further reduction by light to form the latent image. The existence of specially sensitive spots in the grain of silver halide is now well established. Liippo-Cramer and Renwick supposed these spots, centers, nuclei, or specks, as they are variously termed, to consist of colloid silver.23 However, the work of Sheppard and his co-workers on the sensitizing substances in gelatine seems to prove beyond all doubt that the sensitivity centers are composed of silver sulphide, which is produced as a result of a chain of reactions between the sensitizing substance and the silver halide (see page 152). 24 These submicroscopic specks of silver sulphide are distributed at random in, or on the surface, of the grains. It is believed that they cause a strain in the crystal of silver halide creating around them a sphere of weakness in the crystal structure which increases the sensi- tivity of the grains. We will leave to a later chapter a discussion of the manner in which these sensitivity centers function in the formation of the latent image. Grain-Size Distribution and its Relation to the Photographic Prop- erties of Emulsions. — Investigation having shown that the individual halide grain is the photochemical unit of the photographic plate, the properties of the emulsion representing simply the sum of the prop- erties of the individual grains modified by their positions in layers, a study of the effect of grain-size distribution in emulsions and its rela- tion to photographic properties is of great importance. For if emul- sion sensitiveness is merely a matter of grain-size distribution the emulsion maker has only to provide the conditions favorable to the growth of crystals of the proper size in order to produce emulsions of the highest possible sensitiveness or having any other required prop- erties. On the other hand, should it be shown that photographic 21 Svedberg, Phot. J., 1922, 62, 183. Slade and Higson, Proc. Roy. Soc, 1920, A 98, 154. Trivelli, Righter and Sheppard, Phot. J., 1922, 62, 407. Trivelli, Brit. J. Phot., 1922, 69, 687. 22 Sheppard, Phot. J., 1921, 51, 400. Renwick, ibid., 1921, 51, 333. 23 Liippo-Cramer, Kolloidchemie und Photographic. Renwick, Brit. J. Phot, 1921, 67. 2* Phot. J., 1925, 65, 380. THE PHOTOGRAPHIC EMULSION 165 properties are not wholly, or only partially, controlled by grain-size distribution but by other factors as well, the line of investigation must naturally be directed along entirely different lines. Before 1895 Haddon and Banks called attention to the probability of some relation between the size of grains and the sensitiveness of an emulsion and Mees in 1915 suggested that " inasmuch as emulsions are not homogeneous, but contain grains of all sizes, the sensitiveness of the emulsion will depend upon the distribution of the different sizes of grains, as will also the shape of the characteristic curve." 25 Slade and Higson as the result of some investigations on the action of light on an emulsion containing grains of very nearly the same size and only one layer thick also concluded that the properties of the emulsion are determined mainly by the relation of the different sizes of grains to one another and the quantity of each present.26 Svedberg found that for every class of grains of nearly the same size in the emulsion there is a distinct characteristic curve.27 The matter was not fully investigated in a quantitative manner until 1 92 1 when Sheppard, Wightman and Trivelli of the Eastman Re- search Laboratory published the first of a series of papers on the subject (see bibliography). They attacked the problem by photo- micrographing the grains of various emulsions at a magnification of 2000 times and then enlarging the negative five times, so that the actual magnification equalled 10,000 times. The developed grains of a given area were then measured and divided into classes accord- ing to size. The data secured in this manner may be represented graphically by plotting the number of grains of each class as ordinates against the sizes of the grains as abscissae. In Fig. 119 are shown photo-micrographs of the emulsion of a portrait film and a Standard slow lantern plate together with curves showing the size- frequency distribution of each. It will be observed that the grains of the posi- tive emulsion are all comparatively small and uniform, the range be- ing from about 0.2 to 1 micron. The high speed portrait film, on the contrary, shows an extended range of sizes from about 0.2 micron to as high as 2.7 microns with a maximum close to 0.5. A correlation of these facts and the photographic properties of emulsions is to be the subject of further investigation. The data which has been accumulated shows definitely that the relative speed 25 /. Franklin Inst., 1915, lyg, 141. 26 Phot. J., 1919, 59, 260. 27 Z. Wiss. Phot., 1920, 20, 36. 166 PHOTOGRAPHY of an emulsion increases rapidly with an increase in the average size, and range of size, of the grains contained in the emulsion. Several other interesting relations have been indicated in the course of the in- vestigation and these points are now being investigated. At the 9 m** «4 t • V 9 «f •• • v i * «x / y SIZE - FREQUENCY CURVE A = STANDARD SLOW LANTERN SLIDE B — PAR SPEED PORTRAIT FILM "DIAMETER IN U. Pig, uy. Size Frequency Distribution of Silver Halide Grains in a Portrait Film and Lantern Slide Emulsion present time all that can be definitely stated is that there is apparently a very close connection between grain size and size-frequency and the photographic properties of emulsions. THE PHOTOGRAPHIC EMULSION 167 General Reference Works Abney — Photography with Emulsions. Abney— Treatise on Photography. Abney — Instruction in Photography. Burton and Pringle — Processes of Pure Photography. Brothers — A Manual of Photography. Eder — Ausfurliches Handbuch der Photographic Eder and Valenta — Beitrage zur Photochemie. Luppo-Cramer — Kolloidchemie und Photographie. Luther — Die Chemische Vorgange in der Photographie. Mees and Sheppard — Theory of the Photographic Process. Sheppard and Trivelli— The Silver Halide Grain of Photographic Emulsions. Tissandier — History and Handbook of Photography. Valenta — Photographische Chemie und Chemikalienkunde. Wall — Photographic Emulsions. s CHAPTER VII ORTHOCHROMATICS Light and Color. The Spectrum.- — White light consists of a number of wave-movements of various lengths and rate of vibration. When white light is passed through a prism refraction and dispersion take place and the rays are sorted out into waves of different lengths and rate of vibration, producing what is known as the spectrum. The short waves are the most refrangible so that violet is refracted the most and red the least, while green and yellow are refracted to an intermediate extent and occupy a position between the violet and blue on one side and the orange and red on the other. The position of any color in the spectrum in respect to other colors is, therefore, a measure of its refrangibility, or the length of the ether wave. Although the spectrum consists of a continuous band in which the colors graduate into one another, it is customary to recognize seven colors in the visible portion : violet, indigo, blue, green, yellow, orange and red. For purposes of reference, it is necessary to have some recognized means of referring to any desired portion of the spectrum. Such a purpose is fulfilled by the Fraunhofer lines. These are narrow dark lines traversing the spectrum and occurring at fixed points so that they form a convenient means of designation for any part of the spectrum. In Fig. 120 the spectrum is reproduced by the three- color process and the positions of the principal Fraunhofer lines are shown. The numbers beside the lines refer to the wave-lengths in Angstrom units. An Angstrom unit is equal to 1/10,000,000 of a millimeter and is the unit of measurement used in specifying the length of light waves. As we will have occasion to refer to these lines and wave-lengths, the student should study the three-color print care- fully and learn the lines and their relative positions in the spectrum. Visual and Photo-Chemical Luminosity. — Of the colors which form the visible spectrum, yellow is the most luminous to the eye. The relative visual intensities of the various colors of the spectrum 168 INS J. Prismatic Spectrum. — 2. Spectrum produced by diffraction grating. (Showing the principal Fraunhofer lines.) Fio.iwti. — Three-color print of the Solar Spectrum. (20 ORTHOCHROMATICS 169 are illustrated in Fig. 121 from Abney,1 the heights of the curve above the horizontal line giving the relative intensity. It will be ob- served that the maximum intensity is very close to the D line. On either side of this point the visual intensity of the colors decreases, the drop of the curve being especially noticeable in the blue and violet. If a sensitive plate is exposed in a. spectrograph and the densities, which are a measure of the work accomplished by light, are plotted as above, we will find that the silver halides have a totally different DARK VIOLET BLUE &REEN YELLOW RED DARK Fig. 121. Visual Luminosity of the Spectrum after Abney sensitiveness from that of the eye and that the maximum sensitive- ness of the plate is found in the violet, while in the yellow near the D line, where the maximum visual luminosity lies, the plate is prac- tically insensitive. It will be still more instructive if instead of an ordinary plate we use the silver halides themselves. Draper, Hunt, Herschel and, more notably, Abney studied extensively this action of the spectrum on the silver halides and the latter gives the following curves which show the sensitiveness of the chloride, bromide and iodide of silver to the spectrum (Fig. 122). 2 The dotted lines in- dicate the extension of sensitiveness resulting from extreme lengthen- ing of the exposure. The result of mixing the halides is to secure slightly more sensitive- ness in the blue-green but in no case does the increase begin to ap- proach the visual luminosity curve of the spectrum. (See Meldola, Chemistry of Photography, p. 208.) 1 Instruction in Photography, 10th Ed., p. 6. 2 Instruction in Photography, 10th Ed., p. 9. Also see Meldola, Chemistry of Photography, p. 255. 170 PHOTOGRAPHY Since the visual luminosity of the spectrum is so totally different from the photo-chemical activity of the spectrum, it follows that an ordinary plate containing only the silver halides cannot reproduce colors in their proper relation to one another. Blue objects appear much lighter in photographs than they do to the eye, while yellow is reproduced as black. As a typical example, we may take the case of an orange on a blue velvet cloth. Now of the two, the orange is much the lighter, so much so that the blue appears dark in com- M L H G F D C B A m. Fig. 122. Spectral Sensitiveness of the Silver Halides. I. Silver Chloride. II. Silver Bromide. III. Silver Iodide parison. When photographed, on an ordinary plate, what do we get? The brilliant orange is a dark grey, almost black, while the blue has turned out almost white and, therefore, the color rendering is totally false. Many other examples might be given to show the false render- ing of color given by ordinary plates. The incorrect rendering of color was for a long time a serious ob- stacle to the progress of photography but fortunately means have been found which overcome this difficulty and there is now no dif- ficulty in securing proper color values if the proper materials and skill are used. This notable advance has been made possible through the discovery of the fact that certain dyes render the silver halides sensitive, not only to the violet and blue, but also to the green, yellow and red. Chemical Constitution of Sensitizing Dyes. — H. W. Vogel in 1873 discovered that the sensitivity of the silver halides could be extended to the less refrangible rays by certain dyes.3 Vogel found that naph- thalene red, magneta, methyl violet, cyanin were effective as sensi- tizers.4 A number of other dyes were found to act as sensitizers but 3 Ber., 1873, 6, 1302. 4 Ber., 1875, 8, 95, 1635. ORTHOCHROMATICS 171 as these have with but one exception been supplanted by later dyes, we will not stop to consider them here. The exception is eosine, the sensitizing properties of which were discovered by Waterhouse in 1875. This is a powerful green and yellow sensitizer. The fluo- rescein derivative erythrosine is more generally used, however, for the iso and orthochromatic plates of commerce, owing to its greater sensi- tizing range especially in the yellow and the beginning of the orange.5 Prior to 1903 the dyes which had been found to function as sensi- tizers were from widely different classes of dyestuffs. In that year Konig discovered the sensitizing properties of the isocyanine dyes and since that time the principal sensitizers have been among the cyanines. The various dyestuffs used for sensitizing are all formed from benzene which is likewise the source of the organic developing agents. Benzene has the formula C6H6 and was shown by Kekule to have a 5 Eosine and erythrosine are derivatives of the xanthene skeleton O O H which is similar to the parent substance of the acridine dyes, NH NH and the flavine group. NH NH Sensitizers have been found in both of these groups; the cyanines, however, are much the more important. 7 172 PHOTOGRAPHY structural formula which may be represented as a hexagon with the carbon and hydrogen atoms linked together around the six points : CH— CH— CH — CH — CH CH The substitution of nitrogen in the fourth position in the benzene nucleus produces pyridine. The union of a benzene nucleus and C C H CH CH CH CH \/\/ CCN H pyridine produces quinoline. The substitution of the — CH3 group for hydrogen in quinoline leads to quinaldine. C C H C N ch/Vx CH H CH C - CH3 Substitution of the — CHS group produces lepidine. H C C C - CH3 The cyanines result from the linking of lepidine and quinaldine by a ORTHOCHROMATICS 173 chain of one or three CH groups. When the two are united thus = CH— N \/\/ N C2H5 C2H5 I we have the true cyanines. The sensitizing properties of cyanin were discovered by Vogel in 1875. It was for a long time the only sensitizer for red but on ac- count of its tendency to fog and its general unreliability, it has been completely replaced by the wocyanines and carbocyanines. In 1903 the first of the isocyanines, Ethyl Red, was introduced.6 This is ah excellent sensitizer which shows a fairly even band of sensi- tiveness from the ultra-violet to orange. Ethyl Red was followed by a whole series of isocyanine dyes, which surpass it in sensitizing power. These were discovered by Konig and placed on the market by Farbwerke vorm Meister Lucius and Briining.7 These were the sensi- tizers Pinachrome, Pinaverdol, and Orthochrome T. Similar sensi- tizers were introduced by Bayer 8 as Perikol, Isokol and Homokol. In the isocyanines the linking — CH group is substituted in the 2 : 4' position ; consequently ethyl red (1:1 Diethylisocyanjne Iodide) has the formula : = CH- C2H5 N 2HI In 1905 another very valuable series of sensitizing dyes, the carbo- cyanines, were discovered by Konig. These are more powerful red 6 Miethe, Chem. Ind., 1903, 26 (3) 54. 7 D. R. P. 167,159 and 167,770 of 1903. 8 D. R. P. 158,078, 170,648 and 170, 049. 174 PHOTOGRAPHY sensitizers than the wocyanines. The best known sensitizer in this class is Pinacyanol. The carbocyanines differ structurally from the isocyanines in that the linking between the two nuclei is effected not by a :CH- group but by the three-carbon chain, :CH-CH:CH. Thus 2 : 2' — carbocyanine ( Pinacyanol, Sensitol Red, etc. ) has the formula : = CH — CH = CH — N C2H5 1 : 1' Diethyl — -2 : 2'- — carbocyanine Iodide.9 The 4 : 4' carbocyanine = CH - CH = CH-z \/\/ N I C2H5 \/\/ N /\ C2H5 I /' C2H5 I was prepared by Adams and Haller in 1920 and called Kryptocyanine.10 It is a powerful sensitizer for the extreme red and the infra red. Besides the true cyanines, there are related compounds which are formed when one or both of the quinoline nuclei are replaced by other nuclei. In i/w'ocyanine the two quinoline nuclei are replaced by benz- thiazole nuclei. No sensitizers of importance have been found in this group but in the thioisocyanines, Mills and Braunholtz 11 have found a series of dyes which are powerful green sensitizers. — CH : C— S NRX NR 9 Hamer and Bloch, Phot. J., 1928, 68, 21. 10 /. Amer. Chem. Soc, 1920, 42, 2661. 11 Trans. Chem. Soc, 1922, 121, 2004. 12 /. Opt. Soc. Amer., ig26, 12, 397 ORTHOCHROMATICS 175 A cyanine, the composition of which has not been described, was separated from Kryptocyanine by Clarke 12 and introduced by the Eastman Kodak Company as neocyanine. It is the most powerful infra-red sensitizer known. It does not seem possible at the present time to attempt to establish any relationship between the sensitizing properties of a dye and its chemical constitution. Known Facts Regarding Color Sensitizing. — The most valuable work on the theory of dye sensitizing has been done by Eder from which the following facts are summarized : 13 1. The dye must stain the silver halide grain. 2. Vigorous sensitizing dyes are substantive dyes. That is, they dye substances directly without a mordant. Staining of the silver halide grain is no proof of color sensitizing. 3. A dye sensitizes for the rays which it absorbs or more accurately the rays absorbed by the dyed silver halide. 4. The maximum of sensitiveness lies at about the same place as the maximum absorption of the dye, with a slight shift towards the red. Stated more correctly, the maximum of sensitiveness agrees with the maximum absorption of the dyed silver halide. 5. A dye having a narrow band of absorption sensitizes a narrow band while dyes having broad bands of absorption give broad bands of sensitiveness. 6. The brilliancy of the dye appears to have no special influence. 7. The sensitizing power of a dye does not appear to be dependent upon either its fugitive character or its fluorescence. 8. No relation can be found between sensitizing power and the chemical composition of the dye. There are two methods of dye sensitizing : ( 1 ) bathing an ordinary blue-sensitive plate in a solution of the dye and (2) incorporating the dye with the emulsion. In general, greater sensitiveness results from the first method but plates prepared by the latter method appear to keep better. The amount of dye required is very small. The usual degree of concentration varies from 1 part in 1000 to 1 part in 75,000. It is found that in order to sensitize, a dye must combine with the silver halide. Whether there is chemical or molecular combination we do not definitely know. Eder has elaborated the latter theory,14 and 13 Ausfuhrliches Handbuch der Photographie, vol. Ill, p. 150. Grundlage der Photographie mit Gelatine -Emulsionen. 14 Beitr'dge zur Photochemie, vol. Ill, p. 75. 176 PHOTOGRAPHY assumes that the vibrations are absorbed by the colored compound and photochemical decomposition then occurs. The researches of Luppo- Cramer and Traube,15 if they do not prove the existence of chemical combination between the silver halide and dye, show that there is a very close connection between the two. It is found that it is impos- sible to remove the last traces of dye from an emulsion even with re- peated washings. Moreover, the plate after washing still shows the characteristic absorption and sensitiveness of the dyed silver halide. Eder's third conclusion is practically the same as Draper's law, which is the foundation of orthochromatics, and states that only those rays can act chemically on a body which are absorbed by it. Light which passes through a substance or is reflected from it cannot have any chemical action. According to Eder,16 neither the maximum point of absorption of the dye nor the maximum point of absorption of the dye in gelatine agree with the maximum point of sensitiveness of the dyed silver halide. The maximum photographic sensitiveness lies further to the red by about 20 millimicrons than the maximum absorption point of the dye in gelatine.17 That dyes having narrow intense bands of absorption would produce similar bands of sensitiveness is to be expected from the third conclusion (Draper's Law) while the reverse would also be expected. It is also well established experimentally by the work of Von Hiibl,18 Monpillard,19 and Valenta.20 Theories of color sensitizing involving the fugitive character of the dye have been advanced. If this was the case, one would expect the dyes having the least stability to light to be the best sensitizers. Ex- amination does not show this to be the case. For. instance, cyanin is very unstable while erythrosine is quite stable, yet of the two the latter is by far the most powerful sensitizer. Also, dicyanine is extremely unstable and a weak sensitizer, while rose Bengal is fairly stable and yet a good sensitizer. Evidently then there is no connection between the fugitive character of a dye and its sensitizing action. It is easily seen how a dye which is fluorescent, if added to the emul- sion, might produce color sensitiveness but this theory fails when it is 15 Brit. J. Phot., 1907. 16 Beitr'dge zur Photochemie, III, p. 35. 17 This may be explained by Kundt's Law or Wiedemann's theory. See " Re- cent Work in Color Sensitizing," Wall, Brit. J. Phot., 1907, 51, 406-407. 18 Brit. J. Almanac, 1906, 771, and 1907, 744. 19 Bull. Soc. Franc. Photo., 1906, vol. 132. 20 Beitr'dge zur Photochemie, III, pp. 153 and 163. ORTHOCHROMATICS 177 shown that some sensitizing dyes, as erythrosine, are not fluorescent. Many other fluorescent dyes of similar composition are not sensitizers. There is no apparent connection between chemical composition and sensitizing properties. The number of useful dyes is small. Good sensitizers are found in almost all classes of dyes, while dyes differing greatly in stability to light and chemical constitution often show re- markable similarity as sensitizers. While there must be some connec- tion between sensitizing properties and chemical composition such a connection has yet to be discovered. Color Sensitizing Dyes. — Erythrosine is the most widely used sensitizer for green and yellow although there are other more powerful ' sensitizers without the decided depression in the blue-green near 520 /a/a which is characteristic of erythrosine (see spectrogram in Fig. 123). The advantage of erythrosine over more powerful sensitizers, such as pinaverdol, is that the sensitizing action of erythrosine ends abruptly at 580 /t/A while the others continue further into the orange and red. Erythrosine dyed plates can thus be developed in a red light while plates sensitized with the newer isocyanines such as pinaverdol cannot. Pinaflavol, the composition of which has not been published, is one of the few dyes sensitizing specifically for the green. Another is 2-/>-dimethylamino-styrylpyridine methiodide which was pre- pared by Mills and Pope 21 in 1922. The sensitizing curve of pina- flavol reaches from about 580-480 /a/a with a maximum at about 530. The sensitizing action of 2-/>-dimethylaminostyrylpyridine methiodide extends to about 600 /a/a. These green sensitizers are of value more particularly for sensitizing the green plate in three-color photography. Ethyl Red, the first of the isocyanines, has been completely replaced by the more powerful sensitizers, orthochrome T (/>-toluquinaldin- quinolinmethylcyanin bromide), pinaverdol (/>-toluquinaldin-quinolin- methylcyanin bromide) and pinachrome (/>-ethoxyquinaldin-/>-meth- oxyquinolinethylcyanin bromide). Orthochrome T and pinaverdol are essentially green and yellow sensitizers (see spectrograms in Fig. 123). Neither has the depression in the blue-green characteristic of erythrosine. Pinaverdol is a stronger blue-green sensitizer than orthochrome and its action extends farther into the orange, ending at 630 /a/a. The sensitizing action of pinachrome extends even farther into the red ending approximately at 650 /a/a. Pinachrome forms an excellent sensitizer for color-sensitive plates where high red sensitivity is not required. For panchromatic plates with a high red sensitivity, 21 /. Chem. Soc. (London), 1922, 121, 946. 178 PHOTOGRAPHY • ■ -M ■ .jM EASTMAN 40 PINACHROME ■ ; mm^^m ■ ■■....I ■-■ 4- ■■■■■ "• *""nH'^,'i- L_ ■^^■HHHi ACRIOINE ORANGE PINACHROME VIOLET ■ ERYTHROSINE & AMMONIA PANTOCHROME RHODAMINE B PINACYANOLE sj^MsmKmrnmrnmaaMmmm PINAFLAVOL NAPHTHACYANOLE j JB> .*«iMMIIitffll ORTHOCHROME T DICYANINE A PINAVERDOL K RYPTOCYANINE NEOCYANINE Fig. 123. Spectral Sensitiveness of a Gelatino-bromo-iodide Emulsion, color-sensitized with various dyes ORTHOCHROMATICS 179 pinachrome is combined with pinacyanol, an extremely energetic red sensitizer, the sensitizing action of which extends to approximately 690 /a/a (see spectrogram). As pinacyanol does not sensitize to green it is generally used with pinachrome or pinaverdol to fill up the de- pression in the green. Pinachrome violet, a later introduction, com- bines more readily with orthochrome, pinaverdol and pinachrome than does pinacyanol and sensitizes more strongly to red. Pinachrome violet and pinachrome, or orthochrome, make an "excellent sensitizer for panchromatic plates where an even sensitizing action from the blue- green to the red is wanted.22 Pantochrome as a sensitizer resembles pinachrome violet very closely ; the sensitizing action, however, is more even, the minimum near 500 /a/a being less than with pinachrome violet. Pinachrome blue and pinacyanol blue are two later introduc- tions, the sensitizing action of which reaches farther into the red than does pinacyanol itself ; pinachrome blue sensitizing to approximately 680 /a/a and pinacyanol blue to 750 /a/a. Like pinacyanol, neither is a green sensitizer. Napthacyanole (1,1 diethyl di-/?-naphthcarbocyanine nitrate) is a strong red sensitizer which shows a maximum at 690 /a/a in the deep red. As a green sensitizer it is even less energetic than pinacyanol and as it does not combine well with green sensitizers, it is suitable principally for work in which high red sensitiveness is re- quired. Dicyanine was for a long time the only known sensitizer for the extreme infra-red, as with ammonia it has been possible to reach 1000 /a/a. In order to get good sensitizing, however, ammonia must be added to the dye bath and this tends to produce heavy fog. It is a very difficult dye to work, owing to its erratic behavior, and its use has been confined principally to infra-red spectrography. Dicyanine A is an ethoxylated dicyanine which is slightly more powerful as a sensitizer than Dicyanine. Dicyanine is now being replaced even for spectrog- raphy by two later introductions, kryptocyanine and neocyanine. The sensitizing action of the former is greater than that of dicyanine up to 825 /a/a beyond which dicyanine is superior. Neocyanine, however, sensitizes strongly to 900 /a/a while with longer exposures the mercury line at 1 128.8 /a/a has been recorded. Both kryptocyanine and neo- cyanine work clean and free from fog and may be added to the emul- 22 Bloch and Renwick found {Brit. J. Phot., 1920, 67, 304) that aurine (di- methyl-amidoimino-diphenylmethane) which is not in itself a sensitizer increases the color sensitivity of plates sensitized with the isocyanines and carbocyanines and tends to restrain the fogging tendencies of these sensitizers. 180 PHOTOGRAPHY sion before coating or used for bathing. Plates sensitized with krypto- cyanine are furnished by the Eastman Kodak Company as " Extreme Red Sensitive Plates " and the neocyanine plates as " Infra-Red Sensitive Plates." Color Sensitizing by Bathing. — Most sensitizing dyes may be added directly to the emulsion before coating and this is the general practice in the preparation of the color-sensitive plates of commerce. Plates bathed after coating in solutions of the sensitizing dyes usually have a higher degree of color sensitivity but, if the evidence available can be accepted, do not keep so well as those in which the dye is in- corporated in the emulsion prior to coating. The dyes used for sensitizing may be purchased in the solid state or in a solution at a concentration of i : iooo, except in a few cases in which the dye does not keep in solution. For the experimenter, who requires only a small quantity of a dye, it is best, where possible, to purchase the i : iooo solution. In handling the dye solutions, it should be remembered that they are all sensitive to light so that they should be kept away from strong light. The character of the plate is one of the most important factors in successful color-sensitizing by bathing. As a rule, ultra-rapid plates are very prone to give excessive fog after sensitizing and medium speed, clean-working plates are to be preferred. There is, however, no way of estimating the suitability of a given plate except by actual trial. Absolute cleanliness is essential at every point and many of the fail- ures met with in color sensitizing may be traced to chemical con- tamination. Either a tray or a grooved tank may be used for the sensitizing bath; the latter is obviously the more convenient where a number of plates are to be bathed. In either case the solution should be agitated every now and then or it will settle down. Glass trays, or tanks, are to be preferred as they are more easily kept clean. The tray or tank used to hold the dye solution should be used for no other purpose. The time of bathing depends to a certain extent on the temperature and more on the concentration of the dye solution. As one never knows what correction to make for higher or lower temperatures, it is best to maintain a temperature of 20 degrees centigrade as closely as possible. At this temperature and for the concentrations given in the table below, the time of bathing ranges from 1 to 5 minutes. The treatment after bathing varies with the particular dye employed. ORTHOCHROMATICS 181 Some require a brief rinse in methyl alcohol, others immersion in a weak bath of ammonia; some should be washed in distilled water, others should not be washed at all. The precise treatment to follow in each case is indicated in the table which follows. Regardless of the particular dye employed, the plate should be dried as rapidly as possible as the rate of drying- greatly influences the color sensitivity and keeping quality. With some dyes the plate must be dried in from i to 2 minutes if the plates are to keep more than a few days without fog, and in general not more than 5 minutes should be re- quired. A drying cupboard is almost a necessity, but lacking this, one may use one of the electric hot-air dryers as used by hairdressers or an electric fan. The table on page 182 shows the dilution of the sensitizing dye for stock solutions; the composition of the sensitizing bath and the treatment after bathing. The methods indicated are based for the greater part on a paper by Dundon, Color Sensitizing Photographic Plates by Bathing.23 Hypersensitizing of Color Sensitive Materials. — The color sensi- tiveness of emulsions which incorporate the more efficient sensitizing dyes such as pinacyanol, or a combination of pinachrome violet and pinachrome, is sufficient for all ordinary requirements. There are, however, a few special cases, notably aerial photography, which call for still greater color sensitiveness in order to permit of short ex- posures with dense, haze-penetrating filters. To reach this end vari- ous methods of hypersensitizing color-sensitive plates and film have been worked out. From the practical point of view, all are more or less open to the same objection; namely, the material after treatment does not keep at all well and begins to show fog in a very few days after treatment. Jacobsohn 24 recommends for the hypersensitizing of panchromatic material a bath consisting of : Ammoniacal silver chloride, stock solution 4 cc. Distilled water 200 cc. This solution will serve for six 9x12 centimeter (3^2 x 4^ inches) plates, or their equivalent. The time of bathing at 18 degrees centi- grade (65 degrees Fahr.) should be two minutes. The same precau- tions as to cleanliness and rapid drying must be observed as when sensitizing plates with dyes. 23 Amer. Phot., 1926, December. 24 Amer, Phot., 1929, 70; Brit. J. Phot., 1929, 76. 315. 182 PHOTOGRAPHY Composition of Baths for Color Sensitizing by Bathing (The stock solution of the dye referred to in the table is a i : 1000 solution of the dye in methyl alcohol except in the case of Pantochrome, Kryptocyanine and Neocyanine which is i : 2000 and Dicyanine A and Naphthacyanole which is 1 : 5000.) Sensitizing solution Dye Water Stock dye solution Alcohol Ammonia Remarks Erythrosine IOO 20 2 (2 0 Crf \ 5%) Immerse in 2% solution of ammonia prior to bathing. Wash 3 min- utes after sensitizing. Rhodamine B IOO 10 Time of bathing 2 min- utes. Dry without washing. Pinachrome (Sensitol Green) Orthochrome Pinaverdol Pinacyanole Pinachrome violet (Sensitol violet) 100 2 Time of bathing 3-4 min- utes. Rinse well in methyl alcohol after sensitizing and dry as rapidly as possible. Pantochrome IOO 2 Time of bathing 5 min- utes. Rinse 1 minute in methyl alcohol and dry. Naphthacyanole 50 10 40 Time of bathing 5 min- utes. Rinse well in methyl alcohol and dry. Dicyanine A 60 20 20 4 (2 B%) Mix water and alcohol and cool to 500 F. Add dye and allow 5 min- utes to mix with the solution, then stir in the ammonia. Time of bathing 4 minutes with constant agitation. Rinse in methyl alcohol and dry rapidly. Kryptocyanine 500 I 2 (1%) 1 lme 01 [Mining 3 4 min- utes. Rinse in methyl alcohol and dry as rap- idly as possible. Neocyanine 75 I 25 Time of bathing 1 minute. Rinse in methyl alcohol and dry rapidly. Pinachrome violet and Pinachrome Distilled water Alcohol Pinachrome violet 1 : Pinachrome 1 : 1000 1000 500 250 7 7 Time of bathing 3 to 4 minutes. ORTHOCHROMATICS 183 The ammoniacal silver chloride solution referred to is compounded by dissolving 1.5 grams (23 grains) silver nitrate in 50 cc. (about 2 ounces) of distilled water. Hydrochloric acid is then added to this solution as long as a white precipitate of silver chloride is seen to be formed. The clear solution is now poured off, the vessel filled with distilled water, shaken, allowed to settle and then poured off. This operation is repeated a second time, after which the precipitate is dis- solved in 200 cubic centimeters (7 ounces, 20 min.) of ammonia 0.190. This ammoniacal silver chloride stock solution (which keeps well) is used in preparing the hypersensitizing solution, which does not keep. By this treatment the general color sensitivity is increased from 2.5 to 7.5 times, depending on the type of plate or film employed. The Theory of Light Filters. — With our present knowledge of emulsion making it is impossible to make a plate having the same sensitiveness to colored light as the eye. No matter what dye, or combination of dyes, is used the action of the blue and violet remains stronger than it should be. All emulsions are also extremely sensitive to ultra-violet, while this is invisible to the eye. To eliminate the action of the ultra-violet and diminish the action of the violet and blue so as to secure a closer approximation to the sensitiveness of the eye, it is necessary to use colored screens which, by absorbing these colors either completely or partially, cause the less refrangible rays to affect the plate in approximately the same proportion as they do the eye. An orthochromatic filter should, so far as possible, com- pletely absorb the ultra-violet without absorbing any of the visible spectrum completely, but it must absorb the blue and violet to such an extent that the photographic effect on the plate will be equal to the visual effect of those colors. Filters which accomplish these purposes are known as orthochromatic, compensation, ray, or correction filters. While in most cases we desire faithful color rendering, there are times when accurate color rendering will not produce a satisfactory result and it is necessary to deliberately sacrifice truthful color ren- dering in order to bring out the colors satisfactorily. This is due to the fact that there are two kinds of contrast by which objects are picked out from their surroundings by the eye. We may have color contrast where the difference lies purely in color or we may have tonal difference where the color is the same in both cases but the two areas are different in depth. In the latter case, any plate will properly re- produce the contrast provided it is properly exposed and developed. 184 PHOTOGRAPHY In the first case, if the two colors, say green and red, are photo- graphed on an ordinary plate, which is insensitive to these colors, both are represented by black and consequently there is no contrast. If a panchromatic plate which is sensitive to the entire visible spectrum is used with the proper compensation filter, we secure a uniform field of gray without any contrast because of the fact that the two areas are different only in color and not in depth or darkness. Therefore, in order to bring out the contrast between the two colors, it will be neces- sary to sacrifice the correct rendering of either the green or red. If a filter which transmits nothing but green light is placed in front of the lens during the exposure, the green will be reproduced light while the red will be absorbed in passing through the filter and will reproduce dark. If, instead of the green filter, one passing a narrow band in the orange-red is substituted, the red will be reproduced as light while the green is dark because the green rays from the object are absorbed in the filter and fail to reach the plate. Filters which show a narrow band of transmission and are used to pick out colors from their surroundings are known as contrast or selection filters. Color Sensitive Plates with Incorporated Filter Dyes. — Regardless of the particular sensitizing dye, or combination of sensitizing dyes used, the added color sensitiveness to the less refrangible rays does not equal that for the blue and violet. If a yellow filter dye is added to the emulsion, this dye will act as a color screen for the depths of the emulsion when coated on a plate or film base. The screening effect on the grains of silver halide next to the surface will be slight, but the screening effect of the dye will become more apparent as we go deeper into the emulsion. Plates in which a yellow filter dye is incorporated to reduce the sensitiveness of the plate to blue and violet light are known variously as self-screen, non-filter, anti-screen, auto-filter, etc. The dye usually used is filter yellow K. Meister, Lucius and Briining supply a mixture of erythrosine and filter yellow K as Pinortho I, and Pinachrome and filter yellow K as Pinortho II. Ofthochromatic Filters. — A filter which reduces the blue and violet to the point at which the photographic effect of the different colors corresponds to the brightness of the colors to the eye (a con- dition which may be defined as perfect orthochromatic reproduction) is too dense for practical use as the exposure is increased to a pro- hibitive extent. Consequently, it is customary to consider as a fully correcting filter one which absorbs sufficient of the more active rays ORTHOCHROMATICS 185 to render the photographic action about equal for all of spectrum to which the plate may be considered sensitive. To find the absorption curve of a filter which will give correct color rendering on a plate or film we require first to know the sensitiveness of the plate to the various colors. If we photograph the spectrum on the plate in question and express in densities, which are a measure of the work accomplished by light, as a function of wave-length, we ob- tain a curve which shows the sensitiveness of the plate to the colors of the spectrum. Then, if from the ordinates of this curve we subtract those of the curve representing the visual brightness of the spectrum, we obtain a curve showing the absorption which the filter must possess to produce perfect color reproduction with that particular plate or film. The practical consequences are that for best results the orthochro- matic filter for any particular plate or film should be that advised by the maker for his product. Small differences in manufacture lead to differences in color sensitivity even where the same dyes are used; consequently a filter which may be regarded as a fully correcting filter for one plate may not absorb sufficient blue and violet rays to com- pensate for the lower color sensitivity of another plate. Since the color filter absorbs the more active rays, its use necessi- tates an increase in exposure. The amount which the exposure must be increased, under like conditions, when a filter is used is known as the " factor " of the filter. Thus a filter which under the same con- ditions doubles the exposure required by the plate without a filter is said to have a factor of 2. The factor of a given filter depends upon the light source by which the exposure is made and the spectral range of the sensitive material. The greater the relative intensity of the light source in that portion of the spectrum transmitted by the filter, the lower the factor of the filter. The multiplying factor of the usual yellow filter, for example, is much less with clear, incandescent electric light than with daylight, owing to the greater abundance of long-wave radiation in the former as compared with daylight. The expression, " a 2 x filter," is meaningless, therefore, unless the plate and the nature of the light source are specified. Where a filter is stated to be a 2 x filter for a certain plate or film, the use of daylight or an artificial light source approximating daylight, i.e., the white flame arc, or blue incandescent lights, is assumed. The following table will give one an idea of the extent to which the factor of a filter is altered by changes in the light source and the char- acter of the plate. 186 PHOTOGRAPHY Factors for a Yellow Filter (K3) on Ordinary (Portrait Film) on Ortho- chromatic (Commercial Ortho) and Panchromatic (Commercial Pan- chromatic Film) to Daylight, White Flame Arc, Clear Mazda Lamp and Cooper-Hewitt Mercury Vapor Tubes Ordinary Orthochromatic Panchromatic 32 12 4-5 White flame arc 8 8 3 Clear Mazda 3 3 1-5 20 32 5 The use of any filter in glass alters the focus of the lens, so that focusing should be done with the filter in place. For long focus and telephoto lenses the use of filters cemented in optical flat glass is ad- visable as filters of ordinary optical glass may affect the definition of the objective. Orthochromatic Methods in Landscape Photography. — There is by no means complete agreement concerning the value of orthochromatic methods among landscape workers. Some workers pin their faith to an ordinary plate owing to the better representation of atmosphere. Others use color-sensitive plates of the iso type with just enough cor- rection to render the clouds with the landscape while still others in- sist on complete color correction and use panchromatic plates with fully correcting filters. The best methods in practice depend upon the results desired. The pictorialist who revels in atmospheric effects of early morn or late afternoon and evening will find the ordinary non-color-sensitive plate better adapted to his requirements than color-sensitive plates because the very deficiency of the plate causes it to emphasize the features which he desires. The appearance of atmosphere is due to the light rays reflected from dust particles in the air and these rays are always either blue or violet, except at sunset or sunrise when they may be tinged with yellow and orange. Ordinary plates are very sensitive to the blue and violet and also the invisible ultra-violet, which is present in the atmosphere to a considerable extent, and, therefore, emphasize any suggestion of atmosphere. Many workers employ orthocromatic methods only to the extent of securing printable clouds in their landscapes. For this purpose a com- paratively light screen is all that is necessary. Full exposure should be given, otherwise the sky portion of the negative is thin and the foreground has excessive contrast, sometimes appearing as if snow ORTHOCHROMATICS 187 was present. The depth of filter will be determined very largely by the strength of the clouds. If these are strongly marked a very light filter is all that is necessary, while stronger filters are necessary for the thin delicate clouds often observed. Care should be taken not to over- correct the sky (which will be done if a strong filter is used), as the clouds lose much of their delicacy and charm when this is done. For distant views of mountain scenery and over cities with their enveloping haze of smoke and mist and for aerial photography stronger filters are required. Haze results from blue and violet light reflected by tiny particles in the atmosphere and can be removed by the use of filters which absorb the radiation which these particles reflect. The stronger the haze and the lower the initial contrast of the subject, the deeper the filter required. Strong yellow filters requiring from 3 to 5 times increase with panchromatic plates, or film, or the various filters provided for aerial photography enable the photographer to secure practically any details which may be visible to the eye, while the use of deep red filters and highly red-sensitive plates will often enable de- tails to be photographed which cannot be sharply perceived by the eye. Orthochromatic Methods in Portraiture. — Concerning the value of color-correct rendering in portraiture, Dr. Mees says : 25 " In no branch of photography is the reproduction of colored objects in mono- chrome of greater importance than in portraiture and in no branch is it in greater danger of being ignored. The flesh tints, with which the portrait photographer is mainly concerned, are chiefly of a red- dish nature, while the yellow and brown shades of hair and the va- riety of eye-colors apart altogether from the clothing cause every sit- ter to present a distinct problem in color reproduction." Figure 124.4 is a print from a negative made on an ordinary non- color-sensitive portrait plate. Ordinary plates are sensitive only to the violet and to blue, rays which are almost completely absorbed by the skin. The result is that an ordinary plate fails to reproduce the texture of the skin properly and produces excessive contrast which emphasizes all of it's lines and imperfections. The various shades of brown, golden and red hair are difficult to photograph properly and all sorts of dodges are used by operators to secure a passable render- ing of the same. In most cases when the proper tint is secured the de- tail of the shadows in the hair is lost. The wrinkle which exists around the eyes is often a comparatively deep shade of red and is 25 Photography of Colored Objects. 18S PHOTOGRAPHY ORTHOCHROMATICS 189 reproduced too dark with an ordinary plate and the retoucher in lightening the same often destroys the strength of the eye by taking out the wrinkle entirely. In Fig. 1245 is shown a portrait of the same subject under identical conditions excepting that a color-sensitive plate and filter were used in making the original negative. The material used was the East- man Panchromatic film and a K2 filter requiring an increase in ex- posure of 3^2 times. The marked improvement in the rendering of flesh tones and skin texture is quite evident. While the result may not yet be entirely satisfactory and some further retouching may be necessary, considerably less time will be required for this operation since most of the retoucher's work has been done for him, and owing to the comparatively small amount of retouching required on the lat- ter negative there is less danger of losing the facial expression of the subject in that operation. Ultra-rapid panchromatic plates and film of portrait quality are now available so that there is no longer any real objection to the exclusive use of color-sensitive materials for general portrait work. With day- light, light filters may be used without unduly lengthening exposures while with artificial lights, such as the clear incandescent lamp and the open arc with the so-called " panchromatic " carbons, no filter is re- quired for the average subject. With artificial lights, such as those mentioned, having a great abundance of long-wave radiation, pan- chromatic materials are distinctly faster than non-color-sensitive ma- terials having the same speed to daylight because they are especially sensitive to the long-wave radiation of these light sources. It is only a question of time before the use of color-sensitive materials will be- come universal. Photographing Color Contrasts. — We referred to this subject under the subject of color filters but we now wish to devote some space to the application of the same in practice. To photograph a color as black a filter must be employed having an absorption band in the wave-lengths of the particular color to be rendered as black. In other words to photograph any given color as black it must be photographed through a sharp cutting filter which completely absorbs the color of the subject. No rays of light re- flected from the subject will then reach the plate and the color will be as black as it can be made. To render a color as white it must be photographed not in its ab- . 190 PHOTOGRAPHY 'the most valuable publication^ of the year is the * rt Kistorlo de la Deoouverte de la Photographic by Georges Potonniee (published by Montel-Paris ) . In this work the history of photo- graphy is covered completely from its inception to the death of Daguerre in 1851. The second volume [to appear soonjwill complete the work and bring it down to modern times. Another notable work of the year is the Physios of the Developed Photographic ^fmage by.^.E. Ross, which is Number 4 of the series of ^bnographs on the "^heory of "photography issued by the Researoh jfaboratory of the Eastman Ko^ak Company. Many of the more important sensitising and desensitizing dyes are discussed in a work by J.F. Hewitt "I^estuffsjierived from Pyridine, iuinoline.Aoridine and Xanthene". (longsman|»s Green & Co., Hew York) Fig. 125a. Photograph of Manuscript in Blue with Red Corrections using Green Filter Perhaps the most valuable publication of the year is the Historle de la Deoouverte de la Photographic by Georges Po.tonniee (published by Montel-Paris). In this work the history of photo- graphy is covered completely from its inception to the death of Daguerre in 1851. The second volume to appear soon will complete the work and bring it down to modern times. Another notable work of the year is the Physios of the Developed Photographic image by f. S.Ross, which is Number 4 of the series of monographs on the theory of photography issued by the Research laboratory of the Eastman Kodak Company. Many of the more important sensitixing and desensitizing dyes are discussed In a work by J.F.Hewit •Dyestuffs derived from Pyridine, Qulnoline.Aoridine and Xanthene". (longsmanns Green 4 Co., Hew York) Fig. 1256. Photograph of Manuscript in Blue with Red Corrections showing Use of Red Filter ORTHOCHROMATICS 191 sorption band but in its reflection band. In other words, any color will be reproduced as light if it is photographed through a filter of its own color. Red objects absorb blue and green light. Green objects absorb blue and red light. Dark Blue objects absorb green and red light. Yellow objects absorb blue light. Magenta or purple objects absorb green light. Light blue or blue-green objects absorb red light. Suppose, for instance, we have a manuscript typewritten in blue ink with corrections in bright red. We desire to make one photograph showing the manuscript complete with corrections and another show- ing the text without the alterations. What filters must we employ? If an ordinary, non-color sensitive plate without a filter is employed we will probably find that while the alterations in red stand out while the blue of the original text is quite faint. An orthochromatic plate with a compensating filter will make the blue typewriting somewhat darker but for the greatest possible contrast we must employ a contrast filter which completely absorbs both blue and red. Such a filter would transmit a narrow band in the green and would give us the result shown in Fig. 125a. To eliminate the corrections we must reproduce red as white while making blue dark, accordingly we would select a contrast filter transmitting red, such as the Wratten A or F. This would give us the result shown in Fig. 125&. Should it be required to photograph the corrections alone, eliminating the original blue type- written text, this might be accomplished by the use of a dark blue filter, such as the Wratten C. One of the best examples of the value of orthochromatic methods and the application of the principles of color contrast occur in photo- graphing furniture. In Fig. 126 are shown comparative photographs of wood sections on ordinary and panchromatic plates with proper - filters and the immense improvement in results obtained by the use of the latter is at once evident. If red mahogany, for instance, is photo- graphed on an ordinary plate, no trace of the grain is visible, while increasing the exposure merely results in bringing up a large number of scratches imperceptible to the eye. However, by using a panchro- matic plate with an orange-red filter the scratches disappear and the srrain of the wood is brought out. In photographing furniture, success depends chiefly upon the selec- PHOTOGRAPHY {Courtesy of Ilford Lid.) 126. Wood Sections on Ordinary and Panchromatic Plates ORTHOCHROMATICS 193 tion of the proper filter for the subject. For mahogany the greatest contrast is obtained by using an orange-red filter such as the Wratten A. With yellow woods like oak, satinwood, and walnut, the deep yel- low filter as the Wratten G will be of greatest service. Care must be taken not to exaggerate the contrast of inlaid furniture and the mat- ter must be compromised, using either a fully correcting orthochro- matic filter as the K3 or one of deep yellow or orange-red. In general it is best to depart from orthochromatic rendering only when absolutely necessary. Whenever there is doubt, it is good policy to make one exposure with an orthochromatic filter in addition to that made with the contrast filter which is judged to be correct. BIBLIOGRAPHY General Reference Works Baker— Orthochromatic or Isochromatic Photography. Eder — Uber die Chemischen Wirkungen des Farbigen Lichtes. Eder and Valenta — Beitrage zur Photochemie und Spectralanalyse. Hubl — Die Photographischen Lichtfilter. Hubl — Die Orthochromatische Photographic Konig — Das Arbeiten mit Farben empfindlichem Platten. Mf.es — Photography of Colored Objects. Weigert — Die Chemischen Wirkung des Lichtes. CHAPTER VIII THE LATENT IMAGE Photo-Physical and Photo-Chemical Change. — Nearly every body undergoes some change when exposed to light. The change may be slow or it may be remarkably rapid, as in the case of the silver halides, according to the nature of the body, and it may be either physical or chemical in character. In the first case the change consists in an alteration of the appearance or properties of the substance but unac- companied by any change in composition, while in the second case the composition, as well as the properties of the substance, are altered. As an example of a physical change due to the action of light we may take selenium, which in darkness is a non-conductor of electricity but becomes a conductor when exposed to light. Yellow phosphorus, a highly inflammable substance, is gradually converted by the action of light into a red phosphorus with entirely different properties. Pow- dered non-crystalline selenium gradually becomes crystalline upon ex- posure to light. Certain metallic salts, such as the crystalline chloride or iodide of silver, nickel sulphate, and zinc selenate, experience a change in crystalline form under the influence of light. In all such cases it should be observed that no chemical change has taken place. Crystalline and non-crystalline selenium are both selenium and have the same composition, while the same is true of the forms of yellow and red phosphorus and of soluble and insoluble sulphur. The change which has taken place is due to some alteration in the arrangement of the molecules but not to such an extent as to cause a chemical change. Regarding the chemical changes due to light, Eder has made the following general statements : 1. All kinds of light from the ultra-violet to the infra-red, whether visible or not, have some photo-chemical action. The rate of action may vary to a considerable extent, but there is no kind of light that is absolutely without effect on a body if the time is sufficiently prolonged. 