a ea, a as ‘ RAGED nw ae Geta. +h z iE) — © Raymond Pettibon RESEARCH LIBRARY emmy) RESCARCH INSTITUTE N MOORE ANDREAS COLOR CHEMISTRY LIBRARY FOUNDATION mri) | OGRAPHY ITS PRINCIPLES AND PRACTICE A Manual of the Theory and Practice of Photography Designed for Use in Colleges, Technical Institutions and by the Advanced Student of the Science. By eo, NEBLETTE, A.R.P.S. Member of the Faculty of the Texas A. and M. College Formerly Director, Division of Photography, The Pennsylvania State College NEW YORK D. VAN NOSTRAND COMPANY EIGHT WARREN STREET 1927 All rights reserved, including that of into the Scandinavian and other ane” f | Par ae Si, PREFACE 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: (1) 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 Ausfiihrliches 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 iil iv PREFACE 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 codrdination 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 | 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 re ee eet ape en ee, 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 Calotype 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. eter ee ne 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. The Darkroom. Ventilation. Size. Ar- rangement. Water Supply. The Illumination of the Darkroom. The Safelight. The Efficiency of Darkroom Safelights. Trays, Tanks and Graduates. Miscellaneous Features. eet notoriapiic Optics.............. 02.0 eee 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. CuHapTER IV. The Aberrations of the Photographic Objective. . Introduction. Chromatic Aberration. Spherical Aberration. Coma. Curvature of Field. Distortion. Unequal Illumination. Astig- matism. Flare and Flare Spot. Vv vil CONTENTS CHAPTER V. The Photographic Objective. .... 7.90 PartI. THE AsticMats: 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 ANaAsticMaTs: 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 IIJ. THe Terropyective: Principle. The Compound Tele- objective. Early Fixed-Magnification Teleobjectives. Anastigmatic Fixed-Magnification Teleobjectives. Dallmeyer’s Adon. CHAPTER VI, The Photographic Emulsioniyag2 5 a oe CHapTrer Vil. Orthochromatics: ...... 3. he ae 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. The Nature of the Sensitivity Substance. Grain Size and Distribution and Its Relation to the Photographic Properties of Emulsions. Light and Color—the Spectrum. Visual and Photo-Chemical Lumi- nosity. History of Dye Sensitizing. Known Facts Regarding Color Sensitizing. Color Sensitizing by Bathing. Sensitizing for Green and Yellow. Sensitizing for Red. Mixtures of Dyes as Color Sensitizers. The Theory of Light Filters. Orthochromatic Filters. Contrast Filters. Orthochromatic Methods in Landscape Photog- raphy. Orthochromatic Methods in Portrait Work. Photograph- ing Color Contrasts. CuaApTER VIII. The Latent Photographic Image............. Photo-Physical and Photo-Chemical Change. The Latent Image. Artificial Latent Images. Hydrogen Peroxide. Sodium Arsenite. Reversal by Chemical Reagents vs. Reversal by Light. Photo-Re- gression. The Action of Solvents of Silver on the Latent Image. Physical Development of the Latent Image after Fixation. The CONTENTS vil Photosalts. Image Transference. Indoxyl Development. Action of Oxidizing and Halogenizing Agents on the Latent Image. Re- versal by Light. Theories of the Latent Image. The Oxy-Halide Theory. The Sub-Halide Theory. Evidences for the Liberation of Halogen. Do Silver Sub-Halides Exist? Objections to the Sub- Halide Theory. The Metallic Silver Theory. The Molecular Strain Theory. Evidence for the Molecular Strain Theory. The Electron Theory of the Latent Image. The Photo-Electric Effect. Evidence for and against the Electron Theory. The Colloidal Silver Theory. Sheppard’s Orientation Hypothesis of the Latent Image. ESTES G0 226. What is Sensitometry? Resumé of Sensitometric Investigation. Instruments for Sensitometric Investigation. Standard Light Sources. Sensitometers. Instruments for the Measurement of Den- sities. Opacity—Transparency—Density. Exposure and Develop- ment of Sensitive Materials for Speed Determination. 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. Watkins Central Speed Method. Wedge Methods of Sensitometry. The Perfect Negative. Density-Exposure Relation and Correct Reproduction. Latitude of Sensitive Materials. Development 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......... 254 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. ee eee ne: Theory of Development. i .:..5.0... 6.5 266 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. Vill CONTENTS CuHapter XII. Organic Developing Agents................- Developing Power. Classification of Developing Agents. The Source of Organic Developing Agents. The Significance of Group Relations. Slow and Rapid Developers. In Explanation. Adurol. Amidol. Preservatives of Amidol Solutions. Certinal. Edinol. Eikonigen. Glycin. Hydrochinon. Metol. Metoquinone. Mono- met. Neol. Ortol. Paramidophenol. Paramidophenol-Phenolate Compounds. Pyrocatechin. Pyrogallol. Minor Organic Develop- . ing Agents. CuapTeR XIII. The Technique of Development............. 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 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 Colycin. The Efficiency of Time De- velopment. ~ CHAPTER XIV. The Laws of Fixation and Washing.......... 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. CuarteR XV. Defects in Negatives......... 33) 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. CHapTerR XVI. _ Intensification and Reduction.,.. ).9 eee Part I. Repuction. 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 CONTENTS 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......... Part I. DeEvELopING Papers: Characteristics of Development Papers. Adapting the Paper to the Negative. Exposure. De- velopers. The Safelight. Development. Factorial Development. The Proper Factor. The Short Stop. Fixing. Washing. Drying. Alteration of .Contrast. Reduction and Intensification of Prints. The Glazing of Prints. Part II. GetatinE P-O-P: Toning. Instantaneous Toning. Black Tones with P-O-P. Fixing. pee ioe Projection Printing... 0.2.2.2 ee eee 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. Introducing Clouds in En- largements. Enlarged Negatives. Sensitive Materials. _ Exposure. Development. Serer ee | ne foaritern Slide... 0.5... ea. The Lantern Slide and Its Uses. The Negative. Lantern Plates. Printing Frame for Contact Printing. Exposing. Printing by Projection. Developers. Development. Fixing, Washing and Dry- ing. Masking. Spotting. Binding. Advertising Slides. Toning of Lantern Slides by Restrained Development. Physical Develop- ment. Colors on Development with Thiocarbamide. Toning of Lantern Slides. Reduction and Intensification of Slides. CHAPTER XX. The Toning of Developed Silver Images....... 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 1X x CONTENTS 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...... 479 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).... 489 Part I. Historica: Sensitiveness of Chromic Compounds and Bichromated Colloids. The Development of the Carbon and Gum- Bichromate Processes. The Development of the Oil, Bromoil and Powder Processes. The Chemistry of Pigment Printing with Bichromated Colloids. Part II. THe Carson AND CARBRO ProcESSES: Introduction. Car- bon Tissues. Double and Single Transfer. Sensitizing the Tissue. Exposure. Development. Double Transfer. . Transferring to Rough Papers. The Carbro Process. The Bromide Print. Sensi- tizing the Tissue. Transfer. Redevelopment of the Bromide Print. Development of the Carbro. Carbon on Bromide. Multiple Print- ing. CHAPTER XXIII. Printing Processes Employing Bichromated Colloids. II. (Gum-Bichromate and - Allied Processes) . 2. oer 3 eon 511 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- 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. Namias Method of CONTENTS 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. Peewee COPYING)... ee ee Ee eee sits 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............... 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. The Compensating Filter. Handling of the Autochrome Plate. Exposure. Development. Reversal of the Image, Varnishing. After-Treatment of Autochromes. The Agfa Color Plate. Dupli- cating Processes of Screen-Plate Color Photography. The Duplex Method. Appenpix. A List of the More Important Reference Works on EE No ae tee sip ob Piss + ie eee Sena 10) LECHNICAL JOURNALS .........6.-.cecceeeee Xl 3 ‘ t y ‘ ‘ rd , ’ ; 4 - See ¢ A - * % > E s - > ‘ “ > pos 7) ar : s 2 7 ee r aN" W + x . a ae ~ * z . ILLUSTRATIONS Fic. 1. The camera obscura from an old print. (Courtesy of the Smithsonian RRO rr EO i Fk Api wie Va able hs wadsswldseeusecss 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 Société Francaise de Photo- Re eo pt | xc wPinw sw vlalk ¢ sflvibiale dle ala e's wile w © 9 ME EN PU O TCO, ., ) . edie se ck es ec dje donde cede ee ueeucveues 10 i. yA 'y G tate wale ples bis ca a diseesllewneweeeuces 10 Beeenoeranuicuprims by Niepce, 1824. .... 0.6. nce cc ce ce nace enue II ES SE CS EB: fat oy a 12 7. Tomb of Daguerre. (Courtesy of the Société Francaise de Photo- Ss rcs ios wider ule dia Lbs x oF wm dceelS a kp et ee Wye 14 8. Fuming cabinet for the Daguerreotype process.................000008 15 9. Developing box for the Daguerreotype process..............0..0 00008 16 10. An early Daguerreotype portrait. Often stated to be the first portrait EMM Pe ea. Uk sca tle sb Sitia e vis webs Hele wertwemes 7 Bam reer ietiry Ox 1 dIDOt. oo ni is ec ie hae etna eee ane 19 12. Frederick Scott Archer. Drawing from an old print reproduced in J. Werge—The Evolution of Photography.............2.00 cece eens 22 13. The wet collodion process in the field. (From an old manual)........ 24 14. Portable laboratory for field use with wet collodion................... 25 peemeeemtenped each Maddox. ...... 25556250 bee nse leew ececeaaewensunrs 25 16. Typical miniature camera for plates and for roll film................ 36 fy stan cameras tor plates and for roll film..........0-.0....0000.e00 38 18. Professional view camera..................--0200- eM ath Gc, Saude 39 Se evOr tiie Tenee Caticia... 2... 26. cette een eee deeeraes 41 20. Principle of the swing back. Use of the swing back for securing _ TIME LOCUS. oo c sw inp a dian o's Sin cic etek mes cee ase nbn cas 43 21. Ventilation of the darkroom. (Courtesy of Eastman Kodak Company) 47 22. Floor plan of darkroom for amateur use......................-20 ee 48 Sameer er airece Garkroom lamp............- 000-6 s cece eens ne ceeens 50 24. Design for indirect darkroom lamp. (Krug, American Annual of Pho- — I iiss eek a van ete een. y vie'css 40.0.9 'a a eee ee 51 6 GSP Er 52 Peveaten darkroom safelight lamp..:...........600. 25 ccc s eee eee eees 52 Samoavine capinet for plates and films.............-.-.0cccceen ence cctee 57 Seeewnrincipics Of Tefraction...........2 20sec cee cence eet eee eet eee 60 29. Refraction in a medium with parallel sides..................0---. 05. 61 USCS SU 61 TET Fg ga 62 memeeiipal forms of simple lenses............. 0.6.00 e stew eee eee ates 62 33. Image formation: with positive or converging lenses................+. 63 xi XIV ILLUSTRATIONS 34. Course of light pencils through a negative lens....................-. 63 35. Image formation according to the Gauss theory...............+-..+- 64 36. Image formation according to the Gauss theory..................+++-- 64 37. Position of the nodes in common forms of simple lenses............... 65 38. Focal length... 0. ..cc sees te ww upc ca ees + +56 6 alneennnnenn nna 66 39. Table for the calculation of angle of view................sesveeeeee ae 68 40. Graphic illustration of the principle of depth of focus. (Von Rohr).. 72 41. Intensity of the optical image. (Brown)....>.9.a290) eee 76 42. Effect of chromatic aberration on definition.c..7y..9 ee ove 85 43. Chromatic under correction............ 05... oe sienna 86 44. Chromatic over correction, ..5........2e2.-0+ 505 sien eee 86 45. Correction of chromatic correction. (From Beck and Andrews, A Simple Treatise on Photographic Lenses) ...........0e.0e-0eusues 87 46. Irrationality of dispersion. .....3 6.5. ...esess eee) ore ea 88 47. Spherical aberration... 055... «0+» ...0. 4 eee ee 108 70. The Aplanat or R. Row ccc cu wien oc cite oo chen eee ne ene 109 71. Portrait of Petzval uc oy. ss sans seco 0 viper gee ee Iil 72. Petzval’s portrait objectivé...... ++. 52. se oan yee III 73. Modifications of the Petzval portrait objective. ((a@) Dallmeyer, (5) Voigtlander, (c) Zinc-Sommer) ...5.29. koe ee Tis 74. The Goerz Dagor. .. 20. 0esecs ve sas nine ln sien aeons 114 75, Watson’s Holostigmat............% PP ee 115 76. The Collinear and Orthostigmat of Voigtlander and Steinheil....... 4 FTO Deere ee rOtar oeries VITO. Gi. che de lees ccs ee ceeesccence 117 TR ERED a he aes alg ley be be ew's Pmt wh able e dad ee see 118 Meer aNd SYMtOr Of GOETZ. 6.065 ck ck ce eee bea vy ce eee eee e's 118 ET STE i ge og a 119 IEC TEE SAEIHOL OCTIVE fo, gsr isis sa Alene eels pu ee dele ee ca eaees 119 Ea lyicy be sis avis Sv Laake Gc daw dee een bee’ *, ¥120 nen PE ety ST ISCONTIP MAN. Gok ne Slice Psa bs ces vais cack aveeisavcbaae> I21 NE 122 ITO cee se ae RU ds go Mad ie Wie Dv ald Pealev ge ewe e's 122 nn NPR OCGA ed Mars ke ek ae 8 sie wee be vb elew Javea das 123 SOU MCR ee ee eles ees ee avn cab ee ced ivee vie 124 pe eC MME UES. Ses dete eis la el deka evcube dees 125 mae Ree ANE EDA Ge G's. bc aves eae ode e'vsaceedab lescneaneuus 126 MP EE SEIDITIR TIC ose sw sine ps oid ees eae dielesceab sue videwenus 126 Graco eeranoymmetrical Protar i... ci i So lacie cw ceeues 127 eee 5 Uy dic je Pod oe Obie bad edad ve Huewedeee less 128 See ER RE PA iy cs ede lo CR vedas lia faa oe bs owe k vaca: 129 a PME SOG e's, ec Z si coe bs cleus cP ¥ ye bse Oh base a ew Sules 130 a IS ERIE Pe te, oy Oy a's ee ga ews ba veka eee pew leds 131 ee IEEE tei ep 2s ee ey es Pee ova skh ea wea edeens 131 a RINE ET Se ee ol ans db coelp es owl ev dee ead nev Pies 132 encom. te cenital diveroing lens... ee eee cee cee ce ees 133 99. Cooke Aviar—shorter foci—longer foci.....................000 05, eta ASE CIO MS hk ia ke peo tg a he chee vis de Sec snecacdeccsubye 135 SS TT) 8 CI ea a 136 SE, 5 ke ok ie ies Ve edine co dvev ote dehel oboas 136 ee gs Fk. Das gee bs Vc Sb lad ebb ep whe ek ete’ 137 See NE CAPO eels fotos ee en Chae edccsgeasuteteaueces 137 SN Oar Sees yk ge o's Fk as Cea ha bed Oe Coe wee Dae wee 138 Dia alieevers compound teleobjective.. of. 2.0.00. ee ee cc ek eee 140 Bare IGOU Ss ee yee ces cic cw kv ac elvan su ueceurvevevesteres I4I ROC d Gy Gas tke ele ccc ce tec caer epaewusupeaneuwnas 142 eae EE Se igs ye pv ci cleans savacauvcsosspwdeianee te 142 110. Ross Telecentric.......... es ae ere ee eS he Ne Lae, 142 MUTE SATO AON oe. oc ne blk y oy bb alee dake ka eee use baba we Ge ole . 142 i EYRE cy he ttle his Cocuc na cas aiea dua waevusoee wees 143 Ru RT LOTT CCUIV Go. poss vv eo ve Stave nos eb bs hes epee hed awnnrs eee 143 ee ee ee ccp 66 was ek Cee Mae Ge C4 nding hai ad Role es 144 I doe suis gad devises oe toes ae pate wa a wihie ad Hedees we hee 144 Pie oss. 1 ClerOs,.........; OTe Se PII IL REVO See EDR, SER RE AT a 145 OE VS EUS ES EY) se) en miei i 145 MM TIIAMIOICG <1 CLEMO YTIAL So suivoace dino C Wise vis Gag od oes es eve dip eeaedeue aes 145 TEN ey. cg eg moo) Win 4s 9. dw ab elu EON wheal wae a ela eS 146 120. Emulsion washing apparatus. (From Eder’s Ausfiihrliches Handbuch). 162 121. Centrifugal separator. (From Eder’s Ausfiihrliches Handbuch)...... 162 122. The photographic emulsion under a microscope...............000e een, 163 2 ILLUSTRATIONS ay XV1 ILLUSTRATIONS 123. Cross section of a developed emulsion. (Courtesy of Dr. A. P. H. Trivells) 4s. 0s. vee ooert eee bana 20-3 sl she: Otol cecce ble gee 164 124. Hodgson’s preferred points... 2... :ss/s5 ss Shuey «lope eee na 166 125. Size frequency distribution of silver halide grains in a portrait film and lantern slide emulsion... 0... 0.40. 5... 20s ene 170 126. Three-color print of the spectrum...........% + 0s) 172 127. Visual luminosity of the spectrum after Abney....................... 173 128. Spectral sensitiveness of the silver halides after Meldola.............. 174 129. Drying cabinet for sensitized plates..........., .) usu = ena 7 130. Spectrograph of Eosin. Bureau of Standards paper No. 422.......... 178 131. Spectrograph of Erythrosine. Bureau of Standards paper No. 422.... 179 132. Spectrograph of Rose Bengal. Bureau of Standards paper No. 422.... 179 133. Spectrograph of Orthochrome T. Bureau of Standards paper No. 422 179 134. Spectrograph of Pinaverdol. Bureau of Standards paper No. 422..... 180 135. Spectrograph of Pinachrome. Bureau of Standards paper No. 422.... 180 136. Spectrograph of Pinachrome blue.......,-.7.s..7s0sseeae 181 137. Spectrograph of Pinachrome violet..:.. 7... 4. seu: 5 seem eee ee 181 138. Spectrograph of Homocol. Bureau of Standards paper No. 422...... 181 139. Spectrograph of Pinaflavol. Bureau of Standards paper No. 422...... 182 . 140. Spectrograph of Cyanine. Bureau of Standards paper No. 422........ 182 141. Spectrograph of Dicyanine. Bureau of Standards paper No. 422...... 183 142. Effect of ammonia on sensitizing curve of Dicyanine. Bureau of Standards. paper NO, 422.0....4s0a4hienn +» 4's eat had he 184 143. Spectrograph of Dicyanine A......:..2<. Jig ee 185 144. Pinacyanol. Bureau of Standards paper ae ABB: 25) side eee 185 145. Naphthacyanole 2. . 0.56. dss vee ce vale} nals nn ge em ea 186 146. Kryptocyanine 2.0... 6a. e0' oh ooo. «cum «© wohl cui emnn 186 147. Red sensitiveness with bisulphite...........-. 0:5. seme en ae 187 148. Pinachrome and Pinaverdol...........:.... sa 62 ae 188 149. Action of graduated filters... . 2... 1.15.0 is< owas nee nee 192 150. Portrait on ordinary and panchromatic emulsions.................-.-- 193 151. Spectrum of daylight and mazda clear glass bulbs. Bureau of Stand- ards paper No. 422. 0.6.24 sens.ss +s ee 4 cis cin pales enn 195 152a. Photograph of manuscript in blue with red corrections using a green 111 (0 a CPE 196 152b. Photograph of manuscript in blue with red corrections showing use 6f red filter... 6.0.6 i. on plevs sans ys oe eee ie geen ene 196 153. Wood sections on ordinary and panchromatic plates. (Courtesy of Ilford Ltd.) «0.5 66s Seen sa ee piecic eats poke anen 198 154. Illustration of the molecular strain theory of the latent image. (Has- luck, The Book of Photography); «......:s.<0+ssun ee 219 155. Chapman Jones plate speed tester. ........... 0s seus) 54s 228 156. H. and D. sector wheel and exposing apparatus 0.22: 3.400) Gee 229. 1s7. H. and D. densitometer... 0... 0s. 200s «aus wus «oe ee 230 158. Illustrating the relation of opacity, transparency ad eae ; ae $6525 159. The characteristic curve... ois. 565 csv cu eul oes as 55 B36): 2c e 160. Step chart illustrating the theory of the characteristic curve.......... 237- ILLUSTRATIONS Xvi 161. Characteristic curve secured by crossed wedges...............00..000s 241 Seem ttittie ant tne Characteristic Curve... 0006. .ee e cceees 244 163. Development and constant density ratios............ 0.00 ce cee eee eas 245 Preeremeeiinetty Of gamma. CBrowm)...0..2. 0.0.0. b ieee cee eee ee 249 rE ele e a oT a kWh bene s easy eee stander ves. 256 Seems NCTE HIDING 0. a el cele e's See eve ca evi ce ns cecees 257 eM e Me A Ue. ollie faa Ova Dev dvde de vaabesvecaee 257 ee er TRITLCOATE hn yen eee decks ven da caedesv cee evadues 258 eR MEE REI PMIIE RE AND he Ne eg cle sti Ga dele hsb ba bua Gs yeu sane 259 iene invasion phase of development. (Mees)..............-.00 pene 267 171. Growth of density with time of development. (Nietz)............... ei. oe 172. Curve showing growth of density with time of development. ( Nietz).. 273 173. H. and D. method for calculating the time of development for given SN oe clay oxy cia aa uc ceccdcaceestavess 277 foe ate ctied at Calculating the T.C.... ose. lke eee eee 283 Reet eee ie Ceveropment Chatt.. .. ee cece ee ee ee penne 284 176. Effect of a soluble bromide in the developing solution on the plate curve, 285 177. Density depression with a soluble bromide. (Nietz).................. 286 178. Effect of soluble bromide on the densities. (Watkins)............... 287 Poeeoweatunrapig cevyeiopers: (Crabtree)... ...... 2.0... eee eee cee 297 180. Influence of temperature on time of fixation...............0 0000s sues 340 181. Influence of concentration of hypo on time of fixation................ 341 182. Influence of ammonium chloride on the time of fixation............... 343 pert GET ISIGIIN SAONATATUS. 0.022... cee ec ce cs cae ene te ee cue nee 354 enn EE EOL FOU IM... ec ec cc ee vee ee bree ted etedaewene 355 ee SO ils ea ce pices cin vAsley nis caged anwscesawe fees 356 186. Sensitometric action of different reducers. (Nietz and Huse)........ cee: 187. Action of a proportional reducer on the plate curve. (Nietz and Huse), 376 188. Sensitometry of photographic intensification. (Nietz and Huse)...... 387 189. Bench for local reduction. (British Journal of Photography)........ 389 190a@, 190b, 191. Adapting the scale of the printing paper to the negative.... 391 192. Printing machines for amateur and for professional use.............. 397 193. Effect of time of development upon the characteristic curve of paper SMSIONS) 5... es 8s PN ara Ale Sle Pe nse Re SO tg gto pee ee ¢ 401 ieee reetnicay aperated print» washer, (Pako) ....... 000.2000 e cee eee ces 404 195. Centrifugal water pressure type of print washer. (Halldorsen)...... 405 RR PRN EE P(SICKIC) 55. oa fo a cee sas wane vd pena ce sees 406 eet PrOLection PLINting 2... ee ee ee sas eee bee hen ee ce ns 415 er T Ak. kw Vdc do ke Caw Gad ebunaceeseuecanuheye 416 Pee SD eMIAT OAT OCAMCTA ie ck ce ns ees eh eae eho cu sea sawe we ebe wee 417 200. Projection printing apparatus for use with daylight.................. A418 pomeaierene slaern for artificial light. ... 0.6.6 eee ne ee ee ee ee we 419 202. Proper position of the carbons of an arc light for use with alternating aa are aC, Nn in Spb, vk Bane mel SR RV ee eee wie a wpa al slo 8s, oh 421 203. Acetylene burner for projection printing. (Burke and James)........ 422 204. Lamps burning methylated spirit for projection purposes.............. 423 205. Parallax reflector for use with incandescent electric sources......... ee XVili - ILLUSTRATIONS 206. 207. 208. 200. 210. ail. 212. 213. 214. 215. 216. ai7, 218. 210. 220. 221. 222. 223, 224. 225. 226. 227. 228. 220. 230. 231. 232. 233. 234. 235. 236. 237. 238. Securing even illumination with five incandescent electric sources...... 442 Forms for the lighthouse using reflected light. (Wall).............. 425 The function of the condenser...... «0. <:./.:: a0. +s aisle en 426 Conjugate foci in enlarging... .. i... 2s 5 «be eae 427 Adjustment of the light source with condensers..... Pere ere eT 428 Loss of light between condensers due to the use of a long focus lens for projection. ©» (Candy) .....4.45.55+ 00 eanw Pee nee 430 Loss of covering power owing to the use of ae: focus projecting lens | with condensers. (Candy) ......... 00%. «05s Gee stnunnnnnnnanE 431 Ingento enlarging easel for use with printing frame................... 432 Westminister enlarging easel...... ww des'e0:0 lal oie rn 5 ea ie Scatter of light by negatives. (Callies) . <4 ccm 434 Graduating focusing scale for enlarging. (Lockett)................. 438 F and S Lantern slide printing frame... «..<..-. seen ane meee 446 Century lantern slide camera for reduction.«..:... «2+. 55. 448 Slide making by reduction using daylight... ...........:+.+ss eee 448 Device for holding lantern plate in position when using enlarger for lantern slide making by reduction. (Charles).................... 449 Actinometers for carbon printing... .....<.+¢ec se eaeh eee 498 Squeegee board for carbro. (Farmer) ....20. aq ane 506 Curves showing the influence on contrast of variations in proportions of gum, pigment and sensitizer in the gum-bichromate process. (Anderson) | 4 a6 0c b0 5 ok boss aa fiw ain Mine Goorin een 514 Owens’ frame for multiple printing. ..<...<.s<-s-;+shese nee eee 517 Zerbe’s method of registration.<. .....=-«:<+ «srs on eel Proper position of the brush in pigmenting. (Mortimer and Coult- hurst, The Oil and Bromotl Processés) 4.5.4. .-4- ee 530 Results in pigmenting. (Partington) ...... 9.2.45 essen 541 Transfer presses... ..ccce0 06 vue on se ose o's su yee gig tens gneiss nnn 546 Prett’s transfer press... .... 0.550200 se% ss 5 6 ogee Cu cee 547 Copying stand. .......00c0 see ben vececccuw ss epeiie pene Eten nnn Book holder for copying... 0... 0.06.05 «+0 sue sn au Opies 551 Illumination of the copy using daylight........0.. sae ae seen 553 Copying apparatus for artificial light. (Rose, The Commercial Pho- tographer). 60. vcaceakeen utius pa 0 duiphe 5 Soke geen een 553 Method of securing white or black ge (Photo. courtesy of D. J. Pratt) <6. cs onic 0% a igo bonis aoa ta gee eee 567 Sanger Shepherd three-color camera, ...:... 2. «-.4s084 05 eee ve ae Butler’s one-exposure, three-color camera.............-....4++ a) * o Le ee ae a ae THE DEVELOPMENT OF PHOTOGRAPHY 33 before the Photographic Society of London in 1858 a number of carbon prints but received little but ridicule for his efforts, which were faulty, as was to be expected for a first attempt. Gum bichro- mate, so much in favor with certain pictorialists, may be said to date from Pouncy’s work but it was not until brought to the front by Demachy and others at a later date that it became really popular. Carbon printing for the first time became really practical in 1864 when J. W. Swan introduced carbon tissue and the final step was made ten years later when Sawyer invented the flexible support, after which carbon quickly took its place as one of the really great print- ing processes. The modern oil process is a development of the collotype process patented by Poitevin in 1855, the principal difference being in the local application of the ink by brushes rather than a roller. The mod- ern method is due largely to Mr. G. E. H. Rawlings. Bromoil, a method of converting bromides into a condition suitable for pigmenting, was developed by C. Welborne Piper in 1907.?8 Carbro, a method of making carbon prints from bromides, is a de- velopment of the Ozobrome process as introduced in 1905 by Thomas Manly. We will have more to say regarding the history of the vari- ous printing processes in later chapters dealing with the subject of printing. 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, 28 Mr. E. J. Wall has called my attention to the fact that the first suggestion regarding the bromoil process was made by him in the Photographic News for 1907, p. 209. The passage to which Mr. Wall refers is as follows: “Suppose we enlarge direct on to a bromide paper and develop with a non-tanning developer, such as ferrous oxalate, we should obtain an image in the ordinary way in metallic silver. If this image were treated with a bichromate, the gelatine should be rendered insoluble in proportion to the amount of silver present, just as though exposed to light. One would then only have to dissolve out the unaltered bromide and the metallic silver with hypo and ferricyanide to obtain an image in insoluble gelatine, to which the ink or pigment should adhere as in the original oil process.” While to Wall undoubtedly belongs the credit of having first suggested the rationale of the bromoil process, to Welborne Piper belongs the credit of having worked out the details of the same, and having brought. it before the world in a practical form. 34 PHOTOGRAPHY 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. GENERAL REFERENCE WORKS BrotHErs—A Manual of Photography. London, 1899. ; Brown—Who Discovered Photography? Photo-Miniature, No. 60. 1903. CoLsEN—Memories des Createurs des Photographie. Paris. Eper—Geschichte der Photographie. Halle a/S. Eper—Johann Heinrich Schulze. 1917. FouguEe—Sur la Invention de la Photographie. Paris, 1867. Harrison—The History of Photography. London and New York, 1885. LicutTFIELD—Tom Wedgwood, The First Photographer. London. PotonNIEE—Historie de la Decouverte de la Photographie. Paris, 1925. SCHEINDEL—Geschichte der Photographie. TISSANDIER—History and Handbook of Photography. London, 74 WercE—The Evolution of Photography. —e- 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 15¢x2™% to 3% x5 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 4%4 x6 cm. (1.77 x 2.36 inches) to 64% x9 cm. (2.56x3.5 inches), is more expensive, and 35 36 PHOTOGRAPHY 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. In attempting to Fic. 16. Typical Miniature Camera for Plates and for Roll Film make the camera small and light many manufacturers have rather lost sight of stability and their instruments are flimsy and easily de- ranged. While a certain sacrifice in stability is necessary in order to prevent undue weight and size it is desirable that the instrument be sufficiently strong to withstand 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: =~") 2 ] | , q a THE CAMERA AND DARKROOM 37 I. Body of aluminum or better Duraluminum. 2. A platform so that the lens is covered when the camera is folded. 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/6.3 or F/4.5, 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- lature 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 focussing done just as with plates. Film packs thus offer the same advantages as both films and plates; the only objection at present is that film packs are only made in one speed and brand of color sensitiveness. Folding Hand Cameras.—Folding hand cameras are made in sizes from 24x 3% to 4x5 in both film and plate models. There 1s without doubt a greater demand for this class of instrument than any other—a fact which is evident from the wide range of models pro- vided by the various manufacturers. For one thing, the contact prints are sufficiently large to satisfy the requirements 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. 38 PHOTOGRAPHY 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- 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, while there is some uncertainty in general view or portrait work owing to the fact that it is hard to gain a proper idea of the subject from the small Fic. 17. Hand Cameras for Plates and for Roll Film image in the finder, which ‘moreover is seldom in accurate register with the lens. For this reason many prefer to purchase one of the light plate 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 photographing small objects. In addition, the long extension permits THE CAMERA AND DARKROOM 39 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 11x14 inches. In addition to greater stability, the professional cameras have greater bellows extension, and the various adjustments have greater latitude than in the folding compact hand camera already referred to. The lens board is also larger so that long focus lenses of large aperture may be accommodated. In addition there is pro- vision 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 indi- cates, have an arrangement by which the lens may be tilted down- ward 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 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. 40 PHOTOGRAPHY 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 Fic. 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 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 THE CAMERA AND DARKROOM 41 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 Fic. 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. 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 42 PHOTOGRAPHY 31%4xA4% instrument weighs from four to five pounds and occupies a space of approximately 5 x 5 x 6 inches, while the 4x 5 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 214 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 author is of the opinion that the box form type is 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 convenient 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 1s 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- 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 —— = — —— ae THE CAMERA AND DARKROOM 43 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 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 4 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 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 Fic. 20. Principle of the Swing Back. Use of the swing back for securing greater depth of focus 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 44 PHOTOGRAPHY 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 4 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. | 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 te ae es see a eos, | 7 THE CAMERA AND DARKROOM 45 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 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. II. THe DAarKRooM 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 5 46 PHOTOGRAFPAY 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 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. 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 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 employe 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 THE CAMERA AND DARKROOM 47 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 SUGGESTED LOCATION OF INLET & EXHAUST. T SHOULD BE ON OUT SHOE Watt TF POSSIBLE lion Lock VENT. LOCATE JN WALL Jo Zewe IN AUR FROM SurTA8LE PLACE FRUNT INSIDE FLAT INDIAN RED Fic. 21 Ventilation of the Darkroom. (Courtesy of Eastman Kodak Company) 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. The Water Supply (Sinks).—As a large amount of water is re- quired for most photographic operations, the water supply is an im- 48 | PHOTOGRAPHY 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 vi ee) i) Fe Rad | «3 a) Fic. 22. Floor Plan of Darkroom for Amateur Use 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 THE CAMERA AND DARKROOM 49 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 eithér 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- 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. 