2. Photo-chemical action is produced only by such rays as the body absorbs (Draper's Law), so that the chemical action of light is closely related to optical absorption. 3. The sensitiveness of a body towards rays of a definite refrangibil- ity is increased by the admixture of other substances which absorb the same rays. 194 THE LATENT IMAGE 195 4. A substance is, as a rule, decomposed faster by a given color when it is mixed with a body which absorbs one of the products re- sulting from the photo-chemical decomposition. The action of light may bring about either decomposition or com- bination. Examples of the former occur in nearly all photographic processes while a familiar example of the latter is the union of chlorine and hydrogen, to form hydrochloric acid according to the equation : H2 + Cl2 = 2HCI. Moisture is essential to the above reaction and it is possible that a cer- tain amount of water is required for all photo-chemical reactions. Thus the action of light may be either reducing or oxidizing in character, depending upon the nature of the substance under its in- fluence. The Latent Image. — When light is allowed to fall on a photographic plate, or upon silver halide precipitated from solution, the silver bro- mide is altered in some unknown way because a reducing agent, or " developer," is able to darken the silver bromide exposed to light more rapidly than that which has not been exposed. We say that the light has produced a " latent image " because it is invisible to the eye but susceptible to certain reducing agents, and it is our problem to de- termine the nature of this change and of what this latent image con- sists. The nature of the change which occurs when a silver halide is exposed to light is still an unsolved problem, despite much speculation and the enormous amount of experimental work which has been done by the most eminent scientists in an attempt to reach a solution of the problem. While this work has not enabled us to reach any definite conclusion, it has been of very real value as many facts regarding the character and reactions of the invisible image have been established which must of necessity be taken into consideration when forming a working theory of the latent image. Therefore it seems advisable to review some of the more important experimental work by various authorities, which has a definite bearing on the nature and composition of the latent image, before proceeding to a discussion of the theories advanced to explain the same. Artificial Latent Images. — Towards the end of the last century, W. J. Russell found that many substances were able to act on a photo- graphic plate in some manner so as to make it developable without any exposure to light. The number of substances which would act in this manner was very great, and included freshly scratched metals, espe- 1 196 PHOTOGRAPHY daily zinc and magnesium, many fats and volatile oils, and numerous other natural organic bodies like wood, straw, blood and resin. The activity of all these materials was traced to the formation of hydrogen peroxide as a result of the superficial oxidation of the substances in moist air. The vapor and solution of hydrogen peroxide itself ex- hibited the phenomenon ■ to a much more marked degree. Since Russell's first experiments, a vast number of materials have been discovered which, when applied to a plate, make it developable in absence of exposure to light. For instance solutions of many mild reducing agents such as sodium arsenite, very dilute ferrous oxalate, sodium hypophosphite and stannous chloride, dilute acids, certain neutral salt solutions and some dyes can all act on a plate to give some sort of latent image. The materials which have been most investigated in this respect are sodium arsenite and hydrogen peroxide. Their ac- tions on the plate show an extraordinary parallelism with the action of light. A study of the fogging action of peroxide and arsenite should, therefore, be of assistance in shedding some light on the nature and formation of the latent light image, and on the nature of the sensi- tiveness of the grains in a photographic emulsion. Hydrogen Peroxide. — The action of hydrogen peroxide increases with increase in time of treatment 2 of a plate by a solution of definite concentration, and with increase in concentration of the solution, for a given time of treatment, giving rise on development to a density- exposure curve similar in form to the well-known /-shaped character- istic curve for exposure to light. On prolonged treatment with per- oxide the curve shows a definite reversal portion, as in the case of light exposure. The characteristic curve for peroxide treatment varies in the same way with time of development as does the normal curve for exposure to light. Plates most sensitive to light are also most sensitive to peroxide, and the bigger grains in an emulsion are, on the average, more sensitive than the smaller ones to both light and per- oxide. Intensification with Hydrogen Peroxide.— Treatment of a plate with hydrogen peroxide at concentrations below that at which fog is produced increases the developability of the image. This fact, first noticed by Liippo-Cramer,3 has been studied in detail by Wightman,4 2 S. E. Sheppard and E. P. Wightman, /. Franklin Inst., 1923, 195, 337. 3 Liippo-Cramer, Phot. Korr., 1915, 52, 136. * Wightman, Brit. J. Phot., 1027, 74, 447. THE LATENT IMAGE 197 Trivelli,5 Sheppard 5 and Quirk," of the Kodak Research Laboratories by whom it is termed intensification of the latent image. Studies of the effect of hydrogen peroxide on single-layer plates show that the number of developed grains in a given area affected by light following treatment with hydrogen peroxide is greater than the sum of those acted upon separately by light and hydrogen peroxide. If it is assumed that the sensitivity centers of the silver halide grains vary in size and that the effect of light is to increase these centers proportionately, that is to produce latent image centers of various sizes, some of which are large enough to make the grains developable while others are not, then the effect of hydrogen peroxide is evidently to increase the smaller sized latent image centers to the size necessary to render them developable. Just how the hydrogen peroxide acts in bringing this about is still a matter of doubt ; the most likely explana- tion is that the hydrogen peroxide reacts with the small amount of soluble bromide in the plate to set free very small quantities of bromine which according to the theory of Hickman (to be discussed later) re- acts with the silver sulphide of the sensitivity speck in such a way as to produce silver and thus enlarge the development center. The effect of hydrogen peroxide varies with the concentration and time of treatment and is greater for fast plates than for slow. It is prevented entirely by desensitizing the plate in chromic acid or by the removal of the soluble bromide in the plate by the action of silver nitrate. Intensification of the latent image as produced by hydrogen peroxide is not a property of fogging agents generally as other agents such as methylene blue which produce fog are unable to cause intensification. Silver nitrate appears to be the only other substance possessing this property. Sodium Arsenite. — Sodium arsenite gives a characteristic curve similar to that for light, and with a well-defined reversal portion. Plates faster to light seem also to be more sensitive to the action of arsenite. The distribution of the latent image due to arsenite treat- ment has been studied in the same way that Svedberg and Toy studied the distribution of the latent light image due to light, by making statistical measurements on the " reduction centers " shown up by partial development of the emulsion grains. The " reduction centers " in the silver halide grains in an emulsion can be shown up after treat- 5 Wightman, Trivelli, and Sheppard, /. F. I., 1925, 200, 335. 6 Wightman and Quirk, /. F. I., 1927, 203, 261; 204, 731. 198 PHOTOGRAPHY ment with arsenite in a manner similar to that in the case of light, and they are found to be distributed among all the grains, and topographi- cally "on the individual grains themselves, according to the same laws as are found to hold in the case of exposure to light. It is possible, by using the />-phenylenediamine-.silver sodium sulphite mixture, to develop physically, after fixation, the latent image due to sodium arsenite. Treatment of a plate with chromic acid solution desensitizes it to the action of sodium arsenite in the same way as to light action. In the desensitization of a plate to light by chromic acid, it is found that a preliminary exposure to light before bathing in chromic acid greatly accelerates the rate of desensitization. That is, the latent image is attacked by chromic acid much more readily than the sensitive nuclei themselves. The same is found to hold if the " preliminary exposure " is treatment with sodium arsenite solution.6 The formation and reaction of the latent arsenite image are thus very similar to those of the latent light image. Reversal by Light. — With a short exposure to light we get a latent image which on development yields a negative. If the exposure is lengthened considerably the image becomes positive instead of negative when developed, while still further exposure will produce a second negative and it is probable that the cycle may be repeated indefinitely, although owing to the enormous exposures required no one has been able to go past the second negative stage. No photographic process is, strictly speaking, free from the effects of reversal, but rapid gelatino-bromide plates are more subject to the defect than a comparatively insensitive plate, such as wet-collodion. The reactions which result in reversal are still obscure; in view of the action of absorbents of halogen such as derivatives of hydrazine and phenylenediamine and sodium nitrite in delaying reversal, it seems possible that it is caused by excess of released halogen, i.e., bromine, which attacks the latent image and reduces the developability of the grain by rehalogenizing the silver nuclei.7 Reversal by Chemical Reagents. — The function of exposure of a plate is to affect the grains at the points of sensitivity in such a way that nuclei are formed which are sufficiently big to act as deposition centers for the development process. The function of fogging agents such as have been considered must be a similar one. In the reversal process with light it is probable that the function of prolonged ex- 6 Clark, Phot. J., 1923, 63, 237; 1924, 64, 91. 7 Phot. J., 1914, 54, 250. THE LATENT IMAGE. 199 posure is to make the deposition centers inactive again. It is sug- gested by some that this could occur by some sort of " retrogressive " action, the centers reverting to their original state; but actually such a reversion seems to be thermodynamically impossible as long as the light stimulus is acting. The more probable result of prolonged exposure is to bring about some " progressive " action which so changes the centers as to make them no longer able to function as centers for development. How this occurs is not clear. In the case of arsenite, however, a very satisfactory explanation is found in assuming that on prolonged treatment the arsenite peptizes the nuclei formed in the first stages of its action, and so makes them too small to function in de- velopment. This view is supported by the experimental observation that sodium arsenite can peptize colloidal silver in gelatin, and also that it can destroy the latent image left after fixation of an exposed plate, so that it cannot be physically developed.8 It is seen, then, that although latent image formation is similar in the case of arsenite and of light, the reversal process is probably quite different in the two cases. The presence of the latent image alone is a sufficient condition for reversal by arsenite, but for reversal by light, the silver halide itself must also be present. Hydrogen peroxide solu- tion can also peptize colloidal silver and destroy the latent image, so that an explanation of reversal by peroxide solution similar to that advanced for arsenite is satisfactory. In the case of reversal by ex- posure to hydrogen peroxide vapor, however, it is more difficult to con- ceive that it is due to peptization.9 Although it is difficult, to obtain really direct evidence concerning the way in which many fogging agents act, and results and opinions con- cerning their action are somewhat conflicting, enough reliable data have been obtained to indicate that the study is of great importance for the theory of photographic sensitivity. In fact, it has played an important part in leading up to the modern conception of sensitivity as due to the presence on the silver bromide grains of traces of some substance not silver bromide. Photo-Regression. — With a daguerreotype plate, development has to be done immediately after the exposure as the image cannot be retained for more than a few hours and gradually grows weaker after exposure. The same condition of affairs applies to the wet collodion 8 Phot. J., 1924, 64, 363. 9 Phot. J., 1924, 64, 363. Cf . also Wightman, Trivelli and Sheppard, /. Frank- lin Inst., 1925. 200 PHOTOGRAPHY plate, although here the loss of the image may be ascribed to the physical condition of the collodion which requires a certain amount of moisture. With gelatine plates the image is remarkably permanent and instances are on record where gelatine plates have been success- fully developed several years after exposure.10 The gradual disappearance of the image after exposure and before development is termed photo-regression, and appears to be a process exactly the reverse of that which produces the latent image. Accord- ing to Baekeland 11 photo-regression is more apparent on images which have received less than normal exposure. The developing agent used for developing appears to have no effect on the final result. The factors which appear to have the greatest influence on the rate at which the image disappears are temperature and humidity, while the presence of alum or free acid in the emulsion also plays an important part. The higher the temperature and the humidity in which plates are stored after exposure and before development the more rapid is the disappearance of the image. Plates or papers which contain alum, or those in which the emulsion is in an acid state, are more subject to rapid disappearance of the image than plates which do not contain alum, or in which the emulsion is in a neutral or slightly alkaline state. According to Luppo-Cramer the size of grain has an influence, small- grained emulsions showing regression more rapidly than those of coarser grain. The phenomenon of photo-regression is interesting in that it shows that the sensitive plate has a certain faculty of self-recovery from the effects of light and any workable theory of the latent image must satisfactorily explain the reason for the same, before it can receive serious consideration. Action of Oxidizing Agents on the Latent Image and on Sensi- tivity.— The latent image is either reduced or entirely removed by oxidizing agents. Substances such as potassium cyanide, acid po- tassium permanganate, acid ammonium persulphate, chromic acid when used with sulphuric acid or potassium bromide and cupric, ferric and mercuric salts, when used with a halide, completely destroy the latent image.12 Destruction of the latent image by oxidizers, however, does not 10 H. J. Channon, " Effect of Time on the Latent Image," Phot. J., 1917, 57, 72. 72. 11 Zeit. wiss. Phot., 1905, 3, 58. 12 Bullock, Brit. J. Phot., 1927, 74, 590. THE LATENT IMAGE 201 necessarily result in the destruction of the sensitivity of the unexposed emulsion. Chromic acid, to name one substance the action of which has been the subject of considerable investigation, reduces the latent image much more than it does sensitivity. With ammonium persul- phate the difference is even greater than with chromic acid. The re- duction of sensitivity in the unexposed grain by chromic acid varies with the size of the grain, the loss in sensitivity being greater for the large grains than the small.13 If the sensitivity centers of the grains consist of silver sulphide as indicated by the work of Sheppard, and the latent image of colloidal silver, then it is evident that the difference in the action of certain oxidizers on the latent image and sensitivity can be explained as being due to the greater resistance of the latter to attack. Experiments by Clark 14 on silver and silver sulphide solu- tions seem to indicate that this is the case. Clark found that oxidizers which attack silver sulphide have a strong desensitizing action on certain emulsions but that ammonium persul- phate has a much smaller effect. Laboratory experiments on prepared colloidal silver sulphide indicated that it is not attacked by ammonium persulphate, while, of course, it is well known that persulphate dis- solves metallic silver. The slight desensitizing action of ammonium persulphate is explained by Clark as being due to the fact that the sensitivity centers do not consist wholly of silver sulphide but of silver sulphide and colloid silver. Persulphate attacks the latter but not the former thus slightly lowering sensitivity. The sensitizing action of colloidal silver as reported by Renwick 15 and Weigert 16 support Clark's assumption that the sensitivity centers consist not necessarily of silver sulphide alone but possibly silver sulphide and colloidal silver. Physical Development of the Latent Image after Fixation. — If a plate is fixed in hypo directly after exposure one would assume that the image would be destroyed, since hypo is a solvent of the silver halides. Such, however, is not the case for as shown by Young in 1858 with wet collodion and by Kogelmann, Sterry, Neuhauss, Lumiere and Seyewetz and others with gelatine emulsion, the latent image is not destroyed by fixing but may be developed in a physical developer, i.e., a developing solution containing in addition to the 13 Sheppard, Trivelli, and Wightman, Trans. Faraday Soc, 1923, 19, 306. Clark, Phot. J., 1924, 64, 91. 14 Clark, Brit. J. Phot., 1927, 74, 227. 15 Renwick, /. Soc. Chem. Ind., 1920, 39, 156. 10 Weigert, Physik. Z., 1921, 22, 674. 202 PHOTOGRAPHY developing agent a silver salt capable of forming silver in the nascent state.17 Physical development is supposed to be due to the attraction of the nuclei of the latent image for the nascent silver of the developing solution. Assuming this explanation to be correct, then any solution which will reduce a silver salt to the nascent condition should be able . to develop the image, although not a developing agent in the com- monly accepted sense of the term. To confirm this point Lumiere and Seyewetz 18 tried as a developer a solution of silver sulphite in excess of sodium sulphite and formalde- hyde. No image was obtained, however, showing that the nuclei left after fixation are incapable of attracting nascent silver. However, if the fixed-out plate be first immersed in paraphenylenediamine or amidol the nuclei acquire the property of attracting the nascent silver and physical development becomes possible. From this they concluded that the latent image left after fixing can- not consist of a silver halide, as this would be dissolved in the fixing bath, nor can it consist of metallic silver, unless its property of attract- ing nascent silver has been destroyed by unknown factors. Owens 19 has since shown that the latent image after fixing probably consists of silver surrounded by silver sulphide so that no deductions as to the na- ture of the latent image can be drawn from the image which remains after fixing. The Photosalts. — In 1887, a brilliant American chemist, Carey Lea of Philadelphia, succeeding in preparing compounds of silver chloride which contain less halogen than the original chloride, by treating ammoniacal solutions of silver chloride with ferrous sulphate, wash- ing the precipitate, and then treating with hydrochloric acid. A large number of these compounds were prepared by their discoverer and were called " photosalts," because he considered them identical with the compounds formed when silver chloride is exposed to light. The photosalts were considered to be definite chemical compounds by their discoverer, but most investigators took the view that the combination 17 Young, Photographic News, 1858, 1, 165. Kogelmann, " Die Isolierung der Substanz der Latenten, Photographischen Bilder." Graz, 1899. Sterry, Photog- raphy, 1898, p. 260. Neuhauss, Phot. Rund., 1899, vol. 36, 257 and 1904, vol. 41. Lumiere and Seyewetz, Bull. Soc. franc. Phot., 191 1, pp. 264, 373; 1924, vol. 169. Compt. Rendus, 1924, 179, 14. Liippo-Cramer, Phot. Rund., 1924, 61, 780. 18 Phot. Revue, 1925, 37, 48. 19 Phot. J., 1929, 69, 278. THE LATENT IMAGE 203 was more of the character of a " lake," or a physical combination of the altered and unaltered haloids. A theory that the latent image consisted of a solid solution of silver sub-bromide in silver bromide was advanced by Lea himself 20 and was supported by Luppo-Cramer and Lorenz.21 Some very valuable experimental work on the preparation and composition of the photosalts has been done by a number of German photo-chemists in recent years and especially by Reinders and Weigert. Image Transference.— According to Renwick, Eder and Pizzighelli in 1881 were the first to show that an exposed silver chloride plate could be converted by treatment with potassium bromide into silver bromide without any loss of the developable condition. In other words it is possible to transfer the latent image from one halide to another without destroying its capacity for development in those parts where the light acted. This phenomenon is known as " image transference." 22 The Visible Darkening of S^ver Halides to Light. — All three of the halides of silver, the chlorine, bromide, and iodide darken visibly on exposure to light. Scheele showed in 1777 that when silver chloride darkens the halogen is liberated. As a result of the work of Schwarz and Gross,23 and particularly of Hartung 24 using a specially sensitive micro-balance, it is now well established that in the case of all three of the halides, halogen is liberated when the visible darkening takes place. The presence of substances which by absorbing halogen act as " halo- gen acceptors" increases the degree of darkening and Hartung has shown that silver bromide in vacuo in the presence of a bromine ac- ceptor can result in the loss of over 90 per cent of the bromine. The darkening in vacuo in the absence of a halogen acceptor is slight and impermanent. It was formerly believed that the result of the liberation of halogen was the formation of a sub-halide, that is, a halide containing less halo- gen than the normal. Attempts to prepare definite sub-halides of silver in the laboratory have met with conflicting success, and it ap- 20 Lea, American Jour. Sci. (3), 33, 1887, 349, 480. 21 Luppo-Cramer, Phot. Korr. (1906), 43, 388, 433. Lorenz, Phot. Korr. (1901), 38, 166. 22 Brit. J. Phot., 1920, 67, 447, 469. 23 Z. Anorg. Chem., 1924, 133, 389. 24 /. Chem. Soc., 1924, 125, 2198. 8 204 PHOTOGRAPHY pears that the sub-halides of silver which were prepared by Von Bibra25 and Vogel,26 for example, are compounds of finely divided silver and unaltered silver halide. The photosalts of Carey Lea were regarded as definite sub-halides by their discoverer but subsequent in- vestigation has shown these to be absorption-compounds of colloid silver with silver halides. The work of Schwarz and Gross and that of Hartung has at last shown quite definitely that the products of the visible decomposition of the silver halide in light are metallic silver and halogen. The Latent Image.- — It is obvious that if we assume that the latent image differs from the visible image which is formed by longer ex- posure to light, only in degree and not in kind, the latent image could have been regarded as consisting of a sub-halide of silver up to very recent times (1924). Sub-halide theories were supported by a large number of authorities, notably Sir William Abney and Dr. J. M. Eder, who in the earlier editions of his Handbuch der Photographic staunchly defended the sub-halide theory. Others objected that there was absolutely no evidence that a libera- tion of halogen took place with the short exposures required for the formation of the latent image and pointed to the fact that attempts to prepare silver sub-halides in the laboratory had met with inconclusive and contradictory results, and finally that there was no good reason for assuming that what holds true for the visible image is likewise true for the invisible latent image. The sub-halide theory is now only of his- torical interest. Many of those who objected to the sub-halide theory on the ground that the amount of light energy required for the formation of the latent image is insufficient to cause photochemical decomposition as postulated by the sub-halide theory, proposed physical theories which assumed that the result of light was to set up an internal strain within the molecule of silver bromide and cause the atoms to pull apart from each other. The effect of the molecular strain is to render the com- pound less stable so that it is more easily reduced to metallic silver by reducing agents such as photographic developers. Proponents of such theories pointed out that such reactions could conceivably take place with a very short exposure to light which would be very unlikely to result in actual photochemical decomposition of the silver bromide. 25 /. fur Prak. Chem., 2, 12-55. -6 Phot. Mitt., 36, 334. THE LATENT IMAGE 205 The fact that the latent image is formed at extremely low temperatures ( — 1800 C.) was advanced 27 as additional evidence for a physical rather than a chemical basis of latent image formation. The x-ray crystal analysis of crystals, however, leads to the conclu- sion that no chemical molecule, in the sense of a specially combined atom-pair silver bromide exists, but rather a space-lattice of silver ions and bromine ions held together by the electrostatic attractions of the oppositely charged ions, and under such conditions there is little basis for physical theories which involve a molecular strain. Guthrie in 1850 first advanced the theory that the latent image con- sists of metallic silver. He thought that the action of light resulted in the formation of small centers of metallic silver on which the image was built up by the further deposition of silver with the developers used for the wet-plate process. Comparatively little attention was paid to this theory until it was revived by Abegg 28 in 1899. The original con- ception of the metallic silver theory has undergone considerable modi- fication in the past few years and has been transformed into what is termed the colloid silver theory. According to the colloidal silver theory, a sensitive emulsion consists not simply of silver halide in gelatine but a trace of a highly unstable form of colloidal silver in solid solution formed as a result of reduc- tion of some of the silver halide to silver in digestion with heat or ammonia. Renwick advanced the theory that the action of light is to coagulate amicrons of colloid silver to form larger particles which serve as centers for development.29 The Modern Conception of Latent Image Formation. — In recent years it has become widely held that the action of light in forming the latent image is to cause the bromine ion in the silver halide to lose an electron, which is then accepted by a silver ion to form a metallic silver atom, so that the latent image consists of small centers of metallic silver on the surface of the silver halide grains of the emulsion.30 Direct analytical chemical proof of this hypothesis is of course diffi- cult owing to the infinitesimal quantities of silver involved; the only evidence in support of such a hypothesis is that afforded by the cherni- cal reactions of the latent image. These, while not proving that the 27 Dewar, Proc. Roy. Inst., 13, 695. 28 Brit. J. Phot., 1899, 46, 196. 29 Brit. J. Phot., 1920, 67, 447, 463. 30 Sheppard and Trivelli, Phot. J., 1921, 61, 403. Fajans, Chem. Ztg., 1921, 45, 666. 206 PHOTOGRAPHY latent image does consist of silver, are not inconsistent with the view that it is silver.31 Investigations by Toy of the photo-electric properties of the silver halides lend additional support to the hypothesis suggested.32 It has been known for a long time that the silver halides possess both photo- electric and photo-conductivity properties. By the former is meant the complete liberation of electrons from the salt under light action, and by the latter (sometimes called the internal photo-electric effect) the freeing of electrons internally, resulting in a change in conduc- tivity with the illumination. Theories of the latent image based on the photo-electric effect were brought forward by Allen 33 and by Audubert,34 but Toy 35 has shown that the silver halides do not show the true photo-electric effect at wave-lengths greater than 280 ^ while the photographic effect of light, of course, extends to much longer wave-lengths than this, so that there is, apparently, little foundation for a photo-electric theory of photographic exposure. While the silver halides do not exhibit the true photo-electric effect under the conditions of ordinary exposure, there are so many striking similarities between the photo-conductivity effect with the silver halides and the photographic effect that one is inevitably led to the conclusion that the latter as well as the former is due to the loosening of electrons from the halide ions. The photo-conductivity effect is produced with the silver halides by light of practically all wave-lengths to which the same halide is light sensitive. ■ Like the photographic effect, it is influenced but little by extremely low temperatures and can be produced with light of prac- tically the same intensity as is required to produce a developable effect. The photographic reaction can take place in an exceedingly short time, as is shown by the practical use of exposures of 1/1000 of a second and less. It has been shown by Toy that the photo-conductivity effect can be detected within 0.001 of a second after the illumination is applied and in all probability begins instantaneously with the illumina- tion.36 31 Bullock, The Chemical Reactions of the Photographic Latent Image. Mono- graph No. 6 from the Kodak Research Laboratories, 1927. 32 Toy, The Mechanism of the Latent Image Formation. Proceedings of the Seventh International Congress of Photography, London, 1928. 33 Photo-Electricity, 2d Edition, p. 247. 34 Comp. rendus., 1924, 179, 1046. 35 Phil. Mag., 1927, 3, 482. 36 Toy, Nature, 1929, 123, 679. Sheppard, Phot. J., 1928, 68, 397. THE LATENT IMAGE 207 The Role of the Sensitizing Nuclei. — Until the discovery of the sensitivity centers and the general realization of the essentially dis- perse nature of the photographic emulsion, all theories of the latent image were based upon the action of light on a homogeneous grain of silver halide. The discovery of sensitizing nuclei consisting of either silver sulphide, or silver sulphide and colloid silver, has added to the problem of the latent image that of sensitivity. There is no longer any doubt of the existence of these sensitizing nuclei and but little doubt that they are composed of silver sulphide, or possibly, silver sulphide and colloid silver. How the nuclei promote sensitivity is another ques- tion, which is not so clear. First, it may be pointed out that the sensitizing nuclei cannot be re- garded as centers of special sensitiveness, or the spectral sensitivity of an emulsion would be that of silver sulphide and not silver bromide,37 nor does it appear likely that they act as photocatalysts, accelerating the photochemical decomposition of the silver halide. If the sensitiz- ing nuclei are not in themselves photosensitive, nor can they accelerate the photochemical decomposition of the silver halide, how then can their undeniable influence on the sensitivity of the grain be explained? Photographic sensitivity implies developability, i.e., a grain is not sensitive unless it can be converted into silver by a developer. Now in order that a grain may be developable it must have somewhere on, or near, its surface a particle of silver above a certain minimum size. Sheppard is of the opinion that the sensitizing nuclei sensitize for development rather than exposure. In other words, the effect of the nuclei is not to facilitate the decomposition of the silver halide, but to concentrate the photochemically reduced atoms of silver about them- selves so as to form a nucleus of metallic silver sufficiently large to render the grain developable. It is suggested that these sensitizing nuclei cause in their immediate neighborhood, depending upon their size, more or less strain in the silver halide crystal, resulting in changes in the positions of the elec- trically charged atoms, or ions, of silver and bromine in the crystal, thus creating spheres of weakness which serve to concentrate the photo- effect around the sensitizing center. According to this theory, the sensitivity centers do not play an active part in exposure. Their role is regarded as being purely passive and a result of the deformation produced by their presence in the structure of the silver halide grain.38 37 Sheppard, Third Colloid Symposium Monograph. 38 Sheppard, ibid. 208 PHOTOGRAPHY A chemical viewpoint of the action of the sensitizing nuclei has been suggested by Hickman.39 According to this hypothesis the silver sul- phide nuclei serve as an absorber of the bromine set free by the action of light on the silver halide of the grain surrounding it. This bromine attacks the silver sulphide resulting in the formation of free silver so that the total amount of silver produced is considerably greater than would be formed simply by the action of light on the silver halide. While it may not be impossible for such a reaction to take place under the conditions which probably exist in the silver halide grain, in no in- vestigation of the interaction of silver sulphide and bromine in the laboratory has the formation of silver been observed. Still another hypothesis of the action of the sensitivity centers has been tentatively advanced by Trivelli.40 As the silver halides have been shown to be photoconductive, and there is some basis for the as- sumption that in the silver halide grain there may exist a certain amount of finely divided silver as well as silver sulphide, Trivelli sug- gests that when light falls on the grain, the greater photoconductivity of the sensitivity nuclei as compared with the halide produces a differ- ence in electrical potential which results in the electrolytic deposition of silver in contact with the sensitivity center and thus assists in building up a particle of silver sufficiently large to render the grain developable. Useful as these various pictures of the mechanism of the sensitivity nuclei may be as working hypothesis, much experimental work will have to be done before the true facts are established. BIBLIOGRAPHY General Reference Works Andresen — Das Latente Lichtbild. Carey Lea — Kolloides Silber un die Photohaloide. (Translation by Luppo- Cramer, 2d Ed., 1921.) Eder and Valenta — Beitrage zur Photochemie und Spectralanalyse. Luppo-Cramer — Photographische Probleme. Valenta — Photographische Chemie und Chemikalienkunde. 39 Phot. J., 1927, 67, 34. 40 Phot. J., 1928, 68, 14, 67. CHAPTER IX SENSITOMETRY What is Sensitometry? — The merest beginner soon realizes that ex- posure is by far the most important operation in picture making, and the one presenting the greatest difficulties on account of the variable factors which must be taken into consideration in calculating the proper duration of the exposure. One of the most important of these factors is the speed, or the sensitiveness, of the plate to light. Methods by which the sensitiveness of plates are determined come under the heading of sensitometry. While sensitometry is concerned primarily with methods of speed determination, this is not its only value, for in determining the speed of the plate we learn a great deal concerning its characteristics and properties, so that we may define sensitometry, in its broadest sense, as the study of the reproduction of light and shade by sensitive materials. General Resume of Sensitometric Methods. — As early as 1848, Claudet devised an instrument, which was termed a " photograph meter," for determining the speed of the daguerreotype plate. This instrument gave to various portions of a plate exposures which in- crease in geometrical progression as 1, 2, 4, 8, 16, etc. The shortest exposure producing a visible impression on the sensitive material is taken as a measure of the speed of that material. Thus if the light- est visible deposit on one plate is produced by an exposure of 10 seconds, while the time required for another material is double this, or 20 seconds, the relative speeds of the two are as 1 : 2. This method of determining the speed of plates by reference to the lowest exposure which produces a visible deposit is termed the threshold or Schwellen- wert method. While it obviously gives some idea of the relative sensitiveness of different materials to light, it is not very reliable, except where the mere shape of an object is desired, for the test in- dicates the minimum exposure required to produce a visible image and is in no sense a guide to the exposure necessary for the proper rendering of gradation. Moreover it is possible to considerably alter the results by variations in exposure and development. 209 210 PHOTOGRAPHY Such was the state of affairs when Hurter and Driffield, two British amateurs, began their classical researches on plate speed determination which resulted in 1890 in the system of sensitometric investigation named after them — the H. and D. system. Briefly the H. and D. sys- tem differs from the threshold method in that the speed of a sensi- tive material is determined from, several densities rather than one and affords a better indication of the sensitiveness, properties and characteristics of the plate than can be secured from a single density. Further, the final result is not influenced to quite the same degree by variations in development, or other after treatment. It is hard to estimate the real importance of the work of Hurter and Driffield. Their work resulted in much more than merely a method of determin- ing the speeds of sensitive materials. It is hardly too much, to say that it indicated for the first time the rationale of the photographic process and that a large part, if not the greater part, of our present conception of the theory of photography had its inception at the hands of Hurter and Driffield. Instruments, for Sensitometric Investigation. — In plate speed de- termination by the Hurter and Driffield system we need: first, a standard light source for exposing plates ; second, an instrument, known as a sensitometer or exposure machine, for impressing a series of exposures in a definite ratio on different sections of the sensitive material ; and third, an apparatus for measuring the deposits obtained upon development of the exposed material. Standard Light Sources. — Daylight is not suitable for plate speed testing because of its variability both in intensity and color, or spectral distribution. The principal requirements of a suitable light source for plate speed testing are, (1) that it should be accurately reproducible not only in intensity but also as regards spectral distribution in order that the speeds of sensitive materials of different classes as determined in different laboratories may be directly compared, and (2) that it should be reasonably constant in intensity over fairly long periods of time. Hurter and Driffield used the standard English candle; later workers adopted the Harcourt pentane and the Hefner amyl-acetate lamps • These sources are easily reproducible and constant but are deficient as regards the color of the light which, as compared with day- light, is distinctly yellowish to orange and consequently higher speed values are obtained for color-sensitive plates than for non-color-sensi- tive plates having the same speed to daylight. SENSITOMETRY 211 No unscreened light has exactly the same spectral distribution as daylight although the magnesium flame approaches it closely.1 A fairly close approximation to daylight is possible, however, with the acetylene flame or incandescent electric light when used in conjunction with suitable filters. To secure a flame of constant intensity and ac- curate reproducibility, a special burner devised by Mees and Sheppard is generally employed. The light emitted by the acetylene flame is modified by a filter such as the Wratten gelatine filter No. 71 or the Davis-Gibson liquid filter.2 The tendency in recent years, however, has been towards the use of standardized incandescent electric lamps. These score on the point of convenience but require careful control as not only the intensity but the color of the light varies with the voltage.8 Sensitometers. — Sensitometers, or exposure machines, are used for producing on the sensitive material a series of exposures increas- ing in a definite ratio. This series of exposures may be produced either by varying the light intensity acting on the different sections of the plate, or by increasing the time of exposure by definite amounts. A series of exposures produced by variation of the intensity of the light is called an intensity scale while that produced by varying time is called a time scale. Intensity scales may be produced by (1) varying the distance be- tween the light source and the sensitive material for the different ex- posures; (2) by using different areas of a uniformly lighted source; and (3) by the use of a series of screens of known absorbing power. The last method is the only one now in general use. These screens are generally made of pigmented gelatine following the suggestion of Goldberg and fall into two classes : ( 1 ) the so-called optical wedge in which the gradation from light to dark is continuous; and (2) step wedges consisting of a series of steps of increasing density. Such wedges are widely used to expose the sensitive material for speed de- termination where extreme accuracy is not essential.4 The Chapman Jones plate speed tester (Fig. 127) is an example of an intensity scale. The squares numbered from 1 to- 24 are filled 1 For instructions on the use of magnesium see Eder, Brit. J. Phot, 1925, 72, 444; Zeit. phys. Chem., 1929, 141, 321. 2 Brit. J. Phot., 1928, 75, 432- 8 Jouanst, Sci. et Ind. Phot., 1925, 5, 122. Bailland, ibid., 1925, 5, 125. Walsh, Phot. J., 1925, 65, 52. 4 Goldberg, Brit. J. Phot., 1910, 57, 642, 664. Ferguson, Phot. J., 191 1, 51, 405. Renwick, Ibid., 1911, 51, 414. 212 PHOTOGRAPHY with pigmented gelatine of increasing opacity so that each numbered step represents a decrease in exposure to the sensitive material placed beneath as the square root of 2. The plate to be tested is placed be- hind this scale in a special plate holder and the whole exposed to the light of a standard candle at a distance of one meter (39.37 in.). 2 1 10 U 20 21 2 9 12 19 22 3 8 13 18 23 4 7 14 17 24 5 6 15 16 Fig. 127. Chapman Jones Plate Speed Tester Time scales are realized most easily by the employment of a sector wheel. That of Hurter and Driffield (Fig. 128) contains nine aper- tures, each angle being twice the preceding, so that the ratio of ex- posures is in geometrical progression. This revolving wheel is en- closed in a light-tight box carrying at one end the standard light and Fig. 128. H. and D. Sector Wheel and Exposing Apparatus at the other, behind the sector, the sensitive material to be tested (Fig. 128). The objection to a sector wheel is that the exposure is intermittent rather than continuous and the photographic effect of an intermittent exposure differs from a continuous exposure of the same length of time by an amount which depends upon the intermittency and the speed of the sensitive material. For this reason time scales producing SENSITOMETRY 213 a continuous exposure are to be preferred. Exposure machines of this class have been devised by L. A. Jones, G. I. Higson and others and are now extensively employed in commercial plate testing.8 Relation of Time and Intensity in Photographic Exposure. The Schwarzschild Constant. — Bunsen and Roscoe as a result of their investigations on the darkening of silver chloride papers established what is called the reciprocity law' according to which the photochemi- cal effect (or blackening produced) is the product of time and in- tensity, time being a reciprocal of intensity, or vice versa. It was shown by Abney, however, that the photographic plate does not ex- actly obey the reciprocity law and that the effect produced by a given exposure depends upon the actual values of intensity and time and not simply on their product.6 Schwarzschild, as the result of a series of investigations on the re- lation of time and intensity in photographic exposure suggested that the relation might be written as 7 E = IP, where E stands for the effective exposure, or photographic effect; I and t for intensity and time, and p a constant which Schwarzschild thought to be independent of the actual intensities and of the sensitive material. Later work 8 has shown that for each plate there is an optimum in- tensity of light which produces the greatest developable effect. The value of this optimum intensity depends chiefly on the speed of the plate or film. In the region of the optimum intensity the reciprocity law is approximately followed; only for lower intensities than the optimum is Schwarzschild's rule approached. It has also been shown that the maximum density developable for a given exposure depends upon the intensity of light acting, decreasing as this becomes less. Lastly, the magnitude of the reciprocity failure increases greatly as the emulsion speed, or sensitivity of the emulsion, decreases and is ap- parently greater with color-sensitive than with non-color-sensitive ma- terials. 5 On the intermittency error see Abney, Treatise on Photography, p. 391 ; Mees and Sheppard, Investigations ; Formstecher, Phot. Ind., 1927, 25, 575 ; Weinland, J. Opt. Soc. Amer., 1927, 15, 337; Davis, Bureau of Standards Scientific Paper, No. 528. 6 Treatise on Photography, p. 395. 7 Phot. Korr., 1899, 36, 109, 171. 8 Jones, Huse, Hall and Briggs, Proc. Seventh International Congress of Pho- tography, Heffer, Cambridge, 1929. 214 PHOTOGRAPHY There are two consequences of the failure of the plate to obey the reciprocity law which are of importance in sensitometry. The first is that the failure of the plate to obey the reciprocity law is a serious ob- jection to the use of intensity scales such as wedge screens ; the second is that the intensity of the light source used for plate speed testing can- not be ignored. In the sensitometers in common use at the present time, the intensity of light is far below the levels usual in actual photo- graphic practice. A " high intensity " ,'sensitometer has been designed by Jones to meet this discrepancy.9 :::i:::::;:i!:::::i::i::::i;!ii:::i::!ri:i::::: :i;-:n:i-iMffi;:™:!;;!;;;;;;:;;;;n!;;:;;;;;i;;:H;i!=-:;;;::: j::i:::i:; ;i:;:;;:::!;:::i:i:; ::]::!t:i::i:i:i:!;:::;:::::: ' 1 1 ' 1 I'll ! " 1 1 i' ' "! ''iJ Fhj. 133. Characteristic Curve Secured by Crossed Wedges gradation, increasing in density say from o to 6, or an intensity-range of transmitted light from i to 1,000,000, is taken and the plate to be examined is exposed behind this wedge to a standard light source and 22 For a method of expressing plate speeds in terms of the least minimum gradient of the curve, see Jones, Proc. Seventh International Congress of Photog- raphy, London, 1928, and Sheppard, Phot. /., 1926, 66, 190. 228 PHOTOGRAPHY developed to a high contrast. When dry the negative is placed over the wedge used for exposure but at right angles to the same. When observed by transmitted light the characteristic curve is seen as a rather diffused line. To sharpen this line a print is made on a process plate which is developed to the limit in order to secure the maximum contrast and from this a print is made on vigorous gaslight paper, the boundary being reduced, locally if necessary, with a ferricyanide re- ducer in order to obtain a clear, sharp-cut line. The resulting curves may be scaled by impressing on the transparency the necessary scales, one of the unit lines of the log intensity scale being made coincident with a position line on the plate for which the effective exposure is known. The characteristic curve of a sensitive emulsion, as deter- mined by the use of crossed wedges, is illustrated in Fig. 133. The Perfect Negative. — We have now described the manner in which the sensitiveness of sensitive materials is determined and this was the primary object of the researches of Hurter and Driffield, who are largely responsible for the method which we have just described. The most valuable work resulting from the sensitometric investiga- tions of Hurter and Driffield, however, has been the relation of the same to the reproduction of tonal values by the photographic process. The function of photographic processes is to reproduce as faith- fully as possible the shape and tones of natural objects. Accurate drawing is an optical concern and strictly speaking is only indirectly connected with photographic processes. The truthful reproduction of tone and gradation, however, is a function of the sensitive material and is thus distinctly a part of the photographic process. It is along these lines that the work of Hurter and Driffield has been the most fruitful, for they were the first to show the conditions governing the reproduction of tone by sensitive materials and its limitations. A negative is said to be a perfect representation of the subject when its opacities are proportional to those parts of the subject which they represent. Thus with a subject having a range of intensities from 1 to 64 all negatives having the following opacity-ratios would be cor- rect reproductions of the original, because in each case the relations between the various opacities and the corresponding portions of the subject are the same. Light intensities of subject 1 2 4 8 16 32 64 Opacities y2 1 2 4 8 16 32 % ]/2 1 2 4 8 16 SENSITOMETRY 229 The relation between the light intensities, opacities and transpar- encies of a perfect negative may be evident from the following: Light intensities of subject Opacities Transparencies I 2 4 8 a 32 I 2 4 8 16 32 I 1/2 « i/4 i/8 1/16 1/32 The Density-Exposure Relation and Correct Reproduction. — We have previously investigated the relation existing between exposure and density for the purposes of plate speed testing; we are now about to discover the relation which it has to the subject of tone re- production. When we perceive in nature a uniform transition from dark to light we may be sure that the intensity of the light increases more nearly in geometrical than in arithmetical progression, for in the lat- ter case the transition from dark to light is abrupt and harsh. Con- sequently, since in most objects the light intensities increase in geo- metrical progression, the opacities of a negative which is a faithful reproduction of the subject must also increase in geometrical progres- sion. Density we have previously defined (page 217) as the loga- rithm of the opacity, hence with a series of opacities increasing in geometrical progression the densities increase in arithmetical progres- sion. This relation may perhaps be clearer from the following nu- merical example: Light intensities of subject 1 2 4 8 16 32 64 Densities 1 2 3 4 5 6 7 Opacities 1 2 4 8 16 32 64 The mathematician calls each term of an arithmetic series which corresponds to any given term of a geometric series, the logarithm of that term; and the law which alone would produce absolutely true tones in photography would be expressed by saying that the quantity of silver reduced on the negative, or the density, is proportional to the logarithm of the light intensity. From our discussion of the characteristic curve (page 222) it will be remembered that the curve is obtained by plotting density against the logarithm of the exposure. This curve is not a straight line, as would be the case if the densities increased in arithmetical order over the entire range of exposure, but on the contrary has an f shape which was divided into three portions, the lower concave portion, the straight line portion and the convex portion. 230 PHOTOGRAPHY Attention has also been called to the fact that in the lower concave portion the densities increase on the same order as the exposures, or in geometrical progression. The light transmitted is therefore in arithmetical progression, producing a harsh, abrupt transition from dark to light which is characteristic of under exposure. This period is therefore termed the period of under exposure. In the straight line portion of the curve it is evident that the den- sities increase as the logarithm of the exposure, or ' dD/d log10 E = constant. Since this is the condition which has been shown to be essential to proper reproduction, this period is termed the period of correct rep- resentation or the period of correct exposure. In the convex portion of the curve the densities increase in less than arithmetical progres- sion ; consequently, the proper separation of the separate exposures is not secured and the result is flat and lifeless. This period is termed the period of over exposure. The period of reversal is without significance so far as the subject of tone reproduction is concerned. Latitude of Sensitive Materials. — The capacity of a given sensitive material in the matter of tone rendering is therefore determined en- tirely by the length of its straight line portion. It is in this respect that sensitive materials differ widely. Plates to be used for por- traiture, and other work in which a long scale of tones must be ac- curately reproduced, must have a long straight line portion so that the whole range of light intensities can come within the straight line portion of the curve of the sensitive material. Plates for commercial and other work of this nature where greater contrast is required and where the subjects do not possess such a long range of light intensities do not have this long straight line portion with its accompanying power of exact reproduction. The length of the straight line portion of the characteristic curve represents what is commonly termed the latitude of the sensitive ma- terial. Latitude in exposure depends upon two things: 1. Upon the extent of the straight line portion of the sensitive ma- terial. 2. The range of light intensities in the subject. Let us suppose a sensitive material having a long straight line por- tion capable of rendering a range of exposures from 1-64 (Fig. 134). Now if we have a subject with a range of exposures from 1-16 (rep- resented by the arrows) it will be evident that the exposure may be SENSITOMETRY 231 increased four times without forcing any of the tones into the pe- riod of over exposure. However, if the range of light intensities in the subject is increased to 1-32, then the exposure could be in- Fig. 134. Latitude and the Characteristic Curve creased only twice without forcing some of the exposures beyond the straight line portion. In the first case the sensitive material would be said to have a latitude of exposure of 1-4 ; in the second case 1-2. Log Exposure Fig. 135. Development and Constant Density Ratios 232 PHOTOGRAPHY Hence the latitude in exposure possessed by a given sensitive material is a relative term depending upon the range of light intensities in the subject. Development and the Reproduction of Contrast. — While correct exposure is absolutely necessary for correct rendering, it alone is not sufficient, for the time of development also plays a part. It is the function of exposure to secure the proper relationship between the densities and the light intensities which produced them. The densities are, however, only a half-way step towards the realization of a per- fect negative. It will be remembered that it is the opacities which must be proportional to light intensities which produced them. While development is without effect on the relationship of the densities, it does affect very markedly the ratio of the opacities, so that it follows that development is a very important factor in securing correct re- production. Constant Density Ratios. — The effect of the time of development on a series of densities may perhaps be made clear with the aid of Fig. T35- The two series of gradations represent two sensitometric strips which have received identical exposure but different times of development. Series A we will assume to have received 4 minutes development ; series B 2 minutes. The equal rise in the steps of each staircase indicates that the relationship of the densities is the same in both cases and consequently the density ratios are not altered by varia- tion in the time of development. This is what is meant by the law of constant density ratios, which was first enunciated by Hurter and Driffield in 1890.23 In confirmation of the law of constant density ratios we reproduce the following experimental data from an investigation of Hurter and Driffield : Exposures 1 2 4 8 Densityi (4" development) o-775 1. 000 1. 180 I.2S0 Ratio of densities Di 1.0 1.29 1-52 1. 6l 1.260 1.660 1.96 2.08 1.0 i-3i 1-55 1.65 Ratio Di/Di 1.63 1.66 1.66 1.60 23 " Photo-chemical Investigations," /. Soc. Chetn. Ind. (1890), g. SENSITOMETRY 233 Thus it is evident that, within the limits of experimental error, evi- dence supports the law of constant density ratios. Since the ratio of the densities is unaffected by the time of development, it is evident that the ratio is a function of the exposure and that unless the ex- posure has produced the proper relationship between the densities and the light-intensities which produced them correct reproduction is im- possible. An Important Difference. — But while the density ratios are unaltered by the time of development, the opacity ratios are, the effect of an in- creased time of development being to considerably increase the ratio of the opacities. Upon re-examination of the two staircases of Fig. 135 it will be observed that while the progression of the densities is the same in both cases, the amount by which the densities differ, in- dicated by the height of the individual steps, is considerable and that the total range of A is much greater than B. Numerical data, from an experiment of Hurter and Driffield, which shows how development affects the ratio of the opacities, without altering that of the densities, follows : 1 Exposure C. M. S. 2 Density 3 Density ratio 4 Opacity 5 Opacity ratio 1-25 .310 I JO 2.04 1.0 2.5 .520 I.67 3-31 1.62 50 ■725 2-33 5-30 2.59 Strip 2, Developed 8" , . 125 •530 1.0 3-38 1.0 2-5 •905 1.70 8.03 2.37 5-0 1-235 2.33 17.18 5.08 Strip 3, Developed 12" 125 •695 1.0 4-95 1.0 2-5 1. 140 1.64 13.80 2.78 5-0 1.625 2.33 42.17 8.51 It will be observed that all three strips received identical Exposures, but varying times of development. Column 3 shows that the density ratios are practically identical in all three cases, indicating that the increased time of development is without effect on the density ratios. Column 5, however, shows that the opacity ratios have increased con- siderably with increased time of development. Thus in the first strip the ratio of the minimum and maximum opacities is 1-2.59; in the second strip the ratio is 1-5.08; while in the third strip the ratio has increased to 1-8.5 1. 234 PHOTOGRAPHY L Development and Contrast. — We now see clearly the relation be- tween exposure and development and the part each plays in securing a faithful reproduction of the subject as it appears to our visual senses. Exposure is responsible for the proper relationship between the tones, while development determines the differences between the tones. The amount of this difference is determined solely by the dura- tion of development and constitutes what is termed the contrast. Con- trol in development is confined entirely to the length of time which the developer is allowed to act. The growth of no one density may be restrained or increased without affecting the others proportionately. Erroneous exposure cannot be corrected by any alteration whatsoever in development, for if the proper relationship between the densities has not been secured by giving the correct exposure, then no amount of development will supply that relationship which must exist between density and exposure for correct reproduction. Thus there is one, and only one, time of development which will give a technically perfect negative. The proper time of development for a technically perfect negative is that time of development which is required to produce a series of opacities which are directly propor- tional to the light intensities which produced them. In practice, however, owing to the differences in the properties of various printing media, it may be advisable either not to reach this exact proportionality or in other cases it may be advisable to exceed it in order that the visual appearance of the positive print may cor- rectly reproduce the original subject. It must be remembered that the negative is only a means to an end. It is the positive print which is the final result and regulation of the contrast of the negative to meet the requirements of the printing medium is not only proper but necessary. Gamma, as a Measure of Contrast. — Hence we require not only a means of measuring contrast after it is obtained but also a means of calculating the time of development required to reach any given stage of contrast. For this purpose use can hardly be made of the opacities on account of the mathematical complexity which controls their growth and therefore it is common to express the degree of contrast in terms of densities ■ and log exposures, the units of the characteristic curve. To the degree of contrast expressed in terms SENSITOMETRY 235 of density and log exposure, Hurter and Driffield gave the term gamma (y) which has since been generally adopted. Gamma is the ratio of the density range of the negative to the range of the logarithms of the exposures producing them. Or in other terms Difference in maximum and minimum densities of a given series Difference in the logarithms of the corresponding exposures or again J>2 - Pi 7 log £2 - log Ei ' where Dt and D2 are the minimum and maximum densities of the cor- responding exposures Ex and E2. Aside from being an expression of the degree of contrast in the negative, gamma also expresses the relation between the contrast of the negative and the subject which it represents. If the value of gamma is less than one the contrast is less than that of the subject, while if it is more than one the contrast is greater than the subject, provided that in each case the range of exposures fall within the straight line portion of the characteristic curve. The application of gamma as a measure of contrast holds only within the period of cor- rect exposure. Under-exposure produces the effect of high gamma, while over-exposure has the reverse effect, but in both cases the dif- ference in the densities is not proportional to the difference in the logarithms of the exposures and gamma as a measure of contrast fails to have any real significance. Gamma and the Characteristic Curve. — If we connect the various densities of the two staircases of Fig. 135 with a straight line, it is evident that the angle which this line makes with the log exposure base is greater the longer the time of development. In other words the longer the time of development, or the higher the value of gamma, the steeper the slope of the straight line portion to the base. The slope of the straight line portion of the curve, or the angle which it makes with the log exposure line, is thus a measure of the amount of difference between the densities, or, in other words, of gamma. This relation may be expressed in a very simple manner by means of a little geometry applied to the characteristic curve. 236 PHOTOGRAPHY By giving. a plate two exposures denoted at A and B (Fig. 136) on the log exposure scale, we obtain densities denoted by the heights of the vertical lines AC and BD. The horizonal lines OA and OB, therefore, measure the log exposures in like terms. 2.0 sl.6 a 3 o CO CD > o ^3 -a s o •s 0) -a -a 3 o ■9 O CD 1> a o rt & « a n > u .!3 i "d E >■» a &, -a rt J3 > 0 CO u 3 ■ u o s s o tS a, rt x h ,3 0\ +J C3 ■g bo * r7 rt *-< OJ e .a» d r-i CO «^ to u rt _3 O OJ -a bo rt o aj OJ 3 O oj u i-t "JJ *' S to O 3 u u en 3 O - 8. w X oj oj ft rt a) O 5 OJ J3 Na -> Ag + NaBr + D. While apparently satisfactory, this theory really explains very little. For instance, it offers no explanation of the manner in which the de- veloping agent is able to reduce the exposed silver halide to metallic silver. Accordingly later explanations are based on the theory of ions, which can explain more exactly the nature of the reaction which takes place. We know that chemical reaction can take place only in solution and the theory of solutions teaches us that a salt in solution is split up into the so-called ions which are atoms of the elements carrying an electric charge. Metallic or basic ions carry a positive (+) charge and are called cations, while acid ions carry a negative charge and are termed anions. Thus common salt (sodium chloride, NaCl) when dissolved in water is disassociated into the sodium cation and the chlorine anion. In the form of an equation this reads NaCl->Na+ Gl- and in the case of AgBr this becomes AgBr-^ Ag+ Br-. A salt so disassociated is termed ionized. According to the view most generally accepted in the scientific world of to-day, the first stage of the reduction phase consists of the disasso- ciation and ionization of the exposed silver halide and the developing solution which has penetrated the cell wall and dissolved the silver bromide. As the two are both ionized there is an exchange of ions between the two. The silver cation receives an anion from the de- veloper which is sufficient to remove its positive charge and neutralize it. It then ceases to be an ion and becomes metallic silver. The bromide anion is fixed in the form of a metallic or organic bromide 256 PHOTOGRAPHY according to the character of the developing agent. Owing to the ex- ceedingly complex nature of the organic developing agents and to the secondary reactions which take place it is difficult to be more exact on this point. The only reducing agent whose action may be said to be fully understood is ferrous oxalate, although hydrochinon follows a fairly simple reaction when used without a sulphite. The reaction with hydrochinon is as follows : —OH AgBr + 2NaOH- \/ —OH Hydrochinon Ag+ - Br- — 0--Na+ + H20 — 0"Na+ Ionized hydrochinon — 0--Na+ O + 2Ag++Br~ + 2Ag + 2Na++Br — 0"-Na+ The ionized hydrochinon loses two anions which unite with and neu- tralize the two silver cations forming metallic silver, the two oxygen ions combine to form quinone and the bromine anion unites with the sodium cation to form sodium bromide. This completes the first stage of the reduction process and constitutes the reduction phase. The Precipitation Phase. — We may now picture to ourselves the second phase of the chemical reaction within the cell, known as the precipitation phase. The metallic silver formed is in colloidal solu- tion, and as the reaction proceeds more and more silver will be formed until the solution becomes saturated with respect to silver. The re- action must then stop, unless the silver is induced to precipitate. Some germ or nucleus is necessary in order to induce precipitation and the production of this nucleus is the function of the exposure. The sub- stance forming the latent image is thus the germ which induces the silver to deposit and by so doing produces the image. " As silver is deposited, the concentration of silver solution within the cell is conse- quently lowered, and the reaction is increased, the deposited silver thus acting auto-catalytically (but only for the individual grain). The low THE THEORY OF DEVELOPMENT 257 solubility of silver is sufficient explanation of the localization of de- velopment to the individual grain." 1 Investigation has shown that any trace of a nucleus is sufficient to render all of the silver bromide in that cell developable. Hence, pro- vided the cell has complete access to the developing solution, there is no partial development of any cell ; it is either completely developed or not at all. Development as a Reversible Reaction. — The arrows in the above equation indicate that development from the chemical standpoint may be considered as a reversible reaction. This has been experimentally proven for the iron developer and for quinol. Mees and Sheppard 2 have shown that a solution of potassium ferri-oxalate and potassium bromide act on a developed negative to produce silver bromide. With hydrochinon, quinone and potassium bromide act on an exposed and developed plate to form quinol and silver bromide. This reverse ac- tion is largely prevented by the presence of the alkali and sulphites, always used with organic developers, so that the first oxidation product of the reducing agent is further oxidized by air and by the silver bro- mide and the reaction is then no longer reversible. The Action of Sulphites, Soluble Bromides and Alkali in Organic Developing Solutions. — The Action of Sulphites. — Sodium sulphite is customarily added to all organic developing agents for the purpose of preserving the developer and preventing oxidation and consequent staining of the gelatine by the solution when in use. Notwithstanding its universal application its action is but little understood. There are four possible ways in which the sodium sulphite may aid in prevent- ing oxidation of the developing solution: 1. The sulphite may be more readily oxidized than the developing agent. 2. The reverse may be true ; but the sulphite may regenerate the de- veloping agent. 3. The two may form a complex salt which is less subject to oxida- tion than either alone. 4. There may be no protective action, but only a division of oxida- tion, half of the oxygen going to the sulphite and half to the developer. This, as pointed out by Bancroft, would mean an actual though not theoretical decrease in the rate of oxidation. The first we know definitely to be a fallacy, as many of the organic 1 Mees, Phot. J., 1910, 50, 403. 2 Zeit. wiss. Phot., 1904, II, 5. 258 PHOTOGRAPHY developing agents are more easily oxidized in solution than sodium sulphite. The experiments of Mees and Sheppard 3 with hydrochinon support the second explanation, but they were working under different conditions than those of actual practice. It is doubtful that any re- generation of the developing agent occurs with other developers than hydrochinon. There is at any rate no experimental evidence for any but hydrochinon at the present time. The third and fourth seem to be nearer the truth, for we know that hydrochinon and sulphite enter into combination and it is quite possible for the combination to be less subject to oxidation than either alone. But, like many other matters of everyday photographic practice, this is still an unsolved problem theoretically. According to Rzymkowski,* the first stage in the oxidation of a de- veloper containing sulphite by the oxygen of the air is the formation of a sulphate and a thiosulphuric acid derivative of the developing sub- stance, R-S-S03H, which is then hydrolysed to the hypothetical com- pound R-SOH. The easily oxidizable SOH group is selectively con- verted to the sulphonic group S03H, and in this manner the active groups of the developing substance are protected from oxidation.* The Action of Soluble Bromides. — The addition of a soluble bro- mide slows development by diminishing the degree of ionization of the silver bromide and by lowering the concentration of the silver cations which lowers the velocity with which the reaction proceeds to the saturation point. Hence the precipitation phase is delayed, because of the delay in reaching a saturated solution of silver within the cell. The influence of a soluble bromide is felt chiefly in the earlier stages of development.6 The Function of the Alkali. — According to the theory of develop- ment outlined in the preceding pages the reducing agents used for photographic development are considered as pseudo-acids having very small ionization constants but forming strongly dissociated salts. The function of the alkali is to assist in the ionization of the reducing agent and produce ionized salts, as in the case of hydrochinon —OH NaOH — 0'...Na- + 2H,0. — 0'...Na- 3 Zeit. wiss. Phot., 1904, II, 7. * Phot. Ind., 1928, 26, 627. 5 See also Pinnow, Phot. Rund., 1923, 60, 27 ; Zeit. Wiss. Phot., 1922, 22, 72. 6 See Arch, iviss. Phot., 1900, II, 76. Eder's Jahrbuch, 1904. THE THEORY OF DEVELOPMENT 259 The Physical Chemistry of the Developing Process The Induction Period. — Even with the most energetic developers a short space of time elapses between the application of the developing solution and the first appearance of the image. This period is termed the induction period. The causes which produce this period are in general two : ( i ) the time required for the developer to penetrate the film, including both the macro and micro pr.^ses of diffusion referred to in a previous section, and (2) the time required to saturate the solu- tion with silver in order that silver may be deposited and form a visible image. The actual duration of the induction period is controlled by the nature of the developing agent, metol and other energetic agents having a shorter period of induction than the lower energy developers as hydrochinon and glycin, the concentration of the developing solu- tion, temperature and the presence of a soluble bromide. A soluble bromide such as potassium bromide materially increases the duration of the induction period, particularly with developers of low energy. Soluble iodides on the other hand have an accelerating effect and shorten the period of induction.7 The well-known Watkins method of factorial development is based upon the induction period. Watkins' principle is that for any develop- ing agent the time required to produce the visible image is an accurate indication of the speed of development and is a certain definite frac- tion of the time necessary to reach any given stage of contrast. Any variation in concentration or temperature, etr., which would affect the time of development necessary to reach a given degree of contrast affects the time of appearance proportionately. In other words Td^WTa, where T is the time for density D, Ta the time of appearance and W a constant depending on the developer. This statement is sufficiently near the truth to be of practical ap- plication but both theory and experiment show that this simple relation does not hold exactly. The Watkins method of factorial develop- ment will be referred to again in the chapter on The Technique of De- velopment. The Velocity of Development. — After the induction period is passed the growth of the image may be rapid or slow according to the conditions under which the process takes place. The principal factors 7 For a full explanation of this interesting reaction see Sheppard and Meyer, Phot. J., 1020, 60, 12. 260 PHOTOGRAPHY which determine the rapidity of development are the same as those which influence the period of induction. A knowledge of the velocity of development is essential to the calcu- lation of the time required to reach a given stage of contrast (y) and is most conveniently and accurately determined by the method of Std Log E Log E Fig. 143. Growth of Density with Time of Development. (Nietz) Nietz.8 It has been shown in the chapter on sensitometry that a series of plates exposed under identical conditions in a sensitometer and developed for varying times from t1 to t„ produce a series of H. and D. curves the straight line portions of which meet in a point (Fig. 143). If we take any fixed exposure on the log exposure base D T Dev. CMin.) Fig. 144. Curve Showing Growth of Density with Time of Development (Nietz) and erect a perpendicular line we have the information desired, i.e. the growth of density with development since the time of exposure for each of the densities is constant. By plotting Dlt D2, etc., as a func- 8 Theory of Development, p. 80. THE THEORY OF DEVELOPMENT 261 tion of the time we get a curve of the exponential type (Fig. 144) which shows that density increases rapidly at first and then less and less rapidly as development proceeds until finally a point is reached where development apparently stops and there is no further increase either in density or contrast. This, it will be readily seen, is in agree- ment with conditions observed in everyday practice. The explanation of this progressive diminution in the velocity at which density increases is quite simple, although it is a difficult matter to find a mathematical expression which will cover all conditions. After development for any length of time short of that required to produce the maximum density, we have three kinds of grains present : A. Developed grains. B. Developable but undeveloped grains. C. Undevelopable grains. The A grains thus represent the density already attained; the A and B grains together the maximum density which can be secured exclusive of fog. The B grains, therefore, are the only ones subject to develop- ment and as the reaction proceeds the number of B grains will become less and less until finally when all are developed the process must stop. Thus the density undeveloped at any time t will be (Dx — D), where D is the density developed at any time t and Da the maximum den- sity. Supposing that the rate at which the developer reduces the ex- posed but undeveloped grains is a constant and independent of the number of grains (as is actually the case) and that the rate of diffusion remains unaltered, we can express the rate of development or dD/dt as where k is a constant determined by the rate at which the exposed grains are reduced by the developing agent. This formula fits the case fairly well with acid developers over a moderate range but wide variations are observed with most alkaline developers and other more complex equations have been suggested to account for the various fac- tors involved. A comprehensive review of later work on the velocity of development and development velocity equations will be found in The Theory of Development by A. H. Nietz. The Velocity Constant. — Now while the number of undeveloped grains constantly grows less and less as development proceeds, the rate at which the grains are attacked by the developing agent remains con- 262 PHOTOGRAPHY stant. Thus if we have a total of 100 developable grains present at the beginning of development and at the end of the first minute of de- velopment one half of this number or 50 have been reduced to the metallic state, then at the end of the second minute of development the developing agent will have reduced to metallic silver one half of the grains which remain or 25, and so on as the time of development is prolonged. In other words, the developing agent reduces to the metallic state a definite proportion of the remaining developable grains for each unit of time which it is allowed to act. This proportion is termed the velocity constant of development. It is usually denoted byk. The velocity constant at the same temperature and with the same emulsion varies with the developer. It is different with different plates, being influenced by the conditions prevailing during the manu- facture of the plates. To determine the value of the velocity constant, k, we require to know the values of gamma for two sensitometric strips simultaneously exposed and developed for different times, of which one is double the other. The values of yt and y2 having been found, k may be calculated from the following equation : 9 h 72 — Ti The calculations are rendered simpler by the use of the following table worked out by Mees and Sheppard. To use this table divide 72 by y1 and against the value of this dividend in the table is the value of k for 5 minutes development-. The value of k for any other time of development may be found by dividing 5 by the number of minutes development and multiplying by the value of k for 5 minutes. Thus if in a certain case the value for k is given in the tables as .215, k for 2 minutes development will be k = I X .215 = .538. Calculation of the Time of Development for a Given Gamma. — We are now in a position to calculate the time of development required to obtain a given gamma with any particular developer. While in all sensitometric work it is desirable that plates be developed to a gamma equal to unity, in practical work it is often desirable, owing to the re- quirements of different printing mediums, to develop to lower or even 0 Mecs and Sheppard, Phot. J., 1903, 43, 48; Phot. /., 1904, 44, 297. THE THEORY OF DEVELOPMENT 263 higher values of gamma than unity. Thus negatives to be printed on carbon or platinum require to be developed to a higher gamma than those destined for use with developing-out papers. Then again it is usually desirable to develop different subjects to different gammas and consequently it is an advantage to be able to calculate the time of de- A for A u K 71 71 in k o.oos 1.977 O.OO50 O.OIO 1.952 O.OO48 0.015 1.928 O.OO50 0.020 1.903 O.OO46 0.025 1.880 O.OO44 0.030 I.858 O.OO42 0-035 1-837 O.OO40 0.040 I.8I7 0.0040 0.045 1.797 O.OO38 0.050 1.778 O.OO38 0-055 1-759 O.OO36 0.060 1. 741 O.OO34 0.065 1.724 O.OO34 0.070 I-7I7 O.OO32 0.075 1. 691 O.OO32 0.080 1-675 0.0030 0.085 1.660 O.OO32 0.090 1.644 O.OO32 0.095 1.628 O.OO32 0.100 1.612 O.OO32 0.105 1-596 O.OO32 O.IIO 1.580 O.OO30 0.115 1-565 0.0028 0.120 I KKl O.OO28 0.125 1-537 0.0028 0.130 1-523 0.0026 0.135 1.510 O.0O28 0.140 1.496 O.OO24 0.145 1.484 O.OO24 0.150 1.472 O.OO24 0.155 1.460 O.OO24 0.160 1.448 0.0022 0.165 1-437 0.0022 0.170 1.426 O.0022 0.175 1 -41 5 0.0020 0.180 1.405 0.0018 0.185 1.396 0.0018 0.190 1-387 0.0020 0.195 1-377 O.0OI8 0.200 1.368 0.0020 0.205 0.210 0.215 0.220 0.225 O.23O 0-235 O.24O O.245 O.250 0.255 O.260 O.265 O.27O 0.275 0.280 O.285 O.29O 0-295 O.3OO 0.305 0.310 0-315 O.32O 0.325 0.330 0-335 O.34O 0-345 0.350 0-355 0.360 0-365 0.370 0-375 0.380 0.385 0.390 0-395 0.400 22 71 1-358 1-349 1 -34i 1-332 1.324 [.316 1.308 1. 301 1.294 1.286 1.278 1.271 1.264 1-257 1.251 1-245 1.239 1-233 1.227 1. 221 1. 216 1. 210 1.205 1.200 1.195 1. 191 1. 186 1. 182 1. 178 1. 174 1. 169 1. 165 1. 161 I-I57 I-I54 1. 150 1. 147 I-I43 1. 139 1. 136 A for A 0.001 in k 0.0018 O.O0I6 O.O0I8 O.0OI6 0.0016 O.0OI6 O.OOI4 O.OOI4 0.0016 O.O0I6 O.OOI4 O.OOI4 O.OOI4 O.OOI2 0.0012 0.0012 0.0012 0.0012 0.0012 O.OOIO 0.0012 O.OOIO O.OOIO O.OOIO 0.0008 O.OOIO 0.0008 0.0008 0.0008 O.OOIO 0.0008 0.0008 0.0008 0.0006 0.0008 0.0006 0.0008 0.0008 0.0006 velopment to reach any gamma which may be desired. This is a com- paratively simple matter if we have determined the gammas of two sensitometric strips simultaneously exposed and developed for different times so that one is double the other. These constants having been determined, the time of development required to reach any other 264 PHOTOGRAPHY gamma may be found either by the graphical method of Hurter and Driffield 10 or that of Mees and Sheppard.11 The graphical method of Hurter and Driffield can be most simply explained by an .example. Suppose y1 to be 0.82 and.y2 to be 1.36. Take an ordinary H. and D. chart, such as used for plotting the char- acteristic curve, and call the base line divisions " Minutes of Develop- ments " and the ordinates " Gammas " : then there are three points ARCHITECTURE PORTRAIT .6 e 1 I i 1 a 56 ■ 6.2 ] 3 + S 7 O 30 SO TO 100 200 MINUTES DEVELOPED- 2.5 375 S7S Fig. 145. H. and D. Method for Calculating the Time of Development for Given Gamma through which a curve may be drawn — o, 0.82 and 1.36. Suppose y1 (0.82) to have been produced by three minutes development and y2 (1.36) with six minutes; then yt (0.82) is plotted on the 3 minute line and y2 on the 6 minute line. A curve is then drawn through these points and zero. Then the time of development for any desired gamma may be obtained by drawing a horizontal line from the left- hand scale until it cuts the curve and dropping a perpendicular from the point of intersection to the base. In the example shown the times required to reach gammas of 0.80, 1 and 1.30 are found to be 2.80, 3.75 and 5.75 minutes respectively. For the second method as developed by Mees and Sheppard the values of gamma infinity (y,*,) and the velocity constant of develop- ment (k) require to be known. Methods of calculating these con- stants have already been given : for the former on page 239 and the latter on page 262. The values of these constants having been calculated for the case in 10 Hurter and Driffield, Ore the Control of the Development Factor. Phot. J., 1903, 43, 16. 11 Mees and Sheppard, Phot. J., 1903, 43, 48, 199. THE THEORY OF DEVELOPMENT 265 hand, the time of development for any desired gamma may be ob- tained from the equation 7 < - y»(I - e-kl). The actual calculations are rendered quite simple by the use of the tables (page 266) worked out by Drs. Mees and Sheppard for values of (I — e~ht) and corresponding values of kt.12 From the above * — = (I — or Gamma required _ . kt. Gamma infinity Therefore to obtain the time of development for a given gamma, divide the required gamma by the gamma infinity of the plate. In the second column of the tables find the nearest lower value of (I — e~kt) corresponding to the dividend. Opposite this in the first column of the tables will be found the value of kt corresponding to that obtained for (I — e~ht). Divide the value of kt as given by the value of k, as previously found by calculation, and the result is the time of develop- ment required to attain the desired gamma. For example : Gamma infinity — 1 .6, Velocity constant = .15, Gamma required = 0.8. Then 2|= .5 = (J — «-»)• The nearest value of (7 — e~ut) in the tables which corresponds to .5 is .5034, corresponding to a kt of .700. Dividing this by the velocity constant (k) .15 we obtain 4.7 minutes, or 4 minutes and 42 seconds, which is the time of development for a gamma of 0.80. Effect of Temperature on Development. — In common with nearly all chemical reactions, the rate of development is considerably in- fluenced by temperature. The effect of temperature on the time of development was first studied quantitatively by Houdaille in 1903 13 whose work was followed up with a more complete investigation by 12 Phot. J., 1904, 54, 297-8. 13 Bull. Soc. Franc. Phot., 1903, 19, 256. 266 PHOTOGRAPHY Table of Corresponding Values of kt and i — e~"' for Determination of Time of Development for Required Gamma of Plate of Given k and 7°° (Mees and Sheppard, Photographic Journal, November, 1904, page 297) kt 1 - <-« .000 .000 1 .025 .02 .050 .046 •075 •073 , .100 .0952 .125 .1174 .150 • 1387 ■175 .1600 .200 .1813 1 .225 .2082 .250 .2252 •275 .2422 .300 .2592 ■325 .2769 •350* •2945 •375 •3121 . .400 •3297 ' .425 •3458 •450 •3617 •475 •3776 , .500 •3935 ' .525 .4085 .550 .4234 •575 •4373 . .600 .4512 ' .625 .4641 .650 .4772 •675 •4903 . .700 .5034 .725 •5138 .750 .5281 •775 •5394 . .800 .5507 .825 .5613 .850 .5714 •875 •5827 . .900 •5934 •925 .6031 •950 .6218 •975 .6225 , 1. 000 .6322 1.025 .6485 1.050 •6547 1-075 .6609 , 1. 100 .6671 1. 125 .6741 1-15° .6830 I-I7S .6909 diff. for .01 kt •0095 .0086 .0077 .0071 .0064 .0057 .0052 •OO47 .0042 .0038 .0034 .0032 kt I — e-W 1.200 .6988 I.225 •7059 1.250 •7131 t-275 .7203 1.300 •7275 1-325 •7339 1.350 ■7403 J-375 •7469 1.400 •7534 i-425 ■7592 L.450 •7651 1-475 .7710 1.500 .7769 i-525 .7822 1-55° •7875 1-575 .7928 1.600 .7981 1.625 .8029 1.650 •8077 1-675 .8125 1.700 •8i73 1-725 .8215 1-75° •8259 1-775 ■8303 1.800 •8345 1.825 .8387 1.850 .8426 1-875 .8465 1.900 •8504 1-925 •8539 i-95° •8575 1-975 .8611 2.000 .8647 2.025 .8680 2.050 .8712 2.075 •8744 2.100 .8776 2.125 .8805 2.150 .8834 2.175 .8863 2.200 .8892 2.225 .8919 2.250 •8945 2.275 .8971 2.300 .8997 2.325 .9021 2.350 •9045 2-375 .9069 diff. for .01 kt .0029 .0026 .0024 .0021 .0017 .0016 .0014 .0013 .0012 .00105 .OOO96 THE THEORY OF DEVELOPMENT 267 Table of Corresponding Values of kt and i — e~*' for Determination of Time of Development for Required Gamma of Plate of Given k and 7°° (Mees and Sheppard, Photographic Journal, November, 1904, page 297) (Continuation of page 266) kt diff. for .01 kt kt 1 — e—M diff. for .01 kt 2.400 2425 2.450 2.475 .9093 •9113 .9135 ■9157 ■ .00086 3.200 3-225 3-250 3-275 ■9592 .9601 • .96H .9621 ; • .OOO39 2.500 2.525 2.550 2.575 .9179 .9197 .9217 •9237 . > .00078 p 3-300 3-325 3-35o 3-375 .9631 •9639 .9648 ■9657 . .OOO36 2.600 2.625 2.650 2-675 ■9257 .9274 .9292 .9310 > .00071 3.400 3-425 3-450 3-475 .9666 1 •9W4 .9682 .9690 . > .OOO32 2.700 2.725 2.750 2.775 •9328 ' ■9344 •9360 ■9376 > .00064 3-5oo 3-525 3-55o 3-575 .9698 ' .9706 •9713 •9720 , ■ .OOO29 2.800 2.825 2.850 •9392 .9408 .9412 •y4^° j > .00058 3.600 3-625 3-650 o-u/ 0 •9727 ■9732 •9739 Q7A& > .00O26 2.900 2.925 2.950 2.975 •9450 ' ■9463 •9476 •9489 . .00052 3.700 3-725 3-75o 3-775 •9753 1 •9758 .9764 •9770 . > .OOO23 3.000 3-025 3-050 3-075 •9502 1 •95i3 •9525 •9537 . > .00047 3.800 3-825 3-850 3-875 .9776 1 .9780 .9786 •9792 . 3.100 3-125 3-150 3-175 •9549 1 •9559 •9570 •958i J .00043 3.900 3-950 3-975 4.000 •9798 1 .9807 .9812 .9817 . > .00019 Ferguson and Howard, Alfred Watkins and Mees and Sheppard.14 The ratio of the velocity constant, k, for any two temperatures is a measure of the effect of temperature on the velocity of development within this particular range and for that particular developing agent and is termed the temperature coefficient of development (T.C.). The range of temperature chosen in practice is io° C. (18° F.) so that 14 Ferguson and Howard, Phot. J., 1905, 45, 118. Ferguson, Phot. J., 1906, 46, 182. Mees and Sheppard, Investigations. Sheppard, /. Chem. Soc. (Lon- don), March, 1906. Ferguson, Phot. J., 1910, 50, 412. Mees, Phot. J., 1910, 50, 410. Watkins, Phot. J., 1910, 50, 411. Watkins, Phot. J., 1909, 49, 367. 10 268 PHOTOGRAPHY the expression for the temperature coefficient becomes kt° C./kt io° C. The temperature coefficients of a few of the more common developing agents are as follows : Pyro-soda without bromide 1.5 Pyro-soda with bromide 1.9 Rodinal, azol, certinal, etc 1.9 Metol-quinol (no bromide) 1.9 Glycin 2.3 Rytol 2.2 Hydrochinon 2.25-2.4 As a general rule the temperature coefficient appears to be a char- acteristic of the developing agent, being for the most part unaltered by changes in the proportion of alkali to the developing agent, or by dilution, but it is much higher when bromide is used. Mees and Sheppard have shown that there is also a variation in the temperature coefficient with different plates, so that a calculated T.C. for a given developer will not necessarily hold if a change is made to another brand of plates. The temperature coefficient is ap- proximately constant, however, for different batches of the same plate. With certain developing agents of low energy, such as hydrochinon, low temperature not only slows development but has an action similar to that of a soluble bromide at normal temperature, i.e. the inertia is lowered and the lower tones retarded. Calculating the Temperature Coefficient. — As we have already seen, the time of appearance of the image is an indication of the velocity of development, hence we may calculate the effect of temperature on the rate of development with a given developing agent from the dif- ference in the time of appearance of the image at two different tem- peratures. A plate is exposed and then divided into two pieces (or two identical exposures made). One of these is developed at any convenient temperature and the time of appearance noted. The other is developed at a temperature several degrees higher or lower; 10° C. (18° F.) being a convenient difference. The time of appearance at this temperature is noted. We now have the time of appearance at two different temperatures and from this the temperature coefficient may be calculated by the THE THEORY OF DEVELOPMENT 269 following formula : (log Ta — log ta) X 10 , . -^-2 T„ -Sp = log of T.C. for 10 C. In other words, the difference in the logarithms of the two times of appearance, multiplied by 10 and divided by the difference in degrees Centigrade of the two respective temperatures, is equal to the loga- rithm of the T.C.15 Thus if the times of appearance are 30 and 20 seconds at 17.5° C. (63° F.) and 25° C. (770 F.) respectively, we have Difference = .1761 X 10 10 = 1. 761 -4- 7-5 = -2348 = log of 1.72 = temperature coefficient. A very ingenious graphical method devised by Mr. Alfred Watkins is even simpler and avoids all calculations whatsoever.^ The starting point on which his method is based is the fact that the time of develop- ment required to produce an equal gamma increases in logarithmic proportion while the temperature increases arithmetically. The times of appearance having been found for two different tempera- tures, a slip of paper is laid on the log scale of Fig. 146 and the times of appearance laid off against the corresponding values of the log scale. Beneath the marks are placed the respective temperatures. This slip of paper is then laid on the fan-shaped diagram and adjusted so that the two marks cut the lines of the two temperatures, the edge of the paper falling along one of the horizonal lines. The point where the paper slip intersects the radial temperature lines is marked with the proper temperature coefficient.16 As a result of extensive research, Watkins gives the following T.C. for several common developing agents : Pyro-soda (Watkins thermo formula), no bromide 1.5 Pyro-soda (Watkins thermo formula), with bromide 1.9 15 Ferguson, Phot. J., 1910, 50, 414; see also Phot. J., 1906, 46, 182. 16 For other methods see Ferguson and Howard, Phot. J., 1906, 46, 182. 270 PHOTOGRAPHY « * ■ THE THEORY OF DEVELOPMENT 271 Pyro-soda (Hurter and Driffield formula) 1.48 Pyro-soda (Kodak powders) 1.9 Pyro-soda (Ilford formula) 2.04 Rodinal (also azol, victol and certinal) 1.9 Metol-hydrochinone (Watkins thermo formula) 1.9 Glycin 2.3 Hydrochinon 1.4-2.25 (Sheppard and Mees) find 2.20-2.80 Ortol 2.06 Time of Development at Various Temperatures. — The time of de- velopment required to reach any given gamma and the T.C. for the same plate and developer having been obtained, the time of develop- ment at various temperatures is very easily found. Place the edge of ■a sheet of paper on the horizontal line corresponding to the T.C. of the developer and mark off the points of intersection with the tempera- ture lines. Transfer this paper to the log scale, placing opposite the time of development in minutes or minutes and a fraction, the cor- 20 1.8 16 14 12 id S 4 « 4' E P 3' i r 30* 60° 70° ~80° Temp.F. Fig. 147. Stokes Time Development Chart responding temperature at which the examination was made. This having been done the times of development at other temperatures necessary to reach the same gamma may be written down directly from the log scale.17 - 17 This simple graphical method of drawing up a table for the time of de- velopment at various temperatures was first indicated by Mr. Alfred Watkins. 272 PHOTOGRAPHY There is, however, no actual necessity for knowing the temperature coefficient in order to determine the time of development for various temperatures. Since the time of development for a given gamma progresses logarithmically as the temperature progresses arithmeti- cally, if the time of development at two different temperatures is known, a straight line drawn through these two points when plotted on a log scale of times of development as ordinates against an even division scale of temperatures as abscissae (Fig. 147) will indicate, for all practical purposes, the time of development at all intermediate points. This method is due to Mr. W. B. Stokes.18 The Action of Soluble Bromides in Development. — The customary addition of a certain amount of soluble bromide, which is nearly al- ways potassium bromide, to a developing solution for the purpose of preventing " fog " materially affects the normal course of develop- ment. For an unbromided developer the inertia is constant with increasing times of development, but this is not true in the case of a developer containing a soluble bromide in which case at the same degree of de- velopment there is a lateral shift of the curve to the right. This is illustrated in Fig. 148 19 where the solid lines represent the curves of — — Unbromided Bromided Fig. 148. Effect of a Soluble Bromide in the Developing Solution on the Plate Curve the unbromided developer for three different degrees of development and the dotted lines the curves of the bromided developer for similar degrees of development. It is evident that if the curves of the bromided developer are produced below the log E base they will meet in a common point. As the concentration of bromide is increased ■* 18 Brit. J. Phot., 1921, 68, 97. 19 Sheppard, Photography as a Scientific Implement, p. 151. THE THEORY OF DEVELOPMENT 273 Jthis point of intersection moves slowly downward as shown in Fig. 149. The amount of the downward shift, termed the density depres- sion, produced with a given concentration of bromide, is dependent upon the developing agent, being in general greater with low energy developers as hydrochinon than with those of greater energy such as paraminophenol and metol. J, Fig. 149. Density Depression with a Soluble Bromide. (Nietz) Bromide is without effect on the velocity constant k,20 and investi- gation shows that its effect on the general velocity of development is felt chiefly during the earlier stages; the induction period and that immediately following. Perhaps an even more readily comprehensible method of presenting =r==n — , — ^ — - r-^~i — n A 0 C D Fig. 150. Effect of Soluble Bromide on the Densities. (Watkins) the action of a soluble bromide in development is that adopted by Watkins in the Watkins Manned. Fig. 150 represents a subject of four gradations for a given degree of development in an unbromided 20 Nietz, Theory of Development, pp. 124, 170. 274 PHOTOGRAPHY developer. The lower illustration represents the same exposure de- veloped to the same stage in a bromided developer. It will be observed that, while the contrasts of both are equal, the action of bromide has reduced the tones considerably and this depression is more noticeable in the lower tones than the higher. In fact the addition of bromide has prevented the lowest tone from appearing at all. The effect of bromide is to actually reduce the speed of the plate. As the time of development is increased and a higher gamma is reached, the lower tones will develop out, so that in order to restrain the development of the shadow detail in over exposed plates development must be com- pleted before the bromide has lost its restraining action. The use of bromide for this purpose, however, falsifies the gradation of the negative. Theoretically gamma infinity is unaffected by the reasonable addi-' tion of bromide, but in practice, owing to the absence of fog, the print- ing contrast of a negative developed to the same gamma may be higher for the bromided than for the unbromided developer. The restraining action of bromide is greater on fog than on the image, hence, even in cases of underexposure, a small amount of bromide may be advisable in order to prevent the appearance of fog due to development being forced beyond the usual limits in order to secure all possible shadow detail. The Relative Reducing Energy of Developing Agents. — The effect of a soluble bromide at the same concentration varies with the develop- ing agent but is constant and characteristic of that particular agent. Use was made of this property by Sheppard and Mees to compare different developing agents as to their relative reducing or developing energy known as the reduction potential. For a given concentration of bromide under fixed conditions the depression of density will be dependent upon the ability of the developer to overcome the resistance of the bromide. Developing agents of greater energy will require larger amounts of bromide to produce the same depression of density than those of lower energy ; hence the concentration of bromide re- quired to produce a given density depression will be in direct pro- portion to the energy of the developing agent.21 Taking the bromide concentration required to produce a given de- pression of density as unity, Nietz obtained the following scale rep- resenting the relative energies of the more common developing agents : 21 See Sheppard and Mees, Investigations, p. 188. Nietz, Theory of Develop- ment. THE THEORY OF DEVELOPMENT 275 Ferrous oxalate 0.3 p-phenylene diamine hydrochloride (no alkali) 0.3 p-phenylene diamine hydrochloride (with alkali) 0.4 Hydrochinon 1.0 Standard p-phenyl glycin (glycin) 1.6 Hydroxylamine 2.0 Toluhydrochinon 2.2 p-amidophenol (hydrochloride) 6.0 Chlorhydrochinon (adurol) 7.0 Dimethyl p-aminophenol sulphate 10.0 Monomethyl p-aminophenol sulphate (metol) 20.0 Diamidophenol (amidol) 30-40 In general the higher the value of the reducing energy the higher is gamma infinity, but there are several exceptions which are not yet completely understood. Contrary to what might be expected, there appears to be no direct relation between the fogging power of a de- veloper and its reducing energy or reduction potential. General Reference Works Eder — Ausfuhrliches Handbuch Photographie, vol. IV, 1905. Hubl — Entwicklung der Photographischen Bromsilbergelatineplatte, 1922. Luther — Die chemischen Vorgange in der Photographie, 1899. Nietz — Theory of Development, 1922. Reiss — Entwicklung der Photographischen Bromsilberge'atinetrockenplatte. Seyewetz — Le Negatif en Photographie, 1922. CHAPTER XII ORGANIC DEVELOPING AGENTS Developing Power. — The sensitive emulsion, as we have seen, con- sists of certain halide salts of silver in an extremely fine state of division held in a colloidal medium. We have already considered in the chapter on the latent image the various theories proposed to explain the nature of the change which occurs when the sensitive silver salts are exposed to light. While we do not know the composition of the latent image, we do know that there are certain chemical compounds which possess the property of reducing to metallic silver those grains of silver halide which have been affected by light. Such chemical sub- stances are known as developers since they " develop," or render visible, the latent image formed by light. All developing agents are reducers, but not all reducers are capable of photographic development by any means. We are not yet in a position to say definitely what constitutes developing power; i.e. what must be the chemical composition of a substance in order that it may function as a developer. The general conclusions of Liimiere and Andresen bearing on this subject will be discussed later. While in common speech a developer is taken to mean either the developing agent or the solution used for development, in this chapter we are concerned primarily with the developing agent and all reference to a developer applies to a particular agent such as metol, pyro, etc., and not to a developing solution as applied to the plate. This is al- ways termed the developing solution. Classification of Developing Agents. — A comparatively large num- ber of substances possess the property of developing exposed silver halide but for various reasons only a few of these are of practical value. Eder 1 divides all possible developing substances into three classes : i. Those which develop a definite part of the latent image before fog sets in. (Commoa developers.) 1 Ausfiihrliches Handbuch der Photographie, p. 288 et seq. 276 ORGANIC DEVELOPING AGENTS 277 2. Those which develop energetically with a minimum of alkali but produce serious fog. (Powerful developers.) 3. Those which scarcely develop the latent image at all even with a maximum of alkali and yet develop fog vigorously. A somewhat more comprehensive classification is adopted by Nietz.2 1. Developers having too low reducing energy to be useful practically, e.g. ferrous citrate. 2. Developers giving undesirable reaction products in developing, e.g. hydrazine. 3. Developers too powerful for ordinary use, e.g. triamidophenol. 4. Developers of practical utility, e.g. all ordinary developing agents, metol, pyro, paramidophenol, etc. Only this last class will be discussed in the present chapter although references are made to several of the others in the bibliography at the close of the chapter. The Source of Organic Developing Agents. — Benzene, the source of the numerous aniline and phenol dyes, from which the organic de- veloping agents are derived, was discovered by Faraday in 1825, but it was not until 1866 that its structural formula was determined by Kekule. This consists of a hexagon with the carbon and hydrogen ' atoms linked together around the six points. CH 1 A-CH /\ \/_CH 1 \J 5\J3 4 C H (1) (2) (3) 1. The structural formula of benzene after Kekule. 2. Its abbreviated form generally referred to as the benzene nucleus. 3. The points of substitution. The atoms of hydrogen at any of the six points of substitution may be replaced by atoms of chlorine, hydroxyl, or amino groups, and, as a substance with entirely different properties is formed according to the 2 Theory of Development, p. 14. 