50 PHOTOGRAPHY 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. With the former we are already acquainted, but more will be said on this subject shortly. In-_ direct light, while having been adopted by some large commercial houses, has been neglected by the amateur and even by the average professional. The advantages 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 without 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 are too dark and in the end not so safe as supposed, 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 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) and by Burke and James, but there is nothing to Fic. 23. Eastman Indirect Darkroom Lamp prevent the worker from making his own if he so desires, as the con- struction is quite simple. The light-box itself (see Fig. 24) is made of thin sheet iron and is 8x 10x 6% inches in size. There are two ventilators, one in one side and one in the bottom of the box, which must be made so as to prevent any white light from passing. The in- terior 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 ~ THE CAMERA AND DARKROOM 51 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 down from the wall tap which should be close by. To secure the full advantage of the To Wall Switeh bs 6 | Y | ______—.| -.--}} OE ‘ S Fi ° fs Wiring Diagram g Fic. 24. Design for Indirect Darkroom Lamp. (Krug, American Annual of Photography, 1922) 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 condi- tions the illumination will cover an area about sixteen feet square so that additional lights will be needed for a large room. 73.39 69.38 84.19 84.16 OES 68.93 65.16 83.43 78.86 I = incident light; 1’ — light transmitted. In the Syntor, 100 units of light lose at the first surface 5.47 per cent, dropping to 94.53 per cent. Absorption causes a further loss and reflection at the second surface causes a total drop to 88.77 per cent. -The amount lost in the air space is so minute as to be beyond accurate measurement and passing on we trace the losses through the remain- ing lenses to a final transmission of 65.16 per cent. The last two columns show similar figures for the Dagor. In this case there is a similar loss at the first surface, but as the difference in the refractive index of the cemented glasses is comparatively small there is but little loss of light, except at the four glass to air surfaces, and consequently the percentage of light transmitted is higher than in the case of the Syntor, the actual transmission percentage being 65.16 for the Syntor and 78.86 for the Dagor. As both have the same //value, namely F/6.8, it will be seen that the Dagor at F'/7.5 has the same efficiency as the Syntor at F'/6.8. The losses from absorption do not, in general, amount to as much as those produced by reflection. Attempts have been made to reduce the loss of light by absorption by the use of quartz, which has a much greater transparency to ultra-violet than glass. Except for some particular purposes efforts along this line have not proved of much value. Reduction of the thickness of larger lenses by the use of aspheric curves has been tried, but tntil some means is provided to enable such curves to be ground more readily, it is not likely that much will be accomplished in this direction. It is noteworthy that there is also a theoretical foundation for the oft asserted fact that the small lenses fitted to miniature cameras are faster than larger lenses of the same F'/value. This is due to the fact that as the lens becomes larger the thickness of the glass increases, but in geometrical ratio, and the loss of light by absorption is correspond- 82 PHOTOGRAPHY ingly increased. The loss from reflection, however, is independent of the focal length. 7 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 parallel, the conjugate focal distance, v, is equal to the focal length, | f. The intensity of the image is then expressed as d*/f?. 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 d?/v? rather than d?/f?. 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 d?/v?, 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 x, i : Ly a) he ee ‘ on ee ee |) PHOTOGRAPHIC OPTICS 83 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 v?/f?. 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. Exposures Proportioned Proportion of Image to Proportionate to that Required when Original (Linear) Exposures Copying Same Size 1 A ES a ea abt A re oe L.O7 27 PE 244 0 0 Gh TS aaa 1.10 . .28 ee es ck ey x svar es L.21 3 Re iis shee ewes 1.27 31 ee ay cates 1.36 34 eh Bg RS ee ee 1.56 -39 oe) ERE SS A eee ee eee 2.25 .56 ee ere ks Rive samc 3.06 .76 eNO SISA) Sc Soa ee hess 4 I MI a ort tga. bk id aa 8 a 9 2.25 Lhe ap he 20 SE Sa a 16 4 EER ERE es ey Sieg ao bus 25 6.25 Se ee ee eee 36 9 Oo Sk A See ne eae 40 12.25 SRM MET ac eh aa ¥ dy uae sha 64 16 OO Sain ee eee 81 20.25 0 he ee a 100 25. Of 6 ap “aie, See anne 2 a 121 30.25 EMC he otra 3 Sveeaa ass sb? 144 36 OO a SO ESL a a 169 42.25 OS Gi en a ae 196 49 Mie en ere AS os uae dean oh 225 56.25 UN a Po kao uceee 256 64 RS ea si Ok, oa)a baw «Dees 289 72.24 RL Bi. 6 Si 'a-u 4% 4,8, sens 324 81 MM ene 5 5555 pe be seas weet 361 90.25 NM ln os a ok Goch a hia aneia a wiear 400 100 84 PHOTOGRAPHY GENERAL REFERENCE WORKS BecK AND ANDREWs—A Simple Treatise on Photographic Lenses. BoLes AND Brown—The Lens. CoLte—Photographic Optics. Czapsk1—Theorie der Optischen Instrumente. Eper—Die Photographischen Objectiv. Ig1o. Fapre—Traite Encyclopedique de Photographie. Vol. I and Supplements. _ GLEICHEN—Theorie der Modernen Optischen Instrumente. Hartinc—Optics for Photographers. LuMMER—Contributions to Photographic Optics. MetHie—Photographischen Optik. MoessArp—L’Objectif Photographique. Nuttinc—Outlines of Applied Optics. PirpeEr—A First Book of the Lens. ScHMipTt—Vortrage tuber photographische Optik. SCHROEDER—Die E‘emente der Photographischen Optik. STEINHEIL AND Vorr—Handbook of Applied Optics. SouTHALL—-Geometrical Optics. SouTHALL—Mirrors, Lenses and Prisms. TAYLoR—System of Applied Optics. Taytor, J. Traitt—Optics of Photography and Phothpr annie Lenses. RouHr—Theorie und Geschichte der Photographischen Objectiv. WaLLon—Traite Elementaire de L’Objectif Photographique. WaLLton—Choix et Usage des Objectifs Photographiques. CHAR DER: LY: 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 B B Fic. 42. ‘Effect of Chromatic Aberration on Definition 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 as shown in Fig. 42. 85 86 PHOTOGRAPHY The effect of chromatic aberration on the definition of the lens will be evident upon further study of Fig. 42. In this figure A and A’ are two rays of white light from an illuminated point of the object. L is the lens, uncorrected chromatically, y the point at which the yellow rays are brought to a focus, wv the point at which the violet rays are brought to a focus and BB’ the sensitive plate. In focussing the bright yellow rays are used, since they are the most luminous to the eye; the maximum chemical activity, however, lies in the violet and blue, so that vw corresponds to. what is known as the chemical or actinic focus, y the visual focus. Upon examination of the figure it will be observed that when the ground-glass or sensitive plate is at BB’ it will receive two images of any point in the object, the one formed by the yellow rays coming to a focus at y on the plane surface BB’ and the other a disc from the violet rays which having come to a focus in front of the plate at v are now diverging, and form on BB’ a disk instead of a point. Thus instead of a true point image we have the state of affairs shown on the right. If the ground-glass is placed at v, the chemical focus, better results are obtained because of the Fic. 43. Chromatic Under Correction lesser activity of the yellow rays which are beyond the focal plane. With color-sensitive plates, however, the disk of confusion becomes more prominent. It is in any case sufficient to destroy the clear defini- tion of the objective. Fic. 44. Chromatic Over Correction With a converging or positive lens, chromatic aberration takes the form shown in Fig. 43, 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. 44 and is termed chromatic overcorrection. The method employed in correcting chromatic aberration will now ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 87 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. Perhaps a clearer idea of the principle employed may be gained with the aid of Fig. 45. In this figure A represents a single Fic. 45. Correction of Chromatic Correction (From Beck and Andrews, A Simple Treatise on Photographic Lenses) convex positive lens with the rays of white light entering on the left. Dispersion takes place upon the passage of the ray through the lens and the violet rays are brought to a focus at vw and the yellow rays at y. In the same figure B represents a negative lens made of the same glass and having the same focal power as the above positive lens, but of course in the opposite manner. Dispersion again occurs, the violet emerging from a virtual focus nearer the lens than the yellow. The positions of the foci are exactly the same as those of the positive lens because we assume that the lenses are of the same power and made of 88 PHOTOGRAPHY the same glass. If we combine these two lenses it is evident that the negative lens will exactly neutralize the chromatic difference of the positive lens. However, at the same time the positive character of the lens has been neutralized as well and the lens has no longer the prop- erty of refracting or focussing light. If for the negative lens we use the same type of glass but alter the curves so that it is not as powerful as the positive lens we have the condition shown in C of the figure. The convex positive lens is only partially neutralized and the chro- matic differences persist. However if instead of using a glass of the same refractive index we use for our negative lens a glass having ereater dispersion we may neutralize the error of the positive lens without entirely destroying its refracting power. Thus with a suit- able combination we can secure the result of D in Fig. 45, where the lens, while not as powerful as the positive lens alone, is free from chromatic differences and the various colors are brought to a com- mon focus. For the sake of simplicity we have been considering only two mre 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 throughout the spectrum. Thus the total dispersion of one glass may be twice that of another and lenses from the two glasses may bring A Cc DD E.F G H D’ £ Fr G’ H’ a ae Irrationality of Dispersion 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 varies with the wave-length of the light as shown in Fig. 46. This is known as the “irrationality ” of dispersion. a ae ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE § 89 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 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 is due to the use of spherical surfaces which are the only ones which can be ground simply and with sufficient accuracy. It may be described as the in- ability of a lens to convey the marginal (not oblique) rays to a point at the same distance from the lens as the central rays. In other words, the rays passing through the edge of a spherically uncorrected lens do not come to a focus at the same point as those which pass through near the axis. In spherical aberration we are concerned only with the rays parallel to the axis; when oblique rays are considered the aberration is known as coma. There are two classes of spherical aberration as there are of chro- matic aberration. As in the case of chromatic aberration these are 90 PHOTOGRAPHY produced by positive and negative lenses and known as under- and overcorrection respectively. Fig. 47 represents the condition of spherical undercorrection. ‘The parallel rays h,, h,, hz, h, on passing through the lens form image points at different distances from the lens, those passing through the | | | Fic. 47. Spherical Aberration 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 abscissee we may construct a curve showing the degree of undercorrection present. aft 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. 47. By using h,, h,, hs, h, as ordinates and the image Pek < ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 91 distances as abscissz, 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 many different shapes so that it is possible to take two lenses which have the relative focal lengths necessary for chromatic correction and at the same time make them of such shapes as will correct spherical 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. 47 shows a lens which has the spherical aberration corrected for the center and a zone near the margin. The rays h, and h, do not come to the same focus so that by taking their distances and the distance of the rays from the axis, ; “025 +0.25 mm Fic. 48. Spherical Aberration in Tessar Ic. (Kellner) according to the methods used above, it is possible to construct a curve showing the amount of spherical aberration remaining in the combina- 92 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. Fig. 48 gives an idea of this and shows the amount of departure from perfect spherical correction in the famous Ic Tessar F/4.5. © Coma.—Coma, or zonal aberration, is the name applied to the spherical aberration of the oblique rays of light on passing through the lens. With spherical aberration proper we are concerned with direct pencils of light parallel to the axis and consequently there is symmetrical distribution of the light; that is to say, the course of the light rays is the same on both sides of the axis. For this reason it was only necessary to show one half of the lens in the illustrations of spherical over- and undercorrection. From Fig. 49 it will be seen that the course of oblique rays on pass- ing through a lens is completely unsymmetrical. The rays below the 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 Fic. 49. Coma. (Kellner) 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. 50 a represents the condition known as outward coma, the points of the pear-shaped blur facing away from the axis of the lens. ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 98 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 Fic. 50. Two Forms of Coma. (Piper) have previously used as an expression for spherical aberration (Fig. 49). ‘This curve shows the distances of the different points of inter- 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 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. 50 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 8 94. PHOTOGRAPHY 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 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. 7 With a positive lens (Fig. 51) this curve is concave to the lens since the axial pencils come to a focus at a while the focus of an Fic. 51. 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. Fic. 52. Curvature of Field. (Over Correction) With a negative lens (Fig. 52) the image field is again curved but this time the curve is convex to the lens and the condition is then vas ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 95 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 Jenses 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 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. 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 N, and N, (Fig. 53) be the nodal planes of admission and emergence Fic. 53. Distortion respectively of the lens L, and let BC 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 f. Let +N and +N’ be parallel lines drawn through the 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 96 PHOTOGRAPHY central ones, the image point lies not at d, its true position, but at f, 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. 54. Fic. 54. Under and Over Correction for Distortion 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 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 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. 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. ~— ee eee, ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 97 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. 55, 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. Fic. 55. 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 Fic. 56. Greater Focal Length of Marginal Pencils Resulting in Lower Intensity (Brown) 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 98 PHOTOGRAPHY oblique pencils is at a greater distance from the lens than the central pencil. This is shown in Fig. 56, 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 — 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- larly, as does the central pencils, but at an angle. Thus in Fig. 57 the al b Fig. 57. The Angling of the Oblique Ray. (Brown) 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 mathematically provided there is no obstruction of the oblique rays by the lens mount to be considered. Formulz 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 1866.1 This relation is rep- resented in Fig. 58; ? the angles of view subtended by the diagonal of the plate are marked along the top of the graph, while the numbers 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 les 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 1 Brit. J. Phot., 1866, p. 160. 2 Zschokke, Brit. J. Phot., 1917, 64, 203. ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 99 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 uP LY 33° 44° SS° 62° 70° 717° BH 90° 95° 100° 405° 409° Redes isls)caloof | [a] 1) eee SERRE ee isis ab pe ao +- Ww Exposure at corn nN 2 = G6 (8 40 42 44 146 18 20 22 24 26 28 Diagonal divided by focal Length Fic. 58. 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. 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 ® 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 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.* 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 latter affects the central as well as the marginal definition, while pure astigmatism 8 Martin in the Omnar produced by Emil Busch, Beck in the Neostigmar Series. 4D. R. P. 56,109—April 1890. 100 PHOTOGRAPHY 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. 59.2 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. = D ——S —— Fs 38. or oe Les Cas es Ss WINDOW Bans AT Manan oF Fe LD Fic. 59. Astigmatic Deformation. (Kellner) 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 5 Courtesy of Dr. Hermann Kellner and the Society of Motion Picture Engi- neers, ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 101 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 the astigmatic image points ¢ 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 anastigmat. peek 30 25 i) 20 20 H ! 5 15 10 10 5 ae 0 -3 O+1 -1 0 +3 -4 0 -8 : O+1 Fic. 60. Astigmatic Curves. (Von Rohr) By swinging the window bars nearer the axis the astigmatic differ- ence becomes less and less until finally when the bars reach the axis astigmatism disappears. Taking the different angles which the princi- pal ray may take with the axis, we obtain a series of astigmatic image points which when connected give two astigmatic curves. The char- acter and extent of these two curves afford a means of illustrating graphically the amount and character of the astigmatism present in the lens, The general shape of these curves is shown in Fig. 60, the dotted 102 PHOTOGRAPHY line representing the image points of the meridional rays and the solid line those of the equatorial rays. Occasionally, however, the astig- matic curves take different shapes as shown in a, b and c of the same figure. Where the two curves coincide we have astigmatic correction. The deviations represent what are termed astigmatic zones or zonal errors. ‘The amount of these errors is dependent upon the aperture and the construction of the objective and it is the aim of the designer to remove or to reduce these as much as is possible under the condi- tions. | 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 ona 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 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- 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. 61 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 cand c’. If the focus of the secondary reflected image is near the ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 103 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 2 are eae b a aa a Fic. 61. Optical Flare 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. 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 ITI. The following may be recommended as being especially suitable. Hartinc—Optics for Photographers. (English translation by Fraprie.) 1912. LuMMER—Photographic Optics. (English translation by Thompson.) 1903. Von Rour—Theorie und Geschichte des Photographischen Objectivs. 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. For a complete history of the development of the photographic ob- jective Von Rohr’s monumental work, Theorie und Geschichte des Photographischen Objektivs, should be consulted. Although published in 1899 and therefore antedating many of the anastigmats this work still holds its value as the most complete history of the photographic objective in any language. Eder’s Die Photographischen Objektive is also a very valuable source of reference and is more complete as regards the later lenses. A less comprehensive survey, yet one which is perhaps of more real value to students, is to be found in Optics for Photographers, by Hans Harting; while the chapter on the photo- graphic objective in the English translation by McElwain and Swain of Gleichen’s Theory of Modern Optical Instruments is of value. 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 witha — concavo-convex, or meniscus, lens than with the usual bi-convex:. Wollaston’s meniscus (Fig. 62) with the concave side towards the sub- — ject gives an image of satisfactory sharpness over a limited field when 104 THE PHOTOGRAPHIC OBJECTIVE 105 medium-sized stops are used. Since chromatic abetration 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 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- Fic. 62. Wollaston’s Meniscus 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 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. \\\\\ \ YY \\ A , XN il Up wy AAT AAA AKIN AN LU \\ mig, 02, Lhe Chevalier or French Fig. 64. Landscape Lens Grubb’s Landscape Lens Many lenses of this type were constructed in the early days of photography. (See Von Rohr’s work for a complete account.) As an example of a single achromat composed of a double concave negative lens cemented to a double convex collecting lens we may mention the 106 PHOTOGRAPHS lenses of Chevalier of Paris, Francais of Paris, Busch of Rathenow, Goerz of Berlin, and Voightlander of Brunswick. In practically all of these (Fig. 63) the positive lens is of crown and the negative lens of flint. In 1857 Grubb patented an achromatic lens composed of two con- cavo-convex meniscus lenses cemented together, the glass nearer the diaphragm being of crown and the other of flint (Fig. 64). In 1864 Dallmeyer introduced his ‘‘ Rapid Landscape Lens ” which is similar to the above (Fig. 65) but differs in the introduction of a third me- \ Wy, — _— 3.4... eeee ee ee Tular. .... 455 eee F/6.3 Salmotrach 2.0305 sca 2 Bee Orioti (ica) eee F/4.5 Staebel) i) S25 es aoe ee Kaloplast, Steinheil 3) et a8 coke eee Cassar). ie ee F/3.5 LCisS .... . Poe Oa e Triotar 4. .g<, seo oa eee F/3.5 Development of the Triple Objective after H. D. Taylor—the Cooke Aviar.—In 1918 Arthur Warmisham patented (B. P. 113,590 — of 1918) 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 Fic. 99. Cooke Aviar—Shorter Foci—Longer Foci patent for a modification of the triplet in which the central dispersing lens was developed into two similar lenses of lower individual power (B. P. 12,859 of 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. In the shorter foci (Fig. 994) the two collecting lenses are made of highly refractive dense baryta crown (up 1.6116, v 56.4) and are of feet OL OGRAPHIC OBJECTIVE 135 identical power. ‘The two dispersing lenses are of light flint, but have slightly different values (Lens 2, np 1.5682, v 43.4; Lens 3, up 1.5502, UV 45.8). For objectives of longer focus the collecting lenses are made of dense baryta crown (7p 1.6116, v 56.4) as in the case of the shorter foci objectives, but the dispersing lenses are made of heavy flint (np 1.6206, v 36.2). Thus in the case of the short-focus objectives the refractive index of the collecting lenses is the higher, in the case of the long-focus objectives the reverse is the case, the refractive index of the dispersing lenses being higher than that of the collecting lenses while the dispersion is less. In both cases the shallower faces of all four lenses are turned towards the diaphragm. This necessitates a shallower curve for the inner surface of the back positive lens, which reacts favorably on the curvature of field, marginal astigmatism and comatic correction over the entire field. Development of the Triplet Objective after H. D. Taylor—the Aldis Lens.—Three objectives of this firm demand attention. Series Ila F/6.3 (Fig. 100) may be regarded as a development of the triplet Fic. 100. Aldis Series Ila construction of H. Dennis Taylor as described in British Patent No. 1699 of 1899. 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. 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. 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. 101) the front collecting lens is cemented to the central dispersing lens while the rear collecting lens of low power is placed some distance from the cemented element. In both lenses the collecting lenses are of baryta light flint; the dispersing lenses of baryta flint. Series II has a relative aperture of F/6; Series mot F/7.7. 136 PHOTOGRAPHY Triplets with a Pair of Collecting Cemented Surfaces—the Heliar. —In 1900 Hans Harting calculated for Voigtlander an objective which is essentially a development of the simple triplet of H. Dennis Fic. tor. Aldis Series II and III Taylor, which was introduced commercially as the Heliar under D. R. P. 124,943 of 1900 (U. S. P. 716,035 of 1902, B. P. 22,962 of 1900). In the Heliar (Fig. 102) the triplet construction is plainly evident | Fic. 102. Harting’s Heliar from the general similarity of the design. The development made by Harting consists in replacing the single collecting lens of the original triplet of H. Dennis Taylor by a cemented element of two glasses. The objective thus becomes a five glass system, the two ce- mented lenses being an anomalous glass pair, the outer dispersing lens being of flint (mp 1.54990, mgt 1.56547) and the inner collecting lens of crown (up1.61294, ngt 1.62686), while the central bi-concave dispersing lens is made of flint glass of lower refraction (mp 1.53644, ngl 1.54988). Both cemented surfaces have a collective effect. By increasing the number of elements of construction and the in- troduction 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 sharpacene is greater than that of the simple triplet. al = Peer rOTOGRAPHIC OBJECTIVE for 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. 103) under D. R. P. 143,889 of 1902. The Dynar was made Fic. 103. Harting’s Dynar 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. 104). Both the Dynar and the Pentac are five-lens systems con- Fic. 104. Dallmeyer’s Pentac _ sisting of a double-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. The arrangement of glasses in the Pentac is sensibly the same 4 arrangement as the Dynar, the exterior double-convex collecting. lenses are of highly refractive crown (mp 1.6109) and the three nega- q tive lenses of flint glass with a refractive index of about np 1.5485. 138 PHOTOGRAPHY By rigid calculation an objective has been calculated which has an unusually large aperture of //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. 105) lens of the Fic. 105. 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.® 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 2 Since this was written there has appeared an F/2.7 and F/1.8 by Zeiss. * British Patents 186,917/1921, 191,702/1922, 193,376/1922, 232,531/1924. Klug- j hardt, Phot. Ind., 1924, p. 1008. i 4 | | | | | | i ' : i ; } ; : . | ; tier HOVOGRAPHIC OBJECTIVE 139 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 1851.4 The matter, however, remained unnoticed by the 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. 106 and is chromatically and spherically corrected. The two elements are mounted in a tube fitted with an adjustable screw by which the separation of the posi- 4 Optics for Photographers, English translation, p. 185. 140 PHOTOGRAPHY 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 Fic. 106. Dallmeyer’s Compound Teleobjective 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 fil A where D is the aperture of the positive objective, f, 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- q 4 : ‘ ] a J ‘ ’ ; ~THE PHOTOGRAPHIC OBJECTIVE 141 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- magnification teleobjective was the Orthoscopic lens worked out by Petzval and introduced commercially by Dieztler in 1856. This ob- jective (Fig. 107) consists of a front positive combination similar to Fic. 107. Petzval’s Orthoskop 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 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 11 142 PHOTOGRAPHY This was designed by K. Martin and was composed of two cemented doublets (Fig. 108) and had a relative aperture of F/g and a mag- nification-ratio of 124. Soon afterwards Zeiss brought out the Mag- nar (Fig. 109). This was calculated by Rudolph and Wandersleb and had a magnification-ratio of 3 times with an aperture of F/1o. The positive component was a doublet and the rear a triplet. | : Fic. 108. Martin’s Bis-Telar Fic. 109. Zeiss Magnar 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 the calculations of Stuart and Hasselkus the Telecentric, a fixed-focus teleobjective, the positive component of which was a cemented triplet Fic. 110. Ross Telecentric Fic. 111. Dallmeyer Large Adon and the rear component a cemented doublet (Fig. 110). This was issued in two series, one working at F/5.4 and the other at F/68. 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. 111) 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. | es ae - a S - _ ‘ THE PHOTOGRAPHIC OBJECTIVE 143 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 Fic. 112. 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. 112). This construction was introduced by J. H. Dallmeyer Limited in several series as the Dallon. 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. Fic. 113. Radiar Teleobjective A similar construction was patented by H. W. Lee (B. P. 198,958) and introduced by Taylor, Taylor and Hobson as the Cooke Telic. This has a relative aperture of F/5.5 and a magnification-ratio of 2 times. The Radiar telephoto anastigmat (Fig. 113) introduced by the Gundlach-Manhattan Optical Company is of similar construction. 144 PHOTOGRAPHY 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 Telegor of Goerz differs from the cbjectives described immedi- ately above in that the collecting and dispersing members of the rear ye Fic. 114. Goerz Telegor component are reversed in position, the former being on the interior and the latter on the exterior, separated by an air space having a me- niscus shape (Fig. 114). It has a relative aperture of F/6.3 and a magnification-ratio of 2 times. : In the Tele-tessar of Zeiss the rear component is composed of two cemented menisci and as in the case of the Goerz Telegor 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. 115) (B. Fic. 115. Zeiss Teletessar P. 179,529 of 1921). The Tele-tessar has a relative aperture of F/5.5 and a magnification-ratio of 2 times. 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 and Hasselkus (B. P. 188,621) the rear component is a cemented triplet in which two negative lenses enclose a positive member of glass i ia THE PHOTOGRAPHIC OBJECTIVE 145 having lower refraction and higher dispersion (Fig. 116). The ob- jective has a relative aperture of F/5.5 and a magnification-ratio of | | Fic. 116. Ross Teleros slightly over two times. H. W. Lee has also patented a fixed-focus teleobjective (B. P. 132,067) (Fig. 117) in which the rear component is a triplet with a positive lens of low refraction between two négative members of high dispersion. This is manufactured by Taylor, Taylor and Hobson and has a relative aperture of F/5.5 and a magnification- ratio of 3. As this work goes to press Messrs. Taylor, Taylor and - Hobson announce a new series having an aperture of F/3.5. == Fic. 117. Lee’s T. T. H. Telephoto Fic. 118. Voigtlander’s Tele-Dynar The Tele-Dynar of Voigtlander also has a rear element of three members, two of which are cemented and the other separated by an air space (Fig. 118). 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, 146 PHOTOGRAPHY 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. 119). To maintain the actual F value of the photographic objective for any degree of 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 | ! 1 WW a - - - - — - - —- -- ——----—-—----—-—--—- --- —-—-—— => Equivalent. focal length aoe Negative lens forms virtual image of real image formea by positive lens at f, / ' ‘ ' ' ‘ — f ' ! ' ' ' ' ‘ 1 Fig. 119. Dallmeyer’s Adon principle, however, can only be used with objectives of moderate diameter and for low degrees of magnification. GENERAL REFERENCE WorRKS Eper—Die Photographischen Objectiv. FasrE—Encyclopedique de Photographie. GLeEICHEN—Lehrbuch der Geometrischen Optik. Ay! ; ‘i. Pot " a Se ee eae a ee j 4 i : 7 ¥ * THE PHOTOGRAPHIC OBJECTIVE 147 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.) Hartinc—Optics for Photographers. (English translation by Fraprie.) - LumMMerR—Contributions to Photographic Optics. (The English translation by Thompson contains two chapters on British objectives which are not found in the original.) Puyo anp Putticny—Les Objectifs Anachromatiques. TURRIERE—L’Optique Industrielle. 