278 PHOTOGRAPHY group substituted and the point at which substitution is made, it can be readily seen that a very large number of compounds become pos- sible. By substituting chlorine, hydroxyl or amino groups in the first position we secure : —CI —OH — NH2 \/ Chloro-benzene Hydroxy-benzene Amino-benz^ne or Aniline None of these compounds has any developing power. However, if we replace the two hydrogen atoms at positions I and 2, at 1 and 3, or at 1 and 4, we get three hydroxy-benzenes having the formula C(iH4(OH)o and identical in composition but differing in constitution. —OH —OH -OH —OH —OH —OH \/ V" Ortho- Mela- Para- hydroxy-benzene hydroxy-benzene hydroxy-benzene or Pyrocatechin or Resorcin or Hydroquinone Substitution in the 1 and 2 positions is termed the ortho position, 1 and 3 the meta position, and 1 and 4 is termed the para position. Of the three compounds two are developers, />ara-hydroxybenzene being the agent known as hydroquinone while ort/io-hydroxybenzene is known commercially as pyrocatechin. The third compound, mrfa-hydroxy- benzene or resorcin, has little or no developing power. The developing properties of hydroquinone were discovered by Abney in 1880. As a developer, hydroquinone is rather slow and tends to strong contrast. It is very sensitive to low temperature and should not be used below 50° Fahr. (10° C.) Alone hydroquinone is employed chiefly for copies of black and white originals where a high degree of contrast is essential. A suitable formula will be found on page 287. For general work it is usually combined with metol. Pyrocatechin is a more energetic developer than hydroquinone and less sensitive to low temperature. It is now little used. By replacing the hydrogen atom in the second position of para- hydroxybenzene, or hydroquinone, with chlorine, Hauff produced ORGANIC DEVELOPING AGENTS 279 monochlor-hydroquinone (CeH3Cl(OH)2) which was introduced com- mercially as Adurol. Schering substituted bromine in the same way and obtained mono- fcromo-hydroquinone (C6H3Br(OH)2) which also was introduced as Adurol. —OH —OH —CI —OH 'N— Br -OH —OH —OH Hydroquinone Adurol (Hauff) Adurol (Schering) Adurol is a more energetic developer which works somewhat softer than hydroquinone, standing, in this respect midway between hydro- quinone and the rapid soft-working developers such as metol and para- minophenol. It is not as sensitive to low temperature as hydro- quinone and has a longer useful life. It is usually used in combina- tion with metol. Having dealt with the developing agents formed by substituting two hydroxyl groups in the benzene nucleus, let us see the effect of adding a third. There are three positions also which we can obtain by this treatment : that in which all three groups are close together, or ad- jacent; that in which two are contiguous, and the third separated by one position; and lastly that in which the groups are symmetrically placed. —OH —OH —OH —OH —OH \/-°H Pyrogallol OH— \y— OH Phloroglucinol —OH 1,2,4 Trihydroxy benzene These are known as adjacent, asymmetrical and symmetrical tri- hydroxybenzenes or as pyrogallol, oxyhydroquinone and phloroglucinol. Pyro is the only one of these substances used as a developer. Pyro has been in use longer than any other organic developer and is still extensively used. Its character as a developer depends largely on the concentration at which it is employed; strong solutions develop rapidly with strong contrast while weaker solutions act more slowly and with less contrast. Solutions, of pyro oxidize rapidly and in de- velopment the reduction of the pyrogallol on the silver image results 280 PHOTOGRAPHY in a super-imposed stain image which gives to the pyro-developed negative its characteristic brownish-black color. The intensity of the stain image can be controlled by the proportion of sodium sulphite used. Precisely the same condition of affairs applies when the substitution is made with amino groups instead of hydroxyl groups. Thus we may have ortho, para, or meta aminophenols, or we may substitute instead two amino groups or one amino and one hydroxyl group thus producing a whole series of amino-hydroxybenzenes. Pursuing the same idea further we may replace one of the hydrogen atoms with a methyl group (CH3). For example if we introduce an amino group in place of a hydrogen atom in the fourth position in phenol we obtain para-amino-phenol which is well known as the base of such prepared developers as Rodinal, Azol, Activol, etc. —OH —OH — NH2 Paraminophenol (base) Phenol Paraminophenol when used with the alkaline carbonates is a rapid, soft-working developer which readily produces detail but builds up density and contrast only with prolonged development. When made up in alkaline solution with the alkaline carbonates, the free base is liberated on standing and there is a gradual loss of developing power. Paraminophenol is more generally employed in the form represented by the so-called " Rodinals " which will be noticed later. The introduction of a second amino group produces di-amino-phenol or the familiar amidol. —OH —OH -NH2 NH2 Paraminophenol — NH2 Diaminophenol or Amidol Amidol differs from al^ the developers already mentioned in that it can develop without an alkali and may be used in an acid solution. It ORGANIC DEVELOPING AGENTS 281 is a very rapid developer ; the image appears quickly, but density and contrast are added only on prolonged development. Stock solutions of amidol loose their developing power in from one to two days with- out the visible discoloration which accompanies the oxidation of other developing agents. Amidol is used principally for the development of prints. As it develops fog when used in alkaline solution, care should be taken that the sodium sulphite employed does not have an alkaline reaction. This may be corrected by the addition of sodium bisulphite. Stock solutions of amidol which will keep for several days may be prepared by the use of neutralized sulphite prepared as follows : Sodium sulphite (dry) 2 oz. 91.2 gm. Potassium metabisulphite oz. 22.8 gm. Water to make 20 oz. 1000 cc. Boiling for several minutes improves the keeping quality of this solu- tion. The developer is prepared as follows : Amidol 40 gr. 4.2 gm. Stock solution 4 oz. 200 cc. Water to 20 oz. 1000 cc. A number of other methods of preserving solutions of amidol have been suggested. Namias advised the use of boric acid in the propor- tion of 25 grains to the ounce of solution; Crowther,3 the use of glycollic acid in proportion to 1/10 the quantity of sodium sulphite; Bunel,4 the addition of lactic acid one part to fifty of the solution ; and Desalme,5 the use of tin chloride 1 : 25. There are four more developers which may be regarded as being de- rived from paraminophenol, namely, Edinol, Metol, Glycin and Ortol. Edinol is the sulphate of oxymethyl-paraminophenol ; the relation- ship to paraminophenol being clear from the structural formula: OH CH2OH H2S04 2 NH; 3 Brit. J. Phot., 1920, 67, 642. 4 Bull. Soc. Franc. Phot., 1921, , *90. 6 Brit. J. Phot., 1921, 68, 359. 282 PHOTOGRAPHY As a developer Edinol is fairly energetic and is free from any tendency to stain or cause fog. It may be used alone but is more generally employed in combination with adurol or hydrochinon. If paraminophenol be taken, and one hydrogen atom of the amino group be replaced by a methyl group we secure mono-methyl-paramino- phenol. The sulphate of this is sold commercially as metol. —OH HjSOj 2 J — NH-CH3 Metol Metol was introduced commercially by Hauff in 1891, the metol issued at that time being dimethylparamino-meta-cresol, the cresol base being abandoned later in favor of phenol.6 Metol is one of the most conspicuous members of the class of soft- working developers which include also paraminophenol and diamino- phenol, or amidol. Metol is seldom used alone but generally with hydroquinone or adurol ; the rapid soft-working character of metol being supplemented by the greater density-giving powers of hydro- quinone or adurol. Although metol is much more energetic than hydroquinone, a combination of the two is more rapid than either alone, due to the fact that metol brings out the detail of the image very quickly while hydroquinone adds the density and contrast necessary for good printing quality. The combined metol-hydroquinone developer is one of the most popular in existence. Glycine is produced by inserting the carboxyl group in place of a hydrogen atom in the methyl group of metol, being para-oxy phenyl- glycine. —OH /\ —OH — NH2 Paraminophenol v I NH.CH2COOH Glycine 6 The patents taken out for metol covered only its use as a developer and did not disclose either its composition or preparation. Methods of preparing mono- methyl-paraminophenol, or " metol," however, were discovered in several quarters during the World War. See Ermen, Phot. J., 1923, 63, 223. ORGANIC DEVELOPING AGENTS 283 As a developer glycine is slow but powerful, and is especially notable for the fine-grain of the reduced silver and the freedom from veil even in the absence of a soluble bromide. It keeps well in solution and is especially adapted to continuous use in tanks.7 Ortol is a mixture of hydroquinone and the sulphate of methyl- orthoaminolphenol. The probable formula is —OH + -OH NH-CHs- H,S04 -OH Ortol Used alone, Ortol bears a close resemblance to pyro, not only as respects the color of the image, which is brownish-black, but in the progressive appearance of the image. As with pyro, the highlights appear first, followed by the half-tones and lastly the shadows; this gradual building up of the image is quite different from the action of metol and other rapid developers in which the highlights, half-tones, and shadows appear at very nearly the same time and density is built up later. Ortol, unlike pyro, however, does not stain and in solution keeps much better. There are two other developers derived from benzene, diphenal and paraphenylene diamine ; these, however, are not very important. The last is occasionally used for lantern slides and transparencies on ac- count of the very fine-grained images which is produced and would be useful for line work were its contrast-giving properties greater. NH2 OH- NH5 NH2HC1 Diphenal 7 See, Brit. J. Phot., 1928, 75, 514. _/\_ NH2 Paraphenylene diamine 284 PHOTOGRAPHY If two benzene nuclei are joined together, as shown below, we ob- tain a body called naphthalene. If we introduce into this hydroxyl, amino and sulphonic acid groups we obtain a substance which may be termed j3 amino, /?x naphthol, /?3 sulphonic acid, known to photogra- phers as Eikonogen. NH2 Naphthalene Eikonogen As a developer Eikonogen is rapid, though not so energetic as metol, and tends to give very soft images. It is generally used with hydroquinone in order to secure greater density. Solutions of Eiko- nogen do not keep as well as those of metol and the combination of metol and hydroquinone has almost completely replaced Eikonogen. A chemical compound of metol and hydroquinone was introduced by Lumiere in 1903 as metoquinone. Chloranol, another developer in- troduced by Lumiere, is a compound of hydroquinone and methyl- paraminophenol. Hydramine, a chemical combination of hydroquinone and paraphenylenediamine was also introduced by Lumiere. All of these combinations are of minor importance. The relationship of the various developing agents and some of the methods of derivation are shown in the following family tree of the coal-tar developers as compiled by Dwight R. Furness.8 All of the methods are not shown, only those of importance are dealt with for the sake of simplicity. The Significance of Group Relations. — Most of our knowledge of the structure of developing agents and the relation of the structure to developing properties is due to A. and L. Lumiere and to Andresen. The papers of these investigators have established some general rules for the structure of compounds which possess developing power. It is now generally accepted that the presence of hydroxyl or amino groups, either alone or in combination, is necessary in order that a substance may function as a developer. With substances which contain in one benzene nucleus at least two 8 Phot. J. of Amer., 1918, p. 337. ORGANIC DEVELOPING AGENTS 285 hydroxyl groups, two amino groups, or one hydroxyl and one amino group : 1. The substance is a developer only when the groups are in the ortho or para position. Meta compounds, so far as known, have no developing power. 2. In general para compounds possess greater energy than do ortho compounds. PIT COAL I LIGHT TAR OIL Benzene (Toluene, Xylene n i MEDIUM OIL HEAVY TAR OIL \ Many Products Phenol, Naphthalene , lubricating Oil Nitrob'enzene Benzaldehyde Phenol Sodium-K,amido/3,naphthol(3,sulphonate ("Eikonogen " ) Sodium-amidonaphtholdisulphonate ~ 1 ~TcDiogen") Resorcin Phenyl-hydroxylamine Aniline ParamiSophenol Quinone CRodinol Hydroquinone hydrochloride) NitrophenoT .,1 Paramidophenol Orthoamidophenol p- oxyphenyl- glycocoll ("Sly tin") Methyl - orthoamidophenol (+ Hydroquinone • ^■Orfol") Monomethyl- paramidophenol „ CScalol") (Metor* sulphate) Resorcin Triqmidoresorcin Diamidoresocin ("Reducin") Dinitrophenol — n — Diamidophenol ("Amidol ") Pyrocatechin rKachin") r Hydroquinone "I, Pyroqallic Acid Monobrom- Monochlor- (Hade from Gallic Acid) hydroquinone hydroquinone ("Adurol") 3. The di-oxy-benzenes are more powerful than the amidophenols which are in turn more powerful than the diamido benzenes. 4. The developing power is not destroyed by additional hydroxyl or amido groups. 5. In the naphthalene series it is not necessary that both groups be joined to the same benzene nucleus. The general rules re- garding developing function do not apply to this group. 286 PHOTOGRAPHY N I0«0 On ^t" >- 00 t"» o £1 3 3 S3 ??<2 w « M m Tt" ON I ON j n" « h m mo* o eo »-» — o m n q o o a d m oV t-i q q q q io od u in ir> N no o *t m tn^o 6 h d d d d I I b2 S 8 be u 6 S> oo n n co *fr up » On On 00 . 4- rO io M hi _, ro O OnmD O t^-oo O O 't^ 01 . -j §>> _3 § .2 •a j3 o i S o o « 73 "5 >> o u rs .5 a ^ u a v 'Z- C rt " -5 00 u ■- \o X ^ " * .5 p< "£ - ~ CD i I „ •75 "is c o o : 3 i3 ! X a o •s CO O "3 1) 3 rt bo 0 5 C 01 u '•3 J3 " •a Q CO o -a v _3 •3 ■I i S o •5 J 3 bo tg CD JS H _fi c/i •a 73 0) e tn JJ St ■a. I " B a T3 bo a! rt c 2 53 u ■° g S bo ^ -a "S t; 5 o J3 rt *" J3 S 3 •n u u: 1) .S 3 * "5 K~ rt -o St H "S * § ^ o 8 *> a •Se — ^ .SPl 3 O 1 > . 13 O i £ I :*> 0 ft u cn O 3 ■S P 3 § « s C rt J= b § '3 u CP ORGANIC DEVELOPING AGENTS o A. «m5 1 1 o n o Paramino- phenol »0 W I I 1 O Ortol mi>. \ | 1 o Ills o Metol lO |IC 1 | o H 1 N 1 1 O o Hydro- quinone (contrast) io io 1 m 1 o W N 1 N I o o Hydro- quinone O 1 O 1 I o N 1 O 1 1 O Glycine O 1 O 1 O o N 1 «| hO o Adurol (I) 8 1 8 t 1 § "SI"? 5 •S3 S g § O C C yj u> L- 3 3 2 £ O o I I o 8 K u, o O '3 IslJs-g. £ tn £ u> 2 i_ 3 « 3 2 3 33 w *«-« to a O O o O o !>■ t/5 Oh t/5 cu tn > O i_ C/3 aj o £ a. - — ™ 3 _ JB 8" .. & — V P .2 bs -3 o fa ^ PHOTOGRAPHY o- 0 5£ ~- CU s.s O 00 o 0 ti- cs - a, o iO [ 8 1 c- - a o o 8T o o O (N c ','3 • P o CO 3 o a; 4= .ti *i 0) P ■H +J 5 111 C3 fi co CO .3 3 to to 3 6 >>-g g 3 •»'"p - i- > ctf cu ^ § E o o o o o o o >o >> xt c cu ct) is is - e m u co G 5 co 3 4-J TO "J cs -K'O >/P ° CO _ctf E ctf P< ctf 1/1 - o « a o O cu » != £ CD ** ■ J3 a cs . ctf -S ° 2 ctf a & °- ctf 5 3 > cr* cu cu co 'I 'I CU OJ J3 M •3 -3 Ctf cu C 5 ctf 6 5 J ctf ! 2 I « CU CU Ctf p 3 3 h tt n O m jp cu be ctf cu •G _ ORGANIC DEVELOPING AGENTS 289 6. The substitution of chlorine or bromine for hydrogen increases the developing energy. 7. A substance containing two hydroxyl groups requires an alkali, while substances containing two amino groups or one hydroxyl and one amido group do not require an alkali. In substances containing three hydroxyl or amino groups either alone or in combination : 1. Symmetrical arrangements, as 1, 3, 5, have no developing power. Other arrangements differ in developing energy but no definite rule has been found to apply. 2. Hydroxyl-phenols, containing three hydroxyl groups, can develop without alkali but are not practical when so used. 3. Increasing the number of amino groups increases the energy of the developing agent. Phenolate Compounds. — Towards strong alkalis paraminophenol acts as an acid, therefore if the hydrogen atom of the hydroxyl group is replaced by an alkali metal such as sodium, a phenolate compound results having in this case the formula C0H4ONaNH2 or sodium para- minophenolate. This is actually the substance formed when a solu- tion of caustic soda is added to a paraminophenol developer in order to redissolve the precipitated free base. Advantage is taken of this fact to prepare highly concentrated solutions of paraminophenol which simply require dilution with water in order to be ready for immediate use. It is in such form that paraminophenol has achieved its greatest popularity under the names of Rodinal, Citol, Azol, Activol, Certinal, Paranol and Kalogen. These are all patented substances, but a de- veloper of similar composition can be prepared by the formula to follow : Bring to a boil 250 cc. of pure water and, when just before the boil- ing point, add a few crystals of potassium metabisulphite ; when these are dissolved add 20 grams of paraminophenol hydrochloride and finally 60 grams of potassium metabisulphite. The mixture is stirred until all of the metabisulphite has dissolved, and there is then added, with constant stirring, liquid commercial caustic soda sufficient to re- dissolve the aminophenol base. The mixture becomes thick at first owing to the precipitation of the aminophenol base and as the caustic soda is added gradually clears up owing to the precipitated base going into solution. The addition of caustic soda should be stopped just 290 PHOTOGRAPHY before all of the paraminophenol base is dissolved and the solution made up to 400 cc, placed in a rubber-stoppered bottle and allowed to cool. It is very important that a small quantity of paraminophenol base be left undissolved as the least excess of caustic soda causes the solution to be unstable, rapidly turning brown and losing developing power. This is the only trick in the operation and must be carefully observed. A solution properly prepared will keep almost indefinitely. Should an excess of caustic soda be added by mistake, the matter may be remedied by the addition immediately of a solution of sodium bi- sulphite until a slight precipitate of paraminophenol is formed.15 Fine-Grain Developers. — The degree of inhomogeneity, or as com- monly termed, the graininess of the developed image assumes consider- able importance when the negative is to be subsequently enlarged. It is necessary at this point to draw a distinction between the grain of the individual silver grain and the graininess which results from the ag- gregation or clumping of these. It is the latter which is the cause of the graininess observed in ordinary enlargements ; the effect of the individual grains is evident only upon much higher magnification. Absolute graininess varies with different emulsions, being, in gen- eral, greater with rapid than with slow materials, and increases with the contrast, or y, of the negative. Physical development results in an image of finer grain and the use of paraphenylene was recommended for this purpose by A. and L. Lumiere and A. Seyewetz 16 in 1904. Development with parapheny- lene, however, is a very exacting process and the contrast of the image is too low for ordinary requirements. Furthermore, a considerable in- crease in exposure is required. Later they recommended the use of paraphenylene diamine with a weak alkali, such as borax, which re- sults in more rapid development and higher contrast, with but little in- crease in the size of grain.17 At the present time, however, the most practical developer for secur- ing fine grain for subsequent enlargement seems to be the special metol-hydroquinone-borax formula developed by the Kodak Research Laboratories. This is as follows : 15 Metol and Monomet may also be used to prepare developers of this charac- ter. For full directions see Ermen, Brit. J. Phot., 1920, 67, 611 ; see also pp. 610- 611. 16 Bull. Soc. Franc. Phot., 1904, 20, 422. " 5. /. P., 1927, 7, 107- v ORGANIC DEVELOPING AGENTS 291 Metol Sodium sulphite (anhydrous) Hydroquinone IS gr. 768 gr. 38 gr. 15 gr. 16 oz. 2 gm. 100 gm. S gm. 2 gm. Borax Water 1000 cc. The metol is dissolved first in a small quantity of water; a quarter of the sulphite next in hot water, the hydroquinone added, and this solution "is then added to the solution of metol. The remaining sul- phite is now dissolved in additional water, the borax added and this solution added to the other, water being added to bring the total bulk to 16 ounces or 1000 cc.18 The finer grain is due apparently to the high sulphite content as well as to the low alkalinity.19 On this last account, development is com- paratively slow, requiring 15-25 minutes at 65 0 Fahr. Developers for Continuous Use. — It is the general practice in the larger commercial establishments and in motion picture processing laboratories to develop in large tanks holding several gallons of de- veloper until the solution ceases to function properly. While from a theoretical standpoint, it is far better, particularly with highly sensi- tive panchromatic materials, to use a fresh solution for each batch of plates or film, in practice, satisfactory negative quality is secured by continuous use of the developer provided the solution is discarded when it has reached its effective limit. In the choice of the developing agent for continuous use in tanks, both the keeping qualities of the solution and the rate of exhaustion are of importance. As regards keeping qualities, an exact comparison between the figures obtained by Hiibl and Milbauer (table on page 286) is difficult owing to experimental differences; both agree, how- ever, in assigning superior keeping qualities to glycin, followed by pyrocatechin and adurol. The studies of Strauss 20 on the effect of repeated use of developers shows that metol has the longest useful life followed by paraminophenol, edinol, ortol, adurol, glycin, pyrocatechin, hydroquinone, and, much worse than the others eikonogen and pyro. Metol-hydroquinone, or preferably metol-adurol, metol-glycin, and glycin, therefore, appear on theoretical considerations, to be the most suitable for continuous use in tanks. The widespread use of metol- 18 Brit. J. Phot., 1928, 75, 74. 19 Cf. Veldmann, Das Atelier, 1928, 35, 30. 20 Phot. Ind., 1925, p. 309. 292 PHOTOGRAPHY hydroquinone and glycin in the processing of motion picture film by automatic machinery indicates that in practice these developers have been found best adapted to continuous use. General Reference Works Eder — Ausfiihrliches Handbuch der Photographic Seyewetz — Le Negatif en Photographic Valenta — Photographische Chemie und Chemikalienkunde. CHAPTER XIII THE TECHNIQUE OF DEVELOPMENT Introduction. — The chemical and physico-chemical basis of the process of development and the chemical and photographic properties of the organic compounds used for development have formed the sub- ject of the two preceding chapters. In this, the last chapter on the subject, we will be concerned for the most part with the more prac- tical aspects of the matter — the technique of development. The developing solution as applied to the plate consists of four in- gredients : the developing agent itself ; the preservative, generally sodium sulphite ; the alkali, one of the alkaline carbonates usually ; and the restrainer, potassium bromide. Amidol is an apparent exception to this as no alkali is added, but it is probable that the hydrolysis of the sulphite which is always used furnishes the sodium to form a phenolate which is the actual developing agent. Then again the re- strainer is quite often omitted, particularly in negative developing solutions, but generally we may say that the developing solution con- sists of the developing agent, the preservative, the alkali, and the restrainer. The part played by each of these in the process of de- velopment has already been considered ; there remain, however, a few matters of practical importance regarding the use of the sulphites and alkalis in development. These matters it is proposed to take up in the present chapter, together with desensitizing as applied to photographic development, and finally the three principal methods of development : inspection, factorial and thermo. The Sulphites in Development. — The use of sodium sulphite as a preservative in solutions of the organic developing agents appears to have been first suggested by Berkeley in 1882.1 The theory of its action has already been discussed (page 257). There are two forms of sodium sulphite in general use, the an- hydrous and the crystalline, the latter containing seven molecules of 1 Phot. J., 1882, 22; Brit. J. Phot., 1882, 29, 47; Phot. News, 1882, 26, 41. 293 294 PHOTOGRAPHY water of crystallization having the formula Na2S03 ■ 7H20. Calcu- lating the molecular weights of the two forms we have Na2S03 = 126 Na2S03-7H,0 = 252. Thus 126 parts of the anhydrous are equivalent to 252 of the crystal- line salt, or, in other words, the anhydrous form is just exactly twice as strong as the crystal. Hence when using anhydrous sulphite in a solution calling for crystals only one half of the amount called for by the formula should be used. As the crystal form is in almost uni- versal use in England whereas the anhydrous is in general use in this country, this fact should be borne in mind when making use of formulas from an English source. No matter which salt is used it is rare to find it pure, and com- mercial varieties are likely to contain from 2 to as much as 10 per cent of impurities principally as sulphates or as carbonates. While theoretically one ought to test each batch, this is unnecessary since an excess is always used. Of the two forms the anhydrous keeps better in the dry state and for this reason is to be preferred over the crystalline form. Stock solutions of sulphite do not keep well and it is advisable to prepare at one time no more than it is expected to use within a week to ten days. Specially prepared solutions of sulphite containing al- cohol or potassium metabisulphite, however, may be kept much longer. The addition of 10 per cent of alcohol has a beneficial effect on the keeping properties while the so-called " neutral sulphite " is even more effective. Most commercial sulphite is slightly alkaline and this affects the keeping properties, hence the addition of just sufficient acid to neutralize the sulphite is often advised. Sulphuric is the best of the strong acids for this purpose although oxalic and citric acid are often used. Perhaps the most effective means of preserving sodium sulphite solutions is by the use of potassium metabisulphite as follows : Sodium sulphite (dry) 2 oz. 100 gm. Potassium metabisulphite y2 oz. 25 gm. Water to make 20 oz. 1000 cc. Dissolve at ordinary temperature, then raise to the boiling point and finally allow to cool.2 2 The addition of small quantities of hydrochinon as a preservative of stock solutions of sulphite has recently been suggested. See Journ. Camera Club of London, 1923, I, 3. THE TECHNIQUE OF DEVELOPMENT 295 In place of sodium sulphite the corresponding potassium and am- monium sulphites have been recommended, as have also potassium metabisulphite and sodium bisulphite lye, but owing to the cheapness and efficiency of sodium sulphite none of these compounds have ever been widely adopted.3 In the case of a developing agent producing a stain image, the pro- portion of sulphite controls the intensity of the stain. Thus in the case of pyro, increasing the amount of sulphite will decrease the amount of stain to a point where the image is almost pure black, while if the sulphite is decreased the intensity of the stain will gradually increase, passing from black to warm-black, and finally to yellowish- brown. With non-staining developers the proportion of sulphite does not have any decided influence on the color of the deposit, acting prin- cipally as a preservative of the developing solution. The Alkalis in Development. — When alkaline development was first introduced by Russel in 1862 the alkali in common use was ammonia and pyro-ammonia continued to be the favorite developer for many years. Even as late as 1900 pyro-ammonia was still considerably used for negative work. Owing, however, to its volatile nature its action is erratic and uncertain and at present ammonia has been com- pletely replaced in all but a few instances by the fixed alkalis such as the alkaline carbonates and hydroxides. Of these the carbonates are the most widely used, particularly sodium carbonate. Potassium carbonate is used in some special cases, while the hydroxides are occasionally used with hydrochinon and glycin, or for the production of phenolate compounds from par- amidophenol. There are three carbonates of sodium, the bicarbonate or acid car- bonate, the sesqui-carbonate and the normal carbonate. Only the lat- ter is suitable for development. The normal carbonate exists in three forms, the anhydrous Na2C03, the monohydrate containing one mole- cule of water and having the formula Na2C03-H20 and the crystalline having ten molecules of water of crystallization and consequently be- ing Na2C03- ioH20. Calculating the molecular weights of the three forms : 3 Ammonium sulphite, Eder, Phot. Korr., 1885, 22, ill. Potassium metabi- sulphite, Mawson and Swan, Brit. J. Almanac, 1887, p. 139; 1888, pp. 316 and 346. Sodium bisulphite lye, Gilder, B. J. Almanac, 1891, p. 718. 296 PHOTOGRAPHY Crystalline Na2C03-ioH,0 46 + 60 + 180 = 286, Monohydrate Na2CO:! • H20 46 -f- 60+18 — 124, Anhydrous Na2COs 46 + 60 = 106. Consequently we see that 106 parts of anhydrous sodium carbonate are equal to 124 of the monohydrate and 286 of the crystalline form. The anhydrous carbonate is therefore approximately 2.8 times as ef- ficient as the crystalline. As a matter of fact, however, the pure an- hydrous carbonate is not often met with, the usual dry or so-called " anhydrous carbonate " being in reality the monohydrate, Na2C03- • H20. This is slightly stronger by twice than the crystalline salt as 286 : 124 :: 100 : 43.4. It is quite accurate enough, however, for all practical purposes to take the efficiency of the ordinary dry sodium carbonate as being twice that of the crystalline form. The monohy- drate is generally used in this country, the crystalline form, however, is still used in England — a fact which must be borne in mind when using formulas from that country, as, unless specified as dry, the quantities always represent crystal carbonate. The monohydrate is to be preferred to the crystalline salt owing to the fact that it is more stable, as it does not effloresce or lose its water of crystallization. The crystal carbonate owing to its larger water content is also more apt to absorb carbon dioxide from the air. This unites with the carbonate to form bicarbonate, a substance of no practical value in development. There is no settled proportion of alkali which should be used with any given plate or developing agent. Variations within reason have no other practical effect than increasing or decreasing the velocity of development. The existence of an optimum concentration of alkali beyond which no further increase in velocity occurs when the alkali is increased has been shown by Ermen.4 Except in the case of hy- drochinon, this optimum concentration equals about 1 per cent of the anhydrous salt. Increasing the carbonate above this point does not increase either the velocity of the developer, nor the density, but probably does increase the amount of fog and for this reason should be avoided. Not much use is made of the caustic alkalis, such as the sodium and * Phot. J., t§22, 62, 123. THE TECHNIQUE OF DEVELOPMENT 297 potassium hydroxides, except for the preparation of paramidophenol developers and with such slow acting agents as hydrochinon and glycin. When used with hydrochinon, the caustic alkalis form a more active developer, which works more rapidly and softer than that produced by the use of the alkaline carbonates and one which is less affected by temperature. Owing to their supposed tendency to fog and to their action on gelatine there exists a certain prejudice in the minds of many photographers against the use of the caustic alkalis. While with the more energetic developing agents no real advantage attends the use of the same over the alkaline carbonates, the caustic alkalis may be used with complete success if care be taken to use the proper amount The following table shows the developing equivalence of the alkalis and in what proportion one should be substituted for another. Sodium hydroxide Potassium hydroxide Sodium carbonate NasCOs Sodium carbonate NajCOs-HjO Sodium carbonate Na!C03-loH20 Potassium carbonate K2CO3 80 112 106 124 286 138 I. 1.40 1-325 1-550 3-575 1-725 O.714 I. O.946 1. 106 2-553 I.232 0-755 1.057 I. 1. 170 2.699 1.302 0.323 O.452 O.428 1. I. IRS •557 O.280 O.392 0.371 0.434 1. 0.483 O.580 0.812 O.768 0.899 2.072 1. A number of other substances have been suggested for use in place of the alkaline carbonates and hydroxides but few have come to be used extensively. Sodium tribasic phosphate was suggested by Lumiere in 1906 but has never been widely used. Acetone CH3 CO • CH3 was also recommended by Lumiere and has found some favor, particularly in conjunction with pyro. Its action is to combine with the sulphite to form acetone sulphite, the sodium set free com- bining with the developing agent to produce the phenolate which is the actual reducing agent. Its principal advantages are its freedom from fog, somewhat less stain than when the alkaline carbonates are used and no softening action on the gelatine film. . Pyro-acetone for the latter reason makes a very efficient hot-weather developer. Formalde- hyde was introduced by Lumiere in 1898 but has not proved to be very efficient with any agent except hydrochinon in conjunction with which it produces a contrast working developer especially suitable for line copies and similar work in black and white.5 5 Sodium tribasic phosphate, Eder's Jahrb., 1896, 10, 190. Acetone, Bull. Soc. franc. Phot., 1896, p. 558; 1897, p. 550. Formaldehyde, Eder's Jahrb., 1898, 12, 419. 298 PHOTOGRAPHY The Value of Desensitizers. — One of the most noteworthy addi- tions to photographic technique during the last few years has been the introduction of efficient desensitizing agents which by reducing the sensitiveness of the plate enable development to be- conducted in a much brighter light than that which may be used otherwise. The great sensitiveness of the modern dry plate has made their develop- ment a matter of real difficulty for no light which is sufficiently bright to be of much value in practice may be used without danger of fog. Particularly is this true for plates which are extremely sensitive to color, such as the modern panchromatic for which no light is really safe and time and temperature methods are the only satisfactory manner of development. The use of a desensitizing agent, however, enables plates of the very highest sensitiveness to be developed with comparative safety in a bright yellow or orange light, thus considerably facilitating development by either the inspection or factorial methods. Desensitizing Agents. — -Although there are some scattered refer- ences in photographic literature prior to 1920 on the subject of de- sensitizing agents (see Wall, Amer. Phot., 1921, 15, 651, Dec.) pheno- safranine, introduced as a result of the investigations of Dr. Luppo- Cramer, was the first really practical desensitizing agent. Phenosafra- nine used at a concentration of 1 : 2000 reduced the sensitiveness of an extra rapid plate to 1 /750 of its original sensitiveness without any . effect whatever on the latent image so that no increase in exposure is required. A. and L. Lumiere and A. Seyewetz found that toluylene red 1 : 1000, aurantia 1 : 1000, picric acid 1 : 100 are all desensitizers but not so efficient as phenosafranine.6 Pinakryptol, pinakryptol green and pinakryptol yellow are three de- sensitizers of unknown constitution introduced by the Farbewerke vorm, Meister, Lucius and Briining as the result of the investigations of Dr. E. Konig, B. Homolka and Robert Schuloff. Both pinakryptol and pinakryptol green are colored substances and form highly colored solutions but neither has any staining action on gelatine and are conse- quently superior in this respect to phenosafranine which stains strongly. The desensitizing effect of both is as high, if not higher, than that of phenosafranine. Pinakryptol slows development, but with pinakryptol green the rate of development is unaffected. With both the reduction in sensitiveness is much greater for the blue rays than for the red so that with red-sensitive, panchromatic plates there is danger of fog. This difficulty has been overcome by 6 Brit. J. Phot., 1921, 68, 351, 370. THE TECHNIQUE OF DEVELOPMENT 299 the introduction of pinakryptol yellow which is a more effective de- sensitizer for the rays of longer wave-length. Its desensitizing power, however, is destroyed by sodium sulphite so that it cannot be added to the developing solution as can pinakryptol and pinakryptol green but must be used as a separate bath and followed either by a bath of pinakryptol green or a developing solution containing .005 per cent pinakryptol green. Basic Scarlet N, a red dye made by the Compagnie des Matieres Colorantes of France was found to be an active sensitizer by the Re- search Laboratory of Pathe Cinema. According to Moreau it con- sists of a mixture of chrysoidin and phenosafranine. As compared with pinakryptol green, it is stated to be more effective in preventing fog. Mercury cyanide, the use of which as a desensitizer has been pat- ented by the I. G. Farbenindustrie,7 is the only desensitizing substance of practical utility yet found among inorganic substances. Ac- cording to Mayer and Walter,8 the actual desensitizer is the complex K2Hg(CN)4. As a desensitizer, mercury cyanide is as powerful as any of the organic substances known and is less prone to produce fog. It cannot be used as a preliminary bath, however, and is slowly pre- cipitated in developers. It cannot be used with pyro. In use, 0.3 gram mercury cyanide is added to each liter of developer (4.6 gr. to 35 oz.). Desensitizing in Practice. — Either phenosafranine, pinakryptol, pinakryptol green or basic scarlet N may be used as a preliminary bath, or as an addition to the developing solution. When used as a preliminary bath, phenosafranine is used at a dilution of 1 : 2000, pinakryptol, pinakryptol green and Basic Scarlet N at 1 : 5000. An immersion of two minutes is sufficient, although longer immersion is not objectionable in the least. After this the plate may be removed and development conducted in a bright yellow or orange light. If the desensitizer is added to the developing solution, from two to three minutes should be allowed to elapse before turning on the brighter light. According to Dr. Luppo-Cramer, the action of phenosafranine is complete within one minute and pinakryptol green within practically the same time. Pinakryptol, however, is slower in action and requires about two minutes and in practice it may be well to increase these times slightly in order to be sure that the action is completed. 7 B. P. 280,525 of 1926; Brit. J. Phot., 1928, 75, 233 8 Brit. J. Phot., 1928, 75, 692. 11 300 PHOTOGRAPHY If the desensitized plate or film is exposed to bright orange or red light before it is placed in the developer or before the image has ap- peared the latent image will be bleached. Keep the plate covered the first few minutes of development. Used as a preliminary bath, pinakryptol green increases fog with nearly all developers but particularly so with pyro. With glycin, elon (not metol-hydroquinone) and rodinal, however, there is little in- crease of fog.9 Pinakryptol green is more effective when added to the developer than when used as a preliminary bath; a concentration of 1/25,000 in the developer being equal in desensitizing power to 1/5,000 used in a preliminary bath. There is also less tendency to produce fog when the desensitizer is added to the developing solution.10 On the other hand, most developers tend to precipitate upon the ad- dition of pinakryptol green. This is especially true of hydroquinone, less so with adurol, and pyro, while glycin, rodinal, paraminophenol and metol are not precipitated. With metol-hydroquinone, and prob- ably with other developers, the desensitizing action exists at least as long as the activity of the developing solution. Desensitizing affects the Watkins factor, reducing it in the case of pyro and amidol and increasing it in the case of hydroquinone, which it raises from a low-factor, slow-working agent to a high-factor, soft- working developer. Phenosafranine has, in fact, been advised by Luppo-Cramer as a cheap substitute for metol with a hydroquinone developer.11 According to Hubl,12 with hydroquinone the factor varies from 5.3 with phenosafranine, to 4.6 with pinakryptol yellow and 2.6 with pinakryptol. With a metol hydroquinone developer containing 3 parts metol to one of hydroquinone, the change in the factor is insignificant. With pyro and amidol the factor must be reduced one third. Development by Inspection. — The first and perhaps the still most widely used method of development, certainly by the older workers, consists in visual examination of the plate from time to time. Deter- mination of the time of development by inspection is largely a matter of experience. It is not a method which is founded upon any definite scientific basis, nor one which can be expressed in terms which convey any exact information to another worker. Continuous experience 9 Dundon and Crabtree, Brit. J. Phot., 1926, 73, 404. 10 Ibid. 11 & I. P., 1921, 1, 69; Phot. Ind., 1921, p. 534. 12 Amer. Phot., 1925, ig, 642. THE TECHNIQUE OF DEVELOPMENT 301 under standardized conditions, and this only, will enable one to achieve success in developing by inspection. It is true that there are a large number of tips or dodges which are used by the experienced worker as a guide, and which are often recommended to the beginner when in search of assistance, but these in the absence of similar experience convey no -exact meaning and are applicable only to those conditions under which they originated. Under other circumstances and in other hands such indications may be wide of the mark and positively mis- leading. Actual photometric measurement of several sensitometric strips is sufficient to show how unreliable any attempt to estimate contrast by the eye is likely to be and when it is remembered that in development such estimation is made still more difficult by the presence of an opalescent film of silver halide which increases the apparent opacities, but decreases the ratio of the opacities, or the contrast, it is at once evident that development by inspection is subject to exceedingly large errors and that it is a method which can be practiced successfully only after considerable experience with a given plate and other standardized conditions. With incorrect exposure the matter becomes still more complicated. The delayed appearance of the image and the slow increase in density and shadow detail in the case of under exposure leads to over develop- ment in the hope of securing greater density and more shadow detail. As a result the contrasts, which are already too great, are increased and matters are made worse instead of better. Likewise in dealing with over exposure the rapid appearance of the image and the quick growth of density lead one to remove the plate before the proper stage of contrast has been reached. This again is just the reverse of what should be done as it lessens the contrasts, which owing to over ex- posure are already insufficient. While a large number of experienced photographers develop by in- spection, with results in every way comparable to those obtainable by any other method, nevertheless, it must be said that it is a more or less haphazard, unsystematic method of working which lacks the precision which is required of a process as important as that of development. While it is true that none of the commonly used methods of develop- ment are theoretically more than approximations to the required con- dition, nevertheless we think that it may be said that either the fac- torial or thermo methods are more certain and less subject to error than is development by inspection. It is at any rate not a method for 302 PHOTOGRAPHY the beginner, nor even for the advanced worker who works only at intervals and under varying conditions. For these, either the fac- torial or thermo methods are to be preferred. The Watkins System of Factorial Development. — Considerable in- formation concerning the velocity of development is supplied by the time of appearance of the image. In 1893 Mr. Alfred Watkins, the well-known authority on exposure and development, found that the time of appearance is an indication of the speed of development and that 'any variation in the dilution of the developing solution, the tem- perature, or the alkali, affects the time required to reach a given density, or value of gamma, in the same way that it affects the ap- pearance of the image. In other words TD = WTA, where To is the time for density, D, Ta. the time of appearance and W is a constant. It is this constant, W, which is termed the Watkins factor. With any given developing agent the factor depends upon the degree of contrast or gamma which it is desired to reach. Once the factor is found for any given set of conditions the time of development can always be readily determined by multiplying the time of appearance by the proper factor. What Determines the Factor. — The factor is dependent principally upon the developing agent. The presence of a soluble bromide, how- ever, has a decided effect. The factor does not change for different plates, except in the case of a very few plates containing a much larger amount of iodide than usual. Neither is it altered by varia- tions in the amount of the alkali nor by the dilution of the solution, except in the case of pyro and amidol. With these the factor varies with the strength of the solution. Within the range of temperatures generally utilized for development, there is comparatively little altera- tion in the Watkins factor with most developing agents. There is a slight variation, however, with some agents at extreme temperatures. The following factors are suggested for a start, but the experience of the photographer and the requirements of his particular printing medium may lead him to think an alteration of the factor desirable. If after the first few trials, using the factors as given, the negatives indicate that greater contrast would be desirable, the factor should be increased, while if the negatives show too much contrast a lower fac- tor should be used in the future. Hence the following factors should THE TECHNIQUE OF DEVELOPMENT 303 not be considered as final but on the contrary as suggestive only ; to be used until experience shows a higher or lower factor to be better suited to the requirements of the individual. Adurol 5 Pyrocatechin 10 Hydrochinon (min-KBr) 5 Hydrochinon (max-KBr) 4^ Eikonogen 9 Glycin (Carb. Soda) 8 Glycin (Carb. Potash) 12 Paramidophenol 16 Without Potassium Bromide Factor 1 gr. to the oz 18 2 gr. to the oz 12 3 gr. to the oz 10 4 gr. to the oz 8 Pyro-acetone — about double the above. Certinal 30 Amidol (2 gr. to the oz.) 18 Rodinal 40 Ortol 10 Edinol 20 Metol 30 Quinomet 30 With Potassium bromide Factor gr. KBr per oz 9 y2 gr. KBr per oz 5 Vs, gr. KBr per oz 4^ 1 gr. KBr per oz 4 Pyro-soda The factor of a combination developer containing two or more de- veloping agents depends upon the proportions of the various agents to one another. If in equal parts, the factor is simply the average of the factors of the two agents. But if, for instance, the developer contains 2 parts hydrochinon to one of metol (3 parts in all) the fac- tor is put down for all three parts and the sum divided by 3, or 5+5+30 3 =I3M- A combination developer containing pyro, however, does not conform to this rule and its factor must be determined by trial. Accuracy of the Factorial System. — The principle upon which the factorial method is based is open to, and has been made the subject of, some criticism. Although careful investigation has shown that there is not in all cases that definite and fixed relationship between the time of appearance and the time of development for a given gamma as as- sumed by the Watkins factorial method, in the vast majority of cases the departure from this relationship is comparatively small and with- out any particular significance in practical work. There are in all three sources of error in the factorial system. These are as follows : 304 PHOTOGRAPHY 1. The difficulty in observing the correct time of appearance. 2. Occasional variations in the Watkins factor. 3. Variation of the time of appearance with the degree of exposure. Several years ago Mr. A. Lockett conducted a number of investiga- tions with six different persons to determine the seriousness of the errors in observing the time of appearance and concluded that : 1. What is called the personal equation in factorial development is of comparatively small importance, provided average care be used: being, in fact, much less likely to cause variation in results than with the old system of judging development by inspection. 2. Although a developer with a medium factor is probably prefer- able, there is practically no more fear of variable results with a large developing factor than with a small one — given reasonable care in ob- serving the time of appearance. 3. Some individuals are habitually quicker than others in observing the appearance of the image ; but, as a rule, this variation is uniform and may be allowed for by an alteration in the factor. 4. Within limits a slight error in observing the appearance of the image has no serious results.13 Now that development may be conducted in bright yellow light, or even in white light in some cases, thanks to the introduction of satis- factory desensitizing agents, the difficulties in observing the appear- ance of the image are removed and errors from this source are practi- cally negligible. Desensitizing also removes another objection to the factorial system which was formerly of some importance. To observe the appearance of the image it was necessary to hold the dish close to the safelight and this greatly increased the danger of fog, particularly with color sensitive plates. All danger of fog from this source has, of course, been completely removed by desensitizing. Much has been made by critics of the factorial system of the fact that the Watkins factor is subject to variation with different batches of the same plate and developing agent. While it may be true that such inconstancy in the Watkins factor occurs, it is only fair to say .that in practice it is negligible and cannot be said to constitute a serious ob- jection to the method. The variation of the time of appearance with the degree of exposure is the weakest point of the factorial system. It is a matter of common knowledge that the time of appearance of the image in development is greater for under exposure and less for over exposure than for normal 13 Brit. J. Pfiot., 1906, 53, 464- THE TECHNIQUE OF DEVELOPMENT 305 exposure. Since the time of appearance is directly proportional to the time of development by the factorial method, the time of development varies with the time of appearance. Consequently under exposures receive longer and over exposures shorter development than do normal exposures. As we have already seen, the time of development required to reach a given stage of contrast (y) is independent of the time of ex- posure and hence under and over exposures should receive the same development as a normal exposure. Mr. Watkins advises as a means of lessening this source of error that several plates be developed at one time and the mean time of appearance taken. This is of course better than taking the time of appearance from a single exposure, but is at best only an approximation. With plates which have received correct, or nearly correct, exposure factorial development is perfectly satisfactory and in some respects the most desirable method of development. Where correct exposure can- not be ensured, thermo development is to be preferred. Thermo Development. — Development for a fixed time at a certain temperature was indicated by Hurter and Driffield in their first paper " Photochemical Investigations, etc.," before the Liverpool Section of the Society of Industrial Chemistry in 1892. Mees and Sheppard, in a number of papers constituting in general a comprehensive review of the work of Hurter and Driffield in sensi- tometry and other allied problems relating to photographic theory, de- veloped the mathematical relations between the time of development and gamma and the effect of the velocity constant of development (k) and maximum contrast, or gamma infinity on the time of development. Their investigations, besides extending our knowledge of the physico- chemical factors in development, established accurate means of calcu- lating the time of development for any desired gamma. In 1903, Houdaille made the first quantitative observations on the rate of development at different temperatures. Two years later Fergu- son and Howard gave particulars of a method of calculating the times of development at different temperatures and a year later the former published the results of a more complete investigation with mathemati- cal formulae for the calculation of the temperature coefficient. The fundamentals of thermo development were now established, but it remained for Mr. Alfred Watkins, by his carefully compiled tables of the developing speeds of commercial plates and times of develop- ment at various temperatures adapted for use with any plate, to elimi- nate the necessity fpr the individual to calculate by an exacting lab- 306 PHOTOGRAPHY* oratory test the constants for his own particular case. This handicap removed, thermo development became exceedingly simple and was widely adopted both by amateurs and professionals. The Watkins System of Thermo Development. — The time of de- velopment required to reach a given stage of contrast, or gamma, de- pends upon : 1 . The maximum contrast obtainable (y^, ) . 2. The velocity constant of development (k). 3. The temperature coefficient (T.C.). Methods of calculating these factors and from them the time of de- velopment at various temperatures for any given gamma have already been given (pages 262 to 265) and from these the student can de- termine for himself the proper times of development at various tem- peratures with the particular plate and developer to which he is ac- customed. However, as many have neither the facilities nor the in- clination to make these calculations for themselves and yet desire to use the thermo method, in the following pages we will reproduce Watkins' tables of commercial plates and times of development. The Watkins system takes into consideration the developing speed of the plate, the developer and the effect of temperature on the rate of development. All commercial plates are divided into eight classes ac- cording to the time required to reach a gamma of 0.9. These classes are termed Very Very Quick, Very Quick, Quick, Medium Quick, Medium, Medium Slow, Slow, Very Slow, and designated by the letters VVQ, VQ, Q, MQ, M, MS, S, and VS respectively. Instead of varying the time of development for each class of plates, the neces- sary allowance is made by altering the dilution of the developer so that at a normal temperature of 66° F. plates of each class require the same time of development. This reduces the number of scales of times of development to two — one for tray and one for tank development — and considerably simplifies the system. Six different developers are adapted for use with the tables which will be given shortly. These in- clude pyro-soda, metol-quinone, Rodinal, Azol, Citol and Certinal. Developing Speeds of Commercial Plates. — Barnet — Ultra Rapid Super Speed Ortho Studio 500 Press ^ . Studio 400 . .S . .S .MS .VS .MS Portrait Isonon Anchor Commercial Medium Iso Commercial Isonon . . . Contrast .M .MQ ■MQ .M ,M •Q THE TECHNIQUE OF DEVELOPMENT 307 Studio Ortho 400 M Red Seal M Self Screen Ortho MQ Red Diamond MS Special Rapid MS Ordinary MQ Crantbr — Hi-Speed VS Speed-O-Krome S Crown S Banner-X S Instantaneous Iso M Cine Panchromatic MS Positive Cine MQ Ensign — Film MS Gem — Gold Label S Salon Xpres VS Salon S Salon Isochromatic Q Portrait S Colour Screen M Meteor VQ Isochromatic M Panchromatic Tricol Q Universal M Slow Universal MQ Gevaert — Sensima . . . VS Ultra Rapid MS Orthochrome MS Special Rapid S Anti-Halo MS Filtered Ortho MQ Ortho Anti-Halo MS Cine Film MQ Hammer — Special Extra Fast MS Extra Fast MS Slow Q Commercial Ortho Q Ortho Extra Fast MS Ortho Slow Q Ortho Double-Coated MS Aurora Non-Halation MS Photo Postal VVQ Postal Q Double-coated according to brand Eastman Film — Portrait Par Speed MS Super Speed VS Commercial MS Commercial Ortho MS Panchromatic MQ Non-Curling Speed S Premo Speed S Cine Super Speed VS Cine Ordinary S Screened Chromatic Q Empress Q Rapid Process Panchromatic VVQ Ordinary Q Zenith Extra Sensitive Film VS Zenith Film MS Special Rapid Film MS Empress Film VQ Illingworth — Studio Extra Fast VS Fleet MS High Rapidity S Ortho Fast MS Studio Ortho Fast M Ultra Rapid S Special Rapid S Non Screen . . : MQ Medium MQ Ortho Medium MQ Ordinary MQ Imperial — Eclipse S Flashlight S Special Sensitive S S. S. Ortho MS Panchromatic A Q Panchromatic B VQ Special Rapid MS Special Rapid Ortho MS Non-Filter Ortho Q Sovereign M Ordinary MQ Fine Grain Ordinary VVQ Landscape Q Film MS 308 PHOTOGRAPHY Ilford— Zenith 650 VS Zenith 400 S Press VS Monarch VS Special Rapid Panchromatic MQ Most Rapid Versatile. . MS Versatile Ortho Q Special Rapid M Rapid Chromatic Q King's Own M Versatile Rapid M Chromatic M Q Plavik Film MQ Cine M N. C. Speed S Sigma Ortho S Marion — I so Record Q Record S Panchromatic VS Brilliant MQ P. S MS Instantaneous MS Iso M Portrait M W. B M Ordinary MQ Stanley — 50 M Commercial Q Wellington — Super Xtreme VS Xtreme VS Studio Anti-Screen S Press VS Special Extra Speedy S Extra Speedy MS Speedy Portrait MS Speedy S Anti-Screen M Iso Speedy M Ordinary Q Film M Lumiere & Jougla — Maxima M Portrait Instantanee M Grands Instantanee MQ Ortho, Yellow and Green Q Ortho, Yellow and Red Q Intensive 1 . . M Panchromatic VQ Instantanee MQ Extra Rapide MQ Film MS Reproduction MQ Panchromatic Procede Q Paget— XX MS XXX MS Process Panchromatic M S F. Orthochromatic S Special Rapid S Professional Medium S Portrait S Professional Extra Rapid VS Extra Special Rapid R VS Ordinary Panchromatic MS Hurricane S Extra Special Rapid Ortho S Seed — Graflex VS Gilt Edge VS Color Value M L Ortho M 26 X M Non-Halation L Ortho MS Tropical M Panchromatic Q 23 Q Process Q Standard — Extra Imperial M Orthonon M Polychrome M Wratten & Wainwright — Panchromatic Q Process Panchromatic Q Wratten M (backed) Q THE TECHNIQUE OF DEVELOPMENT 309 Developers. — Thermo Pyro-Soda : gr. 16.7 gm. gr. oz. 35-5 91.2 gm. gm. oz. 1000 cc. B. Sodium carbonate (dry) 2 oz. 182.4 gm. Potassium bromide 40 gr. 8-35 gm. 10 oz. 1000 cc. Thermo-Metol Hydrochinon (as modified by F. R. Fraprie) : 60 gr- 6.25 gm. 30 gr- 3-135 gm. 90 gr- 9-375 gm. Water to make 20 oz. 1000 cc. I oz. 45-6 gm. i oz. 78 gm. 20 oz. 1000 cc. Dilution of developer. VVQ VQ Q MQ M MS S VS Pyro-Soda 1 i}4 3 4 5 Metol-Hydrochinon . . .ilA 2 2% 3V2 6 £ 10 drams of each stock to be diluted to make a total of 3 ounces for tray or 10 ounces for tank. Rodinal, Azol, Citol, Certinal.... 20 26 35 45 60 80 105 135 minims solution to be made to a total of 3 ounces for tray or 9 ounces for tank. In metric measure WQ VQ Q MQ M MS S VS M-Q • 3° 41 55 73 94 125 165 210 for tray 9 12 16 22 28 38 50 65 for tank ■ 41 55 73 94 125 165 210 280 for tray 12 16 22 28 38 5° 65 84 for tank Certinal, Rodinal, Azol, Victol. . . . ■ 13 17 23 30 40 53 70 90 for tray 4-3 5-75 7-75 10 13 18 24 30 for tank The above figures are cc. and are to be diluted to a total volume of IOOO cc. Instructions. — The use of the system is simplicity itself. Deter- mine by reference to the table the developing speed of the plate or film and mix the developer as directed for that class, using water which has attained the same temperature as the room in which de- velopment is conducted. This avoids any variation in the temperature of the solution during*development. The temperature of the developer 310 PHOTOGRAPHY having been determined, find the time of development by reference to the table of temperatures. In a safelight, or total darkness, flow the plate with the developer, or if using a tank, immerse the cage contain- ing the plates in the solution and start the darkroom clock. If a timer is not available an ordinary watch may be used. As there is no necessity whatsoever for observing the plate during development, the tray may be covered with a light-excluding cover and the white light turned on in order to observe the time at which development started. When the time of development has expired, turn out the white light and remove the plate. Table of Time of Development at Various Temperatures Degrees Degrees Cent. Time in Time for Fahr. to nearest half tray tank 80 27.O 12 78 26.O IU 13 76 24-5 14 74 23-5 4 15 72 22.5 4M 16 70 21 \Vi 17 68 20 5 I8M 66 19 5H I9M 64 18 §8 21 62 17 22 y2 60 16 6V2 24 58 14-5 7 26 56 13-5 7V2 28 54 12.5 8 sy2 30 52 n-5 32 50 10 9M 34 48 9 10 37 46 8 10% 40 44 7 11H 43 42 6 mU 46 40 4-5 13M 49 If the first trial does not produce a negative having the proper amount of contrast to suit your individual case, classify the plate one class nearer VS for more or one class towards VVQ for less contrast. The Thermo Method with Glycin. — The following formula and system for the use of glycin according to the Watkins thermo method is due to Mr. Arthur Purdon and was published in American Photog- raphy. Thermo-Glycin Stock Solution Water to make 500 cc. or 10 oz. Potassium carbonate (dry) 30 gm. or 280 gr. Sodium sulphite (dry) 10 gm. or 90 gr. Glycin 15 gm. or 140 gr. THE TECHNIQUE OF DEVELOPMENT 311 Cc. of Stock to be Drams of Stock to be made up to 300 cc. made up to 10 oz. for for Tank or 90 Tank or 3 oz. for Plate Class cc. for Tray Tray VVQ 6 1% VQ 7JA 2 Q ioH 2% MQ \zYz 3*A M 18 A,Vi MS 24 6 S 30 8 VS 40^4 10 Temperature — Time Table Temp. Time Time Degrees Minutes Minutes and Seconds Fahr. Tank Tray 80 10 2 M. 50 S. 78 \\V& 3 M. 10 S. 76 12J6 3 M. 30 S. 74 I3J4 3 M. 50 S. 72 14M 4 M. 13 S. 70 16 4 M. 35 S. 68 17 5 M. 66 1 8$6 S M. 20 S. 64 20 s M. 36 S. 62 2\% 6 M. Temperature coefficient, 2.2. The Efficiency of Time Development. — As has been stated previ- ously the time of development depends upon 1. The maximum contrast of the plate (y«,). 2. The velocity constant of development (k). 3. The temperature coefficient (T.C.). The application of any rules found for one particular batch of a particular plate to a different batch of the same plate must depend on these factors remaining constant. As a matter of fact, however, com- paratively large and unordered variations in these factors occur with different batches of the same plate regardless of the extreme care taken by the manufacturers to secure uniformity in their products. The maximum contrast (yM) of a plate is reasonably constant from batch to batch, but varying circumstances ofttimes introduce considerable variation, amounting in some cases to 30 or 40 per cent. The velocity constant of development varies considerably with different batches of the same plate. This is due largely to the rate at which the plates are 312 PHOTOGRAPHY dried, which even in the elaborate systems used by manufacturers is subject to some variation. In addition an alteration in the character of the gelatine used for a batch of plates may seriously alter the factor. Furthermore, as has already been mentioned, the temperature coeffi- cient is not independent of the plate, consequently a table of times of development at various temperatures which is applicable to one plate may not be applicable to another which develops at the same rate at a normal temperature. The temperature coefficient, however, varies but slightly with different batches of the same plate. . Therefore it appears that thermo development can only be accurately conducted when the values for the controlling factors (y^ and k) are known for each batch of plates. Unfortunately manufacturers have not yet seen their way to do this, nor, except in a few isolated cases, have they adopted the plan of indicating the time of development for each batch of plates. This has been done by a few manufacturers in the case of panchromatic plates, but with the vast majority of plates no information is given of the way, nor the extent, to which they differ from previous batches of the same plate. It would be a decided step in advance if the manufacturers could be induced to indicate for each batch of plates the time of development required to reach gammas of say 0.8, I, and 1.3 with the developers regularly advised for use with the plate. Nevertheless where such information is lacking and development is alike for all batches of the same plate, thermo development is a singu- larly uniform process which yields a surprisingly high percentage of satisfactory results. Such errors as may occur from variations in the governing factors are comparatively small and seldom sufficient to be of serious consequence. It is perhaps this more than anything else which has prevented the individual testing of each batch of plates by the manufacturer, who holds, and it must be admitted with a show of reason, that such variations as do occur are not of sufficient importance to warrant the labor and expense involved in the testing of each individual batch of plates. Nevertheless, in spite of the extreme care taken by manufacturers to keep their product uniform, considerable variations between different batches of plates occur occasionally and consequently laboratory testing of each batch of plates by the manu- facturer would be a distinct gain in scientific accuracy and thorough- ness. Development at High Temperatures. — The important thing to avoid in the developing of plates and film at high temperatures is to prevent undue swelling of the gelatine in the warm solutions. The THE TECHNIQUE OF DEVELOPMENT 313 actual hardening of the gelatine by a preliminary bath of formaline or a combination of formaline and sodium sulphate as in the Ilford tropical hardner (B. P. 128,337 of 1918) is essential only at ex- tremely high temperature. When the temperature of the solutions does not exceed 90° Fahrenheit it is sufficient to simply prevent undue swelling of the gelatine during development. If a rapid developer is used so that development is complete within three to four minutes, ab- normal swelling of the gelatine may be prevented by the addition of one part of sodium sulphate to each ten parts of the developing solution. Sodium sulphate does not harden gelatine, but only depresses swelling, consequently after development the gelatine must be permanently hardened with either a ten per cent solution of formaline, or with alum. If alum is used, the gelatine will be more thoroughly hardened if sodium sulphate is added to the alum solution; due to the fact that the sodium sulphate prevents swelling while the alum is hardening. The following formula is suitable: Sodium sulphate (crystals) 1 lb. 120 gm. Potassium chrome alum 3.84 lb. 30 gm. Water 1 gal. 1000 cc. The film or plate should be rinsed briefly after removal from the developer and before being placed in the hardening bath. To prevent the formation of blisters and alum stains, sensitive material should be kept on the move for about one half minute after immersion in the alum solution. If this is not done, stains will be formed on the sur- face of the gelatine which are very difficult to remove. This will occur also if the bath has been overworked or is old, and a fresh bath should be prepared at least once a "day as solutions of chrome alum do not keep at high temperature. After three minutes immersion in the hardening bath, the plate or film may be transferred to the usual and fixing bath for fixing. After fixing it may be washed for five minutes in running water up to 90° Fahrenheit without danger. General Reference Works Blech — Stand-Entwicklung. Brown- — Developers and Development. Hubl — Entwicklung der photographische Bromsilbergelatineplatten. 1922. Luppo-Cramer— Negativ Entwicklung Bei Hellem Lichte. 1922. Renger-Patzsch— Die Stand-Entwicklung. 1920. Seyewetz— Le Negatif en Photographic 1922. Watkins — Watkins' Manual. 1918. Watkins — Photography — Its Principles and Applications. 1927. Modern Methods of Development. Photominiature, No. 139. CHAPTER XIV FIXING AND WASHING The Action of Sodium Thiosulphate on Silver Halide. — Only a few substances are capable of dissolving the silver halides and a still smaller number are of practical value for fixing. Of these only two are of sufficient importance to justify mention. These are potassium cyanide and sodium thiosulphate, commonly termed sodium hypo- sulphite or hypo, but the hyposulphite is an entirely distinct chemi- cal. Potassium cyanide is much too powerful for use with gelatino- bromide emulsion as it tends to dissolve silver and thus weaken the lower deposits of the negative and for this reason sodium thiosulphate, which is free from such action, is generally used. The use of the thiosulphates is due to Sir John Herschel who drew attention to their solvent action on the silver halides in a paper in the Edinburgh Philo- sophical Journal in 1819.1 The precise chemical reactions involved in the fixing process are still somewhat obscure. According to the earlier theories, based upon the compounds isolated by Herschel,2 the thiosulphates dissolve silver halide by uniting with it to form a compound of silver-disodium thio- sulphate according to the reaction : (1) 2AgBr + Na2S203 = 2NaBr + Ag2S203 (silver thiosulphate), (2) Ag2S203 + Na3S3Os = Ag2S203-Na2S203 ( sil ver-monosodium-thiosulphate ) , (3) Ag2S203-Na2S203 + Na2S203 = Ag2S203-2Na2S203 ( silver-disodium-thiosulphate ) . The silver thiosulphate formed in equation (1) is supposed to be insoluble in water but soluble in sodium thiosulphate, being trans- formed into silver-monosodium-thiosulphate (equation (2)) which, like the first salt, is insoluble in water but soluble in sodium thiosul- 1 Edinburgh Phil. Journ., vol. I, pp. 8, 396. 2 Ibid. 314 FIXING AND WASHING 315 phate, in which it is converted into silver-disodium-thiosulphate (equa- tion (3) ), which is soluble in water. Bainc?,3 as a result of a later investigation of the subject, concludes that it is extremely doubtful that reaction ( 1 ) actually takes place, and as all attempts to prepare the compound Ag2S203-2Na2S203 failed, but instead a compound having the formula Na5Ag3(S203)4-3H20 was found, the following equations are probably more accurate repre- sentations of the reactions which take place in fixing: 1. AgBr + Na2S203 = NaBr + NaAgS2Os, 2. 3NaAgS203 + Na2S203 = Na5Ag3(S203)4. The removal of the unaltered silver halide may therefore be con- sidered to consist of two operations: (1) the conversion of the in- soluble halide into soluble double salts by sodium thiosulphate and (2) the removal of this double salt by washing in water. The Mechanism of Fixing. — The mechanism of fixing has been studied by Sheppard and Mees 4 and by Warwick 5 who by very dif- ferent experimental methods reached substantially the same conclu- sions. Without discussing in detail the experimental methods of these investigators, for which purpose the original papers should be consulted, we propose to deal briefly with their principal conclusions. The fixing bath dissolves per unit of time a constant fraction of the mass of silver bromide existing in the film at the origin of the in- terval of time considered. The amount of silver bromide left in the negative at any time can therefore be expressed mathematically as x = a(I— k)n, where a is the original amount of silver bromide in the negative ; k, the fraction dissolved per unit time; x, the amount remaining after n units of time. The value of this fraction, which may be termed the velocity constant of fixation, depends upon the temperature and the concentration of the fixing bath and is independent of the amount of silver bromide in the film, the quality of the gelatine, or previous tanning of the film by formaline or similar agents but is always greater, under identical conditions, with silver chloride emulsions than with silver bromide emulsions and for the same silver halide is more rapid with a decrease in the size of grain. 3 Phot. J., 1929, 69, 314. 4 Investigations, Phot? J., 1906, 46, 235. 5 Amer. Phot., 1917, pp. 585, 639. 316 PHOTOGRAPHY The simplest explanation of the known facts is that the rate of fixation is determined primarily by the penetration of the sodium thio- sulphate through the film, the chemical action being rapid compared with this. Influence of the Concentration of Sodium Thiosulphate and Tem- perature on the Time of Fixation— The investigations on the theory of fixation by Sheppard and Mees were concerned primarily with the velocity rather than the time of fixing which last is of more interest 10° 10* 30* M<>* Jo* 60 * To* TE.MPEHATUKE OF FIXING. BATHS (" CENTIQ.XWe'I Fig. 151. Influence of Temperature on Time of Fixation to the practical photographer. The influence of temperature and the concentration of the fixing bath on the time of fixing, or more exactly the semi-total time, or the time of clearance, was carefully studied by C. Welborne Piper in 1913 and the results published in the British Journal of the same year.8 The results obtained for the effect of temperature on the time of fixing were plotted in the form of a series of curves (Fig. 151). These show that the time of fixing varies approximately in inverse ratio to the temperature so long as small variations of - temperature alone are considered. The curves also show that the effect of tem- perature varies greatly with the concentration of the fixing bath, a bath of 40 per cent showing less variation with a given range of tem- perature than those of lower or higher concentration. The effect of 6 Brit. J. Phot., 1913, 6o, 59. FIXING AND WASHING 317 temperature is especially noticeable with very strong solutions. Thus from the curve representing a concentration of 70 per cent it appears that at a temperature of 200 C. several hours would be required for the clearance of the film and according to Piper there is doubt that complete fixing would ever take place under such conditions. With high temperatures the differences in the time of fixing for baths of various concentrations become less noticeable and it is probable that at a sufficiently high temperature the time of fixation would be the same for all baths regardless of concentration, since all the curves are of similar character and tend to meet in a point to the right of the graph. This is a matter difficult to prove or disprove experi- mentally owing to the softening of the film at the high temperatures involved. Figure 152, from Piper's paper, gives the curves for the effect of varying concentrations at the same temperature, the time in minutes 15' [30 0 0 40 i 14', I rj- 0 f y 60 0 sol 40 50 0 6d ' 7 5* 10% 20% 30% 4054 50% 60% 70% S0%, Strength of Hypo Solutions ( Per Cent) Fig. 152. Influence of Concentration of Hypo on Time of Fixation being plotted against the concentration and the curves representing the results obtained for temperatures of 14, 20, 30, 40, 50, 60, and 70° C. (57, 68, 86, 104, 122, 140, 1580 F.). Influence of Ammonium Chloride on the Rapidity of Fixation. — Ammonium thiosulphate was recommended as a fixing agent in place 318 PHOTOGRAPHY of the commonly used sodium salt by Spiller in 1868. Although it is much more rapid in action than sodium thiosulphate, it does not equal the latter in general adaptability and owing to this, and to its higher cost, it has never been widely used. The investigations of C. Welborne Piper 7 have well established the fact that for each thiosulphate the rate of fixation is more rapid at a certain concentration which is variable with each of the three thio- sulphate salts investigated. With ammonium thiosulphate and sodium thiosulphate the concentrations at which the maximum rapidity of fix- ing is secured are 15 and 40 per cent respectively. At 15 per cent, ammonium thiosulphate is approximately twice as fast as the sodium salt at 40 per cent. The rate of fixation is practically identical at a concentration of 33 per cent. Above this point the sodium salt is the more rapid while at lower concentrations the ammonium salt is the more rapid. The rapidity of fixing is considerably increased by the addition of ammonium chloride to the solution of sodium thiosulphate. The in- crease in rapidity is probably due to the partial conversion of the sodium thiosulphate into the corresponding ammonium salt, but ap- parently some undetermined factor also plays a part in the reaction, since the reaction is not as fast when the proportion of ammonium chloride is sufficient to completely convert the sodium salt to the corre- sponding thiosulphate as when a lesser amount is used. Thus the in- crease in rapidity of fixing cannot be due entirely to the conversion of the sodium thiosulphate into the ammonium salt. The effect of adding various amounts of ammonium chloride to solutions of sodium thiosulphate at various concentrations was care- fully investigated by Lumiere and Seyewetz in 1908 8 and by C. Wel- borne Piper in 1914. The results obtained by the latter investigator are shown in Fig. 153. From this it will be observed that for each concentration of thiosulphate there is a certain definite proportion of ammonium chloride which produces the maximum degree of accelera- tion. On increasing the proportion of ammonium chloride beyond the optimum point, the acceleration diminishes, the rate of diminution in- creasing with the concentration of sodium thiosulphate. The chart does not show the effect of adding ammonium chloride to a bath of sodium thiosulphate above 40 per cent for, since no acceleration takes place under these conditions, the result would be of no practical value. 7 Brit. J. Phot., 1914, 61, 193, 437, 458, 511. 8 Bull. Soc. f^anc. Phot., 1908, p. 217. FIXING AND WASHING 319 The maximum rapidity of fixing is secured by the use of a 15 per cent solution of sodium thiosulphate containing of its weight of ammonium chloride. The following formula is suitable for a rapid fixing bath for use in newspaper and similar work : Sodium thiosulphate ("hypo") 15 oz. 150 gm. Ammonium chloride 4 oz. 50 gr. 37.5 gm. Water to make 100 oz. 1000 cc. The double salts of silver formed in the fixing bath containing am- monium chloride were shown to be less stable than those formed in a 0 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 0 10 20 Fig. 153. Influence of Ammonium Chloride on the Time of Fixation plain bath of thiosulphate by Lumiere and Seyewetz, consequently rapid fixing baths containing ammonium chloride are more rapidly exhausted than plain solutions of sodium thiosulphate and cannot be used for as many plates or films. The addition of free ammonia increases the rapidity of fixing in much the same way as ammonium chloride. Owing, however, to its disagreeable odor and its tendency to produce a type of fog known as " dichroic " it is never used in practice. When are Plates Fixed? — The time of fixation is not as important in general photographic practice as an accurate means of determining when fixation is complete. A rule found in' many textbooks directs 320 PHOTOGRAPHY that the negative be left in the fixing solution as long again as is re- quired for the disappearance of the opalescent coating from the back of the plate. Recent investigations by Lumiere and Seyewetz 9 have shown that in a fresh fixing bath this extra time is unnecessary as fixation is complete when the opalescent coating has completely dis- appeared from the back. When the fixing bath contains less than 2 per cent of silver salts dissolved from previously fixed plates, fixing is still completed when the opalescent coating disappears, but when the amount of silver salts is in excess of 2 per cent, or 20 grams per liter (96 grains to 10 ounces), the removal of unaltered silver salts is in- complete and the residual salts are not removed by prolonging the im- mersion of the plate in the solution. However, if the plate is then transferred to a fresh bath all residual silver salts will be removed. The use of two fixing baths is therefore advisable unless one cares to make up a fresh fixing bath for each batch of negatives and discards it immediately after use. In the first case, the first bath is used nearly to the point of exhaustion, the negatives being transferred from this solution to a fresh one in which they are allowed to remain for five minutes. The first bath is then discarded and its place taken by the second bath which is in turn replaced by a freshly prepared solution. When the plate is allowed to remain in the first bath until completely cleared, then transferred to the second bath for five minutes, there is no danger of imperfect fixation, hence this plan is strongly recom- mended in preference to the somewhat haphazard methods generally advised. Exhaustion of the Fixing Bath. — While it would be well if photog- raphers could be induced to use a small quantity of a fresh fixing bath for each plate or film, discarding the bath immediately after use, this is seldom, if ever, done in practice, the same bath being used continu- ously until its slow action indicates that its fixing power is exhausted. A knowledge of the number of plates, films, or prints which can be fixed in a given volume of bath is thus of considerable practical value, as it enables one to determine when the bath is exhausted and prevents the risk of imperfect fixation owing to the use of an overworked fixing bath. It is evident that the number of plates which may with safety be fixed in any given quantity and strength of bath is dependent upon the amount of the various halides which can be completely dissolved by a given amount of sodium thiosulphate. The order of solubility of the three halides is chloride, bromide, iodide. Determinations of the 9 Brit. J. Phot, 1924, 71, 172. FIXING AND WASHING 321 solubilities of the halides in hypo have been made by Valenta, Lumiere and Seyewetz, Strauss, and by Lehrmann and Busch.10 There are serious discrepancies in the results and further work is desirable. The earlier investigations, conducted on a basis of silver bromide, indicated that a fixing bath could be regarded as having been exhausted when the silver content equalled 2 per cent. In view of later work, in which the influence on fixation of the small amounts of silver iodide present in most emulsions has been more thoroughly in- vestigated, this figure appears to be much too high and according to the latest determinations, the practical limit seems to be from 0.6-0.7 per cent. With an average single-coated plate, this point is reached, ap- proximately, when a 3^4 X AlA plate is fixed in one ounce of 10 per cent hypo. A 20 per cent solution of hypo has greater fixing capacity but not in direct ratio to the concentration. The point at which the fixing bath is exhausted may be determined with sufficient accuracy for all practical purposes by immersing a strip of film or bromide paper in the bath until clear, washing for several minutes under the tap followed by immersion in a 1 per cent solution of sodium sulphide. The formation of a brown coloration indicates that the soluble halides have not been completely removed by the fixing bath and that a fresh bath should be substituted. The Fixation of Prints. — Warwick in 19 17 11 and Lumiere and Seyewetz again in 1924 have called attention to the very short time required for the fixation of bromide and gaslight prints. Both in- vestigators have shown that fixation is a matter of only a few seconds, being approximately twenty to thirty seconds in 25 per cent thiosul- phate and only slightly greater in a fixing bath containing bisulphite. The rapid fixation of paper prints is doubtless due to the porous nature of the support which allows the reaction to proceed from both sides of the emulsion. It does not follow from the above, however, that such short periods of fixation are sufficient under the conditions of ordinary practice. In the above experiments the prints were separately fixed in fresh fixing solutions, a totally different state of affairs from that presented in practical work where large numbers of prints are added to the same solution. When dozens of prints are being treated at the same time, the time required for fixing is necessarily dependent upon the time 10Berichtet 1894, 103; Bull. Soc. Franc, 1907, p. 104; Phot. J., 1907, 47, tag; Phot. Ind., 1925, p. 881, 911 ; Brit. J. Phot., 1927, 74, 91, 105. 11 Amer. Phot., 1917, 11, 639. 322 PHOTOGRAPHY which the print is exposed to the action of the fixing bath. This is a matter of moving the prints around in the bath individually so that each becomes completely exposed to the solution. It is evident that under such conditions the time required for perfect fixation will be much greater than those advised by Warwick and by Lumiere and Seyewetz whose results were obtained with the use of individual fixing baths. On the other hand, the investigations show that in cases of rush work prints fixed individually in fresh baths for 20 to 30 seconds may be expected to be reasonably permanent. Plain Fixing Baths. — Although fixing is accomplished by thiosul- phate alone, plain solutions of sodium thiosulphate are not much used in practical photography. This is due to the fact that the bath soon becomes discolored from the oxidized developer carried over on the surface of the negatives or prints fixed in it, and these oxidized prod- ucts stain the negatives or prints. There is also a tendency in warm weather for the gelatine to swell excessively and become soft, produc- ing frilling and any number of other troubles either while in the fixing bath itself or in the washing which follows. So far as the first objec- tion is concerned staining of negatives or prints in a plain fixing bath may be largely prevented by immersing the negative or print in a weak bath of acid before placing in the fixing bath. Even when an acid fixing bath is used, the use of a weak acid bath prior to fixing prevents the bath from becoming discolored and lessens danger of stain. A weak bath of acetic acid ( 1 ounce 28 per cent acetic acid to 32 ounces of water) is to be recommended for this purpose, especially in the case of prints or when pyro is used as a developer. From Piper's investigations on the influence of the concentration of the bath on the rapidity of fixation (pp. 340-341) it appears that if a bath giving the maximum speed of fixing and the least affected by temperature is desired this would be attained by the use of a solution of approximately 40 per cent. Various other considerations, however, intervene to make the employment of somewhat weaker solutions de- sirable. The abrupt transition from a strong thiosulphate solution to plain water produces a strong tension in the swollen film, which in hot weather gives rise to blisters and frilling and may even cause the whole film to leave the glass. On this account it is usual to employ a weaker solution of approximately 25 per cent, such as is obtained by adding 4 ounces of crystal thiosulphate to 16 ounces of water. A convenient way of preparing plain fixing baths is to dissolve one pound of thiosulphate in about 16 ounces of warm water and when FIXING AND WASHING 323 dissolved add cool water to 32 ounces. Every two ounces of this stock solution therefore contains 1 ounce of thiosulphate. To make up baths of different strengths it is diluted as follows : Thiosulphate required for each 20 ounces of bath Stock solution Water 8 oz 16 4 6 12 8 5 10 10 4 8 12 3 6 14 2 4 16 Acid Fixing Baths. — The addition of acid to the fixing bath for the purpose of combining the acid clearing bath frequently necessary for the removal of yellow oxidation stain when a plain fixing bath is used with the fixing bath itself and thus avoiding a separate operation was advised by Lanier in 1889. If an acid is added directly to a solution of thiosulphate the latter is decomposed and 'sulphur is precipitated. Lanier showed, however, that if a weak acid such as citric, acetic, formic or tartaric be used the precipitation of sulphur may be avoided by first combining the acid with a solution of sodium sulphite. When this is done the bath remains clear and there is no precipitation of sulphur unless there is an excess of acid present. The most convenient method of preparing an acid fixing bath is by the use of sodium bisulphite which may be regarded as an acid sul- phite, having the formula NaHSOs. The following is an excellent formula : Sodium thiosulphate ("hypo") 16 oz. 228 gm. Sodium bisulphite 3 oz. 42 gm. Water to make 64 oz. 1000 cc. In large establishments the bisulphite may be made up as a 50 per cent stock solution. One part of this stock solution to each 20 parts of plain fixing bath will be in the correct proportion. Potassium metabisulphite may be used in place of sodium bisulphite. The following formula is suitable : Sodium thiosulphate 6 oz. 274 gm. Potassium metabisulphite ^ oz. 22.5 gm. Water to make 20 oz. 1000 cc. A formula using citric acid and sodium sulphite originated by 324 PHOTOGRAPHY Lanier was strongly advised by Sir William Abney. The formula follows : Citric acid % oz. 14.3 gm. Sodium sulphite (dry) J4 oz. 15.6 gm Mix in 1 oz. (30 cc.) of water and add to Sodium thiosulphate 4 oz. 228 gm. Water to make 16 oz. 1000 cc. Acid Fixing and Hardening Baths. — The third type of fixing bath, the acid fixing and hardening bath, is an acid bath to which an alum has been added to harden the gelatine and prevent softening and frill- ing with its attendant troubles in hot weather. The ingredients are generally an acid, sodium sulphite and alum and their function and formula are given in the following table : 12 Constituent Function Formula Sodium thiosulphate Fixing agent proper, dissolves silver halide Na2S203 ■ 5H.O Acid .Clearing agent, promotes swell- ing and increases the speed of fixing, reduces stain and col- oration and regulates hardening agent (4) H2S03 (Sulphurous or organic acid) Sulphite Protects thiosulphate against decomposition by the acid. Anti-stain and anti-oxidant. . . .Na2S03 (Sodium sulphite) Alum Hardens gelatine, prevents frilling and softening K2S04- A12(S04)3, (Potash alum) K,S04- Cr2(SO.), (Chrome alum) The following formula using potassium alum is an excellent one for plates, films and papers: Sodium thiosulphate ("hypo") 16 oz. 228 gm. Water to make 64 oz. 1000 cc. Dissolve separately and add to the above Powdered alum y2 oz. 31 gm. Acetic acid 28 per cent 3 oz. 186 cc. Sodium sulphite (dry) ^ oz. 31 gm. Water to make 5 oz. 312 cc. 12 The chemical theory of the acid fixing bath has been fully discussed in a paper by Sheppard Elliot and Sweet of the Eastman Research Laboratory in the Journal of the Franklin Institute for July, 1923, 19s, 45. FIXING AND WASHING 325 . In mixing the last solution, or hardener, it is best to use two separate solutions. Dissolve the alum and sulphite each in a few ounces amount of water. Then add the acid to the sulphite solution, mix the two and add to the solution of thiosulphate. The hardener may be made up as a stock solution if desired as its keeping qualities are good. Troubles with the Acid Fixing and Hardening Bath. — Since it represents a compromise between certain physico-chemical factors and practical conditions, the acid fixing and hardening bath is frequently a source of trouble owing to the fact that the exact balance between the various substances has not been secured when compounding the same. If the bath turns milky on standing it is due to the acid attacking the thiosulphate and precipitating sulphur. This may result from three causes : 1. Too much acid, or too strong acid. Most formulas call for No. 8 or 28 per cent acetic acid and not the CP. or Glacial. 2. Too little sulphite, bad sulphite, or high temperature of the solution. 3. Incorrect mixing. If the method advised above is followed there will be no trouble on this score. If the milkiness disappears on standing it is due to the use of insuffi- cient acid, or not enough hardener to overcome the alkalinity of the developer brought over on the surface of the prints or negatives. If the bath does not harden this is due to the use of insufficient or impure alum, or to the fact that the bath is alkaline or neutral rather than acid. The hardening action of alum is due to aluminum sul- phate and some grades do not have the proper proportion of this sub- stance and accordingly must be used in greater quantity in order to secure equivalent action. Extra Hardening Baths. — In very hot climates or other exceptional conditions, a bath having an even greater hardening action than that above may be an advantage and in such cases the following bath, as worked out by Mr. J. I. Crabtree of the Eastman Research Laboratory, will be found very satisfactory: Sodium thiosulphate ("hypo") 5 oz. 228 gm. Sodium sulphite (dry) I oz. 45.6 gm. Formaline 2.1A fl. oz. 125 cc. Water to make ' 20 oz. 1000 cc. Although this bath has not the keeping properties of the ordinary acid fixing and hardening bath it will keep for at least a week at a tempera- ture of 100° F. 326 PHOTOGRAPHY Owing to the irritating vapors of formaline, it is well to keep the bath in a tank with a tight fitting cover when not in use. The Mechanism of Washing. — Following fixing, the next step is to remove the thiosulphate from the film. This is most generally effected by simple washing in water, although certain substances known as " hypo eliminators " are occasionally used. The rate of the elimination of thiosulphate from photographic films of gelatine has been investigated a number of times : by Haddon and Grundy, Lumiere and Seyewetz in 1910, Warwick in 1917, Elsden the same year and by Hickman and Spencer in 1922.' 3 It has been found that, in general, thiosulphate diffuses from the film expotentially with time as was stated by Mees and Sheppard in their Investigations. In other words the amount removed in a unit of time is proportionate to the concentration present at the beginning of the period. Thus if the original concentration is 10 grams and at the end of five minutes' washing, one half of this, or 5 grams, is removed, then, if the plate is changed to an equal volume of fresh water or kept in the same flowing stream, in other words if the conditions present in the first period are duplicated, the amount removed in this second period will be one half of that which remains or 2.5 grams. The third period will remove 1.25 grams, the fourth 0.75 gram, etc. This law may be expressed mathematically. Thus the quantity of thiosulphate washed out of the film in a period of t minutes from the start is given by dM L,A ^f=k(A- M), where A is the quantity of thiosulphate originally present, k the elimi- nation constant for the film (50 per cent in the above example). Then and therefore Then k = 7 log t A — M 13 Lumiere and Seyewetz, Bull. Soc. franc. Phot., 1910. Warwick, Amer. Phot., 1917, p. J17; Brit. J. Phot., 1917, 64, 261. Elsden, Phot. J., 1917, 57, 90; Brit. J. Phot., 1917, 64, 120. Hickman and Spencer, Phot. J., 1922, 62, 225. FIXING AND WASHING 327 which may be written ^ I j / initial concentration in film / \ concentration at time t The value of k is independent of the initial concentration and may be obtained from i j / Concentration at time h \ h — h \ Concentration at time h J From which the time required to reach any limiting concentration CL is given by : h = I log + in-u So much for mathematical methods which are interesting as they show theoretically to what extent the thiosulphate may be reduced by a given amount of washing. There is some doubt, however, as to their value when applied to practical work. Naturally the above formulas may only be used when the rate of diffusion from the film follows the exponential law and it is by no means established that this is always the case. Hickman found that under certain conditions the rate of elimination followed the exponential law very closely but under other conditions contradictory results were obtained. The Efficiency of Washing Devices. — The most comprehensive study of the efficiency of various types of washing devices which has yet been made is that by Hickman and Spencer. Owing to the errors in estimating very small amounts of thiosulphate by the usual starch- iodide method and also to the fact that such tests do not necessarily represent the concentration of thiosulphate in the film, since this may be higher than that of the wash water, and also to the fact that the thiosulphate remaining in the film may be localized in spots rather than distributed uniformly throughout the film, it was decided to in- vestigate the matter by using a colored dye which diffuses from gela- tine films in the same way as thiosulphate and hence would indicate not only the efficiency of the washing device by the time required for the disappearance of the dye, but would also indicate whether the action was uniform over the whole plate. A dye having the required properties was found in tartrazine, which upon test was found to dif- fuse from the film in the same manner as thiosulphate but much slower, 14 Hickman and Spencer, Ibid. 328 PHOTOGRAPHY alterations in the rate of washing therefore affect plates dyed in tar- trazine in the same way as those containing thiosulphate, but to a different extent. Investigation of several types of washing devices by this means showed that all are more or less inefficient. While the water chang- ing properties of the washer are of importance, agitation of the water is equally important, for the time required for the elimination of the dye was not always in proportion to the water changing properties of the device, but on the other hand varied considerably, under con- ditions otherwise identical, according to the agitation of the wash water. Thus plates placed in an inclined trough in which a constant stream of water was running from the tap washed more rapidly as the slope of the trough was increased. Since the amount of water which flowed over the plates was the same in both cases, it is evident that the velocity of the water over the plate is a factor of considerable im- portance. Tank washers were found to be of varying efficiency ac- cording to the provisions made for the exchange of water, but none were found to equal in efficiency the simple inclined trough. Fig. 154. Windoe's Washing Apparatus A modification of the trough washer described by Windoes in American Photography several years ago is, in the opinion of the writer, one of the most satisfactory devices to be had for the washing FIXING AND WASHING 329 of plates. The principle and construction of the apparatus are made clear in Fig. 154. The plates are placed film side up on each of the shelves and the whole rack placed under the tap. The water flows in a thin, fast moving stream over the surface of the plate and then over the edge of the shelf on to the next plate and so on down to the bot- tom plate. Of course the top plate will be completely washed a little sooner than those beneath it owing to the fact that it receives fresh water directly from the tap while the others receive water partially laden with hypo from the plates above; this is of no serious conse- quence and the last plate will be washed much more quickly than in the conventional tank washer. For the washing of roll film in the strip the writer knows of no means more efficient than the Trox film washer supplied by George Murphy Incorporated, 57 East Ninth Street, New York City. This little device (Fig. 155) sends a thin spray of fresh water down both sides of the film which results in quick and effective elimination of hypo. In cases where the amount of water available is limited, attention may be drawn to the fact that the greatest efficiency will be obtained by dividing the available supply into as many changes as possible with- out making each change too small to cover the plates or film. Ten gallons of water applied in ten changes of one gallon each is much more effective as regards removal of hypo than the same quantity in two changes of five gallons each. The Washing of Prints. — It has always been supposed that under Fig. 155. Trox Washer for Roll Film 330 PHOTOGRAPHY similar conditions prints could be washed free from hypo in the same or even less time than plates owing to the fact that the hypo would be able to diffuse from both sides. This was based upon the assump- tion that the diffusion of hypo from papers followed the same ex- ponential law as found for plates. Hickman and Spencer, however, have shown that such is not the case.15 While the larger part of the hypo is removed from the emulsion in a comparatively short time (as much as 90 per cent being removed by two minutes' washing under certain conditions and with certain papers) a certain amount is tenaciously retained by the fibers of the paper support and this is difficult of removal. For this reason much longer times of washing are required for prints than for plates or films. Prints on thin papers should be washed at least thirty minutes in a running stream of water while the thicker double weight papers should receive from one to one and one half hours' washing. Increasing the velocity of the water or the flow of water over the prints does not decrease the time of washing correspondingly, as in the case of plates. Provided the prints are kept separated the removal of the hypo retained by the paper base appears to be largely a matter of time and not of amount of water or the velocity employed. Unfortunately washing devices for prints are even more unsatis- factory than those supplied for plates. No really efficient and entirely satisfactory apparatus for the washing of batches of prints has yet been devised. The greatest difficulty arises in keeping the prints properly separated in order that the water may have complete access to the surface of each print. To this end it is advisable to avoid overloading of the washer and to separate the prints now and then by hand if necessary. The additional safety afforded in the case of both prints and nega- tive materials by squeezing after washing so as to remove all surplus water is not often realized. The water adhering to the gelatine film always contains some hypo, even if to a very small degree, and the re- moval of the bulk of this water by squeezing removes much hypo. Furthermore, it prevents the formation of drops which concentrate the small amount of hypo left into centers, in which it may be sufficiently great to cause fading. Chemical Tests for Hypo. — To determine the thoroughness of the elimination of hypo from a batch of films or prints, any of the chemical ™Phot. J., 1925, 65, 443- FIXING AND WASHING 331 tests for the presence of hypo may be applied to the drippings from the surface of a negative, or print, collected for that purpose in a test tube. If to this water a few drops of the following permanganate solution are added, the appearance of a greenish-yellow color is an indication of the presence of hypo : Potassium carbonate . 5 gr. 1 gm. Potassium permanganate ^ gr. 0.1 gm; Distilled water to 10 oz. 1000 cc. The change is rather slow, however, when the quantity of hypo is very small and from a practical standpoint the use of iodine starch in the form advised by Clark and Jelley is preferable.16 Two stock solutions are made : A. Iodide 4 oz. 100 gm. Potassium iodide 4 oz. 100 gm. Water to 10 oz. 1000 cc. B. Sodium azide 4 oz. 100 gm. Water to 10 oz. 1000 cc. A starch mixture is also prepared by mixing 90 grains (6 gm.) of soluble starch with cold water and adding this to 7 oz. (200 cc.) of boil- ing water. On cooling, dilute to 25 ounces (800 cc.) and add 60 minims of Solution A (3.25 cc.) and 235 minims of Solution B (13 cc.) diluting finally to a total of 35 ounces (1000 cc). This stock solution keeps well and for use is diluted with one hundred parts of water. If a small quantity of this solution is placed in a test tube and the drippings from a negative or print are collected in the tube, the blue color of the solution will be immediately discharged if hypo is present. Hypo Eliminators. — Many attempts have been made to find a sub- stance which is capable of destroying hypo or converting it to some substance which can be quickly washed out of the gelatine film. Lumiere and Seyewetz in 1902 as the result of an investigation of the efficiency of a number of oxidizing agents as hypo eliminators recom- mended for this purpose ammonium persulphate, potassium percar- bonate and sodium peroxide, which in solution immediately hydrolizes into caustic soda and oxygen. This is, I believe, the first indication of caustic alkalis as hypo eliminators.17 Gaedicke in 1906 again called attention to the fact that ammonium 16 Brit. J. Phot., 1929, 76, 714. 17 Bull. Soc. franc. Phot., 1902, p. 270. 12 1 332 PHOTOGRAPHY thiosulphate is more quickly eliminated from gelatine films than the corresponding sodium compound and suggested that the negative, after one minute's washing under the tap, be transferred to a tray con- taining a 10 per cent solution of ammonium chloride, then washed in four changes of water.18 The danger in this procedure is the very rapid decomposition of the ammonium hyposulphites formed, this being much more rapid than the sodium compound.19 A. Charriou 20 published some notes on the use of sodium bicar- bonate but owing to the experimental methods employed his conclu- sions are unconvincing and there is some question as to the accuracy of the results obtained by him.21 A. E. Amor 22 gives details of some investigations on the action of caustic soda at various concentrations and on ammonium persulphate, hydrogen peroxide and potassium permanganate. A 0.2 per cent solution of caustic soda and potassium persulphate were found to be the two most efficient eliminators but neither represents a decided su- periority over washing in running water provided that this is properly used. The infinitesimal trace of hypo adsorbed by the gelatine and silver image can be displaced a little more quickly by the use of a mildly alkaline bath, shortening the time of washing by five or ten per cent. There is no saving of time, however, since a minute's longer washing would do as much as an eliminator bath of the same duration. Altogether, then, it appears that there is no advantage whatsoever in the use of the so-called " eliminators " of hypo. Plenty of water properly applied is still the secret of thorough hypo elimination. General Reference Works Eder — Ausfiihrliches Handbuch der Photographie, 1905. Mees and Sheppard — Theory of the Photographic Process, 1907. Valenta — Photographische Chemie und Chemikalienkunde, 1922. 18 Phot. Woch., Jan. 30, 1906, p. 41. 19 Lumiere and Seyewetz, Brit. J. Phot., 1908, 55, 417. 20 Compt. rend., 1923, 117, 482. 21 L. P. Clerc, 5. /. P., 1923, 3, 16. 