1920. (The most complete work on the later anastigmats.) Von Rour—Theorie und Geschichte der Photographischen Objektiv. CHAPTER Wi 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 uence 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 solution 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 148 Pe eee RE Ee OO SN ee TO ee ee Ee ey ee ee eS a ee eee eS Be THE PHOTOGRAPHIC EMULSION 149 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 crystals of silver iodide, or chloride as the case may be, are held in suspension within the silver bromide and neither combine chemically with the latter nor exist separately as individual crystals. The processes of emulsion making are therefore concerned with the formation of a uniform, homo- geneous suspension of a sensitive form of silver bromide in a solution of gelatine. 7 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- 150 PHOTOGRAPHY 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- eral outline of the processes involved in the preparaten of gelatine emulsion is as follows: 1. The gelatine is allowed to swell in cold water ee 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 silver nitrate is added to the colloid medium. 5. The solution of soluble halide is next 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: AgNO; + KBr — AgBr + KNOs3. Silver Potassium Silver Potassium nitrate bromide bromide nitrate 7. The process of digestion; by standing from 10-20 hours at ordinary temperatures or by boiling, or treatment with ammonia, in the case of gelatine emulsions. 8. Washing, combined with shredding, to completely remove the last traces of soluble salts. 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. A second method of preparing emulsions consisting in the addition of washed silver bromide to a solution of gelatine was introduced by Abney. The order of the operations in this case is as follows: 1. The soluble halide is dissolved in water. 2. The silver nitrate is dissolved in water. 3. The two solutions are mixed in the dark, thus producing silver bromide according to the reaction previously given. 4. The mixed solution is allowed to stand from I to 4 hours in order that the silver bromide may precipitate. 5. The silver bromide crystals are then washed until chemical tests show that there is no trace of soluble salts remaining. : . j 3 | j | : THE PHOTOGRAPHIC EMULSION 151 6. The washed silver bromide is added to the solution of gelatine. 7. The emulsified silver halide is digested by boiling or by treat- ment with ammonia. Gelatine.—Gelatine belongs to that class of substances known as col- loids from the Greek xoAXa 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 > Hydrogel, is of paramount importance for photographic purposes and it is in this respect that gelatine is distinctly superior to any other colloid. 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 setiing 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, fogs readily and develops slowly, owing to the fact that the penetration of the film by the developing solution 1s more difficult. Accordingly, in practice the emulsion maker uses a medium gelatine, combining hard and soft gelatines in the propor- 152 PHOTOGRAPHY tions which his experience 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.? 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 months 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 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 ally] mustard oil and to be an allyl isothio- 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. * q ; | qi 4 ; ial’ Ls i ~ ae ae = 162 PHOTOGRAPHY of silver wires. The pressure applied forces the emulsion through the interstices of the wire screen into the running water below, where, owing to its fine state of division, the soluble salts are rapidly removed. (From Eder’s Ausfiihrliches Handbuch) (From Eder’s Ausfiihrliches Handbuch) Fic. 120. Emulsion Washing Apparatus Fic. 121. Centrifugal Separator Another method sometimes employed consists in the use of a spe- cially made centrifugal separator which is similar in general principle to the cream separator used in dairy manufacture. A separator as used for separating the soluble salts from photographic emulsion is shown in Fig. 121. The liquid emulsion is placed in the vessel, 4, in the darkroom and the top screwed on. The vessel containing the heated emulsion may now be brought out into daylight and placed on the spindle, S. The crank is then turned for several minutes, the actual rate of rotation being about 4000 revolutions per minute. Finally the vessel A is removed and taken to the darkroom, the top re- moved and the liquid emulsion poured off. The solid salts extracted from the emulsion collect on the sides of the vessel and may be re- moved and re-emulsified in a fresh lot of gelatine. In using a centrif- ugal separator the soluble salts are removed before the remainder of the gelatine is added: when removed by washing, the gelatine is added first. 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 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 PHOTOGRAPHIC EMULSION 163 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. 122) and of various forms and Fig, 122. 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 shown that these are also crystalline and have substantially the same structure as those of larger dimensions.1* There is no evidence for the existence of non-crystalline silver bromide in photographic emul- sions. 3 The size of the silver halide grains in commercial emulsions ranges from the ultra-microscopic particles of less than one micron to grains 18 Wilsey, Phil. Mag. (1922), 42, p. 262. 164 PHOTOGRAPHY 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. From Fig. 122 it might be assumed that all the halide grains com- posing the emulsion are to be found in one layer. This is not so; there are a number of layers, sometimes as many as ten or twelve, de- pending somewhat upon the character of the emulsion. This is indi- cated in Fig. 123, which is a photomicrograph of a cross-section of a (Courtesy of Dr. A. P. H. Trivelii.) Fic. 123. Cross Section of a Developed Emulsion developed portrait film by Dr. A. P. H. Trivelli of the Eastman Re- search Laboratory.1® The number of grains in a given area of a coated plate is enormous. ‘The number varies with the type of emul- sion but averages from 10 to 25 billion per square inch.”° The Sensitivity of the Silver Halide Grain.— Microscopical investi- 19 Reproduced by permission of Dr. Trivelli. . : 20K. P. Wightman, Amer. Phot.: (1923), 17, Pp. 329. Se | ee | : F | THE PHOTOGRAPHIC EMULSION oELOG gation has shown that in spite of the enormous number of grains of silver halide and their close proximity to one another, each individual erain 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.” 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,?* but if we keep to the same emulsion the larger grains are on the average more sensitive than the smaller. There are, however, some differences in sensitivity among grains of the same size and shape and from the same emulsion. Hodgson ** in 1915 showed that the developer attacked the grain of silver halide at certain preferred points (Fig. 124) and Svedberg, who investigated the subject more thoroughly (for list of papers see bibliography), found that these centers were scattered in a haphazard fashion among the grains and that the larger the grain the more likely it was to have a developable center. He also pointed out that the dif- ferences in the sensitivity of grains of one size was in agreement with the chance distribution of development centers. Svedberg was of the opinion that these development centers were due to the discrete nature 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, Phot. J., 1921, 51, 333. 23 Jour. Franklin Inst., 1917, 184, 705; Brit. J. Phot., 1917, 64, 654. 166 PHOTOGRAPHY of light emission and that the necessary and sufficient condition for the formation of a developable center was that a certain number of light quanta fall upon the grain within a certain minimum area. Sil- berstein, like Svedberg, was of the opinion that these facts might be — Fic. 124. Hodgson’s Pretend Points explained by means of the quantum theory of light emission; the variability of sensitiveness among grains of one size and the increase of sensitivity with the size of grain being due to the fact that it is only necessary for a grain to be struck by a light-dart in order to make it developable. According to this theory the development centers do not exist be- fore exposure and all the grains are of the same kind and substance just the same as if they were fragments of a large crystal; the dif- ferences in sensitivity being due to the fact that the larger the grain the more likely it is to be hit by a light-dart. Others, however, among them Sheppard, Clark and Toy were of the opinion that these development centers were present before ex- posure and were due to the presence of a specific chemical substance other than silver halide and to be largely responsible for the high sensitivity of the grains of modern emulsions. One of the important supports of this theory is the fact that a plate THE PHOTOGRAPHIC EMULSION 167 can be largely desensitized by treatment with an oxidizing substance, such as sodium arsenite or chromic acid, which are afterwards re- moved from the emulsion before exposure. Investigation has shown that chromic acid destroys the latent image and greatly reduces sen- sitivity but that the effect on the former is much more pronounced than on the latter. Hence it appears that the sensitivity centers exist before exposure and are not identical with the latent image. Clark was able to show that pre-exposure to light greatly increased the de- gree of reduction in sensitivity on treatment with chromic acid while Wightman and Sheppard showed that in the same emulsion the smaller grains are relatively reduced more in sensitivity than the larger ones. Accordingly: (1) the conversion of the sensitivity substance of the development centers into latent image substance facilitates the attack of oxidizing agents ; (2) the sensitivity substance is held in a different way in the larger grains; or (3) the conversion of sensitivity sub- stance to latent image substance increases the probability of develop- ment for the larger grains. The evidence, therefore, while it does not exclude the possibility of a discrete light action as suggested by Svedberg and Silberstein, does support the theory that there is present in the silver halide grain a substance other than silver bromide which increases grain sensitivity. The discovery of the sensitizing substance in gelatine by Sheppard still further supports the claims of those who believe in the existence of a foreign substance in the grain, the presence of which in varying amounts or states is responsible for the variation of sensitivity of grains of different sizes and for the existence of the development centers. The Nature of the Sensitivity Substance——Those conversant with the subject of emulsions have long been of the opinion that in the preparation of gelatino-silver halide emulsions a slight reduction of silver halide takes place; resulting in the formation of a sub-halide or colloid silver. Luppo-Cramer ** as well as Renwick *° have supported the hypothesis that the sensitivity substance is colloid silver of high dispersity. Walter Clark on the other hand considered the sensitivity substance to be either silver oxide or hydroxide due to the absorption 24 Luppo-Cramer, Kolloidchemie und Photographie, 2d Ed., 10921. 25 Renwick, J. Soc. Chem. Ind., 1920, 156 T, 39; Brit. J. Phot., 1920, 67. 168 : (PHOTOGRARI of OH ions.2® This theory was based upon the work of Fajan and Frankenburger *” on visible decomposition. | However, as a result of the work of Sheppard in the laboratories of the Eastman Kodak Company it has been well established that the substance composing the sensitivity centers is silver sulphide. In- vestigation of the causes of variation in the sensitiveness of emulsions of similar properties but different gelatines showed that this variation was due to the presence of a sensitivity promoting substance which was identified as allyl isothiocyanate. This reacts with ammonia to form allyl thiocarbamide which unites with a small amount of the silver halide to form a double compound which at higher temperatures and in alkaline solution decomposes to form silver sulphide.”* The exact manner in which the minute specks of silver sulphide are held in the grain of silver halide has not yet been definitely determined. It seems, however, that since the crystal lattice of the silver sulphide interpenetrates with that of silver bromide it is possible that the ad- hesion of the silver sulphide speck to the silver halide grain may vary in firmness and it is conceivable that this might affect its sensitizing power. The relation of the sensitivity centers of silver sulphide to the mech- anism of the formation of the latent image will be left until the next chapter. | 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 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. . 26 Clark, Brit. J. Phot., 1923, 70. 27 Fajan and Frankenburger, Zeitischr. Physik. Chem., 1923, 105, 255, 273, 320. - 28 Sheppard, Phot. J., 1925, 65. THE PHOTOGRAPHIC EMULSION 169 As early as 1895 Gaedicke ®® 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.” ** 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.*? Svedberg found that for every class of grains of nearly the same size in the emulsion there is a distinct characteristic curve.*® The matter was not fully investigated in a quantitative manner until 1921 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 abscisse. In Fig. 125 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 of an emulsion increases rapidly with an increase in the average size, 29 Eder’s Jahrbuch, 1895, p. 200. 80 Tbid. 81 J. Franklin Inst., 1915, 179, 141. 82 Phot. J., 1919, 59, 260. 88 Z, Wiss. Phot., 1920, 20, 306. eaeet. 170 PHOTOGRAPHY 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 80 SIZE — FREQUENCY CURVE A= STANDARD SLOW LANTERN SLIDE Le B= PAR SPEED PORTRAIT FILM FREQUENCY PER 1000 GRAINS > iJ 20 100 250 ee Fic. 125, Size Frequency Distribution of Silver Halide Grains in a Portrait Film and Lantern Slide Emulsion 0.5 1.0 1. 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 171 GENERAL REFERENCE WorKS AsNEY—Photography with Emulsions. AxsNEY—Treatise on Photography. AsneEy—Instruction in Photography. Burton AND PrincLE—Processes of Pure Photography. BrotHerS—A Manual of Photography. Eper—Ausfurliches Handbuch der Photographie. EpER 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. CHAPTER VII ORTHOCHROMATICS Light and Color. The Spectrum.—According to the generally ac- cepted theory, light is an undulatory movement in an elastic medium known as the ether. When this elastic medium is set in vibration, a wave-movement is sent out in all directions from the source at the speed of about 186,000 miles per second. If the vibrations are below or higher than a certain limit, they cannot be detected by the eye but may be detected in various other ways. 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, 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. 126 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 172 2) Wave-lengths in Angstrom Units. 7594 7186 6867 6563 5896 5270 5184 4861 4308 3969 3934 7594 7186 6867 6563 5896 5270 5184 4861 4308 3969 3934 1. Prismatic Spectrum. — 2. Spectrum produced by diffraction grating. (Showing the principal Fraunhofer lines.) Fic. 126.—Three-color print of the Solar Spectrum. ° ' Wave-lengths in Angstrom Units. ORTHOCHROMATICS 173 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 seven colors which form the visible spectrum, yellow is the most luminous to the eye. DARK VIOLET BLUE GREEN YELLOW RED DARK Fic. 127. Visual Luminosity of the Spectrum after Abney The relative visual intensities of the various colors of the spectrum are illustrated in Fig. 127 from Abney,’ 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 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. 128).2 The dotted lines in- dicate the extension of sensitiveness resulting from extreme lengthen- ing of the exposure. 1 Instruction in Photography, toth Ed., p. 6. 2 Instruction in Photography, toth Ed., p. 9. Also see Meldola, Chemistry of Photography, p. 255. 13 174 PHOTOGRAPHY 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.) 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- fi dese Sees G i ee Fic. 128. Spectral Sensitiveness of the Silver Halides after Meldola parison. When photographed, on an ordinary plate, what do we get? The brilliant orange is a dark grey, also 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. History of Dye Sensitizing.—Dye sana dates from Vogel’s discovery of the action of corallin in 1873.* Considerable difficulty was experienced in sensitizing gelatine emulsions with corallin and 3 Vogel, Handbuch der Photograpme, 4th Ed., I, p. 204. ORTHOCHROMATICS 175 it was not until the discovery of eosin in 1882 that a really practical sensitizer for green and yellow was found. Following this encourag- ing discovery, many other dyes were investigated by Waterhouse, Vogel, Schumann, and Eder, the last named examining, together with his students, several hundred dyes which might be suspected to pos- sess sensitizing properties. Very few, however, were found which were of practical value and only two are in use to-day, erythrosin, a strong, yellow-green sensitizer, and cyanine, a fairly good orange- red sensitizer. In 1904-5 Konig introduced pinachrome, ortho- chrome T, pinacyanol and dicyanine, all of which are more efficient than any of the dyes previously known and are now in general use. Quite a number of interesting dyes for color sensitizing were dis- covered by Sir William Jackson Pope in 1920* while three new dyes, naphthacyanole, acetaminocyanole, and kryptocyanine, have been in- troduced by Mees and Gutekunst of the Eastman Research Labora- tory.° 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: ° 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. Photo. J., 1920, 60, 183, 234, 253. 5 Brit. J. Phot., 1920, 60, 474. 6 Ausfiihrliches Handbuch der Photographie, vol. III, p. 150. Grundlage der Photographie mit Gelatine-Emulsionen. 176 PHOTOGRAPHY 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 I part in 1000 to I 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,’ and assumes that the vibrations are absorbed by the colored compound and photochemical decomposition then occurs. The researches of Luppo- Cramer and Traube,® 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,® 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.° 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 Hubl,* Monpillard,!? and Valenta.**. | 7 Beitrage zur Photochemie, vol. III, p. 75. 8 Brit. J. Phot., 1907. ® Beitrage sur Photochemie, p. 35. 10 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. 11 Brit. J. Almanac, 1906, p. 771, and 1907, p. 744. 12 Bull. Soc. Photo. Franc., 1906, p. 132. 18 Beitrage zur Photochemie, III, pp. 153 and 163. ee ee ee Oe ee ne tl all TT ae a ee ote p i.e, ORTHOCHROMATICS ‘Ware 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. [or instance, cyanin is very unstable while erythrosin 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. (MUTT rv LL Bk a, b= Nn l\ al al Fic. 129. Drying Cabinet for Sensitized Plates 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 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.—Plates sensitive to the green and yellow in ad- dition to blue and violet are known as orthochromatic or isochromatic. 178 PHOTOGRAPH. Plates sensitive to all the colors of the spectrum are known as pan- chromatic. In dye sensitizing, absolute cleanliness is essential, as dust or chemi- cal contaminations of any kind will cause spots and streaks. Glass trays are to be preferred, since they are more easily kept clean than trays of other material. Fog in dyed plates may be due to the use of the wrong plate, stray light during bathing, use of dye solutions at too high temperature, or too much ammonia. For sensitizing to the green and yellow it is safe to work by a deep ruby light but for panchromatic sensitizing no red light is safe and the operation should be conducted in total darkness. | Slow drying gives rise to uneven color sensitiveness. The drying cabinet shown in Fig. 129 is almost a necessity. The air is driven through the light-proof passages by means of the electric fan and the constant circulation of air enables the plates to be dried evenly and rapidly. Two doors (not shown in drawing) form the front of the cabinet and opening them gives access to the interior. ‘The dyed plates are placed in ordinary drying racks and distributed along the shelves. Sensitizing for Green and Yellow.—The dyes most generally used for this purpose are eosin, erythrosine, orthochrom T, pinaverdol, pinachrome, and homocol. Eosin (C,H,(COC,HBr,OK),O) is soluble in water, alcohol and ether. It was advised as a sensitizer by Waterhouse in 1875 but has elas dull £osin Fic. 130. Spectrograph of Eosin. Bureau of Standards paper No. 422 been completely displaced by the later dyes which confer greater and more even sensitiveness. Reference to Fig. 130 will show that its sensitizing action extends to about 575 with a decided drop in the blue- green at about 540. | Erythrosin (C,H,(COC,HI,ONa),O).—Also known as iodeosin and bluish eosin. It is soluble in both alcohol and water and a strong sensitizer. The curve of sensitiveness (Fig. 131) extends as far as 580 with a maximum at approximately 540-550. There is a decided depression at about 520 in the blue-green. ee DP ae ee eee Pe ORTHOCHROMATICS 179 In use, one part of the dye is dissolved in one thousand parts of dis- tilled water and this stock solution diluted as follows for the bathing solution : Erythrosin Fic. 131. Spectrograph of Erythrosine. Bureau of Standards paper No. 422 Deer EERO (1 20D0) oi ee. ck bic d eae bes ecvsbeceee 100 parts re io ea beaded acta veasaueade 400 parts Be reer Ge eee Ne en ey Sewanee dee 5 parts (This is a 1: 5000 solution.) Rose Bengal (Potassium-tetra-iodo-chloro-flourescein) is also one of the eosin group and was formerly used for sensitizing. It is soluble Rose Benaal Fic. 132. Spectrograph of Rose Bengal. Bureau of Standards paper No. 422 in both water and alcohol. The band of sensitiveness extends (see Fig. 132) to about 600 but shows the depression of sensitiveness to blue-green characteristic of eosine at about 400. Orthochrome T Fic. 133. Spectrograph of Orthochrome T. Bureau of Standards paper No. 422 Orthochrom T.—This is one of the isocyanine dyes prepared by Konig in 1903-4. It is p-toluchinaldin-p-toluchinalinethyl-cyanine- bromide and is also soluble in both alcohol and water and sensitizes to 180 PHOTOGRAPHY the blue-green, green, and yellow but has practically no sensitiveness to red. (See Fig. 133.) The depression in the blue-green is less notice- able than with dyes of the eosin group. In use, a stock solution is made containing one part of the dye to each 1000 parts of water and this diluted as follows to form the bath- ing solution: Stock dye solution... :....c.cveecacecsucscees 3 50 2 parts Water ......usesw ers cob ne ipipaine so) cllee ew gle Qantas 100 parts (This is a 1: 50,000 solution.) Time of bathing, three minutes. Pinaverdol (Dimethyl-6-methylisocyanine Iodide).—lIts band of sensitiveness extends to about 630 which is just a little farther than Pinaverdol Fic. 134. Spectrograph of Pinaverdol. Bureau of Standards paper No. 422 orthochrom T. The sensitiveness to blue-green is slightly greater also _ than the former dye. It is used in just the same manner as orthochrom T (Fig. 134). Pinachrome (Diethyl-6-ethoxy-6-methoxyisocyanme Bromide) (1- C,H,) (6-OCH,)NC,H, : CH:C,H,N(1—C,H,) (6-OC,H, ) Br.— i bea hietniattens fealiivise U Pinachrome Fic. 135. Spectrograph of Pinachrome. Bureau of Standards paper No. 422 Soluble in alcohol and water. The band.of sensitiveness extends to 640, showing a greater sensitiveness to yellow between wave-lengths 525-625. (See Fig. 135.) Used in the same manner as pinaverdol and orthochrom T. Pinachrome Blue—vThis is a dye of secret composition introduced a a ORTHOCHROMATICS 181 by Konig in 1917. According to Dr. Eder it is an excellent sensitizer for dark red to orange and as far as yellow-green. Its sensitizing cutve is shown in Fig. 136. DOr-™- OW} WM LO w+ + Fic. 136. Spectrograph of Pinachrome Blue Pinachrome Violet—This dye resembles pinacyanol very closely in its sensitizing action. It is a strong red sensitizer through orange and yellow to green. There is a small minimum near line C 4% D anda SOOG oO ©: © = Con) oO NOWMOW O Wo oO Lo oO cocmrnm- co oO iO LO + - st Fic. 137. Spectrograph of Pinachrome Violet pronounced minimum in the green. In comparison with pinachrome blue the action does not extend so far into the red but the curve is more even. The use of ammonia increases the speed from 4-6 times but tends to produce heavy fog (Fig. 137). Homocol Fic. 138. Spectrograph of Homocol. Bureau of Standards paper No. 422 Homocol.—This dye sensitizes to about 650 (see Fig. 138) in the orange and shows greater sensitiveness between 500 and 600 than either pinachrome or pinaverdol, the former more nearly equalling it within these limits. It is used at the same concentration and as the other isocyanines above. | Pinaflavol—This is a green sensitizing dye derived from quinoline and prepared by Dr. R. Schuloff of the Hoechst dye works. In com- parison with eosin dyes it does not show the deficiency in the blue- 182 PHOTOGRAPHY green which is characteristic of the former but gives a comparatively even band over yellow-green and blue. ‘The sensitizing band extends approximately from D to F, having a maximum (Fig. 139) at about SRT: Fic. 139. Spectrograph of Pinaflavol 530. The drop of the curve at D is of interest, since it means that in practice green is represented as brighter than yellow. This, however, is incorrect rendering since yellow has the highest visual luminosity of any color. For this reason, the dye will probably find its widest application in combination with other dyes. The sensitizing bath is as follows: Pinaflavol stock solution (1% 1000) ..<../<);.:9s ae een I part Distilled water. 6.65 i6 0. 62 dss eae chee ole pene ee Ran ee 50 parts Time of bathing, 3 minutes. The addition of alcohol at any time lowers the sensitiveness according to Konig. 2-p-Dimethyaminostyrylpyridine Methiodide—This substance was Cyanin Fic. 140. Spectrograph of Cyanine. Bureau of Standards paper No. 422 prepared by Mills and Pope in 1922 and is stated to be the most powerful sensitizer for green that is known at the present time. Plates bathed in an aqueous solution 1 : 30,000 show an almost uniform sensi- tiveness from blue to 560, falling off from there to about 620, where the action practically ceases. Sensitizers for Red.—Cyanine (C,,H,,N.1) (Diamyleyanine-cya- nine Iodide).—Also known as chinoline blue. Only red sensitizer —s as _——s ORTHOCHROMATICS 183 known for years but very prone to cause excessive fog and now superseded by the isocyanines. Its band of sensitiveness (see Fig. 140) extends to 650. Alizarine Blue.—Investigators disagree as to the value of this dye. Scoble ** states that he has had good success with the following formula: ee ET eas rae ge ay alk iowa e bw od ba a0 T's I part ee eee eater a. ea, SR re ye ink vein neces sneees 500 parts Oe 8 SIRES MORRIS ie 2 a > 10: parts ae ARR Ree, Sn SRS Bale, sold sia vie Pe A eee Gn wee ree cs 500 parts Filter and use at once before the solution turns blue. The green color appears to be necessary in order for the dye to sensitize. The band of sensitiveness extends to 900 and it is, therefore, a useful sensitizer for work in the infra red. Owing to the deep staining of the gelatine, it is not so suitable for general work. 5c°o0 ahaiit ee; Dicya nin Fic. 141. Spectrograph of Dicyanine. Bureau of Standards paper No. 422 Dicyanine.—This is a strong sensitizer for the infra-red, the sensi- tiveness extending as far as 1000 when ammonia is used (Fig. 141). Fig. 142 from Walters and Davis ** shows the effect of the addition of ammonia. The amount of ammonia which may be used is limited owing to the strong fog produced. More care is necessary in the use of dicyanine than most of the other dyes and since plates sensitized with it do not keep, it is seldom used except in spectroscopic work in the infra-red.17 The following is the formula advised by Walters and Leavis: =. 14 Phot. J., 1906, 46, 190. 15 Bulletin of the Bureau of Standards, 1922, No. 422. 16 Bulletin of the Bureau of Standards, 1922, No. 422. 17 See Mees and Wratten, Phot. J., 1908, 48, 25. Dicyanine is now being re- placed even for spectroscopic work in the infra-red by neocyanine, a dye de- veloped by the Eastman Research Laboratory having a sensitizing band from 700 to 900. This is the best infra-red sensitizer now available. 184 PHOTOGRAPHY Water cece ce cect ecceceeceeteectcoevet eens bess: ) aq sitet iii Ethyl alcohol (os per cent)... i... 20.6 0+00 sus olds cee ann ne Dicyanine stock solution (1: 1000) )..+.4..+..s ess eee eee eee Ammonia (28 per cent) (C..P oo cscs cisiee «wie cinp con oe iene te Dicyanin, water, and alcohol : = t i f 4 a i 3 s f £ Dicyanin, water, alcohol, and % per cent am+ monia Dicyanin, water, alcohol, and I per cent ammonia Dicyanin, water, alcohol, and 4 per cent ammonia Fic. 142. Spectrograms showing the difference in the sensitizing action of . dicyanin when used in the baths of various composition The use of sufficient ammonia in the bath not only increases the sensitizing action of dicyanin at its maxima, but extends its action into the infra-red as far as 1000 uu Dicyanine A.—This is a greenish-blue dye prepared by Dr. E. Konig of the Hoechst factory. Its composition has not been published but ee ee CS ee |. Pe ee ne es ORTHOCHROMATICS 185 according to Dr. Eder it is an ethoxy derivative of dicyanine. Ac- cording to the same authority its action extends further into the red and infra-red than dicyanine, to 780 to 630 in the dark red and weakly (SS ae Lae (mf Jad ae RY ao) cs) rs) S HOW O LO oS a) LO © coc#rnr-cOo CO WO (We) + is Fic. 143. Spectrograph of Dicyanine A up to 850 in the infra-red. Its minimum from orange to yellow and green is more marked, however, than with dicyanine. For very weak spectra it is used with ammonia. The plates so prepared do not keep well (Fig. 143). Pinacyanol*® (1:I'diethylcarbocyanine Iodide) C,H,N(C,H;) : CH-CH : CH: C,H,N(C,H,)I.—A plate bathed in an aqueous solu- tion shows a band of sensitiveness extending to about 700 in the ex- treme red. Ammonia increases the red sensitiveness in proportion to amount used. (See Fig. 144.) The use of ethyl alcohol and omit- Pinacyanol Fic. 144. Pinacyanol. Bureau of Standards paper No. 422 ting the washing following bathing result in distinctly greater sensi- tiveness from 230 to 650. According to Wallace,!® rinsing in alcohol after bathing still further increases the sensitiveness. Best concen- tration about I : 70,000. Naphthacyanole (1: I’diethyl Di-B- ML Mn ovineetar aa. Nitrate ).— This is one of the quinoline dyes discovered by the Eastman Labora- tory.2° It is a strong red sensitizer showing a strong maximum at about 690. (See Fig. 145.) It is thus superior to pinacyanol as a red sensitizer. The green sensitiveness, however, is distinctly less than 18 See Mills and Pope, Phot. J., 1920, 60, 254. 19 Brit. Journ. Phot., 1908, p. 10%. 20 Brit. J. Phot., 1922, 69, 474. Also Journal American Chemical Society, 1920, p. 2661. 186 PHOTOGRAPHY pinacyanol. ‘This dye in combination with other green sensitizing dyes would appear to offer great possibilities but at the time of writing nothing has been published along this line. ys. ai. ATM, anit hI Fic. 145. Naphthacyanole Kryptocyanine.—This is another of the dyes discovered by the East- man Research Laboratory and is also a red sensitizing dye. The band of sensitiveness extends from 680 to 850 (see Fig. 146) with the maximum at about 770. Like naphthacyanole it has little sensitiveness P| | | | ah ARAN ARRE,, Piciacaulad Fic. 146. Kryptocyanine to green. It is expected to be of great service in astronomical work with the spectroscope. It is superior to dicyanine for work in the infra-red up to 850, beyond which point dicyanine is still unequalled. For bathing, a concentration of 1: 500,000 is recommended. The ad- dition of alcohol or ammonia is not to be recommended. | Pantochrome.—Obtained by the condensation of iodo-ethylate of dimethyl-aminoquinaldine with dimethylamine-benzaldehyde. It sensi- tizes in a remarkably even band for practically the entire spectrum, showing a small minimum at about 500.7? Red Sensitiveness with Bisulphite-—Capstaff and Bullock of the Eastman Research Laboratory found that bathing an emulsion in a two per cent solution of sodium bisulphite for ten minutes at 65 degrees Fahr. and washing for times, ranging from five minutes to thirty hours, produces remarkable red sensitiveness, which depends on the time of 21 Bull..Soc. Franc. Phot., 1920, p. 182. ORTHOCHROMATICS 187 washing.” The maximum sensitiveness is produced by soaking for ten minutes in a five per cent solution of sodium bisulphite, washing for five minutes and soaking for ten minutes in a 0.2 per cent solution of pure potassium bicarbonate and finally washing for five minutes. Fig. 147 shows the curve of sensitiveness. _ 2 008 Oe ino ini ek et an... deRaE ERASER AERA Fic. 147. Red: Sensitiveness with Bisulphite Treatment of an ordinary emulsion with iodide confers red sensi- tiveness on ordinary plates and films.2* Sheppard finds that the re- sults vary considerably with the emulsion, some emulsions being strongly red sensitized while others are apparently unaffected.