22 Brit. J. Phot., 1925, 72, 18. CHAPTER XV DEFECTS IN NEGATIVES Their Cause and Remedy The Why of Defects. — The troubles of the amateur are due to non- observance of the simple physical and chemical laws upon which the operations of exposure, development and the after processes of fixing and washing are based. Most photographic work is conducted in a haphazard, totally unscientific way. Rule-of-thumb methods, while reasonably certain in the hands of the experienced professional, who works under standardized conditions, are unsuited to the variable con- ditions under which the average amateur works and, when we add to this the carelessness and want of accuracy shown by the average amateur, it is not surprising that he meets with a great variety of troubles. In dealing with photographic products, we are dealing with highly sensitive and complex chemical products which naturally de- mand careful treatment, and we must exercise accuracy and care or we are sure to have trouble. The importance of systematic methods of working cannot be too strongly emphasized. Such methods as have been given in preceding chapters are based upon sound scientific facts and, if carried out to the letter, few will be the chances of error and the usable percentage of work will, consequently, be very high. System, accuracy, careful attention to details, cleanliness, these are the things which every student must observe in practice if he would make a suc- cess of photography. Thin Negatives. — A negative is thin either because it has received insufficient exposure or has been underdeveloped. An under exposed negative lacks density because the action of light has not been sufficient to affect the number of silver grains required to produce a deposit of the proper opacity. Careful examination of an under exposed negative will also reveal an absence of detail in the shadows and further examination that the gradation is also false. Owing to the inaccuracy of the eye, this latter may not be particularly noticeable, but it is nevertheless present and is detrimental to the ex- cellence of the finished product. There is no remedy for an under 333 334 PHOTOGRAPHY exposed negative. The only thing that can be done is to make another negative, if possible, and give at least double the exposure. An underdeveloped negative is thin because development has been stopped before the reducer has had an opportunity to reduce the num- ber of exposed silver grains required to produce the proper density and contrast. We have learned that contrast increases as develop- ment is prolonged, so an under developed negative lacks contrast and should have been developed for a longer time or in a stronger de- veloper. In determining whether thinness is due to under exposure or to under development, the important thing to observe is the shadow detail. If shadow detail is lacking, then under exposure is indicated, while lack of contrast or " snap," with full shadow detail, shows under development. Under exposure cannot be remedied but under develop- ment may be corrected by intensification, which will be treated in the following chapter. Dense Negatives. — Dense negatives may be due either to over ex- posure or over development. In the first case, the negative is flat, ap- pears fogged and perhaps unsharp, while the shadows possess too much detail. The highlights print gray, the half tones only a shade darker and the shadows also print gray, the proper gradation from light to dark is lacking, and the effect is flat and weak. Such a negative may be somewhat improved by reduction in a subtractive reducer of the Farmer type, as described in the following chapter. An over developed negative is very contrasty. The highlights, or dense portions, are so opaque that it is impossible to print through them and in attempting to do this, the shadows are considerably over exposed and become too dark. The remedy is to reduce in a super- proportional reducer. Fog on Negatives. — Fog may be defined as a uniform deposit of silver extending over and either partially or wholly obliterating the image. It may be either general or local and due to the accidental ad- mission of light during the operations previous to development, to the use of an unsafe light in development, or from developers contaminated with foreign substances or used at an excessively high temperature or containing an excess of alkali. In fact it may be produced from any number of causes and for this reason its source is quite often difficult to ascertain. It may simplify discussion of the subject if we differ- entiate between local fog, general fog produced by light and chemical fog. Local Fpg. — Local fog is frequently due to faulty plate holders, DEFECTS IN NEGATIVES 335 the wood having split or, the joints become loose in some places so that light is admitted. Examination of the plate holders and a comparison of the location of the fog area by means of a developed negative placed in the holder in the position occupied during exposure will serve to show if this is the trouble. Fog sometimes results from the use of a plate holder not made for the particular camera on which it is being used. While the majority of modern cameras take what are termed standard holders there are some few cameras made for differently designed holders and when one of these is used with a camera for which it is not designed the fit is imperfect and light leaks in producing local fog. It sometimes happens, particularly with view cameras, that after long use the reversible back will become worn or warped so that it does not fit tightly to the body of the camera and light is able to reach the plate. The same case occurs sometimes when a new reversing back is purchased to replace an old one. A frequent cause of local fog with beginners is due to improper in- sertion of the slide of the plate holder. Beginners have the habit of inserting the slide by the corner but this should never be done as it allows the light to pass through the trap around the edges of the slide and produce fog. The slide should be inserted squarely so that the entire opening is closed at once. It is well as a matter of precaution to keep the camera covered well with the focussing cloth when with- drawing or inserting the slide in order to prevent the accidental ad- mission of light. Fog may sometimes be produced by chemical emanations from the wood or varnish of the plate holders. This is more likely to occur with new plate holders or when the plates are left in the holders for several weeks. If this is thought to be the source of the trouble, the plate holders should be exposed, with the slides withdrawn, to strong sun- shine for several days or the woodwork painted with a solution of potassium permanganate. Fog is often produced in a similar way with metal plate holders as supplied with many foreign hand cameras. The best remedy is to paint the inside of the holder with a weak solution of bichloride of platinum. Exposure to light and air as formerly advised is also effec- tive in many cases. General Fog Due to Light. — General fog all over the plate may be due either to light or to chemical fog. If the edges of the plate which are protected by the rabbet are perfectly clear, fog is due to an opening 336 PHOTOGRAPHY in the bellows or camera front or to the bellows having worn smooth and shiny from constant use so that it reflects light about within the camera. Light leakage by the camera front or bellows may be deter- mined by taking the camera into a dark room and placing a lighted electric bulb in it, using the focussing cloth to make a light-tight en- trance for the cable. By inserting the lamp first in one end of the camera and then the other and examining the same from all angles the leaking of light through any minute crevice may be immediately de- tected. If the bellows has worn smooth and shiny on the inside so that it is suspected of producing fog by reflecting light about in the camera it is best replaced by a new bellows. If any wooden or metal parts of the camera have worn shiny and are suspected of causing trouble in the same way they should receive a coat of dead black paint as supplied by dealers. If the edge of the plate or film which was protected by the rabbet is also fogged, the plate was either loaded or developed in an unsafe light unless the fog was produced by chemical reactions. A plate is very much more sensitive to light when dry than after it has once been covered by the developing solution and for this reason it is very im- portant that the safelight used for loading plate holders be adapted to the plate and that the operation of loading be conducted as quickly and as far away from the safelight as possible. It is far better, and not at all impossible after some experience, to load plates in perfect dark- ness and thereby avoid all danger of light fog at this stage. Extreme precautions should also be taken during development to protect the plate from the rays of the safelight except when absolutely necessary for examination. Once it is seen that the plate has been evenly covered by the developer, there is no need to examine the same for several minutes, or until development is judged to be nearly com- pleted, when the plate may be removed and given a momentary inspec- tion before the light. Prolonged examination of the plate before the safelight is responsible for many fogged negatives as well as stains of various kinds. When desensitizers are used, greater liberty may be taken with re- gard to light during development but the safety of the safelight used for loading and for immersing the plates in the desensitizer must be unquestionable, just as when development is conducted under ordinary conditions. Chemical Fog. — A certain amount of chemical fog is inevitable but with proper working conditions the amount may be reduced to a point DEFECTS IN NEGATIVES 337 where it is of no importance except in sensitometry. The amount of chemical fog is determined by a number of factors : namely, the nature of the emulsion, its age and the conditions under which it has been stored, the nature of the developer, impurities in the developing solu- tion and the time and temperature of development. Ultra sensitive emulsions are more likely to fog chemically than those of lesser sensitiveness owing to their highly sensitive character and to the small amount of energy required to make the silver halide grain developable. Low speed emulsions of the type represented by positive emulsions and process plates are usually practically free from fog unless developed under unsuitable conditions. The age of the plate and the conditions under which the plates have been stored play an important part. Sensitive material should never be stored where it is exposed to heat, chemicals, dampness, or in a room where gas is burned. While the common developing agents differ slightly in their fogging propensities, none produce sufficient fog to be of serious importance in practical work except where improperly used. In the presence of an excess of alkali, or when used in a very concentrated solution, such active agents as metol do tend to develop fog owing to the reduction of unexposed silver halide. The correct proportion of alkali to be used under given conditions can only be determined by experiment. Sulphite, whether in the form of sodium sulphite, bisulphite or potassium metabisulphite, is added to prevent the oxidation of the de- veloping solution by air, the oxidized solution tending to produce fog. If an excess of sulphite is added a particular kind of fog, known as sulphite fog, is produced. The nature of sulphite fog was carefully investigated by Mees and Piper in 191 1 1 and found to be due to the reduction to metallic silver of the silver salt dissolved in the emulsion by the sulphite of the developing solution.2 The oxidation of certain developing agents such as metol and hy- drochinon exerts a powerful fogging action. The brown oxidized product of pyro, however, has little fogging tendency unless in very strong solution. In general, however, it may be said that the oxidized products of all developing agents tend to produce fog when sufficiently concentrated. Oxidized samples of the developing agent used in compounding solu- tions are responsible for fog in many cases. Stale sodium sulphite 1Phot. J., 1911, 51, 226; 1912, 52, 221. 2 For another form of sulphite fog see Zeit. wiss. Phot., 1913, 289. i 338 PHOTOGRAPHY containing sodium sulphate causes fog indirectly by not preventing the oxidation of the developing solution. Intense fog is produced by such metallic substances as copper, tin, and the metallic sulphites even when present in a very small quantity. Less than .01 per cent of copper sulphate added to a metol-hydrochinon developer will produce strong fog even on positive, low-speed emul- sions. The action is probably due to an acceleration of the rate of oxidation. The activity of the developing solution becomes greater with an in- crease in temperature and under suitable conditions this may be suffi- cient to enable it to attack the unexposed silver halide and produce fog. For this reason a temperature above 70° F. (21 0 C.) is espe- cially conducive to fog. As the amount of fog grows incessantly with the time of develop- ment, it is unwise to attempt to force development in order to secure greater contrast or shadow detail as by so doing chemical fog is de- veloped and this being stronger in the shadows than in the higher densities reduces rather than increases the contrast of the negative. The amount of chemical fog may be reduced by the addition of a soluble bromide to the developing solution but it is far better to deter- mine if possible the source of the trouble and take appropriate steps to remedy the same. The use of fresh, properly compounded solu- tions containing the proportions of developing agent, sulphite and alkali advised by the manufacturer of the brand of plates employed, and keeping the developer free from foreign matter and at a normal tem- perature of 60-65° F- wul reduce the danger of chemical fog to the minimum. Dichroic Fog. — Fog which appears green by reflected light and red by transmitted light is termed dichroic (two-colored) fog. It was formerly quite common, but is now comparatively rare due to improve- ments in the manufacture of ultra sensitive emulsions and to the use of the carbonates of sodium or potassium in place of ammonia as an alkali. When examined under the microscope the fog is seen to consist of microscopic particles of metallic silver. The size of the particles de- termines their color by transmitted light, a fog which is red in color consisting of very small particles. Dichroic fog may be formed either in the developer or the fixing bath. Its formation is aided by the presence of any solvent of silver in the developer or the fixing bath. Frequent sources in development — DEFECTS IN NEGATIVES 339 are the presence of such silver solvents as ammonia, or hypo, and pro- longed development. Nowadays it most frequently occurs as the result of using an old or exhausted fixing bath containing an excess of dissolved silver and oxidized developer and a deficiency of acid. The presence of sulphur due to the use of too much acid so that the thio- sulphate is decomposed and sulphur liberated is also a frequent cause.3 Three methods are recommended by Lumiere and Seyewetz as being suitable for removal of dichloric fog. They consider the method which follows the most generally useful.4 Immerse the plate in a solution of potassium permanganate grain to each ounce of water) until the fog is dissolved, then rinse and place in a 5 per cent solution of sodium bisulphite or potassium metabisulphite to remove the brown oxide of manganese which is deposited. Developer Stains. — While in reality a stain may be considered to be any deposit foreign to the photographic image which absorbs light, in general the word stain is usually associated with something colored. A stain may therefore be considered as a deposit on a negative or positive whose color is foreign to that of the photographic image. This definition thus includes colored spots, irregular colored markings and general stain. Practically all developing agents have the property of combining readily with oxygen especially in alkaline solution to form colored oxidation products the staining properties of which are very similar to aniline dyes. Furthermore the process of development is in itself an oxidizing process and this results in an additional amount of oxida- tion Which is of course proportional to the amount of silver reduced in the various portions of the image. Consequently the developed image consists of an image of stain superimposed over one of metallic silver. The amount of the stain is greater with some developers than with others and is considerably influenced by the proportion of the sulphite present in the developing solution. Pyro is a developer which is especially prone to give a strong stain image but there is really no excuse for the habitual production of badly stained negatives when pyro is used as the degree of staining is completely under the control of the worker. By increasing the proportion of sulphite to pyro the color may be decreased almost to a neutral black while de- 3 For a complete discussion of the causes of dichroic fog reference should be made to a paper by Lumiere and Seyewetz, Bull. Soc. franc. Phot., 1903, pp. 501, 529 ; Phot. J., 1903, 43, 223. * Bull. Soc. franc. Phot., 1903, p. 324; Phot. J., 1903, 43, 226. 340 PHOTOGRAPHY creasing the amount of the sulphite will increase the color. Provided the stain is not over intense such staining may not be objectional and may at times be actually an advantage as it increases considerably the printing contrast of the image. Most developing agents form a stain image, though with developers like glycin, the oxidation product of which is readily oxidized by sulphite, the stain image is very feeble and practically negligible. Besides the stain image just considered we may have a general yellow, or other colored stain, which is in effect just the same as if the negative had been immersed in a dye solution. When uniform it has no harmful effect other than to increase the time of exposure in printing. It is produced by the use of an old and discolored de- veloping solution, an insufficient amount of sulphite or the use of impure sulphite and in many cases by the use of an old fixing bath which has lost its acid reaction. Since a developer is oxidized far more rapidly in alkaline or neutral solutions than when in an acid state, as the fixing bath becomes neutralized by the alkaline developer carried over on the surface of the negatives, this developer oxidizes more and more readily so that the fixing bath is converted into prac- tically a weak dye solution and stains the negatives immersed therein. It is very important that the fixing bath be kept fresh and acid in order to prevent stain from this source especially when a developer which tends to stain readily is used. The use of a stop bath of a weak acid between development and fixing is also advantageous. To remove developer stain proceed as follows : First harden the film by immersion for 2 to 3 minutes in a 5 per cent solution of formaline and wash for 5 minutes to prevent the gelatine from swelling and frilling in the subsequent treatment. Then bleach in the following : A. Potassium permanganate 5 gm. 75 Hg2Cl2 + 2AgCl. The resulting chlorides of mercury and silver are transparent and blackening is necessary to secure printing density. With sodium sul- phite the reaction is as follows : Hg2Cl2 + Na2SOa + H20 -» 2Hg + Na2S04 + 2HCI. Blackening in an alkaline developer reduces the deposit to a silver mercury compound whose composition is not definitely known and which probably varies with the developer. On blackening with ammonia the probable reaction is as follows : Hg2Cl2 + 2NH3 -» NH2Hg2Cl + NH4C1. When an image bleached with mercuric chloride is acted on by fer- rous oxalate, the image that remains consists of an amalgam of silver AgHg. If the process be repeated each atom takes up another atom of mercury and we get AgHg3 and consequently greater intensifica- tion. The reaction would therefore be as follows : Hg2Cl2 + 2AgCl + 4FeC204 + 2K2C204 = 2 Ag + 2Hg + 2Fe2 ( C204 ) 3 + 4KCI .» Of the several methods of blackening the last is without doubt the most satisfactory. It gives proportionate intensification, a black de- posit which is permanent, and may be repeated to gain any desired degree of intensification. Sodium sulphite reduces the lower densi- ties, producing . what workers call a clean result, which however is secured at the expense of proportional action and purity of gradation. There is question concerning its permanency. The objection to the use of developers containing sulphite is that already stated as an ob- jection to the use of sulphite alone but there is a further objection to the use of the alkali which can by itself effect a partial conversion of the silver mercurous chloride into the carbonates or oxides. This possibility of two distinct reactions at one and the same time is an 13 Chapman Jones, /. 6". C. I., 1893 vol. XII, p. 983. REDUCTION AND INTENSIFICATION 355 important disadvantage which tends to render the action unpropor- tionate and also impermanent. Sodium sulphantimoniate gives ap- proximate proportional intensification and with the exception of fer- rous oxalate is the most satisfactory of the lot. With ammonia the blackening is not uniform and the reducing action in the shadows is very marked, the original gradation being altered to a considerable degree. The degree of intensification and action of the various black- eners on the tones of the subject will be treated at the end of the chapter under the Sensitometry of Intensification. Monckhoven's Intensifier. — The negative is bleached in mercuric bromide as above and blackened in the following solution of potassium cyanide and silver : Pure potassium cyanide 10 gr. 23 gm. Nitrate of silver 10 gr. 23 gm. Water 1 oz. 1000 cc. The silver and cyanide are dissolved in separate lots of water, and the former solution added to the latter until a permanent precipitate is formed. Then allow the solution to stand fifteen minutes and filter after which it may be used. If the intensification is carried too far the plate may be reduced in " hypo." The reaction according to Valenta is as follows : Hg2Cl2 + 2AgK(CN)2 ^ 2Ag + 2Hg(CN)2 + 2KCI. Mercuric Iodide Intensifier. — Traces of hypo remaining in the film cause stains and spots with any of the above intensifiers and it is necessary that the greatest care be taken to thoroughly wash negatives before intensifying. It is a peculiar characteristic of mercuric iodide, and often a very valuable one, that its action is not affected by any traces of hypo which may remain in the film and the negative may be removed from the fixing bath, washed for a few minutes in water, and intensified at once. A. Mercuric chloride ' 175 gr. 40 gm. Water 10 oz. 1000 cc. B. Potassium iodide 1 oz. 100 gm. Water 10 oz. 1000 cc. Add the larger part of the iodide to the mercury, stirring well. Then add the remainder of the iodide in small quantities until the so- lution clears. The solution changes the negative to a brown color which further changes to orange upon washing in water. Redevelop- 356 PHOTOGRAPHY ment in a non-staining developer such as amidol or metol-hydrochinon will render the negative less liable to yellow in time. The chemistry of the reaction is as follows : 14 2HgI2 + 2Ag = Hg2I2 + 2AgI, • HgJ2 + 2(Na2S03) = Hg + Hgl2 ■ (Na2SOa)2. Silver Intensifies. — The following formula and method for silver intensification is that of J. B. B. Wellington and is the revised formula published in 191 1. The film should first be hardened in the following bath : Formaline 1 part Water 10 parts In this bath the negative should be allowed to remain for five min- utes, after which it should be rinsed for a few minutes and then placed for exactly one minute in the following bath : Potassium ferricyanide 20 gr. 2.3 gm. Potassium bromide 20 gr. 2.3 gm. Water to make 20 oz. 1 liter This bath, which should never be omitted, has the effect of pre- venting stains during the process of intensification. Too long an im- mersion in this bath causes the image to bleach, which should be avoided if it is desired to retain the original gradation. In the time prescribed there is no apparent action, but the clearing agent has done its work. The negative should now be rinsed for a few minutes and intensified in the following: Stock Solutions A. Silver nitrate 800 gr. 83.4 gm. Distilled water to make 20 oz. 1 liter B. Ammonium sulphocyanide 1400 gr. 146 gm. Hypo 1400 gr. 146 gm. Water to make.... 20 oz. I liter (Both solutions keep well.) For use take an ounce of A to one half ounce of B, stirring vigor- ously all the while the two are mixed. If stirring is omitted the solu- tion is apt to be turbid, whereas it should be clear. To this is added 1 dram of a ten per cent solution of pyro solution, preserved with sul- phite of soda, and two drams of 10 per cent solution of ammonia. 14 Seyewetz, Le Negatif en Photographic REDUCTION AND INTENSIFICATION 357 Place negative in absolutely clean tray and pour solution over it. The silver begins to deposit within a minute or so and when sufficiently in- tensified the plate should be removed, placed in an acid fixing bath for a short while, and then well washed. Silver intensification is really physical development, silver being deposited upon the original^ deposit. The action is proportional and the results permanent and a negative intensified with silver may be reduced in any manner. Intensification with Chromium. — This process is largely due to G. Welborne Piper and D. J. Carnegie.15 The negative is bleached in a solution of potassium bichromate and hydrochloric acid and the bleached negative blackened in ordinary developer. The bleached image contains a chromium compound the precise formula of which is unknown but is thought to be Cr02. When this is treated with a de- veloper it is reduced! and part of the chromium is left in the image in combination with the metallic silver. While perhaps not absolutely proportional in its action and thus to a certain extent falsifying grada- tion, the same is very slight, and as the process is easily worked and may be repeated over and over so that any degree of intensification ordinarily desirable may be had, the chromium intensifier is of great practical value. The degree of intensification is controlled to a certain extent by the amount of acid present and it is possible to vary the degree of intensification by altering the amount of acid, the more acid used the less the intensification secured, but on the whole it is more de- sirable to use one of the three formulas given and if the result is riot what is desired after the first application repeat the process. The in- tensifier may be kept in the following stock solution from which either of the three bleaching baths may be compounded according to the degree of intensification desired : A. Potassium bichromate.. B. Hydrochloric acid CP. Baths ready for use. A B C Degree of intensification 4 oz. 8 oz. 8 oz. A — Maximum 3 dr. 2 OZ. 8 oz. B— Medium 16 oz. 10 OZ. 4 oz. C — Minimum Bleach in A, B, or C, wash until yellow stain is removed and then redevelop in a non-staining developer. Amidol is to be preferred, 15 Amat. Phot., 1904, pp. 336 and 397; 1905, pp. 453 and 473- 358 PHOTOGRAPHY especially if by any chance it is likely that the process need be repeated, as the change from acid to alkali is particularly hard on gelatine and by the use of amidol this trouble is minimized, since amidol does not re- quire an alkali and any tendency of the gelatine to soften and frill is always increased in the presence of an alkali. Intensification with Uranium. — If the silver image is treated with a ferricyanide it is reduced to a ferrocyanide, the probable reaction in the case of uranium ferricyanide being: 8Ag + 4(U02):i[Fe(CN6) }a -» 2Ag4Fe(CN),i + 3(U02)JFe(CN6)]3. The silver image is therefore converted into a mixture of silver ferro- cyanide and uranyl ferrocyanide, the dark-brown or reddish color of which being non-actinic considerably increases tlfoe.. density and con- trast of the negative. il Uranium is a great builder of detail and contrast and is perhaps the most suitable intensifier for getting the most out of an under exposed negative — the red deposit being able to build up to printing density all the detail which the exposure has been able to impress on the sensitive material. The following is a suitable formula : A. Uranium nitrate 120 gr. 25 gm. Water to make 10 oz. 1000 cc. B. Potassium ferricyanide 120 gr. 25 gm. Water to make, 10 oz. 1000 cc. For use take: A — 10 parts; B — 10 parts; acetic acid — 2.5 parts. The negative must be perfectly free from hypo or stains will re- sult which cannot be easily removed. When intensification is judged to be complete the negative should be removed and washed well in pure water. Hard or alkaline water cannot be used for this purpose for, as pointed out by Sedlaczek,16 the uranyl ferrocyanide is soluble in alkalis. Should the yellow stain remain after several changes of water its removal may be effected by means of a 10 per cent solution of ammonium sulphocyanide or with Potassium citrate 5 gm. 38 gr. Sodium sulphate 25 gm. 192 gr. Water to make 1000 cc. 16 oz. If for any reason it should be desirable to remove the intensification 16 Phot. Ind., 1924, p. 234. REDUCTION AND INTENSIFICATION 359 altogether this may be accomplished by immersing the negative in a weak solution of ammonia or of sodium carbonate. If the negative is to be again intensified this bath should be followed by a weak bath of acetic acid to neutralize any traces of alkali which might remain in the film. Intensification with Lead. — Extreme intensification is secured with lead. Practically the only case in which such extreme contrast is re- quired in ordinary practice is with line subjects from poor originals. The general outline of the chemical reaction is the same as with uranium : 2KeFe2(CN)12 + 4Ag + 6Pb(NOs)2 = Ag4Fe(CN)6 + 3Pb2Fe(CN)6+ 12KNO3. The following formula is recommended : Lead nitrate 400 gr. 41.6 gm. Potassium ferricyanide 600 gr. 62 gm. Acetic acid 3 dr. 20 cc. Water to make 20 oz. 1000 cc. The stock solution will keep well in the dark. Bleach the negative in the above and then wash carefully in 10 per cent nitric acid — the acid makes the film tender — then in water and then darken in Sodium sulphide. 1 oz. 50 gm. Water to make 20 oz. 1000 cc. Intensification with Copper.— The copper intensifier is also only suited to line subjects. The reaction is as follows : CuS04 + 2KBr -3« CuBr2 + K2S04, CuBr2 + Ag -> CuBr + AgBr. Applying AgN03, CuBr + 2AgNOs -> Cu(N03)2 + Ag2Br The following is a reliable formula : A. Copper sulphate 100 gr. Water to make 1 oz. B. Potassium bromide 100 gr. Water to make 1 oz. A and B are both dissolved in hot water. For use they are mixed 208 gm. 1000 cc. 208 gm. 1000 cc. 360 PHOTOGRAPHY and the negative bleached therein after which it is washed for a minute or two and blackened in If too dense the negative may be reduced by the application of a weak solution of hypo (10 grains to the ounce) or potassium cyanide 2 grains to the ounce. Intensification by Sulphiding. — A very convenient method of secur- ing a limited amount of intensification consists in ordinary sulphide toning of the image. The negative is first bleached in a bath of potas- sium ferricyanide and potassium bromide, then washed well and finally darkened in sodium sulphide. The metallic silver is thus changed to silver sulphide, the brown color of which is less actinic than the origi- nal black. Thus while the negative may actually appear less dense after sulphiding, its printing density has been increased by the process. The operation differs in no way from the toning of gaslight and bro- mide prints by the indirect, or redevelopment process. The image is permanent. The Sensitometry of Intensification. — Until quite recently no quantitative measurements of the character and degree of intensifica- tion secured with different agents had been made. This matter was first investigated by H. W. Bennett in 1903, by L. P. Clerc in 1912,17 and more fully by Nietz and Huse.18 It is not our business here to go into the experimental methods, or consideration in full of the factors involved, for which the original paper should be consulted, but to note more particularly the character of the intensification secured by representative intensifiers and their relative efficiency. In the first place it will be necessary for us to notice the difference between visual and photographic intensification, as the two are not the same and we may have one without the other. If the deposit of the original negative is neutral and the intensified deposit also neutral, then any increase in visual density will be a direct measure of the photo- graphic effect. In most cases, however, these conditions are not ful- filled. Some intensifiers depend entirely upon the change of the silver 17 Bennett, Phot. J., 1903, 43, 74. Clerc, Brit. J. Phot., 1912, 59, 215. 18 Communication No. 58 from the Research Laboratory, Eastman Kodak Company, The Photographic Journal, 1918, 58, 81 ; Journ. Frank. Inst., March 1918. Silver nitrate. . . . Water (distilled) 45 gr. 1 oz. 100 gm. 1000 cc. REDUCTION AND INTENSIFICATION 361 to some material having " a more non-actinic color," as for instance uranium and the sulphide method. The authors distinguish between three general classes of intensifiers : 1. Those giving both visual and photographic intensification, as ura- nium. A second class of the same giving neutral deposits, as mercuric bromide with amidol and chromium followed by amidol provided the deposit of the negative is neutral. The most generally useful class of intensifiers. 2. Visual reduction but photographic intensification. Example. Re- development with sodium sulphide where the visual density is less, but the non-actinic color gives photographic intensification. 3. Visual intensification with photographic reduction obtains only when intensifiers having a bleaching effect are used on negatives of high color. Example, Chromium-amidol on a badly stained pyro negative. A special case only. In Fig. 158 the percentage increase in density is plotted as the 300 g-200 L. 1 & i 100- A .6 X// XVI 1.0 1.2 "0 .2 Original Density Fig. 158. Sensitometry of Photographic Intensification. (Nietz and Huse) I. Mercuric chloride + ammonia. VI. Chromium + amidol. VII. Mercuric bromide + amidol. X. Mercuric iodide + paramidophenol. XII. Uranium. XVI. Mercuric iodide + Schlippe's Salt. XVIII. Cupric chloride + sodium stannite. ordinates against the original densities as abscissae. A line parallel with the base would thus indicate proportional intensification. No in- 362 PHOTOGRAPHY tensifier reaches absolute perfection in this respect although several approach it very closely. By plotting the densities of the intensified and original plates against log E in the usual manner employed in sensitometry we get two char- acteristic curves the difference of whose gammas is a measure of the increase in contrast. Thus P^otoSraP^c gamma 01 intensified plate y ip photographic gamma of original plate y op' gives the degree of intensification. The data for a few representative intensifiers is given in the following table taken from the paper of Huse and Nietz : y it Intensifier Blackener y op LIS Potassium bichromate and HQ . . . .amidol 1-45 LIS Potassium ferricyanide and potassium bromide. . . . . . .sodium sulphide L33 1-93 2.05 2.50 The careful study of this and the preceding table will give the stu- dent much valuable information regarding the characteristics of the different intensifiers and their suitability for employment in particular cases. Local Reduction and Intensification. — Local reduction or intensi- fication is of great assistance at times in bringing out certain details in the shadows or in reducing the density of an over-dense highlight. If the negative to be reduced or intensified has been allowed to dry it should be first soaked for fifteen to twenty minutes in water, while if the negative has been handled it may be well to add to the water a small amount of sodium carbonate to remove any grease present on the film. It must be remembered that many intensifiers and some reducers (the latter, however, to a minor extent) alter not only the density but also the color of the deposit and this makes it hard to judge accurately the actual amount of reduction or intensification secured. Preference should therefore be given to intensifiers which do not produce a colored image such as chromium or mercury and ferrous oxalate. For reduc- tion, the iodine-cyanide reducer is well adapted but potassium perman- ganate or Farmer's ferricyanide-hypo reducer may be used. The negative to be reduced is placed in a horizontal position on a REDUCTION AND INTENSIFICATION 363 sheet of glass where it will be well illuminated by transmitted light. A convenient reducing bench described by a writer in the British Journal of Photography is illustrated in Fig. 159. The solution should then be applied to the desired portions with a soft brush or with a wad of absorbent cotton. Use only a weak solution, otherwise the action may be so rapid as to get beyond control while should any of the strong solution be accidentally carried over on undesired areas, it will be impossible to prevent them from being reduced. Local intensification may be carried out very simply by the use of colored dyes. These may be applied in a very dilute state to the de- sired portion and allowed to dry. If too strong the negative may be washed in water to weaken the dye. Suitable dyes for this purpose are erythrosine and the Agfa preparation known as Coccine Nouvelle. Fig. 159. Bench for Local Reduction. (British Journal of Photography) Namias has recommended that the negative be immersed in a 1/1000 solution of potassium permanganate for a few moments and the yellow stain removed from the portions which it is desired to darken by paint- ing over such portions with a solution of bisulphite of soda. Some workers find it advantageous to apply to the parts of the negative not to be acted upon by the reducing or intensifying solutions a water-resisting mixture which protects such portions from the action of the solution. The negative can then be immersed bodily in the solution. A varnish suitable for this purpose may be made by adding to benzol or chloroform a very small quantity of masticated rubber or pure white wax (not pararfine wax). General Reference Works Bennett — Intensification and Reduction. 13 CHAPTER XVII PRINTING PROCESSES WITH SILVER SALTS I. Printing on Bromide and Gaslight Paper Characteristics of Printing Papers. — It is the primary object of the photographer to reproduce in the print the gradations of light and shade observed in the subject photographed. The negative is an in- termediate step; perfect reproduction of the brightnesses of the sub- ject on the negative is of no importance unless the printing process is capable of producing from this negative a print which corresponds to the visual impression of the subject photographed. Accurate repro- duction of the gradations of the subject involve, therefore, (i) the production of a negative in which the opacities are inversely propor- tional to those portions of the subject which they represent, and (2) the production from this negative of a print in which the light reflected from the various parts of the image reproduce the visual impression of the subject photographed. The possibility of accurately reproducing the tones of any subject by photography depends, therefore, not only on the accuracy with which we are able to translate subject bright- nesses into opacity in the production of the negative, but also on the extent to which we are able to make from this negative a positive print in which these opacities are correctly rendered as shades of black re- flecting amounts of light proportionate to the corresponding portions of the subject. There is no difficulty in securing correct reproduction in the nega- tive, for, as shown by Hurter and Driffield, this is possible if the straight-line portion of the curve of the sensitive material is used, and with negative materials the straight-line portion is sufficiently long to take care of the range of light intensities in all ordinary subjects. Printing papers show the same type of curve as do negative mar terials and in both cases the length of the straight-line portion deter- mines the range of light intensities over which correct reproduction is secured. Now assuming that the gradations of light and shade in the original subject are correctly represented by the opacities of the nega- tive, perfect reproduction of the original subject will be obtained if we 364 PRINTING PROCESSES WITH SILVER SALTS 365 use the straight line portion of the curve of the printing paper in mak- ing the print. The length of the straight-line portion of the curve of a printing paper, however, is much shorter than that of negative materials. The greatest range of light intensities possible with a printing paper is the ratio between the amount of light reflected from the white paper and that reflected from the deepest black which the paper is capable of pro- ducing. The maximum black which a paper is capable of producing varies with the emulsion and with the surface of the paper. With a matt paper the difference in the amount of light reflected from the white paper and the maximum black of the paper may be in the ratio of I: IS; with glossy papers the ratio is somewhat greater, varying with different papers up to about i : 50. Since few subjects have a greater range than this, it follows that it is just possible, by using the entire scale of the paper, to include the scale of brightnesses in the subject photographed. The limits of correct rendering, however, are fixed by the range of brightnesses included in the straight-line portion of the characteristic curve of the printing material. The range of the straight-line portion of the curve, however, is considerably less than the total scale of the paper. Consequently, the total range of bright- nesses in the average subject cannot be included in the straight-line portion of the curve; the total scale of the paper must be used at the expense of correct reproduction at both ends of the scale. Aside from differences in total scale, printing papers differ in the slope of the curve, or what is designated as gamma, and in exposure range. With papers, the maximum contrast, or gamma infinity, of the paper is reached at a very early stage of development so that prints are always developed to gamma infinity and the contrast of the print is not varied by development, as in the case of negative materials. The dif- ference in the time of exposure necessary to produce the faintest visible impression and that required to produce the maximum black which the paper will yield constitute what is known as the exposure-scale of the paper. The" exposure scale varies with different papers, ranging from 1 : 5 in the case of a vigorous chloro-bromide paper for amateur print- ing to 1 : 60 or more for carbon or platinum papers. In other words, with a paper such as the first mentioned, an increase in exposure five times that required to produce the lightest visible grey will result in the deepest black the paper is capable of producing, while in the case of a bromide paper 40 to 50 times, and with platinum or carbon, from 366 PHOTOGRAPHY 60 to 80 times as much exposure will be required to produce the full range of the paper from white to black. Papers having a short exposure scale are generally spoken of as contrast papers and those of long scale as soft papers; actually the contrast of a printing paper is not determined exclusively by its ex- posure scale but by its maximum black and gamma infinity as well. In printing, the range of light intensities transmitted by the opacities of the negative cannot be greater than the exposure scale of the paper or the gradations in either the highlights of shadows will be lost. Thus if the light transmitted through the various opacities of the negative range from 1 : 50 then to reproduce all of these differences in the print will require a paper having an exposure scale of 1 : 50; if a paper hav- ing an exposure scale of 1 : 25 is used only one-half of the scale of the negative will be reproduced in the print and the differences in either the deeper shadows or brighter highlights will be lost. In practice, therefore, we use for printing a paper whose exposure scale is ap- proximately equal to the range of the opacities of the negative from which the print is to be made. The range of opacities in the negative is under the control of the photographer as it is dependent upon the degree of contrast, or gamma, to which the negative is developed. Consequently, two courses are open : ( 1 ) the negative can be made to fit the exposure scale of a given paper by development to the proper stage, or (2) the negative may be developed to a given contrast and the print made on a paper having an exposure scale suitable for the negative. In practice, both methods are followed although the wide- spread use of developing papers, the exposure scale of which varies widely in different grades, has caused the latter method to be the more widely adopted. Adapting the Paper to the Negative. — In Fig. 160, photograph a and its accompanying graph illustrate the condition resulting from the use of a long scale paper on a negative with a shorter scale of tones. It will be observed that since the available scale of the printing medium is so much greater than the negative, the use of such paper restricts us to a scale ranging from a white to a grey. Thus, if the densest high- light of the negative is rendered as white, the deepest shadows are grey, rather than black, while if the exposure is adjusted so as to render the shadows black, the highlights of the print are degraded. In either case, the result is smudgy, smoky, with a washed-out appear- ance lacking in contrast and vigor. In the same figure, b and its accompanying graph represent the PRINTING PROCESSES WITH SILVER SALTS 367 368 PHOTOGRAPHY 370 PHOTOGRAPHY condition resulting from the use of a paper having approximately the same scale of tones as the negative. In this case, we have the densest highlight of the negative reproduced as white in the print and the deepest shadow as black, together with a full scale of intermediate tones, the whole resulting in a print with a pleasing gradation from light to dark which impresses us as being natural and proper. If, however, on a negative with a long scale we make use of a short scale paper we have the result represented in Fig. 160c. In this case, we must loose some of the tones, for the limited range of tones available with the particular printing paper is insufficient for the long scale which the negative possesses. Hence we must reproduce the middle tones and shadows correctly and sacrifice the highlights, or we must expose long enough to render the highlights properly and sacri- fice all detail and tone in the shadows. We see therefore that for faithful reproduction of the tones of the subject the scale of the printing medium must approximate to that of the negative and allow us to make the most of the full scale of tones available in a paper print. Now, the three negatives from which these prints were made represent respectively short, normal and long-scaled negatives produced by short, normal and prolonged de- velopment. They are, in other words, what would be termed " flat," " normal " and " hard " or contrasty negatives. Now for a we have used what is termed in ordinary parlance a soft paper. The result is, as can readily be seen, a lack of vigor owing to the fact that the range of tones in the paper is greater than that of the negative. Conse- quently we must employ a paper having a shorter scale, or one termed " medium," " normal," or even " contrast " or " hard." In c we have a negative with a long scale, or what would be termed a " contrasty " negative, which we have printed on a short scale paper, or in every- day language, a " contrast " paper, the result being excessive contrast together with the loss of proper gradations in the highlights or in the shadows. We must therefore employ a paper with a longer scale of tones in order to make use of the full range of tones in the negative. The golden rule for selecting the proper grade of paper is, there- fore : Observe closely the degree of contrast in the negative. If the contrasts are correct, use Medium or Normal paper. If the contrasts are excessive, use Soft paper, while if the negative is lacking in con- trast, use a Hard, Contrast or Vigorous paper. Developing Papers. — Developing papers for positive printing may PRINTING PROCESSES WITH SILVER SALTS 371 be broadly divided into two classes ; bromide and chloro-bromide. The former is a fast emulsion consisting essentially of silver bromide and corresponds very closely to that of negative emulsions except as re- gards speed; the latter is a much slower emulsion in which silver chloride is the predominant halide. Aside from the use of silver chloride as one of the sensitive halides, the essential difference in nega- tive and positive emulsions for development is the lower sensitivity and finer grain of the latter, arising from the fact that in the prepara- tion of emulsions for positive printing, digestion plays a relatively un- important part and from the much lower concentration of the emulsion. Exposure. — While daylight may be used for exposure, artificial light is preferable, owing to its greater uniformity and also to the fact that daylight is much too rapid for the best results, except where very dense negatives are involved. Practically any artificial light is usually suitable but electricity or gas are naturally more rapid and convenient in use than any of the others. Nevertheless, the common oil lamp, acetylene, or pocket flash lamp may be used when for any reason the former are not available. Magnesium ribbon also forms a very satis- factory illuminant, small lengths of from one half to two inches being used at a foot from the negative. Whatever the illuminant chosen, the distance between the illuminant and the printing frame should be standardized so that it is always the same. This distance must be at least equal to the diagonal of the negative in order to secure even illumination, unless more than one light is used. Far more satisfactory than a printing frame, however, is one of the many types of printing machines which are obtainable in a wide variety of styles and prices. A simple machine made by the Eastman Kodak Company, especially for amateur use, is illustrated in Fig. 161. This printer carries a 60 watt electric bulb and a small ruby pilot bulb. The negative is placed in position on the plate glass top and the paper placed over the same. When the platen is brought forward, the two are pressed into perfect contact and at the same time the ruby light goes out and the white light for exposure comes on. Releasing the platen switches out the white light and turns on the ruby bulb. This machine is one of many similar instruments which work on the same principle varying in details ac- cording to the requirements of the amateur, the photo-finisher or the professional photographer. Correct exposure depends upon: the density of the negative, the speed of the paper, the strength of the light and the distance of the negative from the light. Simple instruments 1 have been devised for 372 PHOTOGRAPHY measuring the density of the negative and from this determining the proper time of exposure but on the whole this is not so simple, nor so accurate as simply a test strip exposed and developed under actual working conditions, since so many varying factors alter the time of exposure. When once the correct exposure is found, this number to- gether with the paper used may be placed upon the negative envelope and will serve as a guide for future exposures so long as the other factors remain constant. Exposure is really determined by development and we will have occasion to again refer to the subject shortly. Developers. — There are countless numbers of formulae for de- velopers for both bromide and chloro-bromide or gaslight papers, but the following two are as good as any, although it is perhaps simpler to follow the formula advised by the manufacturer. The first formula, however, may be considered as a standard developer for gaslight papers since it is that advised by almost every maker of such papers in America. The second formula is that of Wellington and Ward and is designed for use with bromide papers for which it is especially suitable but the writer has used it with various makes of gaslight paper with perfect success. Water 40 oz. 1000 cc. Potassium bromide from 5-20 grains according to tone desired. (.25-1 gm.) For convenience in compounding Mr. L. I. Snodgrass recommends that the developer be made up in three stock solutions : one containing the metol and half the quantity of sulphite, the other the hydrochinon with an equal amount of sulphite, and the third the sodium carbonate ; the three stock solutions being mixed in the proper proportions to pro- duce a developer adapted to the work in hand. This method has the added advantage that the keeping quality is better than when the alkali is incorporated with the developing agents. The following is the formula recommended by Mr. Snodgrass and the manner of dilution for typical soft, normal and hard-working developers : 1 As, for instance, the Sanger-Shepherd Density Meter. Standard Metol-Hydrochinon Developer Metol Sodium sulphite (dry) . Hydrochinon Sodium carbonate (dry) IS gr. oz. 60 gr. V2 oz. •75 gm. 11. 5 gm. 3 gm. 1 1-5 gm- PRINTING PROCESSES WITH SILVER SALTS FlG. 161. Printing Machines for Amateur and for Professional Use 374 PHOTOGRAPHY A B C Metol Sodium sulphite (dry) Sod. carbonate (dry) . Water 2.5 gm. 45 gr. 18.0 gm. % oz. 500.0 cc. 20 oz. 18.0 gm. % oz. 10.0 gm. 180 gr. 500.0 cc. 20 oz. 36.0 gm. l)4 oz. 500.0 cc. 20 oz. For A B C Water 3 parts 1 part 1 part 7 parts 1 part 1 part 1 part 3 parts 1 part 3 parts 3 parts 5 parts These proportions may be further varied within reasonable limits to secure the effect desired. If too much of stock solution B is used the print will have a brownish tint, while if too much carbonate (Solution C) is used fog will be produced. Within these limits, however, the developer may be varied to the degree demanded by the work in hand. Wellington Amidol Developer for Bromide Paper Sodium sulphite (dry) 325 gr. 17.2 gm. Amidol (Diaminophenol) 50 gr. 2.8 gm. Potassium bromide 10 gr. .56 gm. Water to make 20 oz. 500 cc. Amidol does not keep well in solution and the above developer should be used if possible the same day or at least within three days of mixing. The Safelight. — Development should be conducted in a safe light. If there is any doubt concerning the safety of the light, lay a sheet of paper under the same in the position ordinarily occupied by the de- veloping tray and expose the same for a minute, then develop for a minute in total darkness. If there is any indication of fog, the light is unsafe and should be reduced in volume with a sheet of postoffice paper or a new safelight should be introduced. An excellent lamp for developing is shown in Fig. 25. For gaslight paper, the proper screen is the Wratten Series 00 and for bromide Series o. Plenty of light may be used but it should be safe. Either of the above screens when used with a 16 candle power electric light will be found perfectly safe and will give an ideal light by which to work. Development. — The development of prints is not essentially differ- ent from that of negative materials but certain factors which are relatively unimportant in negative development become of considerable moment in the development of emulsions on paper. The factors of PRINTING PROCESSES WITH SILVER SALTS 375 diffusion and absorption of the developing solution by the gelatine film become much less important, while development takes on more of the character of the so-called " physical " development, i.e., the reduction of the silver from a solution which interacts with the developer. This alteration in the character of development is not due, however, to the use of an accessory solution depositing silver, as in actual physical de- velopment, but to the solvent action of the sodium sulphite of the de- veloper on the fine-grained silver bromide and silver chloride. As silver chloride and the fine-grained silver bromide of paper emul- sions are more easily reduced by developers than the coarser grained silver bromo-iodide of negative emulsions, the reducing energy of the developer must be less in order to prevent fog. Hence the use of a soluble bromide in paper developers. The use of a soluble bromide in the developer has much the same effect as with negative materials. The inertia point shifts to the left with the time of development and the shape of the characteristic curve changes. Owing to the fact that the ma"ximum contrast is reached very quickly, there is a tendency, particularly in bromide printing where development proceeds more slowly, to remove the print too soon so that the maximum richness of the deposit is lost. The alteration which takes place in the character of the curve is graphically shown in the following Fig. 162 taken from the paper by Mees, Nutting and Jones of the Eastman Research Lab- oratory.2 It will be observed that there is a vast difference between the curves. One is seriously distorted and the straight line portion is very short, practically non-existent, while full development has given a curve showing a straight line portion of considerable length." This condition obtains when bromide and the slow grades of professional gaslight papers are under developed. The condition is somewhat different in the case of a rapidly developing paper such as Velox or Cyko in which case the maximum contrast is reached in a very short time and times of development shorter than this show serious mottling and irregu- larity. The golden rule in developing both gaslight and bromide papers is then : Develop to finality or as far as development may be carried without producing fog. The exposure will then determine the darkness of the print. Factorial Development. — This condition is most easily secured by factorial development in the case of papers which develop slowly as 2 " The Sensitometry of Photographic Papers," Communication No. 21, East- man Research Laboratory, Abridgments, vol. I, p. 68, Phot. J., 1914, 54, 342. 