** Mixtures of Dyes as Sensitizers.—Pinacyanol and Homocol.—tThis combination has been recommended by Monpillard.*®° Without am- monia the gain in the red and green is slight but with increasing amounts of ammonia the sensitiveness increases. The curve follows a fairly straight line with maxima at 580 and 640 to 680 where the sensitiveness ends. The formula advised by Wallace ** is as follows: ee ere eT OCUS EPS o Soc js. dicy. d ates th ais%as ¥-'e lee bye aie ws ae ee 3 parts ee ee ee, eS hdc wg v shiv sle wade wae serine eden 3 parts Ir eS te eke eins cl dis Soe teh ve Oe dds wee e's 75 parts ONES SS OUD TE A oe a ee 5 parts a ee aio k giao nin a oem ey dea eee 5 Mapas 100 parts Pinacyanol and Pinaverdol.—These two dyes in combination produce a very good color-sensitive plate in which the gap in the blue-green characteristic of pinacyanol is greatly benefited. Pinachrome and Pinacyanol.*7—The curve of this combination is shown in Fig. 148. The first maximum at about 560 is that of pina- 22 Brit. J. Phot., 1920, 67, 710. 23 Renwick, Phot. J., 1921, 61, 12. 24 Phot. J., 1922, 62, 88. 25 Bull. Soc. Franc. Phot., 1906, p. 132. 26 Brit. J. Phot., 1908, 55, 102. 27 From Daur, “ Mixtures of Dyes as Sensitizers,” Brit. J. Phot., 1900, 56, 504. The three curves show the effect of three different times of exposure. 188 PHOTOGRAF A chrome shifted about 10 wave-lengths to the red. The second at 590 runs into the other and is probably the pinachrome maximum intensi- fied by the pinacyanol. The color sensitiveness is a little less than the Fic. 148. Pinachrome and Pinaverdol blue sensitiveness and the band of sensitiveness the same as that of pinacyanol. Pinacyanol-Pinaverdol-H omocol.2®—According to Wallace this is an ideal combination, leaving little to be desired in the way of color sensitiveness and freedom from fog. When made up in an ammoni- acal solution, it sensitizes for practically the whole visible spectrum to 720. The usual gap in the blue-green is well closed and the curve is fairly smooth throughout. The following formula is advised: Pinacyanol, 1% 1000. ¢. 0... sce % 01+ 9 oe sd eer ee er 2.5 parts Pinaverdol, 1.3 TO00. ... 654/00: sic + «+ nny olsen 2.0 parts | Homocol, 1: 1000... feuds ence 2 ay 00s op cyte eee 2.0 parts AmMoMia 2.00. ccc cece kd eek Helps de ts ek ee seen 6.0 parts Alcohol 2...'.95 sss «oat a oe Pee ee 1.5 parts Water: oii icn cdi bance bee we arto Se ae he 100.0 parts Time of bathing, 4 minutes. Afterwards immerse in alcohol bath for 30 seconds. 3 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 28 Brit. J. Phot., 1908, 55, 119. Ce ee ne £6) Gat tip ORTHOCHROMATICS 189 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 greater approximation to the sensitiveness of the eye, it is necessary to use colored screens which, by absorbing these colors either completely or partially, aid the less refrangible rays in affecting the plate in approximately the same proportion as they do the eye. An orthochromatic filter should, so far as possible, completely absorb the ultra-violet without absorbing any of the vis- ible spectrum completely, but it must absorb the blue and violet to such an extent that the photographic effect of the plate will be equal to the visual effect of those colors. Filters which accomplish these purposes are known as orthochromatic, compensation, 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. 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. Ifa 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. lf 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 14 190 PHOTOGRAPHY 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. Orthochromatic Filters—From what has been said before it must be evident that the filter must be adjusted to the plate so that the proper amount of blue and violet is cut out to make the photographic effect approximate the visual effect of color. To find the absorption curve of a filter which will give correct color rendering on a particular plate we require to know first the sensitiveness of the plate to the various colors of the spectrum. If we photograph the spectrum on the plate in question and express the densities, which are a measure of the work accomplished, as a function of the wave-length, we obtain a curve showing the sensitiveness of the plate to the colors of the spectrum. Then if from the ordinates of this curve we subtract the ordinates of the curve representing the visual intensity of the corre- sponding colors of the spectrum we obtain a curve showing the ab- sorption which the filter must possess in order to secure correct color rendering on that particular emulsion. 3 In practice, it is not always possible to use a fully correcting filter, since the time of exposure may be increased to such an extent, owing to the absorption of the active blue and violet rays by the filter, that movement will occur. In such cases it is better to be content with partial color correction. As an example: three orthochromatic filters are supplied by Eastman for the Wratten panchromatic plates and panchromatic film. These are designated as K1, K2 and K3 and re- quire an increase in exposure of 114, 3 and 4 times respectively. These multiplying factors denote the number of times the normal ex- posure without a filter must be increased when the filter is used and depend on the filter and the plate on which it is used. For instance, the multiplying factors of the K1, K2 and K3 screens on a Wratten panchromatic plate are 112, 3 and 4% but on the Seed L Ortho plate the factors for the same filters are 3,15 and 25. Thus the multiplying factor of a given filter is always higher with an orthochromatic plate sensitive to green and yellow than with a panchromatic plate which is sensitive to the entire spectrum. Of two plates having the same speed without a filter, in practice, the panchromatic is faster because a lighter screen is required to correct it while at the same time better color correction is secured because the latter is sensitive to red while the former is not. Most plate makers either make filters for their 4 | ORTHOCHROMATICS 191 own products or specify the multiplying factor to be used when an- other filter is used. 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, other than these, employ orthochromatic methods only to the extent of securing printable clouds in their landscapes. For this purpose a comparatively light screen is all that is necessary, for example a Ki on a panchromatic or a 3 or 4 times filter on an orthochromatic plate. When a filter is used in this way, 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 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 clouds (which will be done if a strong filter is used), as they lose much of their delicacy and charm when this is done. Graduated filters are of great value in photographing clouds in con- junction with the landscape. These are filters whose multiplying power or depth increases uniformly from o to 10 or 15 times. The more actinic blue and violet rays from the sky are made to pass 192 PHOTOGRAPHY through a deeper portion of the filter than the foreground (see Fig. 149) so that greater correction may be obtained in the sky and the extreme distance than in the foreground. By this means, the ap- pearance of over-correction in the foreground is avoided while the distance and clouds are sufficiently corrected to enable them to print with the proper contrast. : For panoramic work of mountain scenery and great distances, com- paratively strong filters are necessary in order to eliminate the haze in the atmosphere. Sometimes on very clear days a K3 filter may be Fic. 149. Action of Graduated Filters sufficient but in most cases stronger filters will be required, such as the G “strong yellow ” requiring an increase in exposure of 6 times with panchromatic plates and 30 times with the Seed L Ortho plate. In extreme cases,.even deeper screens will be necessary as for ex- ample the “ B ” screen and the Orange Red “ A ” which require an in- crease of 10 and 12 times with panchromatic materials. The multiply- ing factor of the former on a Seed L Ortho plate is 12 while the latter is not suitable for use with plates of this type since they are insensitive to the light which it transmits. Orthochromatic Methods in Portraiture-—Concerning the value of color-correct rendering in portraiture, Dr. Mees says:7® “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 150 is a print from a negative made on an ordinary non- 29 Photography of Colored Objects, 193 ORTHOCHROMATICS SUOISINWIA Ij}eWOIyIUegG pue AJeUIpPIQ UO eI}10g oSI 3) a | 194 PHOTOGRAPHY color-sensitive portrait plate. Ordinary plates are sensitive only t 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 its 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 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. 150b 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 Ka filter requiring an increase in ex- posure of 3% 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. It is difficult, on face of the above facts, to see why color-sensitive plates are not used more widely in portrait photography. Formerly, the greatest objection to their use was that in order to secure any advantage over the ordinary plate a filter was necessary and this increases the exposure several times so that there is greater danger of movement. Now, both orthochromatic and panchromatic plates of high speed are readily obtainable and with the general adoption of artificial light there is no real reason why color-sensitive products and filters cannot be used with complete success. For all ordinary work the Ki filter requiring an increase of 50 per cent in exposure will give sufficient correction on panchromatic plates while with certain sub- jects a K2 filter which requires about three times more exposure will be necessary. As panchromatic plates of high speed are now avail- able, these exposures should not be unduly long and the advantages ORTHOCHROMATICS 195 secured by the use of color-sensitive products far outweighs the oc- casional loss due to movement of the subject. Most artificial lights are deficient in blue rays and have greater intensity in the red than daylight. Fig. 151 shows the distribution of energy in the spectrum of daylight and gas-filled (tungsten) electric 490 soo 699 4 Pahtria Gionibidlats voller aniPleree Voen Sunlight Tungsten lamp Fic. 151. Spectrum of Daylight and Mazda Clear Glass Bulbs lamps. Under these conditions color-sensitive plates are distinctly faster than ordinary plates owing to their greater sensitivity to the green and red. Furthermore, owing to the spectral distribution of light from artificial sources of this type, filters are not needed except where extra correction is necessary. For all ordinary portrait work with gas-filled tungsten lamps a panchromatic plate without a filter will give sufficient correction and is faster than an ordinary non-color sensitive plate having the same speed in daylight. 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. 196 PHOTOGRAPHY ney Rt most valuable publication’ of the year is the "mistorie de la Decouverte de la Photographie by Georges Potonnice [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 notabte work of the year is the Physics of the Developed Photographic Tuace by f.E.Ross,which is Number 4 of the series of Yonographs on the jae ot DunORFaiiE issued by the Research Levoratory of the Eastman Kodak Company. Many of the more important sensitizing and desensitizing dyes are afsnueaed in a work by JeF.Rewitt "Dyestuffs « derived from | Pyridine, quinoline ,Acridine (Snd 3 Xanthene". (Longemanbs Green & Co., New York) Fic. 152a. Photograph of Manuscript in Blue with Red Corrections using Green Filter Perhaps the most valuable publication of the yet the Historie de la Decouverte de la Photographie by Georges Potonniee , [published by dontercbenteie 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 notabie work of the year is the Physics of the Developed Photographic image by f.E.Ross,which is Number 4 of the series of monographs on the theory of photography issued by the Research laboratory of tie Eastman Kodak Company. Many of the more important sensitixing and desensitizing dyes are discussed in a work by J.F.Mewit ‘Dyestuffs derived from Pyridine,Quinoline,Acridine and Xanthene". (Longsmanns Green & Co., New York) Fic. 152b. Photograph of Manuscript in Blue with Red Corrections showing Use of Red Filter ORTHOCHROMATICS 197 -To render a color as white it must be photographed not in its ab- 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, 152a. 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. 152b. 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. 153 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- eraphed 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 erain of the wood is brought out. 198 PHOTOGRAPHY RCASSIAN WALNUT | CIRCASSIAN Fic. 153. Wood Sections on Ordinary and Panchromatic Plates FEAST INDIAN WALNUI ff INDIAN TEAK > = — tae 4 = co A ws .> fs a me - al pa 7" oo ae i mike “ ee ae ee ee ee eee ee ee ae ee: ee ee ee en. ee ee ee ae Ordinary Plate On Panchro- matic Plate (Courtesy of Ilford Ltd.) ORTHOCHROMATICS Lug In photographing furniture, success depends chiefly upon the selec- 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 WorRKS BakeR—Orthochromatic or Isochromatic Photography. Eper—Uber die Chemischen Wirkungen des Farbigen Lichtes. Eper AND VALENTA—Beitrage zur Photochemie und Spectralanalyse. Husrt—Die Photographischen Lichtfilter. Husit—Die Orthochromatische Photographie. Konic—Das Arbeiten mit Farben empfindlichem Platten. Meres—Photography of Colored Objects. WEIcERT—Die Chemischen Wirkung des Lichts. 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. | ee ee ee Se ee Le a Pe ee OR ee a eR Oe ne ee Se ee ee 200 THE LATENT IMAGE 201 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: H, + Cl, = 2HC. 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.—Light is not the only agent to which the silver halides are sensitive and several other agencies are known to form latent images. According to Namias? a 1: 20,000 solution of crystallized stannous chloride (SnCl,°2H,O) in distilled water will 1 Phot. Korr., 42 (1907), p. 155. Jour. Phys. Chem., 14 (1910), p. 326. 202 PHOTOGRAPHY produce a latent image which may be detected with ordinary developers. The action seems to be exactly the same as that produced by light. The action is stronger in more concentrated solutions, while if the solu- tion is very strong the plate fogs badly and the effect is very similar to that of over-exposure. If the action is prolonged beyond this stage, a visible image is formed which appears to be analogous to that pro- duced by the continued action of light. Towards the end of the last century, W. J. Russell found that many _ substances were able to act on a photographic plate in some manner so as to make it developable without any exposure to light. The num- ber of substances which would act in this manner was very great, and included freshly scratched metals, especially 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 exhibited 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? 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 S-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 2S. E. Sheppard and E. P. Wightman, J. Franklin Inst., 1923, 195, 337. THE LATENT IMAGE 203 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.* ‘ In view of this extraordinary parallelism it is difficult to believe that the actions of peroxide and of light in forming the latent image are not ultimately the same. In fact, theories have been proposed to account for the action of peroxide as due to the emission of radiation as a re- sult of the decomposition of the peroxide.* There is evidence, how- ever, which is contrary to this chemiluminescence view, so that it is by no means generally accepted. Owing to the uncertainty as to the action of peroxide, the study of the fogging action of sodium arsenite is of interest, as in this case the emission of radiation cannot so reasonably be postulated. An aqueous solution of mono-sodium arsenite (NaH,AsO,) reproduces the action of light on a plate as faithfully as does hydrogen peroxide, and it has been studied in more detail in certain respects.® 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- 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 p-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, 8 Luppo-Cramer, Phot. Korr., 1902, 643; 1003, 89; 1908, 548. Graetz, Z. Physik., 1902, 5, 160; 1903, 9, 271. Svedberg, Z. wiss. Phot., 1920, 20, 37. 4 Sheppard and Wightman, J. Franklin Inst., 1923, 195, 337. 5 Bancroft and Perley, J. Phys. Chem., 1910, 14, 292, 648. Clark, Brit. J. Phot., 1922, 69, 462; 1923, 70, 717. Clark, Phot. J., 1924, 64, 363. 204 PHOTOGRAPHY 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.® The formation and reaction of the latent arsenite image are thus very similar to those of the latent light image. Now, these observations with sodium arsenite are of vital impor- tance, if it can be accepted that there is no interaction between sodium arsenite and silver bromide. For then we are faced with the fact that silver bromide does not react with sodium arsenite, and yet is made de- velopable by it. If we neglect the improbable suggestion that the arsenite acts by breaking down the protective action of the gelatin on the grains, the only conclusion we can come to from the observations is that the latent arsenite image is due to reaction of sodium arsenite with something at the grain surface which is not silver bromide. Now, it is clear from a study of the action of arsenite that it is extremely probable that both sodium arsenite and light act at the same points in the grains, so that it appears that light acts at points where there is some material other than silver bromide—that is, the sensitiveness of grains in an emulsion is due to the presence of traces of material not silver bromide, which is distributed in spots haphazard among the grains. ; As stated, these considerations are only valid if there is no interac- tion between sodium arsenite and silver bromide. It has been found that in presence of alkali the monosodium arsenite, which under these conditions is mixed with the higher sodium salts, can react with silver bromide, giving a complex which is unstable and slowly deposits silver on standing. With the monosodium arsenite, prepared from arsenious oxide and caustic soda, some workers claim to have shown an interac- tion of silver bromide, although in the cases studied by Clark no sug- gestion of any action could be demonstrated, and yet a very marked effect was obtained on the plate.’ 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, 6 Clark, Phot. J., 1923, 63, 237; 1924, 64, 91. * Luppo-Cramer, Phot. Ind., 1923, 456. Clark, Brit. J. Phot., 1923, 70, 717. ———— eee - - LHe LATENT IMAGE 205 although owing to the enormous exposures required no one has been able to go past the second negative stage, so far as the writer is aware. 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 conditions leading to reversal and the peculiarities of the phe- nomenon have been studied by many, among whom may be mentioned Abney,® Janssen,? Crowther ?° and Preobrajensky."! It has been determined that atmospheric oxidation is probably necessary, also, that a preliminary exposure to light aids reversal, while oxidizing agents also facilitate reversal, but reducing agents either prevent it altogether or retard its appearance. The red rays were found by Abney to be more active in producing reversal than those of shorter length. (See Treatise on Photography, 1oth Ed., 1918, pp. 93 to 97.) 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- 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 8 Abney, Instruction in Photography, pp. 33-35, 10th Ed., 1901; Treatise on Photography, pp. 93-97, 10th Ed., 1918. ® Janssen, Compt. rend., June, 1880. 10 Crowther, Phot. J., 1914, 54, 253, and Phot. J., 1915, 55, 186. 11 Preobrajensky, Bull. Soc. franc. Phot., 1906, pp. 124, 281; Phot. J. (1906), 46, 371, and (1907) 47, 335. 15 ' “tet am 206 PHOTOGRAPHY that it can destroy the latent image left after fixation of an exposed plate, so that it cannot be physically developed.*? _ 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 2 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.'* Although it is difficult to obtain really direct eee 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 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. 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** photo-regression is more apparent on images 12 Phot, J., 1924, 64, 363. . .* 18 Phot. J., 1924, 64, 363. Cf. also Wightman, Trivelli and Sheppard, J. Franklin Inst., 1925. oll > Wak F Chavaom “ Effect of Time on the Latent Image,” Phot. Ty 1917, 57, 72, 15 Zeit. wiss. Eat 1905, 3, 58. | a a j TL)? oY Pee ee a a ee een CE , eee e ee Oe e ee al oe pe ere ee ee ee leet ee ek nee, Coren id Me AC RIIN Er WEAN 207 which have received less than normal exposure, and 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. The action of alum no doubt explains why developing-out papers are more subject to photo-regression than plates, since alum is always added to the former in order to render the gelatine less soluble, while plates do not need additions of alum unless made for use in very hot climates. According to Luppo-Cramer the size of grain has an in- fluence, 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. The Action of Solvents of Silver on the Latent Image.—The theory has been advanced that the latent image consisted of “germs” of metallic silver which were produced by the reduction of the silver bromide by light. In combating this theory Dr. Eder contended that if the latent image consisted of metallic silver, it should be soluble in a silver solvent, as nitric acid, and that this was opposed to the ex- perimental evidence which he had obtained. Since the effect of silver solvents on the latent image had an important bearing on the metallic silver theory, the question was studied rather carefully both by those who opposed and by those who favored this theory. Luppo-Cramer using collodion emulsion found that 33 per cent nitric acid had little or no effect on the latent image if applied before exposure, but if applied after the exposure the image was partially destroyed and only the highlights remained. Using concentrated 65 per cent nitric acid the image was completely destroyed. In supporting the metallic silver theory Abegg also came to the conclusion that the latent image was destroyed by nitric acid. 208 PHOTOGRAPH: Later Dr. Eder modified his original statement that the latent image was not destroyed by nitric acid and came to the following conclusions : *° 1. The normal briefly exposed latent image on collodio-bromide plates is completely destroyed by nitric acid. 2. The longer exposed latent image is not totally destroyed but is weakened. | 3. The solarized latent image is only slightly attacked by the strongest nitric acid and develops as a thin negative instead of a positive. It is therefore now generally admitted that in the case of a short exposure at least the latent image is destroyed by nitric acid. According to Mercator" other acids which are not solvents of silver will also destroy the latent image. Physical Development of the Latent Image after Fixation. 4g 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 | ; . : . - a ee ae, ee ee ee ee eee le developing agent a silver salt capable of forming silver in the nascent state.*® Lumiere and Seyewetz found that the latent image is partially de- stroyed by prolonged fixing in hypo and by acids. By fixing five min- utes in a 30 per cent solution of hypo with the addition of one per cent of ammonia and washing in water made distinctly alkaline the latent image is not reduced and the process can be worked with but little more exposure than that required for normal exposures. 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 16 The most convenient reference is Brit. J. Phot., 1905, 52, 950, 968. 17 Brit. J. Phot., 1899, 46, 628. 18 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, p. 257, and 1904, p. 54. Lumiere and Seyewetz, Bull. Soc. franc. Phot., 1911, pp. 264, 373; 1924, p. 169. Compt. Rendus, 1924, 179, 14. Luppo-Cramer, Phot. Rund., 1924, p. 780. THE LATENT IMAGE 209 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 Lumiére and Seyewetz *° 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 which we may conclude that the nuclei left after fixing can- not be silver bromide, for hypo is a solvent of the silver halides, nor from the above experiment can they be regarded as metallic silver un- less its property of attracting nascent silver has been destroyed by un- known factors. 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 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 *° and was supported by Luppo-Cramer and Lorenz.?? 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. (See bibliography.) Image Transference.—According to Renwick, Eder and Pizzighelli in 1881 were the first to show that an exposed silver chloride plate 19 Phot. Revue, 1925, 37, 48. 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. 210 PHOTOGRAPHY 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.” ?° | In his Hurter lecture before the Liverpool Section of the Society of Chemical Industry, 1920, Renwick showed that if after exposure the plate is immersed in a 1 per cent solution of potassium iodide, which contains 2 to 3 per cent of a neutral sulphite, all of the silver salts are converted into silver iodide. After washing in a dilute neu- tral sulphite solution, the silver iodide image may be developed with a ¥%4 per cent solution of amidol and 1o per cent each of crystal car- bonate and sulphite of soda. Owing to the insensitiveness of silver iodide when precipitated in an excess of a soluble iodide it is possible to develop plates after iodizing in strong white light. However, if any of the original silver bromide remains it will immediately blacken, owing to the superior sensitiveness to light. That such does not hap- pen when the plates have been thoroughly iodized may be taken as evidence of the fact that the latent image has been transferred from one halide (the bromide) to another, which is in this case silver iodide. Indoxyl Development.—The oxidizing properties of the latent image have been used repeatedly as an explanation of the process of de- velopment. Thus Eder says: 7° “ Chemical development is character- ized by a reduction process, in which exposed silver halide is con- verted into metallic silver ; the unexposed is, however, left intact.” In 1907, Dr. Homolka investigated the oxidizing powers of the latent images on organic compounds, other than the common developing agents, in order to determine if the latent image was an oxidizer in the widest sense of the term.?* After an examination of several com- pounds Dr. Homolka settled on indoxyl, which is an intermediate product between indol and indigo. Indoxy] dissolves freely in water and the resulting solution is completely and instantaneously oxidized to indigo by the mildest oxidizer. If.an exposed plate is placed in a two per cent solution of indoxyl a visible image develops in from five to ten minutes. This image can 22 Brit. J. Phot., 1920, 67, 447, 469. 23 Ausfiihrliches Handbuch der Photographie, 5th Ed., vol. III, p. 289. 24 Brit. J. Phot., 1907, 54, 136. ee ee ee Se ee ae ee a Oe, ee ee ee 54 eh “sy Pe ~~ THE LATENT IMAGE 211 be shown to consist of two separate images, one of metallic silver and another of indigo, either of which may be separated from the other. A solution of potassium cyanide will remove the silver image leaving behind the indigo image, or a solution of sodium hydro-sulphite (Na,S,O0,) will remove the indigo image without affecting the other. Since indoxyl is reduced to indigo in the presence of the latent image, the latent image may be considered as an oxidizer. The latent image formed by light is evidently different in some way from that formed by stannous chloride, for as Dr. Homolka states indoxyl is reduced to indigo by the one but is unaffected by the other.2° A plate bathed in a 1: 200,000 solution of stannous chloride, and washed for an hour in running water, develops rapidly in indoxy]l, but an examination after fixing and washing shows that only a silver image has been formed and that there is no trace of ari indigo image as is the case with a latent image which has been formed by light. Homolka found that with exposures sufficient to produce reversal, only the silver image was reversed, the indigo image remaining nega- tive.*"° Crowther, however, disputes this statement and maintains that both images are reversed to approximately the same extent.?’ Action of Oxidizing and Halogenizing Agents on the Latent Image. —The latent image is either partially or wholly destroyed by oxidiz- ing or halogenizing agents. Thus, if a latent image is treated in a 5 per cent solution of potassium cyanide (KCN) the image is com- pletely destroyed and development either physically or chemically is impossible. Substances which readily give up halogen, as ferric chloride, FeCl,, cupric chloride, CuCl,, mercuric chloride, HgCl,, and the halogen acids, as HCl, HBr, and HI, act in a similar man- ner. Strong oxidizing agents as ammonium persulphate, K,S,O,, and potassium permanganate, KMnOQ,, in an acid solution also depress the latent image. Theories of the Latent Image.—Theories of the latent image may be divided into two classes, physical and chemical; the one considering the change in a silver haloid on exposure to light to be a physical modification, while the other believes that a chemical change has taken place. The chief chemical theories are three in number: 25 Brit. J. Phot., 1907, 54, 210. °6 Brit. J. Phot., 1907, 54, 267. ~ “hot. J., 1915, 55, 186. 212 PHOTOGRAPHY 1. The oxy-halide theory, 2. The sub-halide theory, 3. The metallic silver theory (Silberkeim theorie). The leading physical theories are also three in number; viz., 1. The molecular strain theory, 2. The electron theory, 3. The colloidal silver theory. Each of these, together with its supporting evidence and the chief objections to its general acceptance, will be considered as briefly as possible. The Oxy-Halide Theory.—Many things seem to indicate that the presence of oxygen is essential to the darkening of silver chloride by light, a fact which seems to have been distinctly stated first by Robert Hunt in his Researches on Light.”® Abney found that silver chloride remains unchanged for several months when exposed to light ina vacuum. A similar conclusion was reached by Carey Lea. In an experiment of Tugolessow 7® in which silver chloride was exposed to light in presence of stannous chloride, which is a strong reducing agent and would therefore tend to prevent oxidation, it was found that silver chloride did not darken, even if ex- posed to the action of strong light for several days. The opposite effect is observed when an oxidizing agent, as hydrogen peroxide, is used instead of a reducing agent; the silver chloride then darkens more rapidly and completely than in either air or water. The fact that the darkened silver chloride contains oxygen may be regarded as fairly certain from the work of Hodgkinson and Baker. Dr. Hodgkinson examined the darkened product and as a result of his analysis gave it the formula Ag,OCI,. Baker also found that the darkened product contained oxygen and settled upon the formula Ag,OCL. The equation for the action of light on silver chloride would there- fore be: ; 8AgCl + O, == 2Ag,O0Cl, + 2Cl, or : 8AgCl + 2H,O = 2Ag,O0Cl, + 4HCl. 28 Second Edition, p. 80. 29 Phot. Korr., 40 (1903), 584; also J. Phys. Chem., 1911, 15, 331. THE LATENT IMAGE 213 Baker’s equation would be: 4AgCl + O, = 2Ag,OCl + Cl, or 4AgCl + 2H,O = 2Ag,OCl + 2HCl. Since silver chloride does not darken visibly when exposed to light, provided it is protected from oxygen, while the addition of an oxidiz- ing agent accelerates the rate of darkening, and the presence of oxygen in the darkened product can be shown, it would appear that oxygen is essential to the darkening action. If we assume that the latent in- visible image is different only in degree, and not in kind, from that which is visible, and which is formed by the continued action -of light, would it not be both possible and probable for the latent image to con- sist of a combination of the silver halide with oxygen? Such a theory has been brought forward several times by a few investigators but has failed to elicit much response from the scientific world at large. It will be observed that all experimental evidence in support of the theory has been obtained with pure silver chloride, and that all deter- minations which indicate the presence of oxygen have been made on a product which has darkened wisibly. This is an entirely different state of affairs from the latent image with which we are concerned, for in practice we have to deal not with a pure silver halide, but with a gelatino-silver halide complex, nor do we deal with a visible darken- ing but an invisible one which is only rendered visible upon the ap- plication of certain reducing agents known as developers. If we as- sume, with Tugolessow,® that the difference in the visible and invisible images is due primarily to a difference in the degree of oxidation, we are faced with the fact that one of two unlikely things must happen. First, it seems decidedly improbable that the combination of oxygen and silver halide can take place completely in the short space of an instantaneous exposure. Second, if the action of the light is catalytic, that is to say if the function of the light is to so alter the silver halide that it is subject to oxidation, which need not necessarily take place during exposure, then the change must be physical rather than chemical. In the opinion of the writer, the first of these theories is decidedly improbable, while the second, if carried to its logical conclusion, is es- sentially a physical theory in which the change may consist of either molecular strain or the emission of an electron, the existence of either being in itself sufficient to account for the latent image without the introduction of the oxidation factor. 30 Phot. Korr. (1903), 40, 584. 214 PHOTOGRAPHY The Sub-Halide Theory.—Carl Wilhelm Scheele laid the basis of the sub-halide theory in 1776, when he proved that chlorine is liberated when silver chloride is exposed to light under water. However, ac- cording to Waterhouse,*! Berthollet was the first to suggest the for- mation of a sub-halide containing less halogen than the normal salt. The sub-halide is thus the oldest theory of the latent image and it is likewise one which has at some time been supported by ds every authority of eminence. The existence of a single definite sub-halide has been supported by a number of authorities, notably Sir William Abney. According to this opinion the action of light on a silver bromide plate would be Ag,Br, = —= Ag,Br + Br. It is difficult to explain the phenomena of solarization on the as- sumption of only one compound, regardless of what formula be as- signed to it, consequently Trivelli brought forward the idea of several sub-halides Ag,Br,, Ag,Br,, Ag,Br,, Ag,Br,. None of these com- pounds have been prepared, and as there is no satisfactory evidence of their existence, Trivelli’s hypothesis has not been widely accepted. While experimenting with indoxyl, Homolka found that the oxidiz- ing properties of the latent image were greater than those of silver bromide itself. He therefore concluded that the latent image could not be composed entirely of sub-halide, otherwise how could its oxidiz- ing powers be greater than silver bromide which contains more halo- gen’ In his opinion, there is, in addition to any sub-halide which may be formed by the light, a per-halide containing more halogen than the normal salt. The fact that a latent image formed by light is able to oxidize indoxyl, while that formed by the action of stannous chloride can not do so, was explained by Homolka as being due to ait existence of per-halide in one case and not in the other.* Carey Lea,** Luppo-Cramer, and Reinders *! believe that the latent image is a phase of variable composition with silver bromide as the end term. There has been a good deal of discussion as to the con- stituents of the phase, some claiming that they are silver bromide and silver and others silver bromide and some sub-bromide. As we have no way of distinguishing between these two. hypotheses, this matter is as yet unsettled. 31 Phot. J., 1903, 47, 59. 32 Brit. J. Phot., 1907, 54, 136, 216, 267. 33 Carey Lea, Amer. Jour. Sci. (3), 33, 349 (1877). 34 Reinders, Zeit. Phys. Chem. (1911), 77, 363. ee Se oe ge a eee ee Cee ee ee eee ee en i ee i me . ee : ~oyn ata us Soy rk eee en ) Pinas ee —_ THE LATENT IMAGE 215 Evidences for the Liberation of Halogen.—lIf silver bromide is re- duced to a sub-bromide, containing less halogen than the normal bro- mide, bromine must be liberated and it should be possible to determine the presence of free bromine by chemical means. Abney mentions the fact that in the case of silver bromide long exposure to light produces an odor characteristic of bromine and that chemical tests indicate its presence. Scheele had previously shown that chlorine is liberated when silver chloride is exposed to light under water because chemical tests indicate the presence of chlorine in the water. The following is less conclusive but affords indirect evidence that there is a liberation of halogen: “ If the existence of sub-halides is assumed, it must neces- sarily follow that, if we restore to the sub-halide the bromine which has been lost, normal bromide must be reconstituted. As silver bro- mide may be formed by simply adding a solution of potassium bromide to silver nitrate, let us ascertain if an exposed film of silver bromide may be reconverted to normal bromide in the same manner. The ex- periment confirms the assumption. The impression formed by light disappears completely and the image cannot possibly be developed.” *° The student will please note that owing to other undetermined fac- tors the last experiment may not be entirely conclusive, and subject to different interpretations, while the other facts differ from the true latent image in that they are the result of prolonged exposure to light and also do not consider the possible consequences of any influence on the part of the colloid medium in which the grains of silver bromide are imbedded. The real question is whether there is a liberation of halogen under the conditions of ordinary practice. At the present time no definite answer can be given. Any liberation of halogen which might possibly occur is so infinitesimally small that no chemical tests: of which we are at present aware will indicate its presence. Do Silver Sub-Halides Exist?—The ‘isolation of a definite sub- halide of silver in the laboratory would naturally lend considerable support to any theory of the latent image which calls for the reduction of silver halide into a sub-halide, and it is well that we find what success has attended the attempt to prepare sub-haloids in the labora- tory. Wetzlar, in 1828, claimed to have obtained silver sub-chloride by treating solutions of ferric and cupric chloride with silver leaf. In 1839 Wohler obtained a product which gave on analysis Ag,O by pass- ing hydrogen over heated silver citrate. This would be a sub-oxide, corresponding to the sub-haloid Ag,Cl. Von Bibra *° isolated a body, 35 Mercator, “ Nascent Silver and Sub-Haloid Theories,” Brit. J. Phot., 1899, 46, 620. 36 Jour. fur Prak. Chem. (2), 12-55. 216 PHOTOGRAPHY Ag,Cl,, by treating Wohler’s sub-oxide with hydrochloric acid. The majority of scientists who have attempted to repeat the experiments did not meet with success. The existence of silver sub-fluoride is fairly certain from the work of Guntz who passed a current of electricity through a solution of silver fluoride using silver electrodes. Guntz also prepared a sub- stance which he considered to be silver sub-haloid by treating silver sub-fluoride with hydrochloric acid. Some authorities are satisfied with the evidence while others are not. Otto Vogel *” attempted to prepare silver sub-haloids by treating silver nitrate with cuprous chlo- ride, bromide or iodide. The analytical results agree very closely with the accepted formulas Ag,Cl,, Ag,Br,, Ag,I,. Waterhouse, however, considers it extremely doubtful that the sub-haloids can be prepared in this manner and is of the opinion that these compounds consist of finely divided silver and unaltered silver haloid. Thus the existence of a definite sub-haloid is still open to question, and as late as 1911 Heyer, in attempting to confirm Luther’s earlier experiments which pointed to the existence of Ag,Cl and Ag,Br, was unable to find any conclusive evidence of their existence.** Objections to the Sub-Haloid Theory.—The principal objections to the sub-haloid theory are three in number: 1. Lack of evidence for the liberation of halogen with short exposures. 2. The questionable evidence for the existence of silver sub-halides. 3. The improbability of chemical change resulting from the energy incident on the plate during exposure. The first objection has already been raised in a preceding paragraph, while the second has also been suggested directly above. There is absolutely no evidence at present that there is a liberation of halogen with the short exposures which form the latent image. We are forced to assume that what applies to the visible image also holds true for the latent image, an assumption which may or may not be true, although the similarity in the chemical reactions. of two images is in its favor. The only silver sub-haloid which can be said to exist is the sub- fluoride. The existence of Ag,Br,, Ag,Cl, can only be inferred from the existence of the sub-fluoride but it may be questioned if this evi- dence is sufficient. The third objection is that to which the opponents of the theory have attached the most importance. The amount of energy required 87 Phot. Mitt., 36, 334. 38 Heyer, J. Phys. Chem. (1911), 15, 557, 560. THE LATENT IMAGE 217 to produce the latent image is exceedingly small, equalling, according to Dr. P. G. Nutting, about 10-' ergs, or possibly much less.*° It is difficult to accept as true a theory which calls for a chemical change when the available energy incident on the plate during exposure is so small. It seems very improbable that the small amount of light necessary to form the latent image could result in a complete chemical change. A chemical change may occur, but most investigators are of the opinion that this chemical change is preceded by a physical change. A physical theory can certainly afford a better explanation of the last named objection than can the sub-halide or any other chemical theory. The Metallic Silver Theory. (Nascent Silver or Silberkeim the- orie.)—According to the sub-halide theory, the effect of exposure to light on a silver halide is to cause some of the halogen to be liberated, resulting in the formation of a compound containing less halogen than the normal salt, or a sub-halide. The metallic silver theory differs from the sub-halide in that the liberation of halogen is assumed to be complete ; that is to say the halogen is entirely set free upon exposure resulting in pure metallic silver, the small grains of which act as a germ or nucleus in inducing development. According to Weisz,*° the metallic silver theory was first suggested by Scheele in 1777. Arago, in presenting the details of the daguerre- otype process to the French Academy of Sciences, again expressed the same opinion but the modern conception of the theory is due to Abegg ** and Oswald.*? In 1880, Eder found that upon touching an unexposed plate, im- mersed in a developer, with silver wire, reduction took place where there was contact between the wire and the surface of the plate. Ac- cording to Abegg metallic silver-is the cause of the extensive reduc- tion of unexposed silver halide contained in the film. It was soon proved, however, that reduction took place only when pressure was applied, and that any metal, or hard substance, even the corner of a sheet of paper, drawn across the sensitive surface, would cause reduc- tion in just the same way. Consequently the experiment fails to prove that metallic silver is the cause of the reduction, but on the contrary 39 Since the above was written Sheppard and Wightman have investigated the subject. Their paper will be found in the Journal of the Optical Society of America for November, 1922, pp. 913-916. 40 Zeit. Phys. Chem. (1906), 54, 311. 41 Abegg, Archiv. Wiss. Phot., 1 (1899), 268 et seq. Also Brit. J. Phot., 1890, 46, 1096. 42 Oswald, Lehrbuch der Allgemeine chemie. 218 | PHOTOGRAPHY shows that the image so formed is only an example of an artificial latent image due to pressure. _ Abegg was also able to show that the latent image was destroyed by nitric acid under certain conditions, and he brought this forward as a proof that the latent image consists of metallic silver because it is dis- solved by a silver solvent. This evidence, however, is inconclusive, since other acids and substances which do not dissolve silver are capable of destroying the latent image, while certain substances which destroy metallic silver are without effect on the image.** According to the metallic silver theory, the development of the latent image depends upon the presence of microscopic particles of metallic silver. Regarding this point Mees and Sheppard say: *4 “Proceeding from the fact that a ‘germ’ of metallic silver will induce the deposition of further silver upon it from a super-saturated solution, it provides the most satisfactory theory of development, but fails to account for the resistance of the latent image to oxidizing solu- tions of such potential as destroy metallic silver.” However, it has not been definitely proved, to the writer’s knowledge at least, that the “germ” or nucleus which acts in promoting development cannot con- sist of some other substance than metallic silver, so while the explana- tion is satisfactory, it may not be the only interpretation. The original conception of the metallic silver theory has undergone modification during the past few years and many of those who formerly supported the theory in its original form are now of the opinion that the latent image consists of colloidal silver in an ultra-microscopic form. A further development of this idea is found in the colloidal silver theory of Renwick and others, to be treated later. The Molecular Strain Theory.—There are many scientists who deny that any chemical change takes place when a silver haloid is exposed to light to form the latent invisible image. They are ready to admit that chemical change may occur when the exposure is greatly pro- longed and a visible image is formed, but contend that the amount of energy present during the ordinary photographic exposure is insuffi- cient to produce photo-chemical decomposition. This has always been regarded as a weak point of the sub-halide, or for that matter any chemical theory of the latent image, and has caused many authorities to bring forward theories resting on a physical rather than a chemical basis. . 43 Mercator, “ The Nascent Silver and Sub-haloid Theories,” Brit. J. Phot., 1899, 46, 628. 44 Investigations on the Theory of the Photographic Process, p. 199 (1907). THE LATENT IMAGE 219 The molecular strain theory is essentially a physical theory. Ac- cording to this theory the action of the light sets up an internal strain within the molecule of silver bromide and causes the atoms to ‘pull apart from each other. The effect of the strain is to render the com- pound less stable, so that it is more easily reduced to metallic silver by the reducing agents known as developers. Fig. 154 is an attempt to @) QL Gelatine still under Strain ae ES Ss, oe : New Molecule Actual Photodecomposition WPS, eae Ag.Br 2Aq Br, AqOCl, ete, 7 TN rt i ‘s ‘ \ ‘ Absorbed —{o') ae er) H \ / by Sensitiser rv / Se FIG. -15§4. Cieratton of the Molecular Strain Theory of the Latent Image (Hasluck, The Book of Photography) show graphically what is supposed to take place upon exposure ac- cording to the molecular strain theory. In (1) the original unex- posed molecule of silver bromide consisting of silver and bromine atoms is shown. On exposure, the bromine atom begins to pull away from the silver atom (2) and as exposure is prolonged the strain is increased (3, 4,5). If exposure is sufficiently prolonged the molecule may be completely shattered and part of the bromine may be lost, re- sulting in the formation of a sub-halide, or oxidation Hay, take place, resulting in the formation of an oxy-halide. Evidence for the Molecular Strain Theory.—Photo-regression, or the relapse of the latent image with time, can be very easily explained by the molecular strain theory. The strain which has been set up between the atoms of the molecule of silver: bromide gradually les- sens with time until the point is reached where the strain ceases and the atoms again assume their proper places in the molecule of silver bromide. . The activity of a photographic plate at very low temperatures is _ also more easily explained by a physical than by a chemical theory. 220 PHOTOGRAPHY Prof. Dewar **> found that at a temperature of —180° C. a highly active substance like potassium does not show any action when immersed in liquid oxygen, while the photographic plate was still fairly sensitive at a temperature of —200° Centigrade. It is rather difficult to see why light should produce a chemical change on such a relatively inactive silver halide at such a low temperature when no chemical change is produced with two active elements at a tem- perature twenty degrees higher. It is much easier to accept a physical change in the structure of the molecule than a chemical change, in face of this evidence. The remarkable stability of the latent image is often advanced as an argument for a chemical rather than a physical theory of the latent image, but is there any reason why a physically modified molecule of silver bromide should be less stable than a sub-halide? The Electron Theory of the Latent Image.—Like the theory of molecular strain the electron theory attempts to explain the latent image from a physical rather than from a chemical standpoint. It, perhaps, may be regarded as a logical development of the molecular strain theory, bringing it into conformity with the newer ideas of mat- ter and chemical action established by the theory of electrons. .Ac- cording to the electron theory, when light acts on a molecule of silver bromide, one, or possibly more, of the electrons composing the mole- cule are set free, leaving the molecule in an unstable condition which is readily reduced by the reducing agents known as developers. The Photo-Electric Effect—The action of light in setting free the electron with certain substances, or the photo-electric effect, has been rather extensively studied during the past few years, and advocates of the electron have repeatedly called attention to the bearing of this effect on the latent image.** It was ascertained by Hertz and his successors that light, especially of short wave-length, has a remarkable power of discharging nega- tive electrification from the surface of bodies and this effect can be proved to be due to the emission of negatively charged electrons. For instance a polished metal plate, when illuminated by ultra-violet light, is unable to retain a negative charge because of the discharge of the negatively charged electrons. It will, however, retain a positive charge. In addition to many metals, light can also liberate negatively charged 45 Proc, Roy. Inst., vol. 13, p. 605. . 46 See Photo-electricity, Allen; Photo-electricity, Hughes. ia 2 ie ee es ae THE LATENT IMAGE 221 electrons from the haloid salts of silver. As a matter of fact the silver haloids are vigorously photo-electric; their order to sensitive- ness being bromide, chloride, and iodide, which, it may be mentioned, is the order of their sensitiveness to light under identical conditions. Like the latent image the photo-electric effect is but little affected by temperature and the reaction may take place at the temperature of liquid air. The velocity with which the electrons are emitted de- pends upon the wave-length of the light: light of a short wave-length as ultra-violet produces a high velocity, while light of longer wave- lengths, as orange or red, produces a lower velocity, or the electrons may not be liberated, as a certain critical frequency is necessary to cause the electron to leave the atom. Compare these reactions with the sensitiveness of the photographic plate to light of various wave- lengths. Is not the similarity apparent? Evidence for and against the Electron Theory.—We know that ionization is easily produced by the X-ray tube and the rays of radium and as these act vigorously on the photographic plate it may be pos- sible for the latent image itself to be the product of ionization—or the splitting up of the molecule, or atom, into parts that are oppositely electrified. The relapse of the latent image with time can be satisfactorily ex- plained as being due to the gradual return of the electrons to the parent atom. The phenomena of reversal is explained by the electron theory in the following way: ** “If we suppose that, on the light continuing to act upon the silver halide grain, the electrons continue to be emitted, it follows that there will be a greater and greater accumulation of electrons surrounding the parent grain (of silver halide). The posi- tive charge on that grain will increase, and the negative charge sur- rounding it will increase also.*® If that goes on, a point will be reached at which neutralization takes place, this particular grain re- verting to its original condition.” With very long exposures chemical change very likely takes place and the change is no longer entirely a physical reaction. 7 Chapman Jones, as well as others, objects to the explanation of re- versal because it seems illogical that a short exposure should cause 47“ The Formation of the Latent Image on the Photographic Plate,” H. Stanley Allen, Phot. J., 1914, 54, 179. 48 Because of the continuous liberation of negative electrons—Author. 16 222 PHOTOGRAPHY electrons to be liberated while a longer exposure causes the electron to return to the atom. It also appears that either the attraction between the silver bromide grain and the liberated electrons would increase until it balanced the separating force of light, and electrons would cease to be emitted; or the number of electrons returning to the parent atom would equal the number being liberated by light. In either of these cases we would have a state of equilibrium, and not a complete return of the liberated electrons to the original atom. Ac- cording to Dr. Allen ** this difficulty is removed when it is shown that before this stage is reached the accumulation of negative electrons outside the grain becomes so large that the insulating power of the gelatine, or dielectric, gives away and neutralization takes place. “That is, the force required to tear the electrons from the gel is, as a tule, less than that required to prevent the electrons from leaving the grain.” Our knowledge of the electron and its action is at present hardly sufficient to enable us to either accept or reject the electron theory. There is certainly a close similarity between the photo-electric effect and the latent image, while the liberation of negative electrons from the silver haloid on exposure to light seems to be a more satisfactory explanation of the conditions of exposure, and the small amount of energy required to produce the latent image, than any chemical theory could possibly be. | The Colloidal Silver Theory.—A theory which was apparently first advanced by Lorenz *® and one which has quite recently been ably seconded in a somewhat modified form by Renwick * is that known as the colloidal silver theory. According to this theory a sensitive emulsion is not composed, as many have thought, of grains of silver bromide, but of grains which contain, besides a trace of the gelatine in which they are imbedded, a highly unstable form of colloidal silver in solid solution and that on exposure to light this unstable form of colloidal silver first undergoes change. Renwick *? has shown that a solution of commercial collargol or positively charged colloidal silver has no effect on an emulsion regardless of the stage of preparation at which it is added. Luppo-Cramer ** has shown that if the colloidal 49 Phot. J., 19014, 54, 184. 50 Phot. Korr. (1901), 38, 166. 51 Brit. J. Phot., 1920, 67, 447, 463. 52 Ibid. 53 Das latente Bild, 27, 29; Kolloid Chemie u. Phot., 99; Phot. Korr., 1907, — p. 484. . THE LATENT IMAGE 298 silver has been previously coagulated, or rendered neutral, by an acid it is able to convert an emulsion directly to the developable stage without the necessity of exposure to light. According to the colloidal silver theory then the first action of light is photo-electric in character, consisting in the liberation of negative electrons and the production of the neutral form of colloidal silver, the presence of which is re- garded as the “ germ” necessary to induce development. The photosalts of Carey Lea have always been considered to have an important bearing on the nature of the latent image and have a close connection with the colloidal silver theory because the work of Reinders ** especially has proved that these photosalts, which their discoverer regarded as complexes of sub-halide and normal halide, are really solid solutions of. colloidal silver in silver halide. ‘“‘ One cannot fail to be impressed (says Renwick) with the extreme readi- ness with which silver in the colloidal, or even in the finely divided metallic state, goes into solid solution in a silver halide, and also by the great resistance offered by these solid solutions to the attack of silver solvents and to reduction by reducing agents.” It has long been known that ordinary gelatine emulsions may at times possess a slight red sensitiveness although undyed and since Eder has observed that colloidal silver has a color-sensitizing effect on silver chloride and bromide, it appears reasonable to assume that this red sénsitiveness is due to colloidal silver. Solarization is explained as being due to the formation of a rela- tively insensitive and undevelopable photo-halide by the interaction of the liberated bromine or hydrobromic acid on the latent image. The colloidal silver theory has the merit of reconciling the views of those who demand merely a physical change, those who require the formation of free silver, or a material chemically different from that originally present to act as a nucleus for development, and it should also meet with the approval of the exponents of the electron theory, since it is apparently well established that colloidal solutions of silver are negatively charged and are precipitated when the charge is lost. Sheppard’s Orientation Hypothesis of the Latent Image.—Until the discovery of the sensitivity centers and the general realization of the essentially disperse nature of the photographic emulsion, all theories of the latent image were tacitly based upon a homogeneous emulsion. 54S. Physik. Chem., 191. 224 ~ PHOTOGRAPHY The discovery of the sensitivity centers, their origin and distribution and the conception of photographic sensitive materials as aggregates of grains of silver halide of various sizes and sensitiveness has added to the problem of the latent image that of sensitivity. It is now gen- erally accepted that for a grain to be developable it must contain a nucleus. In other words the photo-effect, whatever its nature, is localized in the grain. The study of the latent image, therefore, be- comes a study of the mechanism of the reaction which takes place within the grain. 3 The sensitivity centers were first conceived of as points of special — sensitiveness of a substance other than silver bromide on the grain of silver halide and facilitating catalytically the decomposition of the grain by light.°®> If the sensitivity centers consisted of specially sen- sitive points of a substance different from silver bromide, their spec- tral absorption should determine, or at least powerfully affect, the spectral sensitivity of the emulsion. In this case, the removal of the sensitivity centers by desensitizing in chromic acid would be accom- panied by a change in the wave-length sensitivity from that of the sensitivity centers to that cf the silver halide. Investigations on the spectral sensitivity of high speed emulsions before and after desensi- tization, however, shows that there is no appreciable change.®* This would indicate that the spectral sensitivity of the sensitivity centers does not differ to any observable extent from that of the silver halide; a conclusion which has been confirmed by the investigations of Toy and Edgerton ** on the number of developable centers produced in the silver grain by light of different wave-lengths, whereby they found that the number of centers produced was proportional to the light absorbed by silver bromide at that wave-length. It appears, therefore, that it is the silver halide and not the silver sulphide of the sensitivity centers that is the real photo-sensitive ma- terial. If the sensitivity centers then are not photo-sensitive and do not facilitate the decomposition of the grain by light, how can their undeniable influence on sensitiveness and developability be explained ? Sheppard °° suggests that the presence of the sensitivity centers of silver sulphide in the crystal lattice structure of the grain sets up a 55 Clark, Brit. J. Phot., 1923, Dec. 14. Renwick, J. Soc. Chem. Ind., 1920, 39, 156. Sheppard and Wightman, Science, 1923, 58, 80. 56 Sheppard, Third Colloid Symposium Monograph. 57 Phil. Mag., 1924, 48, 947. 58 Third Colloid Symposium Monograph. ee ee Pee ey ee er ne ee ee ee ee a THE LATENT IMAGE 225 deformation of the ions nearest the sensitivity center, the amount of this deformation decreasing as we leave the sensitivity center. The sensitivity centers act by concentrating the photo-effect so that the reduction of silver halide by light is localized around the sensitivity center, at which point the actual decomposition takes place. The first silver atoms formed as a result of this decomposition are sup- posed to act as a catalyst and facilitate the increase of the number of silver atoms deposited on the sensitivity center. In a paper on “ The Visible Decomposition of Silver Halide Grains by Light” (J. Phys. Chem., 1925, 29, 1568) Sheppard and Trivelli have shown that the visible decomposition of silver bromide proceeds in such a way as to indicate the existence of an orienting action. Whether the same case applies in the formation of the latent image, as implied in the Sheppard hypothesis, is yet to be shown. According to this hypothesis the sensitivity centers of silver sulphide do not play an active part in exposure. Their role is regarded as being purely passive and a consequence of the deformation which they set up in the lattice structure of the grain of silver halide. Sheppard has lately suggested, however, that the action of the silver sulphide centers may not be entirely passive and that the increased amount of light absorp- tion of silver sulphide, as compared with silver bromide, may result in “a certain amount of re-radiation, irradiated about the nucleus, and falling in the absorption region of silver bromide.” °° It is too early as yet, however, to do more than give the general out- lines of this the very latest hypothesis on the oldest and the funda- mental problem of photographic theory—the mechanism of the photo- chemical reaction and the formation of the latent image. BIBLIOGRAPHY GENERAL REFERENCE WorKS ANDRESEN—Das Latente Lichtbilde. Carey Lra—Kolloides Silber un die Photohaloide. (Translation by Luppo- Cramer, 2d Ed., 1921.) EpER AND VALENTA—Beitrage zur Photochemie und Spectralanalyse. Luppo-CrRAMER—Photographische Prob!eme. VALENTA—Photographische Chemie und Chemikalienkunde. _ $8 Brit. J. Phot., 1926, 73. - CHAPTER 1x 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 ag I, 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 I0 . seconds, while the time required for another material is double this, or 20 seconds, the relative speeds of the two areas 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 Schellen- 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. - 226 " ae ——— ee, i a a a a ee oe oS eee ee ve SENSITOMETRY 227 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. Several workers, notably Mees and Sheppard, have repeated the work of Hurter and Driffield using improved apparatus, and while these workers have done much towards the development of the sys- tem, their conclusions have always been substantially in line with those of the earlier investigators. Although the H. and D. system is not perfect, it is the most comprehensive and accurate method of plate speed determination which we have and is in general use throughout the world. 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.—Although daylight is used for the ma- jority of photographic exposures, in plate speed testing it is necessary that all plates be exposed to a light of exactly the same strength in order that the speeds may be comparable, for which purpose day- light, on account of its variability, is not suitable. The principal re- quirements of a standard light source are that it should be reasonably constant in intensity over fairly long periods of time and that it can be easily reproduced whenever and wherever desired. 228 PHOTOGRAPHY The other important essential is a spectral distribution, or color range, comparable to that of daylight. Given light of the proper color range it is possible by the employment of suitable screens to secure light of very nearly the same color as daylight. The English standard photometric candle was employed by Hurter and Driffield but on account of its variability later workers have pre- ferred other sources such as the Harcourt pentane lamp, the Hefner amyl-acetate lamp, acetylene as employed by Mees and Sheppard or incandescent electric sources fun at a constant voltage. While the amyl-acetate lamp is still employed in some quarters acetylene or in- candescent electric sources are now more generally employed, usually in connection with a light filter so as to reproduce daylight as nearly as possible. Sensitometers.—Sensitometers, or machines used for impressing the sensitive material with graded exposures of a definite ratio, may be divided into two classes: (1) those which vary time while keeping intensity constant and (2) those which vary intensity, keeping time constant. The former are known as time scales, the latter as intensity scales. The Chapman Jones plate speed tester (Fig. 155) is an example of an intensity scale. The squares numbered from I to 24 are filled with pigmented gelatine of increasing opacity so that each numbered Fic. 155. Chapman Jones Plate Speed Tester 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.). The — Eder-Hecht scale is very similar and is extensively used in Germany and on the Continent generally. oan SENSITOMETRY 229 The chief objection to intensity scales of this kind for accurate investigation is the failure of the sensitive plate to obey what is termed the Bunsen-Roscoe reciprocity law, according to which ex- posure is regarded as the product of time and intensity (JT), and one factor may replace the other. Abney and others have shown that this law does not hold, at least over a very wide range, and that, to produce a given effect, time is not the reciprocal of intensity nor vice versa. Time scales are realized most easily by the employment of a sector wheel. That of Hurter and Driffield (Fig. 156) contains nine aper- Fic. 156. H. and D. Sector Wheel and Exposing Apparatus 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 at the other, behind the sector, the sensitive material to be tested (Fig. 156). 