376 PHOTOGRAPHY bromide and professional chloro-bromide papers and by simple time methods in the case of the rapidly developing gaslight papers. As in the case of plate development the factorial method takes care of the variation' in temperature of the developer, and it also affords an ac- curate indication of the rate of development. Since it is customary to develop several prints, one after another, in the same volume of de- veloper which thus becomes weakened by use the time of development 4 8 10 12 14 [g 10 2 0 2E 24 2fi 2 0 Fig. 162. Effect of Time of Development upon the Characteristic Curve of Paper Emulsions grows longer and this is a factor difficult to determine by the method of development by inspection as commonly employed. Another point in favor of the factorial method is that, provided the proper factor is chosen, development is carried to infinity and the maximum quality which the paper can give is obtained. Still another point in its favor is that it makes correct exposure absolutely necessary as a print which is over exposed will be too dark when developed by the factorial sys- tem, while in development by inspection, the print would be removed from the developer when the proper depth had been reached, thus re- sulting in under development and loss of print quality. The Proper Factor. — The proper factor seems to be entirely a matter of the developer and does not seem to be influenced particu- larly by the paper used. Thus, Kodak, Wellington, Barnet, and Illingworth bromide papers have been developed in the Wellington formula, given above, with perfect success using a factor of 15. In fact, factors from 10 to 20 give practically identical results except PRINTING PROCESSES WITH SILVER SALTS 377 that less exposure and longer development is required for the higher factors and in practice 15 may be chosen as a good average, since it is midway between the minimum and maximum useful factors. The proper factor for any developer may be estimated by exposing strips of paper under the negative for various times and developing the same in the developer for various times and observing accurately the time of appearance of the image. The time of development divided by the time of appearance gives the factor: Time of development r •™ 7 - ■ = factor. 1 lme 01 appearance The following is taken from tests conducted with the Eastman Amidol formula. Print No 1 II III IV v 1 1 15 18 27 38 15 14 14 13 9 Time of development 300 210 168 IO4 54 Factor (nearest) 20 15 12 8 6 Prints I, II, and III are practically identical, while IV and V show marked falling off in richness of blacks and in contrast. The proper factor then is somewhere between 10 and 20, so 15 may be used as a standard since it is the average of the two. Time and material spent in determining the factor for any developer will be well repaid in the shape of better and more uniform print quality. With very rapidly developing gaslight paper the factorial method may be used but owing to the rapid appearance of the image in the developer it is rather more difficult to employ and simple development for the times indicated by the manufacturers in their instruction sheets inclosed with the paper is perhaps the best solution. Care should be taken, however, to keep the developer as nearly 65° F. as possible and to use the same for only a limited number of prints. The Short Stop.— While prints may be rinsed in water immediatelv following development and then placed directly in the fixing bath, in commercial establishments and other places where it is desirable to develop several prints before transferring the same to the fixing bath, the prints upon removal from the developer are immersed in a bath 378 PHOTOGRAPHY of acetic acid, which is termed the " short stop." In this bath, de- velopment is instantly checked and the print may be left while several others are developed and then the entire batch transferred to the fixing bath at one time. In some large commercial establishments, it is customary to develop prints and leave them in the short stop until a considerable quantity have collected, when they are fixed together and washed. Such a " batch " may number from one to three hundred prints and is usually governed by the size of the fixing tanks and the capacity of the automatic washers. The formula for the acid short stop is as follows : Water to make 64 oz. 1000 cc. Acetic acid 28 per cent (Comm.) 4 oz. 62.5 cc. Fixing. — Prints require to be thoroughly and completely fixed. Ten to fifteen minutes' immersion in a standard acid fixing bath is sufficient, provided the " hypo " has complete access to the surface of each print. To ensure the latter condition, the prints should be con- stantly turned over and over so that the hypo may be able to reach each and every print. Merely leaving the prints immersed' in a suf- ficient quantity of fresh acid fixing bath of proper strength for an indefinite time is not fixing and is to be heartily condemned. The golden rule for perfect fixation of prints may be stated as follows : Use a fresh acid fixing bath and keep the prints in motion for the- entire time of fixation, which should require at least fifteen minutes. In some commercial establishments, where large numbers of prints are handled in each batch, two fixing baths are used, the prints being fixed in one for ten to fifteen minutes and then transferred to the second for a similar length of time. This is a capital plan and is one which might well be adopted by every amateur finisher. Attention might well be called to the fact that the fixing bath should be acid ; otherwise, the developer carried over upon the surface of the prints will soon cause it to discolor. Careful draining of the prints as they are removed from the developer and the use of an acid short stop be- tween development and fixing will do much towards keeping the fix- ing bath clean. There is more danger of overworking the fixing bath with prints than with negatives, since in the latter case the disappear- ance of the milky backing is an indication of the speed of fixing; whereas, there is no such indication in the case of the fixation of prints. For this reason, it is advisable to keep accurate record of PRINTING PROCESSES WITH SILVER SALTS 379 the number of prints fixed in a given volume of solution, in order that the latter may be discarded as soon as the limit of its fixing powers has been reached. One gallon of any standard fixing bath should fix at least a gross 5/7 prints or approximately 5000 square inches of paper surface. As soon as this amount is reached, the bath should be discarded and a new one substituted. Never add new fix- 1 I FlG. 163. Electrically Operated Print Washer. (Pako) ing bath to a used solution. Pour out the old bath and replace with new. The following is a good formula for the fixing bath : " Hypo " 16 oz. 288 gm. Water to make 64 oz. 1000 cc. Dissolve and then add the following hardening solution, which may be made up in stock solution : Sodium sulphite dry /2 oz. 7.1 gm. Acetic acid 28 per cent 3 oz. 49 cc. Powd. alum y2 oz. 7.1 gm. Water to make 5 oz. 78 cc. Washing. — Prints should be washed for one half to one hour in running water and must be kept separated or they will cling together and the hypo will not be thoroughly eliminated within this time. Sev- 380 PHOTOGRAPHY eral ingenious machines have been introduced for this purpose. Fig- ure 163 shows one of the turbine washers such as are used in large fin- ishing plants. The machine is operated by electricity and has a ca- pacity of about 250 to 300 average size prints, which are kept sepa- rated and in motion very efficiently. Another type of washer which is more suited to the amateur and small worker is that illustrated in Fig. 164. The prints "are given a swirling motion by the water, which Fig. 164. Centrifugal Water Pressure Type of Print Washer. (Halldorscn) enters on the side so that they are kept well separated. The water passes out through the siphon arrangement in the center which re- moves the hypo-laden water from the bottom and at the same time owing to its conical shape prevents the prints from collecting in the center. The perfect washing machine does not exist and after all, perhaps the most efficient method consists in constantly transferring the prints by hand from one large tank to another in both of which fresh water is kept running. At any rate, it is well to take no chances with imperfect washing and where absolute permanency is desirable, tests for the presence of hypo should be made by any of the methods previously given in the chapter on Fixing and Washing. The starch- azide test may be recommended as convenient and sufficiently reliable. Drying. — Small batches of prints may be laid out on blotters to dry or stretchers covered with cheesecloth or muslin may be used. Prints which are allowed to become bone dry in such condition will curl con- siderably and many remove prints when nearly dry and place them between blotters under slight pressure in order to cause them to dry flat. For commercial establishments one of the many forms of drum dryers illustrated in Fig. 165 is recommended. The prints are placed between two broad belts which carry them around a heated drum about three or four feet in diameter. The revolution requires about three to five minutes and when the prints reach the starting point they PRINTING PROCESSES WITH SILVER SALTS 381 are thoroughly dried. In some models the drum may be run at dif- ferent speeds so that single and double weight or thick papers may be dried in one revolution of the drum. There are several dryers of this type on the market. The one illustrated is the Sickle and is shown not because it is any better than any other but merely as an example. Each of the commercial machines has its own distinctive features which should be carefully studied by the intending purchaser in order that he may be sure that he is securing the best apparatus for his particular requirements. Alteration of Contrast. — With a metol-hydrochinon developer it is possible to secure varying degrees of contrast with the same paper by varying the proportion between the two developing agents. Metol being a member of the soft-working class of developers, increasing the amount of metol leads to softer results while increasing the pro- portion of hydrochinon (which is a contrast developer) leads to greater brilliancy. This is at times very convenient when dealing with negatives having too much or too little contrast for the particular paper required. The three-solution metol-hydrochinon developer worked out by Snodgrass is especially suitable for this purpose as it af- fords a convenient means of preparing directly from stock solutions a soft-working-, normal or contrast developer as required. With some bromide papers increased contrast may be secured by using a hydrochinon-caustic soda developer as employed for process and photo-mechanical plates but with some papers and, particularly with most gaslight papers, this leads to images of poor color. FiO. 165. Rotary Belt Dryer. (Sickle) 382 PHOTOGRAPHY When it is required to secure the best possible results from exces- sively contrasting negatives, without resorting to persulphate reduc- tion or other manipulation of the negative, Sterry's method may be used.3 By its use soft results may be obtained with the very hardest negatives. The exposed paper is bathed for two or three minutes be- fore development in a solution of potassium bichromate and then de- veloped in the ordinary way. The following stock solution is made up: Potassium bichromate I oz. 91 gtn. Strongest ammonia (.880) 1 dr. 12.5 cc. Water to make 10 oz. 1000 cc. For use take one to two drams of the above stock solution to ten ounces of water (12.5-25 cc. to 1000 cc). Determine the exposure required to secure the proper detail in the highlights (neglecting the shadows) when developing in the usual way. Then make up the solution as above and immerse the exposed sheet of paper in the solution for three minutes. Wash for half a minute and develop in the regular developer. Development is some- what slower than ordinarily but the shadows are held back while the highlights come out to proper depth sooner so that the print is softer and has a better scale of values. An acid fixing bath must be used for fixing to avoid stains from the bichromate solution. Various de- grees of softness may be secured by altering the strength of the bi- chromate solution; the stronger the solution the softer the result, other things being equal. Reduction and Intensification of Prints. — It is not often that one desires to go to the trouble of reducing or intensifying prints as it is usually as simple and more satisfactory to make them over. There are times, however, as in the case of a big enlargement, where expense is an item of importance, when it may be desirable to attempt reduction or intensification bef ore going to the time and expense of making a new print. As in the case of negatives, prints may be reduced so that the con- trasts are increased (subtractive reduction), diminished (super-propor- tional reduction) or unaltered in contrast, the depth of the print alone being reduced (proportional reduction). For proportional reduction the following permanganate reducer is quite satisfactory : 3 Phot. J., 1907, 47, 170. PRINTING PROCESSES WITH SILVER SALTS 383 Potassium permanganate 7 gr- Sulphuric acid (10 per cent solution) 350 min. Water 16 oz. 1 gm. 50 cc. 1000 cc. This acts rapidly; from 45 to 120 seconds is the average time and it reduces any slight fog which may be present. For increased contrast (subtractive reduction) the iodine-cyanide re- ducer is suitable. Owing to its poisonous nature it must be handled with care. Iodine (10 per cent solution in potassium iodide solution) 403 min. 57-5 cc. Potassium cyanide (10 per cent solution) 70 min. 10 cc. Water 16 oz. 1000 cc. For reduction of contrast ammonium persulphate appears to be the only suitable reducing agent. Ammonium persulphate .560 gr. 80 gm. Sulphuric acid 8 min. 1.06 cc. Sodium chloride (salt) 6 gr. 0.8 gm. Water 16 oz. 1000 cc. For use, dilute with two parts of water.4 The most satisfactory agent for intensifying prints is chromium. While other intensifies will produce some increase in depth, the color is nearly always affected and the whites of the print tinted while none is so dependable in use as chromium. In general the operation is ex- actly the same as for negatives. Particular attention, however, must be paid to the thorough removal of every trace of bichromate before development. The time of washing necessary for this may be con- siderably shortened by immersing the print for five minutes in a dilute solution of potassium metabisulphite. Redevelopment may be with amidol or with the developer used for the original print, provided it is fresh and does not contain a large amount of soluble bromide. Should the amount of intensification secured the first time be insufficient the operation may be repeated. In this case it is well to use amidol for redevelopment as there is less danger of frilling or excessive softening of the gelatine. The Glazing of Prints. — Prints with a highly glazed surface can be produced on any of the glossy grades of P-O-P or developing papers. Such prints are best for reproduction and for work of a scientific na- 4 The above formulae are taken from the paper of Jones and Fawkes on the " Reduction of Developed Prints," Brit. I. Phot., 1921, 68, 275. 384 PHOTOGRAPHY ture where it is necessary to preserve fine detail. For producing the highly glazed surface, specially prepared iron plates coated with a hard brilliant enamel are generally employed. These are commonly termed ferrotype plates or squeegee tins. The wet prints are placed on the polished surface of the ferrotype plate and brought into perfect con- tact with the same by the use of a wringer or a hand roller. When dry, they can be stripped from the plate, having acquired as a result of this treatment a highly glazed surface similar to that of the plates on which they were dried. In place of the usual ferrotype plates, celluloid or glass may be used. The highest glaze is secured with glass, but owing to the greater danger of prints sticking to the surface so that they cannot be removed without damage, either the ferrotype tins or celluloid are preferred. The first aim of the worker should be to learn the characteristics of the paper used with respect to glazing. Some papers, which are har- dened in the process of manufacture, may be removed directly from the wash water and placed upon the tins ; with other papers this would re- sult in complete failure and some form of prehardening is necessary to prevent the prints from sticking to the plates. If the prints are dried and then resoaked until limp in cold water, the gelatine becomes con- siderably harder and the danger of prints sticking to the plates is largely obviated. Either alum or formaline may be used for harden- ing the emulsion, in order to prevent the necessity for an intermediate drying, but of the two formaline is undoubtedly the better. In the first place it does not endanger the permanency of the prints, as if not completely washed out in the few minutes wash which should follow immersion in the formaline solution it will evaporate entirely. In ad- dition, it possesses the advantage over alum in that it has no tendency whatsoever to produce iridescent markings on the prints. A solution of I oz. formaline to 20 oz. water is sufficiently strong. If glass is used the plates are first cleaned by soaking for several minutes in weak sulphuric acid (1 oz. commercial H2S04 to 10 oz. water), then rinsed under the tap and scrubbed with plain washing soda, again rinsed and allowed to dry. When dryithe glass is coated with perfectly fresh ox-gall (1 oz. ox-gall to 40 oz. water). Old ox- gall is worse than useless and will actually cause the prints to stick. The prints are placed face down on the prepared surface, a blotter placed over them and pressure applied by means of a roller until the prints are seen to be in perfect contact with the surface. The glass is then placed in a cool dry place and the prints allowed to become thor- PRINTING PROCESSES WITH SILVER SALTS 385 oughly dry before trying to remove them. Some may leave the glass completely when dry, those that do not may be removed by inserting a finger nail under one corner and pulling away from the glass. Lately Callier has recommended that the glass be coated first with a 2 per cent solution of gelatine. When this is dry, a thin film of collodion is superimposed. This collodion is prepared as follows : Pyroxyline (soluble) 756 gr. 45 gm. Vasoline oil 96 min. 2 cc. Amyl acetate to make 35 oz. 1000 cc. When the collodion has dried the prints are applied as usual and when dry will drop off. There is absolutely no danger of sticking. Ferrotype tins, however, are more generally employed in this coun- try. With these the danger of prints sticking to the plates is much less than with glass. It is necessary, however, to keep them absolutely clean and well polished. For this purpose a solution of benzol and spermaceti wax, or turpentine and beeswax, is usually employed. Suitable formulas follow : Beeswax 20 gr. 45 gm. Turpentine 1 oz. 1000 cc. Spermaceti wax 20 gr. 45 gm. Benzole 1 oz. 1000 cc. A few drops of this are rubbed on the plate with a piece of clean flannel and the tin polished with another piece of flannel or other soft cloth. Celluloid forms a very suitable substance for glazing although the gloss is not so high as that produced by glass or by ferrotype plates. A special brand known as Glazine, manufactured by the Glazine Pad Company, Hillsborough, Sheffield, England, is supplied for this pur- pose. With this the danger of prints sticking to the plate is practically negligible. Neither is polishing of any kind required. The writer has used them with perfect success for two years and believes them to be, all things considered, the most satisfactory glazing substance obtainable. II. Printing-Out Processes Gelatine Printing-Out Papers. — Gelatine printing-out papers (called P-O-P for short), once almost universally used for positive printing, have been rendered almost obsolete by the modern developing papers and are now seldom employed. There remain, however, some 386 PHOTOGRAPHY purposes for which they are still unrivaled as, for example, for prints from which half-tone plates are to be made and for photo-mechanical reproduction processes generally. It seems well, therefore, to' include some brief directions for the manipulation of such papers. Printing itself is very simple and calls for but little comment. The printing frame may be loaded and the progress of printing observed in an ordinary room if one takes the precaution not to stand close to a window and does not leave the paper exposed to the direct rays of light any longer than absolutely necessary. For the best result the negative must be one which has received full exposure and development ; weak, under exposed or under developed negatives will not make good prints on gelatine printing-out paper and such negatives are best printed on developing paper. Care should also be taken that the negatives are perfectly dry, otherwise a silver stain may be formed which it is al- most impossible to remove. Exposure should be to the strongest daylight available, excepting direct sunlight, which must not be used except with very dense and contrasty negatives. As there is a certain decrease in depth in the processes of toning and fixing, printing must be carried considerably further than would appear necessary from an examination of the pic- ture in the printing frame. The depth to which printing must be carried to allow for this falling off in toning and fixing is easily learned after a few trials and thereafter gives no trouble. After being removed from the frame the prints are placed away from light and under pressure until a number have accumulated and one is ready for toning and fixing. Exposed prints, however, should not be kept from one day to another. Toning. — Before toning, the prints are washed in running water for a quarter of an hour, or in several changes of water, care being- taken that washing is thorough and that each print receives its proper share of washing. Prints which are left lying on one another do not wash properly no matter how long the period of washing ; it is essen- tial that they be kept moving, so that each print is exposed to fresh water. Some methods of toning, however, do not require the previous washing. There are almost innumerable formulas for the toning bath and many variations in the processes of toning. As ordinarily practiced, toning is an operation which requires considerable practice in order to be able to secure satisfactory and uniform tones. There is a method of controlled toning, however, with which even the most inexperienced PRINTING PROCESSES WITH SILVER SALTS 387 person can secure agreeable tones and with a high degree of uniform- ity. The solutions required are a 10 per cent solution of ammonium sulphocyanide, a 10 per cent solution of common salt, a 10 per cent solution of hypo and a gold bath containing I grain of gold chloride to each dram of water. The principle is to use a definite weight of gold for a given number of square inches of paper and leave the prints in the bath until the gold has been used up. The toning bath is made up as follows : Measure out 10 ounces (iooo cc.) of water and add two drams (25 cc.) of the sulphocyanide solution and 1 ounce (100 cc.) of the salt solution. Mix, and add 1 dram (12.5 cc.) of the solution of gold chloride. Label the bottle Gold Toning Bath. Each ounce (28.4 cc.) of this solution contains 1-10 grains (.0064 gm.) of gold which is sufficient for'two 3/4 x 4% prints. For warm brown tones half to three quarters of an ounce (14. 2-2 1. 3 cc.) is enough: for blue tones a little more may be needed. Other sizes may be handled by taking the proper amount for the size of the print. Now suppose you have ten 3^ x 4J4 prints to tone. Measure out 5 ounces (142 cc.) of toning bath and put the prints directly into it without washing. Continue to move them around in the bath until no further change of color can be observed. The final stage is when the surface looks cold and slaty blue. They are then removed, washed, fixed and again washed and dried. Instantaneous Toning. — Another certain method of securing uni- form tones is the so-called instantaneous method. Four stock solu- tions are required : A. Ammonium sulphocyanide 1 oz. 100 gm. Water to make.... 10 oz. 1000 cc. B. Gold chloride IS gr. 133.3 gm. Water to make 7l/2 oz. 1000 cc. C. Sodium phosphate 1 oz. 100 gm. Water to make 10 oz. 1000 cc. D. Saturated solution of borax. Mix for ten 4x5 prints (200 sq. inches), A 1 dr. 10 parts Water 1 oz. 80 parts B yi oz. 5 parts C 1 dr. 10 parts D 2 dr. 20 parts 388 PHOTOGRAPHY The prints, which should be only one shade darker than the desired shade, are put directly in the toning bath without previous washing. On entering the bath they first turn red, then a dark purple which is almost black in the deepest shadows. No matter how much longer they are left in the bath no further change takes place. As soon as toning is seen to be complete, the prints are fixed or removed to a tray of clear water until ready for fixing. The advantages of these methods over those commonly advised are obvious as all uncertainty is removed and the operation can be worked at night by artificial light which is impossible in the ordinary way. The editor of American Photography remarks that a large number who have given it a trial found it to work perfectly.5 Black Tones with P-O-P. — Black tones can only be secured with P-O-P by toning with platinum. The following bath should be used : Meta-phenylenediamine 7 gr. 1.4 gm. Potassium chloroplatinite 7 gr. 1.4 gm. Water to make 10 oz. 1000 cc. This solution must be prepared directly before use as it does not keep at all well. As soon as the desired tones are secured remove the prints and then fix and wash thoroughly. Bluish-black tones may be secured by first toning in a gold bath, washing well and retoning with platinum ; the color depending on the depth to which gold toning is carried. The more gold deposited the bluer will be the final result after platinum toning. When a platinum toner is used, the prints should be immersed before toning in a 5 per cent solution of salt for five minutes, then rinsed and toned. After toning the prints should be immersed for 5 minutes in Salt lyi oz. 140.4 gm. Sodium carbonate (dry) p4 oz. 46.8 gm. Water 16 oz. 1000 cc. Then rinse, fix and wash thoroughly before drying. Fixing. — After toning the prints are washed for several minutes, then transferred one by one to the fixing bath which consists of a 10 per cent solution of hypo. The prints must remain in the fixing bath at least 10 minutes during which time they must be separated con- stantly by hand in order that the fixation of each print may be thor- ough. Fixing is followed by a thorough washing for y2 hour in run- 5 The writer is indebted to Fraprie's work Practical Printing Processes for these methods of toning. PRINTING PROCESSES WITH SILVER SALTS 389 ning water, or in 6 changes of water, allowing 5 minutes for each change, after which the prints are ready for drying. General Reference Works B urban k — Photographic Printing Methods, 1896. Glover— Print Perfection— How to Attain It, 1924. Hanneke — Das Arbeiten mit Gaslicht und Bromsilberpapieren, 1918. Hinton— P-O-P. Mercator — Der Entwicklungs-druck auf Bromsilber, Chlorbrom und Chlor- silber Gelatineemulsion Papieren, 1907. Hawkins — Photographic Printing, 1930. Stenger — Neuzeitliche photographische Kopierverfahren. Stenger — Die Kopierverfahren, 1926. Snodgrass — The Science and Practice of Photographic Printing, 1923. Valenta — Die Behandlung der fur den Auscopir-Process Bestimmen Emul- sionspapiere, 1896. Wheeler — Photographic Printing Processes, 1930. CHAPTER XVIII PROJECTION PRINTING Introduction. — There has been an increasing tendency on the part of both amateurs and professionals in recent years to abandon the large, bulky and cumbersome camera and its necessary impedimenta and rely entirely upon projected prints from the small negatives where large prints are required. This course has much to recom- mend it. Photographic materials are now so perfect that given proper care small negatives can be made which will stand consider- able enlargement and still compare quite favorably in quality with contact prints from larger negatives, while the convenience of the small, compact hand camera is not to be belittled. Perhaps the principle of projecting will be clearer from an examina- tion of Fig. 1 66, in which A represents the illuminant (either daylight Fig. 166. Principle of Projection Printing or artificial light), B the negative, C the lens, and D the easel. The rays of light from A pass through the negative B, an image of which is formed on the easel D by the lens C. The distance between the lens and the easel determines the degree of enlargement, the greater this distance the greater the degree of enlargement. Fixed-Focus Enlarging Cameras. — The simplest form of apparatus for projection printing consists of a box carrying at one extremity the negative and at the other the sensitive paper, with a lens between (Fig. 167). Such cameras are an article of commerce and are in- 390 PROJECTION PRINTING 391 tended primarily for the amateur or occasional worker. As they are fitted with cheap single lenses with a small aperture, the time of ex- posure is relatively long and may easily run into minutes unless day- light and a fast bromide paper are employed. The fixed-focus Fig. 167. Box Enlarging Camera camera forms a very convenient and satisfactory instrument for the man who desires to make a larger print now and then and who does not care to go to the trouble and expense of employing a more elabo- rate form of apparatus. While such instruments may be obtained commercially, there is no reason why the worker cannot make one of his own if he is at all handy with tools, as the construction is quite simple. For this pur- pose one may make use of the lens attached to his regular camera, providing the fixed-focus enlarging box with a lens flange to take the same, or, if the lens cannot be removed from the camera, fastening the entire camera on a platform within the enlarging camera. As the commercial forms of. such apparatus are equipped only with single lenses having a very small aperture it will be readily seen that with but a little time and materials the worker may provide himself with apparatus which is actually superior to the commercial article. The 392 PHOTOGRAPHY total length of the camera is, obviously, the sum of the distances separating the lens and negative and the lens and sensitive paper. If the positions of the nodes of the lens are known the two conjugate distances can be precisely measured and the partition carrying the lens placed in position without preliminary testing. When the posi- tion of the nodes is unknown it is necessary to find the proper position for the lens by experiment. In fact to ensure critical focus it is well in all cases before fixing the lens to make some preliminary tests on a sheet of ground glass placed in the position to be occupied by the sensitive paper. Apparatus for Projection Printing with Daylight. — There are cam- eras made especially for enlarging and reducing. These are provided Pie. 168. Daylight Enlarging Camera with long bellows extension so that the degree of enlargement, or re- duction, may be varied, within certain limits, as required. In Fig. 168 we illustrate a typical camera designed for this purpose. Such cam- eras are heavy and expensive, occupy considerable space and, more- over, the size of enlargement is limited by the size of the camera. For these reasons other forms of apparatus are to be preferred. The most satisfactory method of employing daylight for projection printing is illustrated in Fig. 169. A window which receives clear unobstructed light from the sky is blocked up, except for a small opening about twice as large as the largest negative to be used, and provision made for attaching an ordinary camera, the reversing back having been previously removed. A platform is built to support the camera and guides or markers placed to show the proper position of the easel. Provision must of course be made for holding the nega- tive and one or two sheets of ground or opal glass which serve to diffuse the light and prevent uneven illumination of the negative. Where clear unobstructed light cannot be secured it is necessary to PROJECTION PRINTING 393 employ a reflector, as shown in the figure. This reflector must be at an angle of 450 so that it will cast the unobstructed light from the sky onto the negative. This reflector may be of any enameled surface as white paper, or wood painted with a glossy white paint. A mir- ror is to be avoided. In place of a reflector the ribbed glass known FlG. 169. Projection Printing Apparatus for Use with Daylight as prism glass may be used, being placed two or three inches before the negative in order that there may be no danger of it being in focus. Daylight has several positive advantages and at the same time disad- vantages which prevent its general use. In its favor it may be said that the light is rapid and owing to the perfect diffusion any retouch- ing or handiwork on the negative does not show so prominently as when the light comes from a concentrated source. For this same reason there is less granularity apparent with a high degree of enlarge- ment when daylight is used than when condensers and a concentrated light source, such as the electric arc, are employed. On the other hand, daylight is never constant and exposures are therefore liable to sudden change, which occasions considerable waste of time and material. Apparatus for Projection Printing Using Artificial Light. — Owing to the inconstancy of daylight practically all enlarging is now done with artificial light. The typical lantern for enlarging with artificial light is illustrated in Fig. 170. It will be observed that this consists of the illuminant with its light-tight lamp house, either condensers or reflectors for securing the maximum efficiency of illumination from 394 PHOTOGRAPHY the source used, the negative carrier, the bellows with a projecting lens and lastly the easel. While this is the schematic plan of prac- tically all enlarging lanterns on the market, they naturally vary a good deal in minor details. The construction of such apparatus is not above the capabilities of the average worker who is handy with tools Fig. 170. Enlarging Lantern for Artificial Light and should he care to use the lens and camera which he has already, the expense of the apparatus may be made almost negligible. Direc- tions for making such equipment have been published many times in nearly all of the journals to which the worker who wishes to build his own lantern is referred for further information. Where condensing lenses are not employed and the light source is one which does not require attention while in use, it may be convenient under certain circumstances to place the light outside of the room used for enlarging. This arrangement has the advantage of increasing the amount of floor space and lessening the effect of the heat liberated by the lights. Self-Focusing Apparatus for Projection Printing. — A new era in apparatus for projection printing began with the introduction by the Eastman Kodak Company in 1920 of self-focusing projection ap- paratus in which the image is kept in focus automatically regardless of the degree of enlargement. Other manufacturers have followed the Eastman Company in the field and a number of models are now available. Among these may be mentioned the Callier ; lea ; Border- tint ; Aldis-Ensign ; Sichel's Overton ; Butcher's Autoprint ; Noxa, be- sides many others. Directions for the construction of self-focusing projection apparatus have been published in several places (see bib- liography). Illuminants for Projection Printing. — The principal requirements PROJECTION PRINTING 395 of a satisfactory illuminant for enlarging are that it should be reason- ably constant in strength, that it should be strong enough to allow of rapid exposures, and be easily adjusted and convenient in use. Day- light is the least suitable of the illuminants because of its variability. It varies not only from day to day and from hour to hour but may even vary considerably in the short space of a few minutes. For this reason daylight is entirely unsuited to the professional or commercial enlarger who must produce prints of uniform quality, while its use by the amateur means the wastage of much material that might other- wise be saved. The worker who has access to electricity will of course use some form of electric light, and, while it must be admitted that equally good enlargements may be made by daylight or the weaker light sources as acetylene or gasoline vapor lamps, electric light sources surpass all others in general adaptability, since on the whole they are more constant in intensity, more easily adjusted and possess higher intensities than the other sources. The Mercury- Vapor Lamp. — One of the most satisfactory lights for enlarging is the M-shaped mercury vapor tube supplied by the Cooper- Hewitt Electric Company of Hoboken, N. J. The light is extremely rich in violet rays and is consequently very rapid, so that the slower grades of " gaslight " paper may be used successfully, while for large projected prints the time of exposure is materially lessened. The M- shaped tube gives an even illumination which requires no condensers and, as little heat is produced, it is particularly suitable for summer use. Owing to the M-shape of the tube the illumination is so much diffused that the minimum amount of ground or opal glass is required to secure even uniform lighting. This results in a higher intensity. Undoubtedly the M-tube with a sheet of ground or opal glass as a diffuser forms the nearest approach to daylight of any artificial light and is especially desirable for enlarging from portrait negatives. The mercury-vapor lamp is a rather expensive illuminant for the amateur as its initial cost is rather high, but as it consumes very little current it is not so expensive in the end and in the long run is well worth the cost. Where much enlarging is done and speed and quality and not the initial expense are the prime requisites, then the mercury-vapor lamp may be considered the ideal light. The Electric Arc. — From the optical standpoint no other light so completely fulfills the requirement of the ideal light surface for en- larging as the electric arc. Whereas all other sources radiate light 14 396 PHOTOGRAPHY in all directions, the light from the arc is confined to a small area of about half an inch in diameter on the positive carbon. The light is extremely intense and also very rich in blue and violet rays, especially if flaming carbons arc employed. Direct current is by far the most satisfactory for an arc light source as it is more steady, under better control and more economical of current. If alternating current must be used, then it is well to arrange the light in such a way that the carbons are tilted toward the condensers at an angle of about 300 (Fig. 171). When this is done the highly incandescent tip at the extremity Fir,. 171. Proper Position of the Carbons of an Arc Light for Use with Alternating Current of the lower positive carbon is directed toward the condensers, result- ing in a higher intensity of illumination. Were it not for its incon- stancy and the attention which it requires, the arc lamp might be con- sidered the ideal source for projection printing, but these are factors of considerable importance in practice and consequently we believe that for general purpose work either (he mercury vapor tube or gas- filled mazda lamps are to be preferred. Incandescent Lamps. — Gas-filled lamps of high intensity, such as 5°°. 75° a°d 1000 watts, have been extensively used for projection printing since their introduction several years ago and are steadily in- creasing in popularity. They are very constant and convenient in use, and the color of the light, while not as rich in actinic rays as the arc or mercury-vapor lamp, is nevertheless very satisfactory and the intensity is sufficient for all ordinary enlarging. Special types, known as stereopticon or projection lamps, are now supplied having a small concentrated filament which is almost as satisfactory for use with condensers as the arc. On the whole, however, lamps of this type PROJECTION PRINTING 397 work better with parallax reflectors than with condensers. The prin- cipal objection to the gas-filled lamp is the amount of heat produced and, as in the case of the arc, which develops still more heat, care should be taken that perfect ventilation is secured, otherwise the nega- tive may be melted or warped especially if on film. Although of lower intensity than electric sources, the Welsbach mantle is well suited to projection printing except for high degrees of enlargement, or with dense negatives, when the time of exposure may be rather lengthy. Writing in the British Journal of Photography (1922, 69, 767) W. Gard described how he has used for a 2>Vk x 4% enlarger a 4-volt, 4- watt lamp in connection with a 6-volt, 40-ampere-hour accumulator and for a 4^x6^2 enlarger a 6-volt, 12-watt lamp with an 8-volt, 40-ampere-hour accumulator. The lamp is of lower voltage than the accumulator and thus gives a more powerful light. He states that an exposure of 4 seconds was found quite sufficient when en- larging from 4^2x6*^ to 8x10 or 10x12 using ordinary bromide paper. Securing Even Illumination without Condensers. — Whatever the illuminant employed it is of primary importance that the negative be evenly illuminated. For this purpose condensers are generally em- ployed with the electric arc and other illuminants approximating to a point source, such as the Nernst lamp or the low volt, concentrated filament, locomotive headlight lamp. For more diffused sources, re- flectors are more suitable. For gas-filled mazda lamps of 500 and 1000 watts the Parallax re- flector provides a convenient and efficient means of securing uniform illumination without diminishing the intensity of the source. Fig. 172 shows the construction of this reflector and its appearance when illuminated. It consists of a number of silvered mirrors so placed that the rays of light from the lamp which strike them are reflected out towards the negative in a parallel beam. A very satisfactory means of dealing with incandescent lamps of lower power is to employ the same in series as shown in Fig. 173 The lights at the corners should be somewhat stronger than that at the center which merely serves to fill in the gap in illumination in the center. This arrangement used with one or two sheets of ground glass should provide perfectly uniform illumination. Although less powerful than direct light there are several worth- .398 PHOTOGRAPHY while advantages to the use of reflected light. It is softer, owing to more complete diffusion, and consequently gives softer, more delicate results, with a longer scale of gradation and greater freedom from granularity than a direct source. For the portrait photographer it is FlG, 172. Parallax Reflector for Use with Incandescent Electric Sources particularly advantageous since any retouching or other handwork on the negative does not show up so prominently as when a direct, concentrated source is employed. While theoretically the reflector to be employed should be parabolic © © ® @ Fie. 173. Securing Even Illumination with Five Incandescent Electric Sources in form, in practice any of the forms illustrated in Fig. 174 will be found suitable. The light houses illustrated may be built of thin sheet metal or wood. In the latter case it is well to line the sides adjacent to the lights with asbestos to prevent danger of fire. The reflecting portion should be of sheet iron coated with aluminum paint. Care should be taken that the reflector is not too small or the illumination of the negative will not be uniform. To ensure uniform illumination the reflector should be made at least 3 times the size of the largest negative to be employed. For the smaller sizes a single gas-filled PROJECTION PRINTING 399 bulb on each side may be sufficient. For larger sizes, however, it will be necessary to use more than one bulb on each side, or the long tubular lamps as used in window display illumination may be used. The Condenser in Projection. — With a suitable light source the negative can be uniformly illuminated by means of condensing lenses. □ : LU Fig. 174. Forms for the Lighthouse Using Reflected Light. (Wall) These lenses are of the plano-convex type and are used in pairs, the two convex sides facing one another and separated by a fraction of an inch. The diameter of the condensing lenses must be at least as large and preferably somewhat greater than the diagonal of the largest nega- tive with which they are to be employed. Condensing lenses are sup- plied in pairs either mounted or unmounted in the following sizes and focal lengths : Diameter Focal length in inches Thickness in inches 4 5% 7/s 4% m i%2 4& m 2%2 5 &A I%2 5% 8 I%2 6 10 I%6 6V2 10 i^i 8 12 Ix%2 9 • 14 1% 10 15 i% 12 18 2% 14 21 2% 400 PHOTOGRAPHY The focal length of a pair of mounted condensing lenses can be de- termined from the usual rule governing the focal length of a com- pound lens system. As the plano-convex lens has only one determin- able nodal plane, which lies adjacent to the vertex of the convex sur- face, the distance between the nodal planes obviously becomes that separating the convex surfaces of the lenses. The expression thus reads : Fi X F2 F = FiX Fa- d Where : F = the equivalent focal length. Fx — the focal length of lens I. F2 = the f d£al length of lens 2. d = the distance between the facing convex sides of the mounted lenses. The function of the condensing lenses is to collect the diverging rays of light and condense these on the negative, bringing the light rays to a focus in the projecting lens. Thus in Fig. 175 the light rays Fig. 175. The Function of the Condenser from the source E strike the first condenser and are refracted so that they enter the second condensing lens parallel to one another. In this second condensing lens further refraction takes place, the rays being converted into a converging cone the apex of which (or the focus) lies at, or very near to, the principal nodal plane of the projecting lens, 0. Thus the rays which in the absence of the condensing lenses would have continued on in straight lines and be lost are bent and made to pass through the lens, so that uniform illumination together with a high intensity is obtained. From this it is evident that to secure the maximum intensity, to- gether with uniform illumination of the negative, when condensing lenses are used without a diffusing medium, such as ground or opal PROJECTION PRINTING 401 glass, the light source must be located at such a point on the optical axis that its image, formed by the condensing lenses, is located at the principal nodal plane of the projecting lens. If we ignore the exact nodal planes, this means that the distances AB and CD (Fig. 176) are conjugate foci of the condenser while the distances ED and DF are conjugate foci of the projecting lens. As the degree of enlargement is altered the distances ED and DF will vary according to the rule of I 1 B - - F ' OPT I CA L ■ A — 1 5 C -D C AXIS Fig. 176. Conjugate Foci in Enlarging conjugate foci (Chapter III, p. 74). With every variation in ED :DF there will be a corresponding variation in AB : CD, so that in order to keep the image of the light source in its proper place at the principal nodal plane of the projecting lens the distance AB must be varied each time the degree of enlargement is altered. . The problem is thus one of conjugate foci and as such might be calculated mathematically with the aid of the formulas givea in an earlier chapter (Chapter III, p. 75). However, owing to the fact that condensing lenses are of cheap construction and entirely without cor- rection, the image of the light source is never a sharp one and in prac- tice such calculations are not of much practical value. One can usu- ally determine the proper position for the light source with sufficient accuracy for all practical purposes from the examination of the circle of illumination as thrown on a sheet of white paper. The character and position of colored fringes or areas of uneven illumination indi- cate the steps which should be taken towards securing equality of illumination. This matter is illustrated in Fig. 177. Condensing Lenses with Diffusing Media. — With a light source of small dimensions, such as the electric arc, and in the absence of a dif- fusing medium, such as ground or opal glass, the image formed at the focal point may be smaller than the aperture of the objective. It is obvious that in such cases the effective aperture (on which the speed of the lens depends) is not that indicated on the mount but that of the light beam which passes through the objective. Thus it may 402 PHOTOGRAPHY happen that a lens with relative aperture of F/4.5 is actually being used at a relative aperture of F/li. Therefore the time of exposure would not be altered if the lens is stopped down to F/6.3 or F/8. However, if a diffusing medium, such as ground or opal glass, be interposed in the path of the light rays, either in front of or between Fie. 177. Adjustment of the Light Source with Condensers In Figs. 1 and 2 the radiant, i.e., the crater, needs to he properly adjusted laterally; it is too far to the right or left. In Figs. 3 and 4 it is too high or too low. In Figs. 5, 6 and 7 it is too near or too far from the condenser. Fig. 8 shows it to he in correct position, the field being entire1}' clear. the condensers, this condition no longer applies. One is then dealing not with direct light but with scattered light and in this case the ex- posure is directly proportional to the lens aperture as in ordinary photography. The ground or opal glass may be placed either in front of or between the condensing lenses. Theoretically placing the dif- fusing medium between the condensing lenses results in less loss of light from diffusion, but practically the difference is so small as to be almost negligible, while there are other disadvantages which far out- weigh this slight advantage. The uniformity of illumination is not nearly as satisfactory as when the diffusing medium is placed before the condensing lenses and in the case of ground glass the grain tends to produce a granular effect which may in certain cases be decidedly objectional and which it is always well to avoid as much as possible. This is due to the fact that the ground glass is much nearer to the focal plane of the negative and therefore more nearly in focus. Therefore with ground glass a position slightly in front of the con- densing lenses is advisable. Opal glass being practically free from granularity may be placed between the condensing lenses if desired. Even in this case, however, a position before the condensers is prob- PROJECTION PRINTING 403 ably to be preferred. The use of ground glass makes unnecessary the adjustment of the light source each time the degree of enlargement is altered. However, in order to obtain maximum printing speed, an adjustment should be made when there is a considerable difference in the degree of enlargement. By the use of diffusing media, scattered rather than direct light is employed and this, as already noticed, has the effect of reducing to some extent the contrast of the large print as well as its granular appearance. For these reasons the use of dif- fusing media is always advisable with portrait or pictorial negatives and for those to be enlarged considerably. The Projection Objective. — A large number of the projection ap- paratuses on the market, especially those of foreign make, are fitted with objectives of the Petzval type. Aside from its large aperture and excellent axial spherical correction, this type of lens is by no means the best for the purpose, owing to the rapid diminution in the intensity of the image towards the margin and the pronounced cur- vature of field. Much better is the aplanat, or rapid rectilinear, which, though slower, has a flatter field with more perfect marginal definition. The rectilinear, however, is surpassed by the anastigmat whose su- perior marginal definition combined with an astigmatically flat field renders it particularly well suited for projection purposes. For use with condensers in conjunction with an arc, or similar source, and without a diffusing screen there is no advantage in the use of a lens having an aperture much larger than F/6.8 since this is sufficiently large in most cases to admit the whole of the converging beam of light from the condensing lenses. However, when condensing lenses are used with the ordinary incandescent light^or diffusing media of any kind is inserted in the path of the light rays, a larger aperture may be required in order to secure the full efficiency of the light source, since in this case the image of the light source formed within the projecting lens is larger than before. With completely diffused light sources, such as the Cooper-Hewitt mercury-vapor light, groups of incandescent lamps used with ground or opal glass, totally reflected light or a single incandescent electric light in a parallax reflector, a large aperture is also of advantage in reducing the time of exposure. Except when condensing lenses are employed the only effect of the focal length of the lens is to determine the length of the apparatus necessary. The longer the focal length of the lens the greater will be the bellows length and floor space for a given degree of enlarge- ment. 404 PHOTOGRAPHY With condensing lenses, however, the focal length of the lens must be chosen with reference to the focal length of the pair of mounted condensing lenses. If the focal length of the objective is much longer than that of the condensing lenses, the light efficiency is reduced by loss of light between the condensing lenses as shown in Fig. 178. Fig. 178. Loss of Light between Condensers Due to the Use of a Long Focus Lens for Projection. (Candy) In addition to this loss, under such conditions the size of the light image will be greater than the size of the light, and the aperture of the objective may not be sufficiently large to accept it, which results in further loss. Contrariwise if the focal length of the objective is much less than that of the condensing lenses, the converging power of the latter will be reduced owing to the convergence of the rays between 0 XI yfoN Fig. 179. Loss of Covering Power Owing to the Use of Short Focus Projecting Lens with Condensers. (Candy) the condensing lenses (Fig. 179). Therefore when condensers are employed the objective used for projection should have a focal length approximately equal to the focal length of the pair of mounted con- densing lenses. Exact equality is not required but the two focal lengths should be as nearly identical as possible. Attention may perhaps be usefully called to the fact that with cer- tain lenses in which the front component has a strong condensing PROJECTION PRINTING 405 action, so that the " inconstancy of aperture " (Chapter II, page 79) is pronounced, better results are obtained when the lens is turned so that the front component faces the negative. The Projection Easel. — The easel may be simply a large drawing board of soft wood to which the paper is attached by means of push pins. As a matter of convenience the wood may be covered with heavy " cork lino," a heavy linoleum used for floor covering. This enables the paper to be pinned up with very little pressure. For convenience in placing the paper in position, the easel may be painted white and ruled with heavy black lines for the different sizes of en- largement or in one half inch squares numbered in large figures each way from the center on a vertical and a horizontal line. A further refinement consists in provisions for raising or lowering the easel and for sliding it to the right or left. By this means the portion of the negative used for projection may be brought within the limits of the area marked on the easel for various sized enlargements. Mr. E. J. Wall coats the easel with a mixture of the following com- position which does not- dry but remains tacky so that the sheet of paper placed in position and rubbed down will be held in place for as long as required, after which it may be stripped off without difficulty : Gelatine 407 gr. S3 gm. Golden syrup .' 