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 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. Instruments for the Measurement of Density.—While it would be possible to determine the density of the photographic deposit by actually weighing the silver, such methods would be both tedious and inaccurate owing to the extremely small quantities involved, and it is both simpler and more accurate to determine the density optically. The densitometers, or photometers, used for this purpose may be re- 230 PHOTOGRAPHY garded essentially as light-balances. They serve to bring together two beams of light in order that the eye may more accurately com- -pare their intensities. The H. and D. densitometer (Fig. 157) is of the bench type, based upon the law of inverse squares, the principle | a iz NAAANARAANYZ Fic. 157. H. and D. Densitometer of which is well known to students of elementary physics. The Bun- sen grease-spot serves as an indicator, equal illumination on both sides causing the grease-spot to vanish, thus indicating photometric balance. The grease-spot is shifted first one way and then the other until a balance has been obtained. Then the silver deposit to be measured is inserted in the path of one of the beams of light and the grease-spot indicator again shifted until a balance is secured. The SENSITOMETRY 231 difference between the first and last reading is a measure of the opacity of the deposit. Instead of varying the distance between the light source we may weaken one of them in a measurable way by polarization. Polariza- tion photometers of the type devised by Hufner and Martens are well adapted to the measurement of photographic deposits and are ex- tensively employed. Instead of polarization we may weaken one of the light beams by the use of absorption material. Densitometers of this type employing calibrated neutral-black absorption wedges have been designed by nu- metfous workers and are especially suitable for preliminary work and for instructional purposes. It may be pointed out that the density of a photographic deposit is not a definite, unvarying amount but that it depends to a certain ex- tent on the method of measurement. The photographic deposit is not homogeneous, as assumed by Hurter and Driffield, but is a light-scat- tering medium and consequently the Lambert-Beer law of absorption (to be referred to later) does not hold. The subject has been com- pletely investigated by Callier, F. F. Renwick and F.C. Toy. It has been shown that densities measured by parallel rays differ markedly from those measured by scattered light secured by placing the deposit in contact with opal glass. Renwick has shown that even then the apparent density is reduced by inter reflection between the opal glass and the negative. Opacity-Transparency-Density.—Since we must continually make use of a number of terms having reference to the absorption of light by the developed silver deposit, it is well that we become familiar with the laws governing the absorption of light and the terms used in con- nection with the same. Opacity is the term applied to the resistance of a substance to the passage of light. In other words, it may be expressed as the light which must fall on one side of the substance in order that a light of unit intensity be transmitted. Mathematically, this may be expressed as T/Is, I being the incident and J, the transmitted light. Transparency is just the reverse of this, being a measure of the fraction of the incident light which passes through the substance, or Ie/i; I being the incident and J, the transmitted light as before. 232 | PHOTOGRAPHY In 1890 Hurter and Driffield introduced the conception of density. This they termed the amount of light stopping substance in the de- posit and defined as the logarithm of the opacity or the — logarithm of the transparency. } D = log,, (opacity) ==log,, (/2/), D==— log,, (transparency) ==—log,, (J «/I). This conception of the density of a photographic deposit was based upon the Lambert-Beer law of absorption. Lambert’s law states that, in passing through equal layers of a material, equal proportions of the light which traverses them are absorbed. Mathematically then, if J is the intensity which penetrates the surface, and J, the amount which has escaped absorption at a depth of +, then 2 a Mind where the constant k is the absorption-coefficient for that particular substance. This law suggests that absorption is a molecular effect, each molecule absorbing a definite amount of the light incident upon it. Now in solutions the number of molecules is proportional to the concentration. Therefore the total absorption of a solution depends upon the concentration and the thickness of the layer traversed by the beam of light. If m is the concentration, then the law for absorption in solutions would be expressed as This is known as Beer’s law. . From the above it follows that __ log J — log Ie ae mx k When logs are taken to base 10, k is called the absorption constant, or, as defined by Hurter and Driffield, the density. | Within certain limits density is proportional to the mass of silver per unit area, or D= pM, where D is the density, M the mass of silver and p a constant termed the photometric constant. For an area — of 100 square centimeters having a density of 1, Hurter and Driffield obtained a value for p of .o131 gram of metallic silver. Eder ob- tained .o103 and Sheppard and Mees .o1031. : Perhaps the relation of opacity, transparency and density may be made plainer with the aid of the following step wedge (Fig. 158). In this we have four sections of increasing density, each additional cee ek SENSITOMETRY 233 density being due to the superimposition of an equal density. That is to say, in section I, we have no silver deposit; in section 2, a silver deposit of a definite value; in section 3, two such deposits and so to N Fic. 158. Illustrating the Relation of Opacity, Transparency and Density layers. The following table then shows the relation between the opaci- ties, transparencies and densities of the various sections. Licut oF INTENSITY J No. layers of silver deposit...... 0) I 2 é N 3 Wransparenoy GPAs: : (=) (=) : (=) (=) x I 3 3 3 3 PIenepArenOyaties. See AE Lee. I I a ole: ai I 3 9 27 (3) UC Es es 2 ES gO ee I 3 9 27 (3)N LL pS AEE gi a er 0) 477 954 1.43 alate The first line gives the number of layers of silver deposit. The second line shows the transparency expressed as powers of the fraction which is the transparency of one film. The third line shows these multiplied out, and the fourth gives the inverse of these or the opacity while the last line gives the log to a base of 10, or what Hurter and Driffield term the densities. : Exposure and Development of the Sensitive Material for Speed Determination.—Before proceeding with the exposure of the sensitive material for the purposes of speed determination it is necessary to adopt a standard unit of exposure. The photographic effect of a given exposure depends upon three things: the intensity of the light source; its distance from the sensitive. material, and lastly the duration of the exposure. Hence the standard unit of exposure must concretely specify the unit intensity of the light source, its distance from the sensitive material, and the unit of exposure. The standard adopted by Hurter and Driffield and now accepted as an International standard 234 PHOTOGRAPHY is the Candle-Meter-Second (C. M. S.) which means the exposure of the sensitive material for one second, to a light-source with an in- tensity equal to one candle power, at a distance one meter from the light. . The material to be tested is first cut into strips I x 4% inches, the strips of sensitive material being preferably cut from the center of a large specially coated sheet in order to avoid any irregularity in the thickness of the coating. Two of the strips are placed in a specially made plate holder which is placed in the exposing machine. Only a small section of each strip is exposed, the remainder being reserved as a “‘ fog strip’ which is used to determine the density due to the fog- ging of the emulsion. This fog density is then subtracted from the total density as obtained from the densitometer readings in order to get the true density due to the action of light on the sensitive material. The exposure complete, the strips are ready for development. For - accurate results in sensitometric work it is absolutely necessary that a thermostat be used in order that the strips may all be developed under identical conditions of temperature. It would require too much space for us to go into details regarding thermostats for this purpose and as an understanding of the same is unnecessary to the subject of sensi- tometry from the standpoint of the student we leave the subject, re- ferring those interested further to the sources contained in that por- tion of the bibliography at the end of the chapter. The following pyro-soda developer is considered the standard for plate speed testing; but it is no secret that its use is not universal, probably because some of the later and more powerful reducing agents are capable of giving somewhat higher speed numbers. Pyrogalfol ass. cs.de sees apacase po. v0) ale elkouse oats alee a 8 parts Sodium carbonate’ (cryst.)........ 0. sesnss See ele ne 40 parts Sodium sulphite (cryst.) occ 5. ¢ceos oc 6 «seb eee 40 parts Water to makes... sco. 5 dec seine win’ s-p 4 ok ate pee ene 1000 parts No bromide must be used in the developer used for plate speed de- termination. Bromide delays development in a particular way which will be explained in the chapter on the Theory of Development and prevents concordant speeds from being obtained. Practically it may be said to slow the plate, for with normal development the lower densi- ties are restrained and do not assume their full intensity ; with increas- ing development, however, the lower densities gradually gain until finally the result is almost the same as if no bromide had been added. The uncertainty, however, which accompanies its use makes it unde- | { } : | | - | | | == aie a ee eee ge re es ee eee al a” Sean a SENSITOMETRY 235 sirable for plate speed testing, valuable though it may be in practical work. In practice two strips are nearly always exposed side by side and one of these is developed for twice the time of the other, for a reason which will appear later. After fixing, washing and drying, the densities of the various por- tions of the strips are determined by measurement in the densitometer, every precaution being taken to eliminate all sources of error so as to obtain the most accurate measurements possible. A note of the vari- ous densities and the duration of the exposure which produced them is made as each density is measured and with this information we are in a position to see what has been the reaction of the plate to the various exposures. The Relation of Exposure and the Growth of Density.—A strip exposed in a sector sensitometer contains nine exposures, a range of. 1:512. This range is amply sufficient for the purposes of speed de- termination, but as we wish, for purposes of demonstration, to in- vestigate the effect of increased exposure on the growth of density over an even wider range we will assume that a number of strips have been exposed in such a way that we have obtained a range of ex- posures from 1 C. M. S. to over half a million C. M.S. In the table below we have placed opposite this series of exposures the densities obtained by Hurter and Driffield in an actual test. (See H. and D. Memorial Volume, p. 103.) | | Exposures in C. M.S. Densities Difference ey Ck a Se WOOD heat oua ete eae a PT ee acy nae 100 EVR ar So Sees 100 OS ge 1 ee Scene RAO Ua, Fae ie Maal ee Re 180 OSCE A ee rae BONES oral cpuheeich rete SOR ¢ 160 Se a eos’. 2 eg Oe et, Rr ROR: SO ee 215 AMG SPs vino 00 LOAD actions sae Rae 225 ee Gd es 5 Phe Sey san ie Re 405 Re ee ON ik Ps Leo AP ean aie Ae tir pee D> 530 Me ele iy PGs a A gate oe oe oes coe 415 bh, Ae on a SSS eee ok eee 245 ea) Oe 6 Sa ea IONS? vate away ic teeter 450 TT ENG oo A a NEA POPS ARE. ee aS aa 130 ENA el tas thle pales. oj Fs hiss GB 260- hae ta adiautan aces 165 ES REG SE a eae 8 AGG ey eeu elles 125 eRe OG ie Sots Gok Kd CE OE eS aah lee eho 103 ek cas eek tk Ctl YER I Te ee ere 0.034 Ry ok oe vin sn oe BT ns nee care eee 0.194 PATO ed reas Seek Ss a Be os Neko a ae ig 0.162 MEAN prado vie rch teed ack POMS: Fol a te un ee ee 0.208 PUMA ORs sos cons CH ee Oe ALAS Saeed ate BUN ck Sore eG Aare 0.056 236 PHOTOGRAPHY If you have had the patience to go through the above table carefully, as you should do, you will observe that at first every time the exposure is increased there is practically an equal increase in the density, finally the increase in density for each additional increase in exposure becomes practically a constant and the differences in the last column show no change, excepting of course that due to experimental errors, and finally the growth of density for each additional exposure begins to grow less and less until a point is reached where additional exposure de- creases rather than increases the total density. The Characteristic Curve.—It is rather hard to get these points clearly fixed in the mind when the results are set up in tabular form. It is much easier to comprehend this relation between exposure and density by graphic presentation. This might be done by plotting density against exposure but in practice the density is plotted against CE VET AEE CAE CAE AE LTTE 01 2 4 8 163264 &C up To..... oe a ee Exposure Seconds Fic. 159. The Characteristic Curve 3.0 2.0 Density 1.0 0 the logarithm of the exposure instead. There are two reasons for this: (1) The range of the two variables is quite different, for while the densities run to about 3, the exposures range as high as a half- million C. M. S., so that no real information can be secured from a curve in which density is plotted directly against the exposure. (2) There is no simple law between exposure and density so that no part of the curve will be a straight line representing an equal addition of ~~ <> SENSITOMETRY 237 density for each increase in exposure. Accordingly the density is plotted against the Jogarithm of the exposure instead of against the exposure directly. The exposure-density relation when plotted out in this manner pro- duces what is termed the characteristic curve of the emulsion and takes the general form illustrated in Fig. 159. It represents graphi- Correct 12 4 8 16 328C Exposure Fic. 160. Step Chart Illustrating the Theory of the Characteristic.Curve cally the growth of density with increased exposure and summarizes in a handy and tangible manner most of the physical characteristics of the sensitive material, so that once we have obtained the characteristic curve of any material we are in a position to predict, not only its speed, but its various other characteristics. The Significance of the Characteristic Curve.—In an effort to © ‘bring home to the student in a still simpler manner the real significance of the characteristic curve we will attempt to explain the various relations with the aid of Fig. 160. In this the steps are supposed to represent the various exposures and their height the amount of the corresponding density. We have divided the entire curve into three divisions, the significance of which we will shortly explain. The first period is characterized by a rapid increase in density as the exposures increase, the increase in density being approximately proportionate to the increase in the exposure. The relation existing within this period is shown by the following results of Hurter and Driffield : | Peoocre 20 ©, WM. 5. (1)......-...5- PICHSIEY No pt aG rae ce eee: Relative, 1. Eaovmura 100 ©. M.S. (8)..........:. Denagity, £.005...5-... 2+. Relative, 8.4 With increasing exposures we reach the second period in which the addition of density for each increase in exposure becomes to all in- 17 238 PHOTOGRAPHY tents and purposes a constant. Within the limits of this period, repre- sented by the straight line portion of the characteristic curve (Fig. 159), each time the exposure is doubled there is an equal addition to the density. That is to say that while the exposures increase in geo- metrical progression the densities increase in arithmetical progression, or for example: EXDOSUTES. lua eae I 2 4 8 16 32 64 128 256 Densities .......... o I 's 3 4 5 6 7 8 This relation between exposure and density has a special significance of great importance which will appear later. Passing on to the third period it will be observed that the steps are growing less and less for each exposure and finally a point is reached where there is absolutely no increase in density, after which increased exposure results in a decrease in density. This last portion of the curve (not shown in Fig. 160) known as the period of reversal is of great theoretical importance but, as it is reached only with enormous exposures, it has no significance in practice and thus we leave it, re- ferring the student to the literature of the subject for further infor- mation. Inertia as an Inverse Measure of Speed. ‘aNttip the straight line portion of the characteristic curve is produced so as to cut the log E base line, as shown in Fig. 159, an exposure is indicated which was termed by Hurter and Driffield the inertia. The inertia is an inverse - measure of the speed of the plate: that is to say, a slow plate has a high inertia while a rapid plate has a low inertia. To obtain numbers which are a direct measure of speed, the value of the inertia, as deter- mined by a sensitometric test, is divided by a factor which in the case of a standard candle as used by Hurter and Driffield equals 34. A plate having an inertia of .54 will thus have an H. and D. speed of 34 54 The precise significance of the inertia as a measure of speed is some- what difficult to define. The exposure which it represents is not the “threshold exposure” (the minimum exposure necessary to produce a measurable density) nor does it indicate the maximum exposure which will give proper rendering of the gradations of the subject, but an ex- posure somewhere between these extremes, and Hurter and Driffield claimed that practically it indicated the beginning of the Pe of ex- posure in which correct gradation is secured. cates SENSITOMETRY 239 Variation of the Inertia—While the precise significance of the inertia is somewhat clouded Hurter and Driffield could have found no other point so stable and so little susceptible to variation from which to calculate the sensitiveness of sensitive materials. Both Hurter and Driffield and also Sheppard and Mees have shown that, provided the plate does not contain free bromide, the value of the inertia is un- affected by variations in the time of development. The value of the inertia is also unaffected by variation in the temperature of the de- veloping solution (except with developers of very low reducing energy as hydrochinon) or by variations in the concentration, or composition, of the developer. Hurter and Driffield also claimed that the inertia was constant for all reducing agents, but Mees and Sheppard were able to show that this was not strictly true. According to the results ob- tained by these investigators there are two general classes of sensitive material, one class gives practically identical values for the inertia re- gardless of the developing agent, while the other class gives a some- what lower value with ferrous oxalate than with organic developers such as pyro, metol, etc.® Although the inertia is constant with increasing time of development this is not true if the plate contains free bromide, or if the developing solution contains a soluble bromide. In this case there is a lateral shift of the curve towards the right with a consequent increase in the value of the inertia and lower sensitiveness. However if development is pro- longed the restraining action of a soluble bromide on development be- comes less and less and the inertia point gradually shifts to the left, finally reaching almost the same value as would have been secured had the developing solution been free from soluble bromide.* It is for this reason that all developers used for speed testing must not contain a soluble bromide, otherwise the speed of the plate will depend upon the degree of development and concordant readings will be difficult to obtain. 1H. and D. Memorial Volume, pp. 119-120. Mees and Sheppard, Jnvestiga- tions, p. 282. Later investigations, however, indicate that this statement is open to question and is not definitely settled as was formerly believed. See Shep- -pard, Phot. J., 1926, 66, 190. 2 Mees and Sheppard, Investigations, p. 283, also 173. 3 Mees and Sheppard, Investigations, p. 284. # There is some question as regards this latter statement. Nuietz in the Theory of Development, the latest authoritative work on the subject, was unable to con- firm the previous statements of Hurter and Driffield and Mees and Sheppard. He remarks, however, that the results obtained were obscured in many cases by fog so that the conclusions may not be correct. 240 | PHOTOGRAPHY Watkins’ Central Speed Method.—Mention should be made at this point of the “central speed method” of Mr. Alfred Watkins, the eminent English authority. In selecting the inertia point Messrs. Hurter and Driffield remarked that for several reasons it would be more satisfactory to take as a measure of speed the beginning of the straight line portion of the curve but the difficulties of determining this point with sufficient accuracy made its choice impractical. Mr. Watkins has worked out a perfectly practical method of obtaining, not the beginning of the straight line portion, but its middle point the value of which he uses as a measure of the speed of the sensitive ma- terial and terms the Watkins central speed.® In the words of Mr. Watkins, “ The central speed method does not indicate the smallest exposure which will give a visible image, nor the minimum which will give truthful rendering, but that giving the best results.” It should be noted that the calculation of the central speed by the Watkins method is not subject to error on account of fog to the same extent as the Hurter and Driffield method. It has never been widely adopted, how- ever, because it indicates much lower speeds than the H. and D. method besides having several theoretical objections of its own.*® Wedge Methods of Sensitometry.—Since the introduction of a simple method of manufacture by E. Goldberg in 1910, neutral tint wedges have been rather extensively used in photographic sensitometry. Luther’s method of obtaining the characteristic curve directly with- out troublesome calculations by means of graded neutral tint wedges is particularly ingenious. A square neutral-gray wedge of known gradation, increasing in density say from 0 to 6, or an intensity-range of transmitted light from 1 to 1,000,000, is taken and the plate to be examined is exposed behind this wedge to a standard light source and 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 5 Watkins, Photography, its Principles and Applications, p. 306. 6 A new method of plate speed determination based upon the characteristic curve as obtained by H. and D. methods, but differing in the measurement of the sensitiveness, has been described by Nietz in Theory of Development, Chap- ter IV. This method has not at present found widespread application and for this reason we will not discuss it further but refer the student to the original source. SENSITOMETRY 241 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. 161. TRE Hee HEATHER Fic. 161. Characteristic Curve Secured by Crossed Wedges 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 242 PHOTOGRAPHY the opacities of its gradations are proportional to those parts of the subject which they represent. It must be distinctly understood that this does not imply that the opacities must be in direct proportion to the light intensities of the subject in order that the reproduction be correct; it is proportional and not direct proportionality which is necessary. Thus with a subject having a range of intensities from I 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 COMES aa portions of the subject are the same. Light intensities of subject..... eS yiatie uiese I 2 4 S.- 56 ai G4 Opacities .....0. ake Gee eee yy I 2 4 a (ee 3 4 WA I 2 4 3° 16 4 ie 2 4 8 The relation between the light intensities, opacities and transpar- encies of a perfect negative may be evident from the following: Light intensities of subject....... a I 2 4 8 16 32 Opacities i545... Bice Sea I 2 4 8 16 32 Transparencies :.:.0s80 ae I 1/2. thal Re 2G ese 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 ie of tone re- production. When we perceive in nature a uniform transition from faa 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 232) as the loga- rithm of the opacity, hence with a series of opacities increasing in geometrical progression the densities increase in arithmetical progres- ston. This relation may perhaps be clearer from the following nu- merical example: Light intensities of subject... .i.. 200%. 5..0255 I 2 4 8 oat 2 ee" Ga Dessities AGcra: le ee = 4 pla SiGe d anegate kon ee ri, ing 3 4 5 6 2 Opacities 2050. spa ica wi «thee tee ee 2 4 Bee ees Le sD w) Toe) ow Toa = te fas eT eh le oo - a a en ae ee ee ek ee ee) ep ae 7 E SENSITOMETRY 243 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 236) 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. 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 log,, 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 1s 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 244 PHOTOGRAPHY 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. 162). aan awe eee eS ane eEer ewan ee ee es 0 1 2 4: 8 “16°32. Ga ee eee Fic. 162. Latitude and the Characteristic Curve 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 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 I-32, then the exposure could be in- 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. Hence the latitude in exposure possessed by a given sensitive material SENSITOMETRY 245 is a relative term depending upon the range of light intensities in th 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 Density B l 2 4 8 16 — ete. ’ Log Exposure Fic. 163. Development and Constant Density Ratios 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. 163. 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 246 PHOTOGRAPHY 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." In confirmation of the law of constant density ratios we reproduce the following experimental data from an investigation of Hurter and Driffield: Exposures I 2 4 8 Dewaty: (4” development)............ 0.775 1,000 — 1.180 1.250 Ratio of densities Din-.. v2 sess 1.0 1.29 1.52 1.61. Density2 (12’’ development)........... 1.260 1.660 1.96 2.08 Ratio of densities dish: sue oes 1.0 1.31 1.55 1.65 Ratio DiJDe ey eee ee 1.63 1.66 1.66 | 1.60 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. 163 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: 7“ Photo-chemical Investigations,” J. Soc. Chem. Ind. (1890), 9. SENSITOMETRY 247 I 2 3 4 = Exposure | Density Density Opacity Opacity Co Mas. ratio ratio Seti eveioped 4). eu. i. 1.25 .310 1.0 2.04 1.0 2.5 520 1.67 se 1.62 5-0 725 2.33 5-30 2.59 Stine aJeveoped O.. .. eek... 1.25 530 1.0 3.38 1.0 2.5 905 1.70 8.03 2537 5.0 1.235 2.33 17.18 5.08 Strip 3, Developed 12”......... 1.25 .695 1.0 4.95 1.0 2.5 1.140 1.64 13.80 2.78 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 I—2.59; in the second strip the ratio is 1-5.08; while in the third strip the ratio has increased to 1-8.51. 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 248 PHOTOGRAPHY 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 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 pe D, ere dD, Y log E, — log E; where D, and D, are the minimum and maximum densities of the cor- responding exposures £, and £,. 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 SENSITOMETRY 249 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. 163 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, — . Densities Fic. 164. The Geometry of Gamma. (Brown) 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. By giving a plate two exposures denoted at A and B (Fig. 164) 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. Now apply the formula which we have previously arrived at from our definition of gamma, that is: tart Es Cech ene . | Gane log Ei — log Ee 250 PHOTOGRAPHY In the diagram draw CE parallel to the log exposure base line OB. Then gamma = ->—— 7 = 73 See Now this ratio DE/CE is one way of measuring the angle CDE or 6 (theta). It is the tangent of the angle 6 (theta), the ratio of the side (in any right angle triangle) opposite one of the other angles to the side connecting this opposite side to the angle: perpendicular base This tangent of the angle 6 (theta), or tan 8, as it is called, is equal the ratio of trigonometry. to gamma, for it is plain from the diagram that the angle DCE is equal to the angle CFA, which is the angle of the slope of the straight line portion of the characteristic curve.® The Calculation of Gamma.—lIt would be possible to measure the angle and find the value of its tangent in the published tables but there is a much simpler way of finding the value of tan @ by using the chart itself. From the point 100 on the log exposure scale draw a line (HG in Fig. 164) parallel to the straight line portion of the characteristic curve (CD in Fig. 164) until it intersects with a perpendicular drawn through the 1000 point on the log exposure scale (G in Fig. 164). It is clear that since HG is parallel to CD the angle KHG is also equal to 6 and therefore tan KHG is equal to tan @ or gamma. Tan KHG, however, equals GK/HK which in turn equals GK/1 since the dif- ference between the log of 100 (= 2) and the log of 1000 (= 3) is 1. Therefore if we mark on the vertical line KG a scale which cor- responds with that on the opposite side of the chart, the point where the parallel line from H meets the scale indicates the gamma without any calculation at all. This method of calculating gamma was de- vised by Hurter and Driffield. A slightly different method but based upon the same mathematical principle is used by Mr. Alfred Watkins. A distance is measured off on the log exposure scale equal to 10 times the value of the inertia obtained by projecting the straight line portion of the curve until it intersects with the log exposure scale. At this point erect a perpen- 8] am indebted to Mr. George E. Brown for the above method which is taken from his “ Hurter and Driffield Doctrine” in the British Journal of Photog- raphy, 1921, 68, 374. . a | ) | | SENSITOMETRY 3 251 dicular line to intersect with the characteristic curve. The density at the point of intersection is equal to gamma. Thus in Fig. 164 the value of the inertia is 0.3 and | 10 X 0.3 = 3.0. Erecting at 3 a perpendicular to the log exposure scale it is found that this perpendicular intersects the characteristic curve at a density of about 0.8 and is identical with the value secured by the previous method. The value of gamma may also be calculated from any two densities qvithin the straight line portion of the curve from the formula Dei Oat low ie — log Ay The graphical methods, however, are much more convenient. Instruments have been devised by which gamma may be obtained without calculations or plotting of densities: consideration of. these, however, is beyond the scope of this work.® The time of development necessary to obtain any given gamma when the time of development for other values of gamma is known will be given later in the chapter on the theory of development. Gamma Infinity.——The amount of contrast, and therefore the value of gamma, since gamma is the numerical expression of contrast, in- creases with the time of development up to’a certain point; beyond this point no further increase occurs, in fact, after this stage has been reached, lengthened development reduces rather than increases the value of gamma owing to the intervention of fog, the effect of which is greater on the lower densities than on the higher. The maximum amount of contrast or, in other words, the highest gamma obtainable with any given material is termed gamma infinity (yo). The value of gamma infinity depends chiefly upon the sensitive material, although experimentally small variations are secured with different developing agents.1° High speed emulsions for portrait work have a low gamma infinity as a high degree of contrast is never required in portrait work: in fact material tending to give a high gamma would be a disadvantage. In commercial, landscape and general exterior work greater contrast is required and sensitive ma- 9 See: Watkins, Phot. J. (1912), 52, 206; also Photography, Its Principles and Applications. Renwick, Phot. J. (1914), 54, 163. 10 Nietz, Theory of Development, p. 102. 252 PHOTOGRAPHY terials made for these purposes are made to develop to higher values of gamma infinity than those made for portrait work. The greatest contrast of all is secured with plates of the process type as used for copying line work in black and white where absolutely clear lines to- gether with a field of the greatest possible opacity is required. ' Gamma infinity may be determined experimentally, but as it in- volves the measurement of very high densities and since these may be more or less affected by the fog produced on long development, the process is subject to large experimental errors and the values of gamma infinity are generally determined by calculation from lower values of gamma. It will be remembered that in exposing the sensi- tive material in the sensitometer two strips were exposed under identical conditions and that these strips were later developed under like conditions, the duration of development, however, varying as Tino | A method of calculating gamma infinity from the values of two sensitometric strips developed so that t, = 2t, was first worked out by Mees and Sheppard in 1903.4 From certain mathematical data based upon the velocity of development they calcu- lated the following expression of gamma infinity in terms of lower gamma : of a Yo" Te T= eth This formula, however, is not so simple as that of Renwick: ” vo aie * one(aryy) eye A graphical method of determining gamma infinity which avoids all calculation has recently been worked out by Renwick and will be found extremely convenient.?® 11 Phot. J. (1903), 43, 190. 12 Phot. J., 51, 213. eae 18 Phot. J., 1923, 63, 331. For two other methods see: Renwick, Phot. J., 1914, 54, 165-6. Krohn, Phot. J., 1914, 54, 166-7. | j : | . | = : i SENSITOMETRY Dia 288 GENERAL REFERENCE WorKS EpER—Systeme der Sensitometrie des Plaques Photographiques. (French trans- lation by E. Belin, 1902.) EpER AND VALENTA—Beitrage zur Photochemie. Fercuson—Hurter and Driffield Memorial Volume. With excellent bibliog- raphy to 1920. MEEs AND SHEPPARD—On the Theory of the Photographic Process. 18 CHAPTER X THE EXPOSURE OF THE SENSITIVE MATERIAL The Problem.—The problem in the exposure of the sensitive ma- terial is to find that time of exposure which is necessary under the prevailing conditions of light, subject and diaphragm to produce for each tone in the subject a proportionate density in the negative, so that the densities representing the tones of the subject may all lie within the straight line portion of the characteristic curve. There are four factors which determine the correct time of ex- posure : . The intensity of the light. . The subject. . The speed of the lens se the diaphragm used). 4. The sensitiveness of the plate or film. Ww NR Light Intensity and Exposure.—The intensity of natural light is de- termined by the time of day and time of year, by disturbances in the atmosphere and by latitude. Based upon investigations of Bunsen and Roscoe, Scott of Dublin in 1880 drew up a series of tables showing the variation in the in- tensity of daylight due to time of year, time of day and latitude. Assuming equal conditions the table on page 255, therefore, indicates relative exposures for different seasons and latitudes. The countries south of the equator have their maximum light value in December, instead of June, therefore, if the positions of the months in the above table are exactly transposed, the table will apply both to the Southern and Northern hemispheres. Atmosphere.—lf the intensity of sunlight was unaffected by the at- — mosphere and physical obstructions the simple table above would be an accurate guide to photographic exposures. But the intensity of sunlight is markedly affected by the presence of clouds or dust par- ticles in the air. Clouds at times may increase the intensity of sun- light by reflection, but more often they decrease its intensity. Such alteration is extremely difficult to estimate except by chemical means 254 THE EXPOSURE OF THE SENSITIVE MATERIAL 255 VARIATION IN EXPOSURE FROM MoRNING UNTIL EVENING FoR DIFFERENT LATITUDES By R. de B. Adamson, B.Sc. British Journal Photographic Almanac Lati- tude June May, July April, Aug. Mar., Sept. 60° Feb., Oct. Jan., Nov. December June May, July April, Aug. 55° Mar., Sept. Feb., Oct. Jan., Nov. December June - May, July April, Aug. EO° Mar., Sept. Feb., Oct. Jan., Nov. December June May, July April, Aug. 40° Mar., Sept. Feb., Oct. Jan., Nov. December June May, July April, Aug. 30° Mar., Sept. Feb., Oct. Jan., Nov. December North Hemisphere tole bole QAR W He eH BOwN eee Se vie WWN SS SA nile Walco loo ee bol hole Pleo mleo woo MORNING December , Jan., Nov. Feb., Oct. South Hemisphere Mar., Sept. April, Aug. May, July June Sa ee ee LO NiR le is ee DW SS eS eS | | nll NR le lH WN Hee OCORNHHH bol bole bole Nile lene NDS SS ee bole bole Rol I I I 2 3 4 4 AFTERNOON December Jan., Nov. Feb., Oct. Mar., Sept. April, Aug. May, July June December Jan., Nov. Feb., Oct. Mar., Sept. April, Aug. May, July June December »Jan., Nov. Feb., Oct. Mar., Sept. April, Aug. May, July June December Jan., Nov. Feb., Oct. Mar., Sept. April, Aug. May, July June 256 : PHOTOGRAPHY and although the eye after experience may be able to approximately determine its visual intensity, it cannot estimate its actinic intensity and it is this with which we are concerned. Towards evening, when the sun approaches the horizon, there is a marked decrease in the ac- tinic power of the light, but the eye detects little, if any, difference and it is difficult to estimate exposures under these conditions. The fol- lowing will give an idea of the relative intensity of light under dif- ferent conditions of cloudiness, but is only approximate, as there are many degrees of cloudiness and the eye cannot readily estimate the extent to which the passage of actinic light is ne by the same. ‘Intense light (best possible light)......... 9 aes seeeane “dt nae Bright diffused light (sun behind pm e but still brights dig ics Light clouds (shadows visible).......... « & seca bite. 0.a:k eae < Heavy clouds (no shadows)......... wee cae scat Vie eS Very :heavy: clouds. « o.4ceeeet Sg ahas ere (die 8 7 ae ...4-5 or more The Subject—The majority of the subjects in general photography may be divided into six classes: Sea and Sky, Sea Views and Ship- Fic. 165. Sea and Sky ping, Open Landscape, Average Landscape, Outdoor Portraits, In- teriors and Indoor Portraits. > Class I. Sea and Sky—A subject, such as Fig. 165, which con- sists of sea and sky, receives the maximum amount of light, since there are no obstructions of any kind, while at the same time the 7 : 4 Z > ] ‘ THE EXPOSURE OF THE SENSITIVE MATERIAL 257 amount and color of the reflected light is high, as few subjects reflect so large a proportion of the incident light as water the blue color Fic. 166. Sea View and Shipping of which is decidely actinic. The degree of contrast is low, since there are seldom any deep shadows near the camera and, therefore, Sa ee ee ee ee ee ee ee, ee ee a ee a ee Fic. 167. Open Landscape : lengthened exposure is not necessary to soften extremes of contrast. Unit Factor 1. ; Class II, Sea Views and Shipping.—Should the subject contain vessels within one hundred feet, the exposure would have to be in- f, " a 258° PHOTOGRAPHY creased on account of the near presence of a deeper shadow than common. Snow scenes, which contain no near dark objects and panoramic views, require about the same exposure as sea views with shipping. This class requires about double the exposure of the pre- ceding. Unit Factor 2. Class III. Open Landscape——Open fields and landscapes contain- ing no objects of importance within a hundred feet require about Fig. 168. Average Landscape twice the exposure of the class above. Such a subject is shown in Fig. 167. Factor 4. Class IV. Average Landscape.—li the figures in the above land- scape should be brought nearer the camera than one hundred feet, the exposure would have to be increased as less light will be re- flected from the subjects. Subjects in which the principal objects, whether persons, animals, or bushes, are about twenty-five feet from the camera fall into Class IV, illustrated in Fig. 168, and require about six times the exposure of Class I. Factor 6. Street scenes require about the same exposure as Class IV, if the —— se eee, eee 7 THE EXPOSURE OF THE SENSITIVE MATERIAL 259 buildings are not close together or high, and if both sides are in sun- light. If the buildings are high, as in most business streets in the larger cities, the exposure must be increased several times. Class V. Outdoor Portraits——Portraits in the shade, as Fig. 169, require from 8 to 10 times the exposure of Class I (SEA AND SKY). Conditions in this class of work vary to such extremes that it is difficult to fix a factor, but that given will serve as a guide. Fic. 169. Outdoor Portrait Class VI. Interiors and Indoor Portraits—For the same reason, it is almost impossible to fix a factor for indoor portraits or interiors. Perhaps the factors of fifteen and twenty respectively will fit average conditions. Under equal conditions of light and color, exposure is unaffected by the distance of the subject and in a clear atmosphere, such as Switzer- jand or our own West, there are times when all objects beyond twenty-four times the focal length of the lens require the same ex- posure. In most parts of the country, however, there is a blue haze in 260 PHOTOGRAPHY the air which possesses high actinic value and consequently shortens the exposure required for distant objects. Summarizing the factors for the different classes of So bi: we have: Class I Sea and sky... 0056 as cile gs aks wen 0/ecs 00ers gee een I Class II Sea views with shipping. ........ «+s seen eee 2 Class"Iii - Open’ landseapes: alee een Oe ce eee bon Pikes 4 Class IV Average landscape. ......,:..+++s4 ene eeeee 6 Class V _ Portraits in shade: 7.9.42.) sea 8-10 Class VI. Indoor portraits, -..2..00 eee ste tean oP 10-15 Class VII Interiors. <4... seis s sien. as + eulul oll eee ee 15-20 Speed of Plate—Owing to the absence of any universal standard in sensitometric methods, the plate speeds of one manufacturer can- not be compared with those of another. At the present time the only reliable basis of speed comparison for the plates of different makers are the tables issued by the makers of the Watkins and the Wynne meters, the American Photography exposure tables and the Bur- roughs-Wellcome Handbook. These lists include practically all the plates on the market in English-speaking countries. TABLE SHOWING CORRESPONDENCE OF SYSTEMS OF PLATE SPEED DETERMINATION By L. P. Clerc, in Revue Francaise from British Journal of Photography, 1922, 69, 200 Scheiner Eder-Hecht H. & D. Relative I 42 9 I a 46 9 1.27 3 48 12 1.62 4 50 15 2:07 5 53 19 2.64 6 56 24 3.36 4 58 31 4.28 ie: 61 40 5.45 9 64 50 6.95 10 66 64 8.86 II 68 82 115 12 71 104 14.4 13 74 133 18.3 14 T¢ 170 23-4 15 80 216 29.8 16 82 276 37-9 2 84 351 48.3 18 86 448 61.6 19 88 570 78.5 20 90 727 100.0 THE EXPOSURE OF THE SENSITIVE MATERIAL 261 Another disturbing factor which fortunately is seldom sufficient to cause serious trouble is the variation in speed of different batches of the same brand of plate. While every possible means is taken to ensure the uniform speed of every batch of plates turned out, it is beyond human skill to secure complete uniformity and even with the best of attention and care to all processes, large variations in speed will now and then occur. Sometimes the variation may be as large as 50 per cent but this is unusual, although variations of Io per cent are not uncommon. It would be a gain in accuracy and a distinct advantage to the practical worker to know the actual speed, secured by a.laboratory test of each batch of emulsion, for each box of plates he uses. Speed of Lens.—In a former chapter it was stated that with the same lens the exposure required by any two stops was inversely as their areas. From an optical standpoint this is correct, but as Abney has pointed out, the effective chemical action of the two is not the same, and, therefore, so far as exposure is concerned the optical relation is only partly true.t_ Apparently the law holds good when rapid plates are in use and the greatest departures appear in the case of very slow plates. Using lantern plates, Abney determined that if ten. seconds was required to obtain a certain density at light of unit strength, 1440 seconds was only sufficient to give one fourth the same density when the light strength was 1/144 of its original unit value. Fortunately this matter is of no account practically, since rapid plates are not affected to any noticeable degree and the latitude of modern plates balances any such tendency. However, from the scientific viewpoint, it is interesting. Further work of a similar nature was published by Abney in his Treatise on Photography.” Determination of the Time of Exposure.—There are three methods in common use by which photographers determine the proper time to expose: 1. The empirical. 2. By the use of tables. 3. Exposure meters. The first method calls for but the briefest comment. The so-called gift of exposure which many photographers claim to possess does not exist. The ability to estimate the time of exposure under given con- 1 Brit. Jour. Almanac, 1894, p. 600. ® Treatise on Photography, toth Ed., pp. 391-405. 262 PHOTOGRAPHY ditions by examination of the image on the ground-glass or other like means consists simply in the comparison of present conditions with past experiences and were it not for the remarkable latitude of sensi- tive materials such methods would end in failure. While it is possible for one as a result of extensive experience under certain conditions to estimate with a fair degree of accuracy the time of exposure under similar conditions, for most workers and especially for the beginner, the occasional worker or for one who works under varied conditions such methods are inaccurate and unreliable. Reliable tables or exposure scales are much more satisfactory and if properly used will yield a high percentage of printable negatives, but here again a certain amount of judgment is needed, which is only ob- tained by experience, in order to properly classify the character of light, whether intense, bright, cloudy-bright, etc. While one gains ability in this respect with experience, even the trained eye is by no means an accurate judge of the actinic intensity of light, so that tables and scales are only another step toward the solution of the problem. The only way to ensure success in exposure is by the use of exposure meters which actually measure the chemical activity of the light at the time of the exposure. Exposure Meters.—There are two general types of exposure meters: (1) the actinometer, which measures the chemical strength of the light by the darkening of sensitized paper and (2) visual meters, which determine the strength of the light by photometric methods. The two standard meters of the first class are the Watkins and the Wynne. Besides these there are several others, the Imperial, Photo- meter M. and V., Haka-Expometer, Metropose Michant, Steadman and Beck. The first named are made in watch form and both de- pend upon the darkening of sensitive paper to a standard tint which, however, differs in the two meters. In practice there is little to choose between them. The Wynne has a lighter standard tint and requires only one fourth the time to make a test of the light as the Watkins, but a separate quarter-tint dial can be obtained from the manufac- turers of the latter. In the Watkins meter, the stop is placed against the plate speed number and therefore the scales do not need to be ad- justed as long as the same plate, or stop, is in use. The Wynne indi- cates at the same time the exposure for all stops, but must be reset whenever there is a change in the light value. Properly used, there is no question as to the accuracy of either. The rule for the use of the meter is to: Test the light in the shadiest 1 ees ee ee ’ % . 7 | } THE EXPOSURE OF THE SENSITIVE MATERIAL 263 part of the subject in which full detail is required. Therefore, if the subject is an open field, take the direct sunlight; if under the shade of trees, take the strength of the light where the subject is seated. Hold the meter to face the light that falls on the subject, not to face the camera nor the subject itself. In most cases, hold the instrument to face the sky but where the main light does not come from the sky, hold the meter so as to face the main source. The time required for the paper to reach the standard tint may be measured by a watch, by a pendulum or may be counted. A\ll in all, the latter is to be preferred, but the worker must be able to count seconds accurately—a matter which is not difficult after a little practice with a watch. One of the best methods of timing seconds mentally is to repeat, audibly if neces- sary, some phrase which one can easily speak in a second, such as, for instance, one-thousand-and-one, one-thousand-and-two, etc. Most people’s seconds are half-seconds. The watch is satisfactory, but with the pendulum it is difficult to watch both the meter and the bob at the same time. The stop-watch meters are accurate but expensive. It is easier to judge the proper matching of the two tints if the instru- ment is held at arm’s length and the tint viewed through half-closed eyes. The important thing to observe, and the whole secret to the successful use of a meter, is the time required for the sensitive paper to reach the darkness of the standard tint. Color is not to be con- sidered. Having found the actinometer time, as it is called, it re- ° mains to set the scales and read off the proper exposure. Full direc- tions accompany each meter and the reader is referred to these for further details. Should over or under exposure occur consistently when all of the above precautions have been taken, it may be corrected by a change in the speed of the plate. Thus, if over exposure occurs using plate speed 180 for Seed L Ortho plates, use a higher speed, say 250, while a lower plate speed, say 130 or 90, would be necessary if under ex- posure occurs with 180. Once the plate speed which gives the re- sults desired has been found, it should be adhered to and used as the basis of all calculations. It is seldom necessary to make smaller altera- tions in plate speeds than 50 per cent. Thus a change from go to 100 would not be noticed and 130 might be used without noticeable alteration. _ In the case of indoor portraits or interiors the time required for the determination of the actinometer time is lessened by the use of lighter tints for purposes of calculation. Thus with the Watkins meter the 264 PHOTOGRALRE y. first visible darkening of the sensitive paper requires exactly 1/16 of the time necessary for the standard tint. One can, therefore, take the time in minutes or seconds for the sixteenth tint, multiplying this value by sixteen to obtain the full actinometer time. In the case of interiors or still life one may expose the plate and meter at the same time, the diaphragm employed being such that the camera exposure is equal to the actinometer exposure for either the sixteenth or quarter tint. Tables for this purpose are given in the instruction booklet accom- panying each meter. Corrections for Special Subjects——For all ordinary subjects, as open landscapes, average landscape views, trees, portraits in shade, buildings, groups and interiors, no correction is necessary and the ex- posures indicated by. the meter will be found about right. There are a few subjects, however, which require alterations in the meter reading because of their high actinic color and on account of exceptional re- flecting power. The following table gives the proper alteration to be made for the more important of these exceptional subjects: Sky or Sea and sky.) jigs sce tease a ee .... 1/10 indicated exposure Snow or glacier scenes, sea views with shipping, black and white prints... 05....VG..464 ¢00keas ete 1% indicated exposure Open landscapes, lake views, river banks from the water, copying half-tone photos..............++- 1% indicated exposure Very dark colored objects as old furniture and dark paintings in a non-actinic color.............0.005 1% indicated exposure Visual Meters—Types, Principle and Methods of Use.—Meters of this type measure the visual intensity of the light reflected from the shadows. Most of them are really extinction photometers and deter- mine the intensity of light reflected by finding the point at which detail in the shadows disappears. Meters of this type consist of two essential parts: (1) A filter to subdue the highly visible but non-actinic rays as yellow and green. : (2) A mathematically and scientifically accurate means of determin- ing the value of the light reflected from the subject. To be absolutely accurate the filter would have to be adjusted for the plate in use, but owing to the wide latitude of commercial plates, this has not been attempted and the meters of this type on the market have a screen which is suited only to non-orthochromatic plates. The most active rays chemically are the invisible violet, known as ultra-violet, ee eee ee ee ee ee eT Se THE EXPOSURE OF THE SENSITIVE MATERIAL 265 and these it is impossible to measure visually so that in this respect a visual meter falls short of the standard set by an actinometer. Mathe- matically considered, the methods employed by the various meters are sufficiently accurate for practical use but there is a personal factor to be considered in the measurement of light intensities by the eye. The author has found that no three people out of a dozen among his stu- dents will obtain the same readings because to each the details in the shadows disappear at a different stage. He has made no tests as to the effect of changes of the size of the pupil of the eye during, or before, the examination but does not doubt that there is a wide vari- ance among different persons in this respect. For instance, a person passing from a darkened room into the sunlight and at once testing the light visually would be almost certain, other things being equal, to obtain a higher reading than one who had been outside in the sunlight for.a half-hour or more. It is this personal error—this variation of the size of the pupil under different conditions—that tends to destroy the accuracy of the instrument and render its readings fluctuating. The writer is aware that many use these meters with good success and that this may in some measure prove their value, but he is equally convinced that both scientifically and practically the actinometer is superior, and this conclusion has been reached not:only from his per- sonal experience but from large numbers of students, whom it has been his privilege to instruct. Detailed methods of use are included with each instrument and, as _ they vary considerably, the reader is referred to the descriptive matter issued by the firms manufacturing the same, rather than encumbering these pages with matter which may readily be obtained elsewhere. Prominent meters of this type for sale in this country are the Heyde, McMurty, Trilux, Diaphot and Justaphot. GENERAL REFERENCE WorKS BoursAuLtt—Calcul du temps depose en Photographie. CLrEMENT—Methode practique pour determiner exactment le temps depose en photographie. FrApriE—The Secret of Exposure. Outdoor Exposures, Photo-Miniature No. 54. Exposure Indoors, Photo-Miniature No. 157. STEADMAN—Unit Photography and Actinometry. Watxins—Manual of Exposure and Development. . Correct Exposure—How to Secure It, Photo-Miniature No. 105. Viwat—Calcul des temps de pose et Tables photometriques, i, i ees i a CHAPTER XI THE THEORY OF DEVELOPMENT Introduction.—Like nearly all photo-chemical reactions, develop- ment is a complex and many-sided process. It is neither entirely chemical, nor physical, nor physico-chemical, but is a composite of all three. The first step in the process of development is the diffusion of the developing solution through the gelatine which carries the exposed silver halide grains in suspension. This constitutes what is termed the invasion phase and is entirely a matter of physics, being controlled by the physical laws of diffusion. Once the developing solution has reached the silver grain which has been acted on by light a reaction takes place in which the exposed silver halide is converted to metallic silver. This stage is chemical in character and may be termed the reduction phase. ‘The silver so formed, we will find, is in solution and before the image is formed precipitation must take place. This is termed the precipitation phase and is chemical in character. The pre- cipitation of silver results in a density and the difference between the densities produced by the action of varying intensities of light produces contrast. The growth of density and the growth of contrast are con- trolled by both the physical and the chemical phases of development and hence these are physico-chemical in character. Thus we find that development may be broadly divided into three ~ divisions : 1. The physical viewpoint. 2. The chemical viewpoint. 3. The physico-chemical viewpoint. We will accordingly investigate the theory of the subject in this order. The Invasion Phase.—The general properties of gelatine and the structure of the photographic emulsion were considered in the chapter on Emulsions, where we found that the photographic emulsion con- sists essentially of exceedingly minute particles of silver bromide held in colloidal suspension in gelatine. The exact structure of gelatine is still an unsettled matter, but it will suffice for our purposes if we rep- resent by Fig. 170 the structure of the gelatine in which the grains of silver bromide are imbedded. The structure is of course very ir-_ 266 | | : | 3 | THE THEORY OF DEVELOPMENT 267 regular, probably there is no definite structure, but the illustration will serve to illustrate the physical conditions of development. From an examination of this figure it will be observed that a jelly consists of a large number of cells which are intersected in all directions by pas- ASS E EXE | WWF We we SVBEX Dd ‘« vic POMS So SRI OSES BS Ws w4N), > NZS [SE LESSOR: ee Fic. 170. The Invasion Phase of Development. (Mees) sages. The cells and passages contain a weak solution of jelly while the walls consist of a very much stronger film of gelatine, the whole resembling a sponge filled with water. In b of the figure we have - indicated by a black dot the grain of silver bromide in each of these cells of gelatine. Remembering that the individual grain is the limit- ing factor in development, we are now in a position to trace the course of a molecule of developing solution which is passing through the jelly on its way to the exposed grain of silver bromide. Beginning at the surface of the film, a molecule of the developing solution rapidly diffuses through the passages and arrives at the cell wall. Here the gelatine is more resistant and the penetration is less rapid. Once the molecule has passed through the cell wall, chemical reaction proceeds immediately. The rapid diffusion of the developing solution through the passages is termed macro-diffusion, while the much slower penetration of the cell wall, which ‘constitutes the second phase of the physical action in development, is termed micro-diffusion. The first phase is of exceedingly short duration and is complete within a very few seconds after the developer is applied. The second may require from several seconds to more than a minute. Both stages must be accomplished before the grain of exposed silver halide can be converted to metallic silver, and since this is necessary to produce the image, three stages of development, four in fact, have taken place when the image appears. Owing principally to the exhaustion of the de- veloper as it penetrates the depth of the film, the exposed grains which lie near the surface are the first to be reduced, while those which 268 PHOTOGRAPHY are buried deeper within the film develop more slowly. Hence all three phases are taking place at the same time but in different parts of the film. The Chemical Reaction within the Cell—the Reduction Phase.— Development is essentially a process of chemical reduction. Accord- ing to the earliest theory of importance the process consisted in the reduction of the exposed silver bromide to metallic silver by the de- veloping agent, the liberated bromine combining with the alkali to form an alkaline bromide. This reaction may be represented by the following equation in which D represents the developing agent: AgBr + DNa— 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* Cl- and in the case of AgBr this becomes AgBr — Ag? Br-. A salt so disassociated is termed tonized. According to the view most generally accepted in the scientific wentie 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 THE THEORY OF DEVELOPMENT 269 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: _ AgBr — Agt — Br- eee — Nat ©. 4 2Na0H> ae bee + HO —O-Nat Bee ci Ionized hydrochinon. —Nat ich +- 2Ag'° Br = can + 2Ag + 2Nat+Br— YY — Nat Metal 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 term 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 19 270 PHOTOGRAPHY solubility of silver is sufficient explanation of the localization of de- velopment to the individual grain.” + 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 Reveruibis 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 ? 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 that the developing agent. 2. The reverse may be true; but the sulphite may regenmeaee oe 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. ee ee aie ot | ] ee ee ee ee ee ee ee, lel ll oe THE THEORY OF DEVELOPMENT 271 developing agents are more easily oxidized in solution than sodium sulphite. The experiments of Mees and Sheppard * 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. 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.* 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 (ye. INQ ae + = + 2H,.0O. OH NaOH Ora ia Tue PuHysIcAL 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: (1) the time required for the developer to penetrate the 3 Zeit. wiss. Phot., 1904, II, 7. 4See Arch. wiss. Phot., 1900, II, 76. Eder’s Jahrbuch, 1904, 1 272 PHOTOGRAPHY film, including both the macro and micro phases 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.® 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, etc., which would affect the time of development necessary to reach a given degree of contrast affects the time of appearance proportionately. In other words Ta = WT, where T is the time for density D, T, 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 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 (7) and is most conveniently and accurately determined by the method of Nietz.° It has been shown in the chapter on sensitometry that a 5 For a full explanation of this interesting reaction see Sheppard and Meyer, Phot. J., 1920, 60, 12. § Theory of Development, p. 80, THE THEORY OF DEVELOPMENT 273 series of plates exposed under identical conditions in a sensitometer and developed for varying times from f, to t, produce a series of H. and D. curves the straight line portions of which meet in a point Std Log E Log E Fic. 171. Growth of Density with Time of Development. (Nietz) (Fig. 171). If we take any fixed exposure on the log exposure base 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 D,, D,, etc., as a func- tion of the time we get a curve of the exponential type (Fig. 172) 7 Dev. (Min.) Fic. 172. Curve Showing Growth of Density with Time of Development (Nietz) 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. 274 PHOTOGRAPHY 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 ¢ will be (D,.— D), where D is the density developed at any time ¢ and D,, 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 dD as k(D. — D), where F& 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- 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 . 3 . : ; { | ‘ : THE THEORY OF DEVELOPMENT 2795 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 by k. 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 y, and y, having been found, k may be calculated from the following equation: 7 The calculations are rendered simpler by the use of the following ’ table worked out by Mees and Sheppard. To use this table divide y2 by y, and against the value of this dvidend 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 or 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 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- 7 Mees and Sheppard, Phot. J., 1903, 43, 48; Phot. J., 1904, 44, 297. 276 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. determined, the time of development required to reach any other gamma may be found either by the graphical method of Hurter and PHOTOGRAPHY These constants having been Driffield ® or that of Mees and Sheppard.® 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.075 0.080 0.085 0.090 0.095 0.100 0.105 0.110 0.115 0.120 0.125 0.130 0.135 0.140 0.145 0.150 0.155 0.160 0.165 0.170 0.175 0.180 0.185 0.190 0.195 0.200 8 Hurter and Driffield, On the Control of the Development Factor. Phot. J., 1903, 43, 16. 0.205 0.210 0.215 0.220 0.225 0.230 0.235 0.240 0.245 0.250 0.255 0.260 0.265 0.270 0.275 0.280 0.285 0.290 0.295 0.300 0.305 0.310 0.315 0.320 0.325 0.330 0.335 0.340 0.345 0.350 0.355 0.360 0.365 0.370 0.375 0.380 0.385 0.390 0.395 0.400 9 Mees and Sheppard, Phot. J., 1903, 43, 48, 199. — . . be A sc ce ee ce oe ee ee oe oe Pe ae NNHHN YN NNW&® mT ODO U1 .210 .205 .200 -195 .1QI .186 .182 .178 -174 .169 .165 161 -157 -154 .150 -147 -143 .139 .136 .216 7 0.0010 0.0008 0.0008 0.0008 0.0006 0.0008 0.0006 © 0.0008 0.0008 0.0006 ; 4 } . 7 a THE THEORY OF DEVELOPMENT 277 The graphical method of Hurter and Driffield can be most simply explained by an example. Suppose y, to be 0.82 andy, 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 reds O32 O6825 125. 25 s 'o 20 an 30 “62 320 ©46 is 2 15 LANDSCAPE 13 ; ARCHITECTURE. § PORTRAIT .6 | : 0 ae tae 70 100 200 500 1000 i@) 1 Ce le a) '6 7 MINUTES ‘DEVELOPED=28 375 575 m eererere Or 2 #345 7 13 af 3 4 8.7188 Fic. 173. 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 y, (0.82) to have been produced by three minutes development and y, (1.36) with six minutes; then y, (0.82) is plotted on the 3 minute line and y, 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 at Sheppard the values of gamma infinity (yo) 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 in page 252 and the latter in page 275. The values of these constants having been calculated for the case in hand, the time of development for any desired gamma may be ob- tained from the equation A ace Yo(L eh!) 278 - PHOTOGRAPHY The actual calculations are rendered quite simple by the use of the tables (page 279) worked out by Drs. Mees and Sheppard for values of (J —e*t) and corresponding values of kt.