407 gr. 53 gm. Glycerine 1 oz. 65 cc. Chrome alum 8 gr. 1 gm. Water to make 16 oz. 1000 cc. Any grade of cooking gelatine may be used. It is first soaked for Yi hour in about i/\ of the total volume of water to which has been added the syrup and glycerine, after which it is melted in a water bath by heating to 500 C. (120° F.). Dissolve the alum in an ounce of water. Then make the bulk of the solution up to 15 ounces, add the alum solution and strain through linen. Allow 65 cc. of this mixture to every 100 square centimeters of the easel (about 1 oz. to each 100 square inches). The mixture sets in about 24 hours and the easel is then ready for use.1 Perhaps an even more convenient method consists in using a large printing frame. Fig. 180 shows a commercial easel designed for use with printing frames and provided with guides in order that the frame 1 Amer. Phot., 1923, p. 717. 406 PHOTOGRAPHY may be returned to the same position when loaded with the sensitive paper as it occupied when focusing. For commercial use where large numbers of prints must be made rapidly with a projector of the horizontal type an easel such as the " Westminster," 2 illustrated in Fig. iSo. Ingento Enlarging Easel for Use with Printing Frame Fig. 181, is very convenient. The easel itself is swung to the hori- zontal position for inserting the paper which is fastened in place by clamping over it the hinged sheet of glass, after which it is swung to the vertical position for the exposure. Fig. 181. Westminister Enlarging Easel With projection apparatus of the vertical pattern the easel becomes a very simple affair. In this case a flat surface of sufficiently large dimensions with a sheet of clean glass free from flaws or two bar 2 Made by Westminster Photographic Exchange, Ltd., 6i Piccadilly, London, W. C, England. PROJECTION PRINTING 407 weights sufficiently long to keep the sheet of paper flat during ex- posure constitute all the fixtures necessary for speedy and efficient working. Whatever the form the easel takes means must be provided for altering the distance between it and the projecting lantern and in such a way that the parallelism between the plate and the easel may not be disturbed. For this purpose a grooved track may be made or mark- ers may be placed on the floor to indicate the position of the easel for different degrees of enlargement. The Negative for Projection Printing. — It is difficult to give any precise definition of the proper type of negative for projection printing since so much depends upon factors for which no definite numerical expression is available. Of primary importance is absolute freedom from physical defects of any kind such as scratches, pin-holes and spots of all kinds, as they are enlarged along with the rest of the nega- tive and become unpleasantly conspicuous in the finished print. While much may be done towards removing such defects by appropriate handwork, such work requires to be done very carefully as imperfec- tions which would not be seen on a contact print are only too prominent when enlarged. Hunter and Driffield were the first to call attention to the fact that positives obtained by projection possess more contrast than contact prints from the same negative and on the same printing material.3 Seven years later Chapman Jones 4 investigated the scattering of light by the photographic plate and in 1909 Andre Callier in a paper before the Royal Photographic Society of Great Britain 5 showed precisely how this was responsible for the difference in contrast between posi- tives made by projection and those made by contact printing from the same negative and on identical printing media. He says : " In projection there is, of course, a scattering of the light trans- mitted by the negative (Fig. 182). The ray SN coming from the light source 5" is scattered in passing through the negative N, and only a part of the light coming from the negative can enter the lens. As in the transparent parts of the negative the loss by scatter is nearly zero (owing to the relative absence of reduced silver), it follows that the contrast between the non-scattering parts of no density and the scattering parts of high density will be increased by the scatter. In 3 /. 5". C. I., 1891, 10, 98. 4 Phot. J., 1898, p. 102. B Phot. J., 1909, 49, 200 ; Zeit. miss. Phot., 1909, 7, 257. 408 PHOTOGRAPHY contact printing this scattered light is not lost, and consequently the contrast is much less than in the case of projection." It follows, therefore, that negatives for projection printing require less contrast than those intended solely for contact printing. Owing to the intervention of certain factors, for which numerical expression is unavailable, no satisfactory means exist for determining the differ- ence in contrast which should exist in negatives for projection print- ing and those destined for contact printing. Here, as in numerous other cases in practical photography, experience is the only safe guide. Fortunately, however, the differences among printing media and the opportunities for control in the operation of printing are such as to largely remove this disability so that it is, to a certain extent, possible to secure from any average negative an enlargement with virtually the same gradation as a contact print. In general, however, thin negatives, soft rather than hard, free from fog and from physical defects of all kinds, as well as any undue granularity, are best for enlarging. Recent researches have shown that there is virtually little or no difference among the common de- veloping agents with respect to the granularity of the image. The use of any particular developing agent is therefore not so important as the avoidance of high temperature in developing, fixing and washing: in prolonged drying in hot, humid air and in under exposure. All of these things tend to increase the granularity of the image and are to be avoided with negatives destined for projection printing. In enlarging from small film negatives, the granularity of the image and defects due to minute scratches and abrasions of the film base can be reduced by immersing the film during exposure in a liquid of ap- proximately the same refractive index as the film base. Suitable liquids for this purpose are glycerine or toluol. A small quantity of either of these is poured on a piece of clear glass and the negative laid down on the layer of liquid so that no air bells are formed. More glycerine or toluol is then poured on top of the negative and this N Fig. 182. Scatter of Light by Negatives. (Callier) PROJECTION PRINTING 409 covered with a sheet of glass ; both sides of the film being thus covered with the liquid. In this condition the negative is inserted in the en- larger and the exposure made. After the enlargement has been made the film is washed for 15 minutes and dried.6 The Technique of Projection Printing. — Assuming the apparatus to be in order and everything ready for use let us consider briefly the technique of projection printing with different forms of apparatus. With daylight or with completely diffused light sources such as the Cooper-Hewitt mercury vapor lamp, reflected light or incandescent lights with reflectors, the operations are simple indeed. The light is first turned on and then the negative inserted in the carrier with its face towards the easel. The projected image is then focused roughly on the easel in order to determine the degree of enlargement. If this is satisfactory all that remains is to focus sharply, cover the lens, or turn. off the light, place the sensitive paper in position and expose. However, if the projected image is larger or smaller than desired the easel must be moved nearer to, or farther from, the lens, as the case may be, until it is seen that the projected image is approximately the size desired, after which the image is sharply focused and the ex- posure made. These distances from the lens to the easel and from the lens to the negative are conjugate distances and may be readily calculated for any given set of conditions (Chapter III, page 74). The following table, however, will show the conjugate distances for all ordinary degrees of enlargement and for lenses of the usual focal lengths. By " degree of enlargement" is meant linear enlargement. Thus from 4x5 to 8 x 10 is two times enlargement, not four times. When condensing lenses are employed the operations are not so few in number or so simple. In this case the negative should be inserted in the carrier and the image roughly focused to the desired size. The negative carrier should then be removed and the light source adjusted to secure an evenly illuminated field of maximum intensity. These ad- justments have been noticed already on page 400 of this chapter. The light source having been adjusted so as to obtain a uniformly ' illuminated field, the negative carrier is again inserted and the image accurately focused after which the exposure may be made. Then the paper may be pinned in position and where the easel is not provided with means for altering its position in order that the pro- jected image may be brought within certain previously marked lines, 6 Hickman, Brit. J. Phot., 1927, 74, 87. 410 PHOTOGRAPHY Table for Calculating Distances in Enlarging or Reducing From The British Journal Photographic Almanac Focus of Lens Times of Enlargement and Reduction Inches 1 Inch 2 Inches 3 Inches 4 Inches S Inches 6 Inches 7 Inches 8 Inches 4 4 6 3 8 2% 10 2% 12 22/5 14 2% 16 18 2% 2% 5 5 7V2 3M 10 3% 12% 3% 15 3 173^ 27l0 20 277 22% 2»/» 3 6 6 9 4% 12 4 15 3M 18 33A 21 3% 24 377 27 3% 3% 7 7 10U sH 14 4M 17% 4M 21 47s 24% 47m 28 4 31% 37io 4 8 8 12 6 16 5% 20 5 24 47* 28 4% 32 44A 36 4% 4% 9 9 13% 18 6 22% 57s 27 SV« 31% 5% 36 S'/t 4°% SVm 5 10 10 15 7% 20 25 6% 30 6 35 57« 40 577 45 5^ 5% 11 11 16% m 22 7% 27% 6Vs 33 6% 38% 67.2 44 677 49% 6% 6 12 12 18 9 24 8 30 7% 36 775 42 7 48 67? 54 6M 7 14 14 21 10% 28 9% 35 42 8»/« 49 S7« 56 8 63 7% 8 16 16 24 12 32 10M 40 10 48 97* 56 9% 64 9'A 72 9 9 18 18 27 I3tf 36 12 45 11M 54 iq7» 63 10% 72 1 o7, 81 10% The object of this table is to enable any manipulator who is about to enlarge (or reduce) a copy any given number of times to do so without troublesome calculation. It is assumed that the photographer knows exactly what the focus of his lens is, and that he is able to measure accurately from its optical center. The use of the table will be seen from the following illustration : A photog- rapher has a carte to enlarge to four times its size, and the lens he intends em- ploying is one of 6 inches equivalent focus. He must therefore look for 4 on the upper horizontal line and for 6 on the first vertical column and carry his eye to where these two join, which will be 30-7 The greater of these is the distance the sensitive plate must be from the center of the lens ; and the lesser, the distance of the picture to be copied. To reduce a picture any given number PROJECTION PRINTING 411 of times, the same method must be followed; but in this case the greater num- ber will represent the distance between the lens and the picture to be copied, the latter that between the lens and the sensitive plate. This explanation will be sufficient for every case of enlargement or reduction. If the focus of the lens be 12 inches, as this number is not in the column of focal lengths, look out for 6 in this column and multiply by 2, and so on with any other numbers. it is preferable to cover the lens with a lens cap containing an orange light-filter transmitting rays to which the paper is insensitive. This colored screen may be prepared by soaking an unexposed, fixed-out and thoroughly washed plate in tartrazin, naphthol, yellow S, orange G or ammonium picrate, which should be used in saturated solutions and the plate immersed in the dye bath for about 15 minutes, then rinsed and dried. However, where lateral or up-and-down movement of the easel is possible it is perhaps preferable to line the board in squares as previously suggested and. center the same with respect to the projected image when focusing. Then the light may be cut off entirely and the paper placed in position by the aid of the numbered squares. We have already shown on page 405 methods which may be conveniently employed. With automatic focusing apparatus in which the image is always in focus regardless of the degree of enlargement the process becomes as simple as contact printing. In this case one has only to adjust the distance between the proj-ection apparatus and the easel to secure the size of image desired, after which the paper may be placed in position and the exposure made. Focusing— There will be no difficulty in focusing as a rule; how- ever, with dense or fogged negatives and at high degrees of enlarge- ment some trouble may be experienced occasionally. In such a case it is well to take an old negative which is quite dense and make a few ragged scratches on it with any sharp-pointed instrument. This may be inserted in the negative carrier in place of the negative and can be focused sharply with ease, after which it is removed and the negative reinserted. Where the exact nodal plane of the projecting lens is known it is possible to construct a focusing scale for the lens and easel using as a basis the distances given in the table on page 410. It is not often, however, that the positions of the nodes are known and in this case the method indicated by Mr. A. Lockett may be usefully em- ployed.7 All that is necessary to provide any enlarging lantern with an accurate focusing scale, by this method, is the precise determina- 7 Brit. J. Phot., 1924, 71, 171. 412 PHOTOGRAPHY tions of the positions at two different degrees of enlargement, say 3 and 4 times linear, marking the position of the lens standard on the base of the camera for each degree of enlargement. From these two points we can calculate the position for any other degree of enlarge- ment. Since the marks 3 and 4 on the baseboard of the camera indi- cate the actual tested extensions for that degree of enlargement, when set at position 3 the conjugate focus must be F + (F/3) while at 4 it is F -4- (F/4). Eliminating F which occurs in both, the distance 3 to 4 is equal to or Y2 of the focal length of the objective, so that the distance 4-6 on the scale equals the distance 4-3. In a like man- ner we find that Distance 3-4 = distance 4-6, Distance 3-6 = distance 3-2, Distance 2-3 = distance 2-1 J/2, Distance 3-1 J-2 = distance 1 J/2-1, Distance 6-8 = distance 6-4. Accordingly the scale of positions on the baseboard of the camera will appear as in Fig. 183. Projection printing with a lantern thus fitted Fig. 183. Graduating Focusing Scale for Enlarging. (Lockett) is only slightly less rapid and convenient than with the more expensive automatic focusing apparatus, since nothing more is required than to set the position of lens to scale according to the degree of enlargement required. Determining Exposures in Projection Printing. — The usual method of determining the proper exposure is by trial with test strips and this is the only certain method in practice. Several methods of determin- ing the proper exposure have been devised by various writers but on the whole they all demand more work and time than the average worker is willing to spend and in practice the method of exposing test strips is almost always used. Edward S. King 8 and Rev. F. C. Lambert 9 have described methods 8 King, Brit. J. Phot., 1906, 53, 188. ' Lambert, Amat. Phot., 1921, p. 161. PROJECTION "PRINTING 413 in which the image thrown upon the easel is examined by a candle and the distance noted at which the candle must be placed in order to ob- literate all traces of the image. This distance in inches is then squared and the result multiplied by a correction factor which depends upon the aperture, the paper, etc., and is found by trial for any given set of conditions. In practice the writer has found extinction photometers of the Hyde exposure meter and lea Diaphot type useful and convenient as indica- tors of the approximate exposure. The projected image is examined through the instrument and the black-glass wedge turned until the de- tails in the highlights are just visible. A little experience will enable the proper point to be reached quite readily. The exposure indicated on the meter is then multiplied by a correction factor which depends upon the speed of the paper, the stop in use, and the color of the nega- tive.10 Relative Exposure, Scale and Aperture in Enlarging or Reduc- tion.— When the best exposure has been found for a given negative at a certain size of enlargement, the time of exposure at any other degree of enlargement or reduction is easily found, provided condensers are not used. Perhaps the most convenient method of making such calculations is by the charts described by Capt. S. M. Collins.11 :I5 a HO r8 -6 -4-5 • c -3~ H b 50- 40 30- l20\ S3 : £ 15- d : S> io: 8-- 7- 6-= 5- 4- 3.5J c I5n.50e -40 10- 8 6 S5 "5 4 c - 3 30 720 V 15 E 'T--5 :-4 lJ-3.5 d 4-pl 5- 6 8 f2 10- ■3 8 l5>4 I 5 £ -6 ,20- 3o-:fl 40^10 50^ 60-M5 In the general problem of exposure there are 3 factors which may all vary; these are : the size of the picture, the aperture of the lens, the duration of the exposure. Any two being known the other can be 10 For a very complete system of exposure calculation based upon measure- ment of the highest density of the negative in a wedge photometer, see article by J. M. Sellors in the British Journal of Photography, 1923, 70, 349. 11 Brit. J. Phot., 1923, 70, 31. 414 PHOTOGRAPHY found. The following scales provide a means of making these calcula- tions. From Chart I is read the alternation in the F/number required when varying the size of reproduction to obtain the same exposure. In Chart II, C and D show the mutual relation of size and exposure if the same F/number is retained : and scales E and D connect the F/number and the exposure when it is necessary to change either without varying the size. To find the proper diaphragm when it is desired to alter the size of reproduction, but not the exposure, using Chart I run the edge of a ruler from the size of the image to the aperture used. Mark the posi- tion at which the ruler cuts the index line. From this position run the ruler to the new size of reproduction. The other end cuts the aperture scale at the proper aperture to employ. To determine the time of exposure with the same aperture for an- other degree of reproduction we make use of scales C and D of Chart II. The straight edge is applied to the scales so that it cuts scale C at the size of image for which the proper exposure is known and scale D the time of exposure. Mark on the index line the point of intersec- tion as before and join this point to the corresponding size of image on scale C. The other end of the ruler will then cut the scale D at the proper time of exposure for the new size of reproduction. The aperture required for any given exposure when that for a given exposure is known may be calculated from scales D and E. With the edge of a ruler join the aperture on scale E with the corresponding exposure on D. From the point of intersection with the index line transfer the ruler so that it passes through the exposure required on scale D. Then the other end cuts scale E at the proper aperture to use. Introducing Clouds in Enlargements. — In the case of landscape negatives with a bald-headed sky, it is often possible to improve the pictorial effect considerably by printing in clouds from another nega- tive. While there are several ways of doing this, the method to be described is as satisfactory as any and is perhaps the most generally useful. The image of the landscape negative is first focused on a sheet of thin cardboard placed on the easel. On this sheet of cardboard the outline of the skyline is traced with a soft pencil. The cardboard is now removed and cut along the pencil line. The upper piece will serve to mask the sky portion while making the exposure for the land- scape while the lower portion will serve to mask the landscape while the cloud is being printed. PROJECTION PRINTING 415 This much done, the bromide paper is placed in position and the proper exposure given for the landscape portion masking the sky por- tion by the sheet of cardboard. It is not often that the sky portion re- quires any masking, but when it does the appropriate mask is held close in front of the bromide paper and kept in slight up-and-down movement with the cut-out outline in close register with the image. Now replace the orange cap on the lens and trace lightly on the bromide paper with a soft lead pencil the outline of the sky. Then, without moving the paper, replace the landscape negative bv the cloud negative and adjust the latter so as to secure the clouds in the desired position. Next bring the other mask in position so as to cover the landscape portion and make the exposure for the clouds, keeping the mask moving up and down as before. If the proper times of exposure for the two negatives have been accurately determined by exposing pairs of test strips and developing the same together for the same time, this procedure should result in a satisfactory result. Of the aesthetic factors in the combination of landscape and sky from separate negatives, it is not within the province of this work to speak. The worker's sense of the aesthetic and his knowledge of na- ture must ever be on guard in combination printing in order that the result be true to nature and satisfactory to the artistic sense. Enlarged Negatives. — Where a large number of prints are required it is sometimes more convenient to make an enlarged negative and print from it by contact, while certain printing processes as gum,' carbon, and oil require an enlarged negative if prints larger than the original negative are desired. There are two practical methods : In the first a contact positive is made from the original negative using either the carbon process or a slow dry plate and from this positive the enlarged negative is made in the ordinary way. The second method consists in making an enlarged positive of the size required and from this making the negative by con- tact printing. Other methods involving the reversal of the image have been advised but as they are hardly suitable for practical use they will not be discussed. Sensitive Materials. — The sensitive materials used are an important factor in securing satisfactory results. Most inexperienced workers make the mistake of selecting for this work plates of the transparency or lantern slide type. While admirably suited to positives for visual examination, such plates are not well adapted for making either the 416 PHOTOGRAPHY intermediate positive or the final negative. Far better results are to be had from the use of a medium speed, clean-working plate of normal contrast. The writer has used successfully Eastman Commercial Film, Imperial Fine Grain Ordinary and Cramer Slow Iso plates. The class of plates of which the above are typical representatives are rather faster than ordinary enlarging materials, especially the. East- man film, which is a comparatively fast yet clean-working emulsion. They consequently demand greater caution in handling and exposure, but give better gradation than the contrast-working transparency plates which tend to produce blocked highlights and clear shadows, causing a loss of detail at both ends of the scale, together with excessive con- trast as a whole. Extra rapid plates are of course somewhat more difficult to handle and do not give contrast or density quite so readily as those of slower speed. This property may of course become an advantage in dealing with contrast originals, for which plates of the rapid class may be used with advantage, just as transparency plates may be serviceable at times with very weak originals, but on the whole it is preferable to select a clean-working, fine-grained plate with an H. and D. speed ranging from 100 to 150, as the contrast of the final result may be controlled to the extent usually required by alterations in the time of development. Exposure. — The conditions of exposure will naturally vary -with the materials chosen and the equipment of the individual worker. Care t must be taken during all operations to see that both the negative and the sensitive materials are completely free from dust, otherwise there is likely to be a fine crop of small transparent spots which completely ruin' the result. It is well to call attention to the fact that perfect contact between the two surfaces is essential. The slightest want of contact which may not be observable in the small positive will become serious when enlarged, particularly if the degree of enlargement is considerable. For this reason the light amateur printing frames should not be used for this purpose, but rather the heavy professional frames which are equipped with much stronger springs. It is also of equal importance that the focus be accurate .when enlarging. The simplest way of ensuring accurate focus is to replace the usual easel with one consisting of a removable sheet of ground-glass on which the image can be focused by transmitted, rather than reflected light. The use of a ruled test plate in the negative carrier is also to be advised. The time of exposure will naturally vary with conditions and must be determined by test. For this purpose the first plate should be ex- PROJECTION PRINTING 417 posed in strips, giving a range of exposures from which the proper time of exposure may be determined after development. The proper exposure is that which is just sufficient to penetrate the deepest de- posits and produce a deposit on the sensitive material. No part — excepting perhaps a small point — of the intermediate positive should be clear glass. Even the highest highlight should show a slight de- posit. Nor should the deepest shadows be of any great density. What is required is a soft, almost flat-appearing positive of full detail and delicate gradation. Under exposure is to be avoided and par- ticularly worthy of serious attention is that slight under exposure . which tends to give a brilliant, clean-cut result. Invariably this bright, snappy, clean-cut result, for which the inexperienced worker .is quite enthusiastic, is the result of slight under exposure and is ac- companied with a loss of gradation at both ends of the scale, but more particularly in the highlights. The strip which has received from two to four times the exposure of the snappy-appearing strip (which suggests a good lantern slide) is a better indicator of the proper ex- posure than the brilliant strip. Development.- — To accurately reproduce th'e original negative both the intermediate positive and the negative must be developed to a de- gree of contrast equal to that of the original negative, or what is termed technically a gamma of unity. If it is desired to lessen the contrast of the original, the time of development of either the positive or the negative, or both together, may be shortened so that each is developed to a stage of contrast less than unity. This is a matter far more easily accomplished with exactness by time development than by either inspection or factorial methods. The Watkins thermo system of development is perfectly adapted to the development of both the intermediate positive and the final en- larged negatives. The thermo system, however, is calculated for a degree of contrast of less than unity (.9), hence if it is desired to secure an accurate reproduction of the contrast of the original nega- tive a longer time of development than that indicated by the tables is required. While I have not calculated its mathematical accuracy, I have secured results sufficiently exact for all practical purposes by developing the positive as directed by the tables, but classifying the plate or film used for the negative one class higher than listed on the table of developing speeds. This opposition of effect, while perhaps not mathematically exact, gives approximately the same degree of 418 PHOTOGRAPHY contrast as the original negative. To reduce the contrast of the final negative both the positive and the negative may be developed as in- dicated by the tables, or in the same way, to increase contrast both may be developed as if listed one class higher. In either case, the main point is that one is working under standardized conditions, which enable the source of trouble to be located and the necessary changes in procedure made in a calculable way. General Reference Works Bayley — Photographic Enlarging, 1923. Fraprie — How to Alake Enlargements. Smith— Enlargements— Their Production and Finish. Snodgrass— The Science and Practice of Photographic Printing, 1923. CHAPTER XIX LANTERN SLIDES AND TRANSPARENCIES The Negative. — In no branch of photography is better technical work required than in making lantern slides and the proper place to begin is with the negative. The clever worker may scrape and make numerous alterations on his negative in such a way that no one will be able to tell the difference in the print, but practically no handwork may be done on a negative from which a slide is to be made, for every touch is magnified from fifty to a hundred times and becomes painfully evident on the screen. Therefore if one is making negatives which may be used for slides it is important to choose a plate of fine grain and reasonably free from mechanical defects. The interior of the camera and the plate holders should be kept free of dust and the plates carefully dusted before loading the holders. Fixing and developing solutions should be fresh and filtered and it is better to fix in an up- right tank rather than a tray. After washing, each negative should be cleaned with absorbent cotton to remove adhering sediment and the negative placed in a dust-free place to dry. Too much trouble can hardly be taken in securing the finest quality negative as practically nothing can be done to remedy faults in technique, except, of course, reduction or intensification. Lantern Plates. — Nearly every plate manufacturer makes at least two varieties of plates for lantern slides. We may divide the com- mercial plates into three classes : 1. Fast plates for reduction in the camera. 2. Slow, " gaslight " plates for contact printing. 3. Plates of extreme contrast or made especially for warm tones. Representative brands of the first class are the lantern plates of Cramer, Hammer, Eastman, Agfa, Hauff, Gevaert, Ilford, Barnet, Illingworth, Imperial and Wellington. Slow lantern plates, handled in almost the same manner as " gas- light " paper, are almost entirely of foreign manufacture. Prominent among plates of this type are the Wellington 5". C. P., Imperial Gas- light, Illingworth Slogas lantern plate, and Gevaert. 419 420 PHOTOGRAPHY Various tones may be secured on most of the lantern plates named above by proper manipulation but there are a few plates which are made especially for the production of warm-toned slides, namely, the 11 ford Alpha, Gevaert IV arm-tone and Grieshaber's Varicta. Printing Frame for Contact Printing. — The beginner is advised to start by selecting negatives from which slides may be made by contact printing and, after he has mastered this method and can make a good slide from any reasonable negative, he may take up reduction. For Fig. 184. F and S Lantern Slide Printing Frame contact printing the negative must not be larger than 3 x 3^4 inches but it often happens that only an area of these dimensions is really wanted from a larger negative and contact printing is then possible. While an ordinary frame may be used it is better to either purchase a special lantern slide frame such as the one illustrated (Fig. 184) or to make one from the following description. Select a frame two or three sizes larger than any of the negatives from which slides are to be made, say 8x10 for 5x7.. In this is placed a piece of plain glass and over it a piece of opaque paper in the center of which has been cut an opening 3x4 inches. Instead of the usual hinged back one of a piece of flat wood, its under side covered with felt, is fitted in. This has an opening in the center measuring 3% x 4. A little door of the same dimensions is hinged to one side of the larger back and one of the springs attached to fasten the door and secure contact between the negative and the lantern plate. To use the entire back is taken out and the negative inserted, the desired portion being placed exactly over the opening in the black paper. The back LANTERN SLIDES AND TRANSPARENCIES 421 is now replaced and fastened down. The small door may then be opened, the lantern plate inserted and the exposure made. Any number of slides may thus be made from the same portion of the negative without readjustment. Exposing. — For exposing use any artificial light, preferably elec- tricity. The bulb should be frosted or an image of the filament is liable to fall on the frame. A board should be marked off so that the distance from the frame to the light may always be the same, and variations in the strength of "the light eliminated. It is impossible to give any idea as to the length of exposure since lights differ and no two brands of plates have the same speed. The best plan is to make a series of trial exposures for different times on the same plate and from this series pick the one giving the best results. To do this hold a card in front of the frame and uncover an inch of the plate for say two seconds, then shift the card so as to expose another inch and give another two seconds and so on until the whole plate has been exposed and we have four strips the exposures of which are 2, 4, 8 and 16 seconds. In order to make the strips equal and regular the positions of the card may be marked on the outside of the frame. After de- velopment and fixation the plate may be examined and the exposure giving the best result readily determined. Before treating develop- ment, however, we will discuss the advantages and methods of making slides by reduction. Printing by Projection. — The writer firmly believes reduction to be superior to contact printing. The definition is better, there is less Fig. 185. Century Lantern Slide Camera for Reduction danger to the negative, or the lantern plate, and any shading to lighten or darken parts of the slide is more easily done. There is also the great advantage of being able lo make a slide from a negative of any size when either wet or dry. This last feature is particularly desirable when an advertising slide is wanted in a hurry. 422 PHOTOGRAPHY Slide making by reduction is simply rephotographing the original negative on the lantern plate. Special cameras are available for this purpose (Fig. 185) having at one end a set of nested kits for holding the negative and at the other an adjustable back taking a plate holder of lantern slide size, the lens being fixed in the central compartment. Except for slide making in quantity, their expense is not justified. Another method consists in blocking out a window in the same way as described on page 392 in the chapter on projection printing and placing the camera on a sliding track facing the negative as shown in Fig. 186. For slide making in quantity, however, artificial light of some kind is far more satisfactory than daylight owing to its uniform- Fig. 186. Slide Making by Reduction Using Daylight ity. If desired, artificial light may be used in a similar way, condens- ing lenses or reflectors being used to secure uniform illumination. It is more convenient to use for this purpose a camera which focuses from the rear. With a camera focusing from the back it is possible to set the lens at the conjugate focus with respect to the original and the degree of reduction required and focus without disturbing this relation. This is not possible with a camera which must be focused from the front as any movement of the lens to secure accurate focus naturally disturbs both conjugates. . Those provided with a good enlarging lantern may use it for reduc- tion by using a supplementary lens over the regular lens to shorten the focal length, or better by building an extension cone sufficient to pro- vide the amount of bellows extension required. The length of the cone will depend upon the focal length of the lens, the degree of reduc- tion and on the length of the bellows fitted to the machine. The tables of focal distances given in the chapter on projection printing will be of assistance in determining the length of the extension cone. LANTERN SLIDES AND TRANSPARENCIES 428 The most satisfactory method which we have seen for holding the plate in position for the exposure is that described by Mr. D. Charles in the British Journal of Photography 1 and illustrated in Fig. 187. The details of construction will be apparent upon a thorough examina- tion of the illustration. In the writer's opinion, sharper focus may be obtained when focusing is done on a ground glass rather than by re- FiGi 187. Device for Holding Lantern Plate in Position When Using Enlarger for Lantern Slide Making by Reduction. (Charles) fleeted light. When a ground-glass screen is used methods of Parallax focusing as described on page 524 in the chapter on Copying may be employed to advantage. As in the case of contact printing, exposures are best determined by test plates exposed in sections. Calculations on the order of those described previously in the chapter on Projection Printing, however, may be useful as a first approximation. Developers. — A developer suitable for lantern slides should be non- staining, free from fog, and produce an image of good color, fine grain and comparatively high contrast. No developing agent has ever surpassed the old ferrous oxalate in meeting these requirements and it is, therefore, the developer par-excellence for lantern slides and trans- parencies. It is now seldom used, however, having been replaced by the more convenient and more energetic organic developing agents. Next to ferrous oxalate one of the best developing agents for trans- parencies is glycin. The following formula is suitable (Hub!) : Sodium sulphite (dry) 600 gr. 125 gm. Warm water to make 4 oz. 400 cc. Glycin 480 gr. 100 gm. Mix well and add gradually : Potassium carbonate 5 oz. 500 gm. Water to make y]4 oz. 750 cc. 1 Brit. J. Phot., 1922, 6g, 232. 424 PHOTOGRAPHY For use dilute with 8-12 parts of water. Slides must be well washed before placing in the fixing bath or stain will appear. Perhaps the most generally used developer for lantern slides and transparencies is hydrochinon. It has the advantage of giving good contrast and satisfactory color but the quality of the image is not so good as glycin. The following formula is suitable (Lanier) : A. Hydrochinon 77 gr. 10 gm. Sodium sulphite (dry) 154 gr. 20 gm. Potassium ferrocyanide 922 gr. 120 gm. Water to make .16 oz. 1000 cc. B. Caustic soda 384 gr. 50 gm. Water to make 16 oz. 1000 cc. For use take 10 parts of A to one of B. For general use with American plates, however, the developer as made up above may be further diluted with an equal part of water. This developer works with medium contrast. If a hydrochinon developer giving maximum contrast is required the hydrochinon-caustic soda formula given on page 287. For very fine grain images with the hydrochinon developer add 50- 300 grams (385 grains — 5^ ounces) ammonium chloride to each 1000 cc. (16 oz.) of the above developer. Other developers suitable for lantern slides are amidol and metol- hydrochinon. Pyro may be used, but excepting for warm-toned slides, is not so convenient, nor altogether as satisfactory, as the non-staining agents already noticed. Development. — The writer expresses a decided preference for the Watkins factorial method in developing lantern slides and trans- parencies. The density of a slide should be determined solely by ex- ' posure, and development regulated so as to produce the degree of con- trast required. Once the proper factor has been found for the de- veloper and plate in use it is a simple matter to locate errors in ex- posure and development. The following table shows how to deter- mine the principal defects of exposure and development and to remedy the same. Fault Cause Remedy Too much density, Over exposure Give less but develop correct contrast. to same factor. Too little density, Under exposure Give more but develop correct contrast. to same factor. Too much contrast, Over development Develop to lower factor. proper density. - Give same exposure. Too little contrast, Under development Develop to higher factor. proper density. Give same exposure." LANTERN SLIDES AND TRANSPARENCIES 425 The proper factor can be found only by experiment. It is dependent upon the plate, the developer and the degree of Contrast required. In all cases it is less than that required for negative development and as a rough guide a factor about % of the regular Watkins factor may be taken for the first trials. The proper safelight adds greatly to the accuracy in observing the first appearance of the image and in determining the course of develop- ment. For the very fast lantern plates an orange safelight, such as the Wratten Series o, should be employed; for the slower gaslight plates the Series oo, a bright-yellow, is safe. Fixing, Washing, Drying. — After development, rinse well in run- ning water and place in an acid fixing bath, those given in the chapter on fixing being entirely suitable. It is important that the fixing bath be kept fresh and acid at all times, otherwise there is liable to be a slight stain on the slides, which, while not particularly noticeable when the slide is held in the hand, will injure the transparency and brilliancy of the highlights when projected. Wash for fifteen to twenty minutes in running water and on removing wipe the surface with a piece of wet absorbent cotton to remove adhering grit and dirt. The slides should then be placed in the rack to dry, leaving at least two inches between each plate in order to ensure a good circulation of air. The use of an electric fan is advisable where possible. It is very important, however, particularly where a fan is used, that the atmosphere be free from dust and perhaps one of the best ways to prevent dust from settling on them is to place a piece of newspaper over the drying rack. Then, if the air is not filled with dust stirred up by cleaning or some similar operation, they will dry comparatively free from dust particles. Masking. — This is a very important operation, particularly where the subjects are pictorial, as the composition of the finished picture is largely determined at this stage. Of course as much of this should be done in the process of reduction as possible in order to secure the largest possible picture on the slide plate. In nearly all cases the matts used for masking off the undesired portions of the picture should have square corners. It is only occasionally that circular and oval shapes may be usefully employed. Ready cut matts are fur- nished commercially in a wide variety of shapes and sizes but with most of these the corners are rounded and except in some cases this is nearly always objectionable. Adjustable matts with which any de- sired size or shape may be obtained are far more generally useful, for, 426 PHOTOGRAPHY although they cannot be so quickly applied, there is no need to keep on hand a large number of various sized matts since the proper pro- portions and size for any requirement may be prepared from the single block of adjustable matts. Spotting. — After the mask has been fixed to the slide with a little glue and has set under pressure until there is no danger of move- ment, the slide may be spotted. Spotting is employed to guide the operator in placing the slide in the lantern correctly. The rule to fol- low in placing the spot is to hold the slide as it should appear on the screen and place the little square of gummed white paper in the lower left corner. The lantern operator places the slide in the lantern up- side down with the spot acting as a thumb mark. Binding. — This is the final operation and one at which the beginner is not usually successful. Not that it is a difficult operation, but there is a little knack to it which is readily acquired with experience. The easiest method is to employ four strips, although many experi- enced hands prefer the continuous strip method. To use the former, begin by cutting a number of strips of binding paper and 4 inches in length. The former are used for binding the ends ; the latter the sides of the slide. Then take a piece of absolutely clean cover glass (a cleaned-off slide plate) and place it against the masked side of the slide. The binding strip is then moistened and applied to the edge of the slide. First see that the strip of binding tape is placed evenly so that it will fold over regularly and uniformly on both sides of the slide. Proceed in a like manner with the other four sides, then place the slide under pressure for an hour or so. Advertising Slides. — One is often asked to produce a slide showing both printed matter and an illustration. If the two are copied on a plate suitable for the photograph, then it is impossible to obtain suf- ficient contrast in the legend, while on the other hand if a process plate is used to reproduce the legend correctly the photograph is excessively contrasty. There are two ways of overcoming this. One way is to make the copy on a plate of low speed which will give fair contrast so as to obtain the very best result for the photograph ignoring the legend. When the negative is dry, cover the photograph with any waterproof varnish and immerse the negative in a ferri- cyanide-hypo reducer until the lines of the legend are clear. Then after a thorough washing, intensify in Monckhoven's intensifier to LANTERN SLIDES AND TRANSPARENCIES 427 secure the proper contrast. When dry the negative may be printed in the ordinary way with good results. Another way consists in making one negative for the photographic portion and one for the line portion, using the appropriate sensitive material for each. The portion representing the photograph on the process negative is then blocked out by means of opaque while the legend is blocked out on the negative made for the illustration. Means of registration having been provided it is then a simple mat- ter to secure a slide of the proper quality by double printing, first for the illustration and then for the legend. Toning of Lantern Slides by Restrained Development.— Warm tones of black, brown, red, purple and sepia may be obtained by the use of a developer heavily restrained with soluble bromides and the colors so obtained are usually, in the writer's opinion, superior to those obtained by processes of after-toning! The principle consists in the over exposure of the slide plate followed by development in a developing solution highly restrained with potassium or ammonium bromide or a combination of the two. Not all plates produce satis- factory tones with such treatment and the best tones are secured on the plates advertised as warm-tone by the makers and on the slower brands of slide plates such as Eastman Slow, Wellington S. C. P., etc. The following formula is recommended for the Wellington S. C. P. and when properly used will be found satisfactory for other makes of like nature : A. Metol 20 gr. 2 gm. Hydrochinon 60 gr. 6 gm. Sodium sulphite (dry) 350 gr. 35 gm. Sodium carbonate (dry) 350 gr. 35 gm. Potassium bromide 20 gr. 0.6 gm. Water to make 20 oz. 1000 cc. B. Ammonium carbonate 1 oz. 91 gm. Ammonium bromide I oz. 91 gm. Water to make 10 oz. 1000 cc. Without solution B the tone is blue-black. By increasing the ex- posure and adding correspondingly larger amounts of restrainer warm-brown, sepia, purple and red .tones may be obtained. The fol- lowing table gives the approximate increase in exposure and the com- position of the developing solution for the various tones: IS 428 PHOTOGRAPHY Color Exposure x normal ' Developer Black Normal A I part, B — part Warm-Black Normal xi^ A I part, B % part Brown Normal x 2 A 1 part, B % part Warm-Sepia Normal x 3 A 1 part, B part Purple Normal x 4 A 1 part, B y2 part Red Normal x 6 A 1 part, B % part The appearance of the slide as it lies in the tray is not an accurate indication of its final color and the best results are secured when de- velopment is conducted factorially. The exposure determines the tone to a large extent and development must be regulated accordingly. The proper factor varies from three to five with the above formula. For the Wellington S. C. P. lantern plate a factor of three is recom- mended, while the writer has found five to be the best for some other brands of plates. • Physical Development. — -The advantages of physical development are: facility of obtaining soft slides from harsh negatives, the trans- parency of the shadows, the unique bluish-black tone and the ex- cellent results obtained by sulphide toning. The precautions to be observed are: 1. Use only fresh plates. 2. Give about double the exposure ordinarily demanded for regular developers. 3. Keep all trays absolutely clean. The following is the formula of Dr. Mees : A. Metol 96 gr. 5 gm. Citric acid 96 gr. 5 gm. Acetic acid, glacial 1 oz. 25 cc. Distilled water to 20 oz. 500 cc. B. Silver nitrate 1 oz. 10 gm. Water to make 10 oz. 100 cc. To develop a slide 1 ounce of A is poured in a clean glass graduate and 50 minims of B added. The exposed slide is placed in the tray, the mixture poured on and kept in motion. During development the silver may be deposited all over the plate but this can be removed by rubbing with wet cotton wool. As soon as the developer becomes brown it should be discarded. Fresh developer must be used for each batch of slides and the trays and graduates cleaned thoroughly each time before mixing in order to insure absolute cleanliness. Physical LANTERN SLIDES AND TRANSPARENCIES 429 development is not a process for commercial use but is an interesting method for the amateur who desires unusual effects. Colors on .Direct Development with Thiocarbamide. — The addition of thiocarbamide to a developing solution of metol and hydrochinon restrained with ammonium bromide for the purpose of obtaining a wide range of colors ranging from a delicate violet through red, blue, blue-black and black on lantern slides by direct development was first advised by Wratten and Wainwright, the English firm of plate makers, in 1909. The resultant image is of a very fine quality, with an unusual transparency in the lower tones which is obtainable in no other way, while the range of colors obtainable on slow lantern plates by modifi- cation of the exposure, developer or temperature is unsurpassed by any method of toning by direct development. The process is a very difficult one and it is recommended that fche. student studiously avoid the same, not only until he has mastered ordinary slide making in black and white, but also until he is thoroughly familiar with the pro- duction of warm-tone slides by the methods previously described. Ex- perience alone can enable the worker to master the process.2 The Toning of Lantern Slides and Transparencies. — With the ex- ception, of the hot hypo-alum method, the methods of toning described in Chapter XX may be used for lantern slides as well as prints. Ex- cellent brown tones may be secured by the usual process of indirect sulphide toning ; the copper and uranium processes are also widely em- ployed. The toned image, however, leaves something to be desired as regards transparency, the tones produced by after-toning processes never equalling those produced by direct development in this respect. Probably the finest results in after toning are secured by dye-toning processes. Dye toning has found an extensive application in the mo- tion picture industry but does not seem to be widely employed for lantern slides. For further information on dye-toning methods the reader is referred to Lantern Slides — How to Make and Color Them obtainable free from the Eastman Kodak Company, Rochester, N. Y. Reduction and Intensification of Lantern Slides. — Neither the re- duction nor intensification of lantern s'ides and especially of warm- tone slides is to be recommended as a general rule. For reduction, a weak solution of ferricyanide-hypo may be used, but in the case of warm-tone slides only a slight clearing action should be attempted as substantial reduction alters the color. 2 Johnson, Phot. J., 1926, 56, 159. 430 PHOTOGRAPHY Where greater contrast is necessary it is preferable to intensify the negative rather than the slide. However, if for any reason this is un- desirable the slide itself may be intensified. The chromium intensifier is satisfactory and convenient for this purpose. For warm-tone slides, however, preference should be given to the following silver intensifier which has the advantage of not altering the color : A. Metol 88 gr. 9.2 gm. Glacial acetic acid ! 1 oz. 50 cc. Citric acid 176 gr. 18.4 gm. Water . .-