*° From the above Yt = bee UF ene Yo ( z ) or Gamma required _ bensicccstentcolle Sid buckets NG ge Gamma infinity Cee 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 (J — e~**) 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— et). Divide the value of kt as given by the value of R, as previously found by calculation, and the result is the time of develop- ment required to attain the desired gamma. For example: Gamma infinity ==T1.6, Velocity constant == .I5, . Gamma required 08. Then 0.8 aah ne Bees: By tases Bs (I — e~**), The nearest value of (J —e"*) in the tables which corresponds to .5 is .5034, corresponding to a kt of .7oo. 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 * whose work was followed up with a more complete investigation by Ferguson and Howard, Alfred Watkins and Mees and Sheppard.” 10 Phot. J., 1904, 54, 207-8. 11 Bull. Soc. Franc. Phot., 1903, 19, 256. 12 Ferguson and Howard, Phot. J., 1905, 45, 118. Ferguson, Phot. J., 1906, 46, 182. Mees and Sheppard, Investigations. Sheppard, J. 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. . a a EEE THE THEORY OF DEVELOPMENT 2 lx ( 9 TABLE OF CORRESPONDING VALUES OF ki AND I — e~*' FOR DETERMINATION OF TIME OF DEVELOPMENT FOR REQUIRED GAMMA OF PLATE OF GIVEN k AND y® (Mees and Sheppard, Photographic Journal, November, 1904, page 297) kt .000 025 .050 075 .100 (725 150 175 .200 225 250 275 .300 325 350 375 .400 425 .450 475 .500 © 525 -550 575 .600 .625 .650 675 .700 725 -750 775 .800 825 .850 -875 .g00 925 .950 975 1.000 1.025 1.050 1.075 1.100 1.125 1.150 1.175 diff. for .O1 kt .0095 .0086 .0077 .0071 .0064. .0057 .0052 .0047 .0042 .0038 .0034 .0032 kt 1.200 1.225 1.250 1.275 1.300 1.325 1.350 1.375 1.400 1.425 1.450 1.475 1.500 1.525 1.550 1.575 ' 1.600 1.625 1.650 1.675 1.700 1.725 1.750 1.775 1.800 1.825 1.850 1.875 1.900 1.925 1.950 1.975 2.000 2.025 2.050 2.075 2.100 2.125 2.150 2.175 2.200 2.225 2.250 2.275 2.300 2.325 2.350 2.375 7 — (eke .6988 -7959 He oh -7203 7275 -7339 -7493 -7469 7534 -7592 .7651 -7710 -7769 .7822 -7875 -7928 -7981 .8029 .8077 8125 8173 .8215 .8259 .8303 -8345 .8387 .8426 .8465 .8504 8539 8575 S61! .8647 .8680 .8712 8744 .8776 .8805 .8834 .8863 .8892 .8919 8945 .8971 .8997 9021 9045 .9069 diff. for .OT Rt .0029 .0024 -0022 -002I .OOI7 .OO16 0014 0013 -OOI2 .OOT05 280 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 y® (Mees and Sheppard, Photographic Journal, November, 1904, page 297) (Continuation of page 279) diff. for ae - kt I — e-kt .o1 Rt kt 7 vaneake 2.400 9093 } 3.200 9592 2.425 .QII3 3-225 -QOOlI 2.450 9135 -00086 3.250 .g61r { ‘20039 2.475 .9157 3.275 .9621 2.500 .9179 ) 3.300 9631 2.525 “9t07 | oheys 3-325 9639 | 49036 2.550 .9217 | 3.350 9648 2.575 9237 3-375 ey 2.600 .9257 3-400 -966 2.625 9274 3-425 = -9u/4 2.650 .9292 eute 3 3-450 .9682 noe 2.675 .93I0 ; 3-475 .9690 2.700 .9328 3.500 .g698 2.725 9344 00064 ° 3-525 ae .00029 2.750 .9360 ; 3-550 9713 2.775 9376 3-575 9720 2.800 .9392 3-600. .9727 2.825 9408 00058 3-625 9732 .00026 2.850 9412 ; 3.650 9739 2.875 9426 3-675 9746 2.900 .9450 3-700 9753 2.925 .9463 .00052 3-725 9758 .00023 2.950 .9476 ; 3-750 9764 2.975 .9489 3-775 -9770 3.000 .9502 3.800 9776 3.050. .9525 3.850 —.9786 3-075 9537 3-875. 9792 3.100 9549 3.900 .9798 3-125 9559 |} oo043 3.950 9807 | goor19 3-150 .9570 3-975 .9812 3.175 .9581 4.000 .9817 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 10° C. (18° F.) so that the expression for the temperature coefficient becomes Rt C./ Re 10” 4, The temperature coefficients of a few of the more common developing agents are as follows: id able THE THEORY OF DEVELOPMENT 281 PMeNCRMPPELIIOUC DIOMNde, 0. fdas cia reve ad Cee eluvc see ees 1.5 PTO COINOE, i. isa uv a ev tle cave eh es Sev ewes est 1.9 eR MRN POT OCRTINAL CLG, ccs sas ccs ed ya warns Ale a so hin ewe ees 1.9 RE eIIIMPC TMS OL OMIC) oo au a bee vg) v0 os 9,050 vo sega canes 1.9 ts, hc 5 fo siereelhs ie ee law ie eee ure ches Pen yart 2.3 eee y ol ag gis.o fis. eck and win binleele) Sk gw wee wee ceed 2.2 ee oa.) td a AD Ee eR GA picesch g.4 x Pacals bye. wogelg 8 b's 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 following formula: (log T, — log ta) X 10 T° —¢° In other words, the difference in the logarithms of the two times of appearance, multiplied by 10 and divided by the difference in degrees = log of T.C. for 10° C. 282 PHOTOGRAPHY ‘Centigrade of the two respective temperatures, is equal to the loga- rithm of the T.C.%* Thus if the times of appearance are 30 and 20 seconds at 17.5° C. (63° F.) and 25° C. (77° F.) respectively, we have log 30 = 1.4771 log 20 = rela from log tables. Difference = .1761 xX 10 10 . = 1.761 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. 174 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.’ 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 Pyro-soda (Hurter and Driffield formula).................. 1.48 Pyro-soda (Kodak - powders) ......... 2% «atm mane ee 1.9 Pyro-soda (Ilford formula) ...........2 2205 aoc 2.04 Rodinal (also azol, victol and certinal). << 7.0.0 ee ee £05. Metol-hydrochinone (Watkins thermo formula)............. 1.9 Glycin 0.6. e pesos ne oe ama pone nase glp palatine manne een 2.3 Hydrochinon, 6.0). 606 sicales s.00 « td boule: eos one im ea 1.4 -2.25 (Sheppard ‘and Mees). find. ......5..%..4 she ee 2.20-2.80 Ortol ssos sce pore 00 a0 eet duce a6 up ece Sbm call el neem 2.06 13 Ferguson, Phot. J., 1910, 50, 414; see also Phot. J., 1906, 46, 182. 14 For other methods see Ferguson and Howard, Phot. J., 1906, 46, 182. ——o.. 7 283 THE THEORY OF DEVELOPMENT ‘) ZL 2y} Buryepnosjed JO poyyyy suUDpeM ‘VLI “SI zIwIS) = DI WH 1tYv907 ( oricsosgL 09 0S OF OF o Si O16 3 JTW Ly 50S £9 OL ers) A1v9S JyNLVYIdWIL ; b ; 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- tnent at various temperatures is very easily found. Place the edge of a sheet of paper on the horizontal line corresponding to the T.Ce6is the developer and mark off the points of intersection with the tempera- ture lines. Transfer this paper to the log scale, p!acing opposite the time of development in minutes or minutes and a fraction, the cor- responding temperature at which the examination was made. This liaving been done the times of development at other temperatures necessary to reach the same gamma may be written down directly from the log scale.*® There is, however, no actual necessity for knowing the temperature 284 PHOTOGRAPHY Time in minutes 0° Temp. F. Fic. 175. Stokes Time Development Chart 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 15 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. THE THEORY OF DEVELOPMENT 285 on a log scale of times of development as ordinates against an even division scale of temperatures as abscisse (Fig. 175) will indicate, for all practical purposes, the time of development at all intermediate points. This method is due to Mr. W. B. Stokes.'® 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. 176%" where the solid lines represent the curves of 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 Unbromided ereren Bromided Fic. 176. Effect of a Soluble Bromide in the Developing Solution on the Plate Curve , in a common point. As the concentration*of bromide is increased this point of intersection moves slowly downward as shown in Fig. 177. 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. 16 Brit. J. Phot., 1921, 68, 97. 17 Sheppard, Photography as a Scientific Implement, p. 151. 20 286 PHOTOGRAPHY Bromide is without effect on the velocity constant k,’* 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 the action of a soluble bromide in development is that adopted by Watkins in the Watkins Manual. Fig. 178 represents a subject of Fic. 177. Density Depression with a Soluble Bromide. (Nietz) four gradations for a given degree of development in an unbromided 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 éxposed 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. 18 Nietz, Theory of Development, pp. 124, 170. THE THEORY OF DEVELOPMENT 287 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 a ma _ a= A B c D Fic. 178. Effect of Soluble Bromide on the Densities. (Watkins) 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.’® 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: ee iy oe ba ove 6 die ie DA wane a eee bw ble bes 0.3 p-phenylene diamine hydrochloride (no alkali).................. 0.3 p-phenylene diamine hydrochloride (with alkali)............... 0.4 ay eee MN oe occ isin u,v o eighties pt aa best awe ne ves 1.0 Standard ee PIV CMI ots ek nea eis tose stb eneensds tesevavens 1.6 A ERRNO MIO ots cin Gs tee cee bes a's Sas ces pe sa ws SA ete eee 2.0 OT a ea eg eee 2.2 19 See Sheppard and Mees, Investigations, p. 188. Nietz, Theory of Develop- ment. eres 288 PHOTOGRAPHY p-amidophenol (hydrochloride) ........iciss0.42-05589 0a ral Sie OO Chlorhydrochinon. (adurol) ........0:. «0.0 «se 206 = 9 sy apn ne 7.0 Dimethyl p-aminophenol sulphate............+0.... see 10.0 Monomethyl p-aminophenol sulphate (metol)..............00- . -20.0 Diamidophenol’)(amiidol) ..2.:0. 54 28s) ek be > boas ie 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 WorxKS Eper—Ausfuthrliches Handbuch Photographie, vol. 1V, 1905. Hust—Entwicklung ‘der Photographischen Bromsilbergelatineplatte, 1922. LuTHER—Die chemischen Vorgange in der Photographie, 1899. Nietz—Theory of Development, 1922. Reiss—Entwicklung der Photographischen Bromsilberge' ater ee SEYEWETzZ—Le Negatif en Photographie, 1922. i i a el CHAPLER. 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 Lumiere 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? divides all possible developing substances into three classes : 1. Those which develop a definite part of the latent image before fog sets in. (Common developers. ) 1 Ausftihrliches Handbuch der Photographie, p. 288 et seq. 289 290 PHOTOGRAPHY 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.? 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. triaminophesak 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.—One of the most fertile fields of the research chemist in modern times has been that branch of organic chemistry which is concerned with the products re- sulting from the destructive distillation of coal. Among the long list of organic compounds which science has prepared from what was formerly considered to have little or no value are found practically all of our modern developing agents. Benzene, the father of the numerous aniline and phenol dyes, and likewise of our organic developing agents, was discovered by Faraday "in 1825, but it was not until 1866 that its structural formula was de- termined by Kekule. This consists of a hexagon with the carbon and hydrogen atoms linked together around the six points. CH CH (1) 1. The structural formula of benzene after Kekule. 2. Its abbreviated form generally referred to as the benzene nucleus. 3. The points of substitution. 2 Theory of Development, p. 14. —aV es eee el eee eee ——— = 7 a . ' i 4 : ORGANIC DEVELOPING AGENTS 291 ‘The atoms of hydrogen at any of the six points of substitution may be replaced by atoms of chlorine, hydroxyl or amido groups and, as a substance with entirely different properties is formed according to the 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 amido groups in the first position we secure: Cl —NH, Chloro-benzene eS Bea aes Amido-benzene or Phenol or Aniline None of these compounds has any developing power. However, if we replace the two hydrogen atoms at positions 1 and 2, at I and 3,. or at. 1 and 4, we get three hydroxy-benzenes having the formula C,H,(OH), and identical in composition but differing in constitution. OH OH OH —OH vie | OH Ortho- Meta- Para- hydroxy-benzene hydroxy-benzene hydroxy-benzene or Pyrocatechin or Resorcin or Hydrochinon 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, para-hydroxybenzene being the agent known as hydrochinon while ortho-hydroxybenzene is known commercially as pyrocatechin. The third compound, meta-hydroxy- benzene or resorcin, has little or no developing power. By replacing the hydrogen atom in the second position of para- hydroxybenzene, or hydrochinon, with chlorine, Hauff produced mono- chlor-hydrochinon (C,H,C1(OH).) which was introduced commer- cially as Adurol. Schering substituted bromine in the same way and obtained mono-bromo-hydrochinon (C,H;Br(OH),) which also was introduced as Adurol. 292 PHOTOGRAPHY —OH —OH —OH 6 3 Br —OH —OQH —QH © Hydrochinon Adurol (Hauff) Adurol (Schering) 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 3 —OH —OH —O04e —OH —OH onl —OH ' OT Pyrogallol Phloroglucinol 1,2, 4 Trihydroxy- benzene These are known as adjacent, asymmetrical and symmetrical tri-hy- droxybenzenes or as pyrogallol, oxyhydrochinon and phloroglucinol. Pyro is the only one of these substances used as a developer. Precisely the same condition of affairs applies when the substitution is made with amido groups instead of hydroxyl groups. Thus we may have ortho, para, or meta amidophenols, or we may substitute instead two amido groups or one amido and one hydroxyl group thus produc- ing a whole series of amido-hydroxybenzenes. Pursuing the same idea further we may replace one of the hydrogen atoms with a methyl group (CH,). . For example if we introduce an amido group in place of a hydrogen atom in the fourth position in phenol we obtain para-amido-phenol which is well known as the base of such prepared developers as Rodinal, Azol, Activol, etc. aE OH NH. Phenol Paramidophenol ae ee a a en re q ORGANIC DEVELOPING AGENTS 293 The introduction of a second amido group produces di-amido-phenol or the familiar amidol. OH : NH; 2 NH, - —OH Paramidophenol Amidol There are three more developers formed from paramidophenol, metol, ortol and glycin. If paramidophenol be taken, and one hydrogen atom of the amido group be replaced by a methyl group we secure mono-methyl-paramidophenol. The sulphate of this is sold commer- cially as metol. u 3 H. NH—CHs3. Paramidophenol Metol (base) Ortol is a mixture of hydrochinon and the sulphate of methyl-ortho- amidophenol. The probable formula is a \Ounia a Za Glycin is produced by inserting the carboxyl group in place of a hydrogen atom in the methyl group of metol, being para-oxyphenyl- glycine. NH.CH,COOH Glycin 294 PHOTOGRAPHY There are two other developers derived from benzene, diphenal and paraphenylene diamine; thése, 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 it produces and ‘would be useful for line work were its contrast-giving properties greater. | NHe eee a NH; NH.HCl Diphenal (Andresen) Paraphenylene diamine If two benzene nuclei are joined together, as shown below, we ob- tain a body called naphthalene. If we introduce into this hydroxyl, amido and sulphonic acid groups we obtain a substance which may be termed 8 amido, 8, naphthol, @, sulphonic acid, known to photogra- phers as Eikonogen. JO Aa Naphthalene Eikonogen 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.* 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 (see bibliography at end of chapter) have established some general rules for the structure of compounds 8 Phot. J. of Amer., 1918, p. 337. 4 ORGANIC DEVELOPING AGENTS 295 which possess developing power. It is now generally accepted that the presence of hydroxyl or amido groups, either alone or in com- bination, is necessary in order that a substance may function as a de- veloper. PIT COAL LIGHT TAR OIL MEDIUM OIL HEAVY TAR OIL Many Products Benzene (Toluene, Xylene Phenol, Naphthalene, Lubricating 01! Sodium-«,amido 8, naphthol 2, sulphonate Eikonogen”) Sodium-amidonaphtholdisulphonate ~ (Diogen”) Nitrobenzene Benzaldehyde Phenol Resorcin Trigmidoresorcin Diamidoresocin Phenyl-hydroxylamine Aniline (“Reducin”) Paramidophenol Quinone (“Rodinol "= Hydroquinone hydrochloride) eas Nitrophenol Dinitrophenol Seed Diamidophenol Paramidophenol Orthoamidophenol (“Amido!”) Methyl - _ pzoxyphenyl- | orthoamidopheno! glycocoll (+H lydroquinone = (“Glycin”) “Orto!”) Monomethyl- Resorcin Pyrocatechin Hydroquinone paramidophenol (‘Kachin’) « (“Scalo/”) p ! ‘ : me Pyrogallic Acid Monobrom Monochlor - pean ey eye) (Made from'ballicAcid) hydroquinone hydroquinone (“Adurol") With substances which contain in one benzene nucleus at least two hydroxyl groups, two amido groups, or one hydroxyl and one amido 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. 3. The di-oxy-benzenes are more powerful than the amidophenols which are in turn more powerful than the diamido benzenes. 296 PHOTOGRAPHY 4. The developing power is not destroyed by additional Aside or amido groups. 5. In the naphthalene series it is not necessary that both groups ie joined to the same benzene nucleus. The general rules re- garding developing function do not apply to this group. 6. The substitution of chlorine or bromine for hydrogen increases the developing energy. . A substance containing two hydrowy groups requires an alkali, while substances containing two amido groups or one hydroxyl and one amido group do not require an alkali, =a In substances containing three hydroxyl or amido 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 amido groups increases the energy of the developing agent. “Slow ”. and “ Rapid” Developing Agents.—It is necessary that we consider at this point the nature of the difference between the so-called “slow ” and “ rapid” developers. With a developing agent of the type represented by metol, amidol and the paramidophenol compounds, the time of appearance is very short, but the image sub- sequently builds up very slowly. With one of the so-called “ slow ” developing agents, such as hydrochinon or glycin, the time of appear- ance is much longer, but the image builds up rapidly. This difference in the two types of developing agents is shown in Fig. 179. Graded strips which had received the same exposure were de- veloped in metol and in hydrochinon side by side and a strip removed from each solution at regular intervals. After 1 minute all the steps were visible on the metol strip but only the heavier exposures on the hydrochinon strip. At the end of two minutes, the lower tones cor- responding to the shadows were just visible on the hydrochinon strip, but at the end of four minutes both strips looked almost alike. It is evident, therefore, that in the first case the image has appeared in full detail at an early stage, while contrast has only been built up with pro- longed development. In the second case, the image appeared com- 4 | : q 9 4 ; | q ORGANIC DEVELOPING AGENTS 297 paratively late in development while contrast built up steadily from the beginning. Therefore when the progress of development is judged by inspection, the tendency is to remove the plate from the de- fou bd 2MIN. 2MIN. 4min. 4min. Fic. 179. Slow and Rapid Developers. (Crabtree) veloping solution too soon when using developers of the first class and too late when using developing agents of the latter class. Owing to this fact the former are frequently referred to as soft-working de- velopers and the latter as hard-working, although this is not strictly correct.: : In Explanation.—The following pages are devoted to the character- istics of the modern organic developing agents. Each of the more important of these has been treated in the following outline: Chemical name, Appearance and solubilities, Characteristics as a developer, Representative formulas. The developers have been classified in alphabetical order by trade names. Several developers no longer on the market, but once popu- lar, have received brief treatment. Fe ae ay adele “a i é x as . *] 2 ae oe * 298 PHOTOGRAPHY -¥ Adurol.—Mono-chlor-hydrochinon, C,H,(OH).Cl; mono-bromo- hydrochinon, C,H,(OH),Br: OH | OH 5) Cl | Br OH we: OH In 1898 Hauff, and independently Luppo-Cramer, investigated the 4 possibilities of securing a better developer by modifying hydrochinon by the substitution of bromine, chlorine or other halogens for one of the hydrogen atoms in the hydrochinon nucleus. These investigations resulted in a developer introduced by both Hauff and Schering as Adurol. The Adurol of Hauff is mono-chlor-hydrochinon while the mono-brom product is made by Schering. Both are alike in general properties although Lumiére in 1914 found that the mono-brom product is the more energetic of the two. It is a white crystalline powder which dissolves readily in water. As compared with hydrochinon, Adurol keeps better in solution, is not so sensitive to temperature, gives density more readily, even when used with alkaline carbonates instead of caustic alkalis and is more energetic in action; standing in this respect about midway between hydrochinon and the rapid soft-working developers such as metol and paramidophenol. The color of the deposit is blue-black and very suitable for lantern slides or transparencies. A formula follows: | : T. Adurol sic Vien. « otaidataca tag Rie re 85 gr. 19.5 gm. Sodium, sulphite -Cdry)s4.ci onic eee ee 382.3 gr. 87.5 gm. Water to make... 25.5 accusers eee 10 08; 1000 cc. Il.. Potassium. carbonates: .s2-) 6.5 1% oz 125. gm Water to makes 4 Wace 3y ee ee 10 Oz 1000 cc For studio and instantaneous exposures take equal parts. For full exposures outdoors take: : Solution. Ii... ce v cee selene oc) esas «nine orten ne I part Solution IID... ns.¢sabeees one ne eho sey Sten I part Water ....cccceccctcugaet ss piltiely ive Mee inti I part ORGANIC DEVELOPING AGENTS 299 For a concentrated single solution the following is recommended : PE TLV ) onc ec bce nacre kecsccescsees 2 OZ. 200 gm. PMI CATODOAIC Sele. cate ca vice cevecsvocsecs s OF. 300 gm. Oe UTS SR ee a IO Oz. 1000 cc. When completely dissolved add: SM CGT tie vs ce sieve kav ccects Y oz. 50 gm. For studio and instantaneous exposures dilute with three parts of water. For full exposures take one part to five parts of water. Amidol.—Diamidophenol hydrochloride, C,H,(OH) (NH,),2HCI: OH NH. ree NH, Amidol was introduced by Hauff in 1892 and is now made by a number of firms. Dianol (Lumiére), Acrol (Eastman), Nerol (Schering) and Amidol-Johnsons are all products practically identical in composition. The commercial product takes the form of steel-blue, needle-like crystals which remain clear and colorless in solution, the © solution loosing its developing energy without the visible coloration which accompanies the oxidation of other developing agents. One part of amidol is soluble in four parts of water and the solubility in- creases rapidly as the temperature is raised. Amidol differs from most other developers in common use in that it develops without alkali. It may also be used in an acid solution. It belongs to the class of rapid working developers, the image ap- pearing very quickly with full detail and density growing slowly with time. It is rather sensitive to temperature and the working solutions should be kept as near as possible to 65° F. It is comparatively in- sensitive to potassium bromide, small amounts of which act only as a clearer, while quite large amounts are required to restrain develop- ment. Amidol is recognized as the finest developer for bromide papers owing to its rich velvet-blue-black and black tones and its free- dom from fog or stain. It is also ideally suited to the development of lantern slides and transparencies, for which purpose it is excelled only by ferrous oxalate. 300 PHOTOGRAPHY The following are excellent formulas. Owing to the rapid loss of developing energy in solution it is advisable to add the dry amidol immediately before use, care being taken to dissolve the same thor- oughly in order to prevent spots. On no account can the solution be kept over a day. Wrater: 2.07 5. 5G wan be ee eee a 20 02. 500 —s cc. Sodium stlphite (dry) o..ra.dees5 2 ee ee 325 gr. Io gm. AMAIMOL Cos sins vee bos oe eeioea yee ee ee 50 er. a) Potassiam. bromides... i550 tenes eee 10 gr. 75 gm. Absolutely pure sulphite should be used in preparing the above. A sodium sulphite solution to which potassium metabisulphite has been added will keep in good condition for several days and by adding amidol a fresh developer can quickly be made as required. ‘The fol-— lowing is the B. J. Almanac formula for the neutralized sulphite solu- tion : Sodium sulphite: (dry)i¢...0. 4... 2 be ee 2 02: 100 gm. Potassium metabisulphite. . 2... .s.esaeg ee 1% oz. 25. gm. Water to make., ...swss écieeus Jao s eee ee re a 20 oz. 1000 cc. After the chemicals have been dissolved it is well that the solution be boiled. Boiling is not essential but it improves the keeping quality of the solution. The developer is prepared as follows: Amnidol is) 04 «5 aisinisit ne die iety . . occc ae ys eos ten «Gna 4 Oz 200 cc Water” i.e. cen Coie go ates ay 20 02. 1000 cc. Preservatives of Amidol Solutions——A number of methods have been suggested for preserving solutions of amidol so that stock solu- tions may be prepared for future use. Namias has advised the use of 25 grains (.05 gm. per 1000 cc.) of boric acid per ounce of solution as an effective preservative and ac- cording to Crowther glycollic acid in proportion to 1/10 the quantity of sodium sulphite is even more efficient.* According to Namias a de- veloper of the following composition retains its activity for a long time owing to the preservative action of metol:® Amidol 22. <2... +sauculs seiner sie se sae 5 gm. a. er, Metol ....... s «¢ ve a) Gas dele eaters ne Seal I gm. 4.5 gr. Sodium sulphite (dry) .. acc. - a) es ee 20 gm. 87.5 gr. Potassium bromide... 7.70.24 a0) evens eee 2 gm. 88 gr. Water to make: . is occ Aa eee ".. 1000 cc. I0~ oz. 4 Brit. J. Phot., 1920, 67, 642. 5 Prog. Foto., 1921, p. 45. —— ee ee ORGANIC DEVELOPING AGENTS 301 Bunel has advised the use of lactic acid as a preservative of amidol. To each 1000 cc. of the following amidol developer are added 5 cc. (1:50) of lactic acid (sp. gr. 1.21) :° MA le pan veccaccesececcsssee 5 gm. 229%, Pe IIBSM SULT TICE. HCUTY os ois F cs leecle Govan secdus 30 gm. 131 gr. A PRMITIR EU NG See Ghirs aici eds ga es Hales dale ga eu 1000 cc. IO Oz. M. J. Desalme has recommended the use of stannous (tin) chloride as a preservative of amidol. About one part of the following stock solution is added to each 25 parts of developer: 7 CCITT IO COV SIA Le. os ves ns vey ma vee vne se 5 gm. T.6..-oz, RIUM PEC OW UCL foe 5 cst ob eae cons cena ven. 7 gm. 2.03 Oz. RNS et Wee yl cee wx o's sch ees cedte's 30 Cc. IO Oz. After cooling slowly to nearly room temperature the above mixture is poured slowly and with constant stirring into a cold solution of PO ACNE (ULV) ole. sista cee cose II gm. I 0z., 44 gr. RNP ec cin vic eds Geevcu wavvede 60 cc. 6 oz. The whole is then made up to a total bulk of 100 cc. (10 0z.), al- lowed to stand for twelve hours and filtered. In using amidol, which is not suitable to the employment of an alkaline salt, the stannous tartrate solution is first neutralized, or rendered slightly acid, by addition of sodium bisulphite, litmus paper being used to determine the point at which a slightly acid condition is reached.” Amidol prepared from 2: 4 dinitrophenol by reduction with tin and hydrochloric acid retains small quantities of tin chloride and thus when made up with sodium sulphite forms a solution which keeps well. The observation has been patented, No. 2070 of 1922.8 Certinal—This is merely a trade name for a paramidophenol de- veloper similar to Rodinal. It is marketed by Ilford Limited, London. Edinol—_C,H,OHCH,OHNH,, sulphate of oxymethylparamido- phenol: OH CH,OH HSO, ot ssp aba tne 2 NHe 6 Bull. Soc. franc. Phot., 1921, p. 290. 7 Bull. Soc. franc. Phot., 1921, p. 192; Brit. J. Phot., 1921, 68, 359. 8 Brit. J. Phot., 1922, 69, 81. 21 302 PHOTOGRAPHY Edinol was introduced as a developer by Bayer in 1901. It comes in yellowish crystals which are very soluble in water. The solubility in plain water is 15.9 grams per liter and in a five per cent solution of sodium sulphite plus an equal amount of sodium carbonate the solubil- ity is 9.7 grams per liter. It is free from tendency to fog and is non- staining. Neither does it affect the skin as do some of the other or- ganic developing agents, notably metol. It is not very sensitive to bro- mide and may be used alone or in combination with hydrochinon or adurol. As a developer it stands midway between the rapid soft- working developers such as metol and paramidophenol and the slow contrast developers as hydrochinon. It is particularly suited to the development of lantern slides and transparencies and in combination with hydrochinon forms an excellent developer for bromide and gas- light papers. For soft portrait negatives: Sodium. sulphite. (dry) ...00 4% 555.4 suse = 2 0z 100 gm Eedanol 20, icds «gine os ce toh cele eee 100 gr. II gm. Sodium carbonate (dry)c..c stesso ae ee tot 50 gm. Water to make... dis baasunn's see ee 20 Oz. 1000 cc. For contrasty negatives and general outdoor work: Acetone sulphite. os sc..s eeu acee peo ee 288 gr. 99. Po, Sodium sulphite (dry). ... 6.25 en eee 14 oz. 275 gm. Water to make: .5:3 .7sas tus sas ee 10 Oz. 1000 Cc. For use take 34 dram each of I and II and make up to 1 ounce with water. In metric measure 94 cc. of each to 1000 of water. The Wat- kins factor is 15. Further dilutions are given in the following chapter. The action of the desensitizer phenosafranine on hydrochinon is such that it raises it from a slow, contrast working agent to one of high speed and tending to softness in much the same way as metol. Safra- nine has therefore been suggested by Dr. Luppo-Cramer as a cheap substitute for metol. He gives the following hydrochinon-safranine formula: A. Hydrochinon 2.5 cae, ies ee 12 gm. 52.5 gr. Sodium gulphite (dry) ..........60.-s0000 40 9 OO poem ee Potassium: bromide... .."...5....4s.0 Pl AVET SN iis ee as bps es MO BHecial Navid... esa.k ve hak wes S Gevaert— PMT MCE GEN oes cs, ies ff Bs ws Ae MQ Se CUE Ota a 2 Say ee VS I We CELA gS maps pi coder ROME a MQ WA tiers Rue wpe + ow « MS Oreho Medium 6... +. 20. wears Re MQ PATEOPUTOMN Ge ws edges ee MS ee Eh 0 oe ne ee Ne oe Tk ae MOQ Pepin me. Paes. S Imperial— BNR me tse foc Soa 4) Cla ours MS PECISE PSPs ots ey Hiei See Baa S PUPEPEREMENO, 668062 ov kee es MQ PPR RRIIO Ta alee vic Geek ess oink he" SOUOVABE TIAGO hoc eas eee MS special! Senstivesuau.! Wve es = Un tte: U0tirs Oi a eee a MO Res a AE ag | Ca ee Reo a he SW MS Hammer— PEnCDrogi atic Faas ace ee sure O Se dae eS MS Pamenromiatic, Bs ica tae cle: VQ ED) a hee a, Coe Ue a MS SPOCIOT RADI 0.8 oe Gok ke ae aes MS REN BRS ee il a dials ea 4 3 O Speciotiaon Orthoy so yeaeh MS oiimerea Orthod has QO None titer Grihaes. pole ae 2 QO (eto exted Fast. 2 eco. es MS POIMESORIT biter als woseet sees M Sperm: ti 0 BY Giga aa @) CECE a eee ast cs Ortho Double-Coated.......... MS Fine Grain Ordinary... 2. ss. VVQ Aurora Non-Halation.......... MS Latiisea pes ey Se ae QO Phoeos Ostalwwie oc cs. VVQ Balmer ical ee ee oy MS Ilford— Lumiere & Jougla— OE a oy swiss VS MaRS v0 RO alah Bees ws M PAE T SI O S ee a > Portrajtuinstantanees 6 yim...’ M OC ua snl) VAS Ss ee VS (Grande Ingstantanee..). 455.4%... MQ CT ET fe An ah ee a VS Ortho, Yellow and Green....... O Special Rapid Panchromatic..... MO Ortho, Yellow and Red .o....... O Most Rapid Versatile........... MS ARES tie eee OE ei wig hip S05 mead a0 M oo2 PHOTOGRAPHY Panchrotiatigu. acs ek cee VQ Film... 5.2.3) coe ee M Inetantanee.s4 OAS eni. eee MQ Paget— Extra Rapide.. eieiee es tere: MOQ SOX 5 ss 0.0 ok ae ae ee Fils 3: oie oer eee MS ». 0.0. Gwe Reoroductionoiesa i 1s ok ee MQ Process Panchromatic.......... M Panchromatic Procede.’.....>.%5. Q S. F, Orthochromaticy= +. S Plavik- Pugs oes. 20 areas eee MQ Special. Rapid. 3. 3 oe oe eee 5 COONS ears ee Se ee ee M Professional Medium........... Se NiiGe Speed. 625 sa seer SS) Portrait... sisasow as se S Sigma, Ortho.) os a.0o eee eee S Professional Extra Rapid....... Marion— Extra Special Rapid R.......... Iso Record .. 45 Syoce eee Q Ordinary Panchromatic......... Retord ). ee eee ee 5 Hurricane, ./72 sine eae 5 Panchromatic: .ca0 ase eee VS Extra Special Rapid Ortho...... Ss Brilliant 2 .4\coiesiaee bape ee MQ Rajar— Po So cone dae ee eee ee MS Film, |..44 435, 33 lristantaneouss sara. eee ea ee MS Seed— [sts 2. 2 eee ee eee M Graflex. 0. 5.4.55 #aeee eee VS Portrait 2 ¢0.ssa sae oe M Gilt Edge. 0.2.2 ee WB cd: Sere ee ee M Color Value. 32s yae see eee M Ordinary .-i55i ea ene a ee MOQ L, Ortho «<5 vin cic Paden eee M Stanley— 26 XX. Oo reete cst. be eee M BOs ob bia uace oy Cale eee M Non-Halation L Ortho......... Commercal, 2... os5 eee Q Tropical.) sc sa8's eee M Wellington— Panchromatic(: 35) -inaaereeee ~Q Super Ktreme. kaneis tees VS 23 0 cce weue xb Se We ee Q Rtremes Vac emnts eee VS Process. vx:a.5 55s gnneiyeee ee Q Studio Anti-Screen............:5 de Standard— Préses oc ves see oes ee ee VS Extra Imperial... ..1oc3 ee ote M Special Extra Speedy. ..-2...-2- | Orthonon ©. 0.42.94. eee M Extra Speedyioy ssa oe MS Polychromeon,...i5 os pe ae M Speedy Portrantic, Unies ae MS Wratten & Wiasmeoviakaes Speedy.5 oo Civ ta eee eee S Panchromatic...........-0+.0: Q Anti-Screeit. 2s4.5 v.54) ore ee M Process Panchromatic.......... Q Iso Speedy 2.23.03 see eee M Wratten M (backed)........... QO Ordinary sic pa hone ee Q » Developers.—Thermo Pyro-Soda: A. Potassium metabisulphite...................05 80 gr 16 gm. Pyro ees tact bedew mma ee eager 160 gr 32 gm. Sodium -sulphite-(dry)oee 1,4. da eee I Oz. 100 gm. Water to make. ....555770és 6x04 pees ee 10 oz. = 1000 CC. B. Sodium carbonate (dry) ..s ...aces see 2 Oz. 200 gm. Potassium’ Bromide.) 0000", .w sss ee ee eee 40 gr. 8 gm. Water t6 makes oo vse as «10's 0 de ae 10 Oz 1000 cc. a [a _ oa ¢ = » NN ee ee en ee ee ee ee ee ae ee eC Um em Ke ee = THE TECHNIQUE OF DEVELOPMENT 333 Thermo-Metol Hydrochinon (as modified by F. R. Fraprie) : Peer Oraenitiiy Met abDISUIphite.......cs0ce esac cae: 60 gr. 6 gm. Vr coach ce oe ae le il eae a0.’ -er, 3 gm. RIE oP eye bk eos oS cece Pele vanes gO gr. 9 gm. MRE ATTN co ice scydle choca deeevss ses TEU aya 1000 cc. OS SEO 00 2) Tee Oz, 50 gm. meemmentmonate Cdry) io. .is desks d seek oes 14 oz 75 gm. ES Se ee a ee 20.5. OZ. 1000 Cc. Dilution of developer. VVQ VO QO MQ M MS S VS PYTO-SO0G ok oa so I 14 134 244 3 4 5 634 Metol-Hydrochinon...114 2 224 3% 44 6 Bon STO drams of each stock to be diluted to make a total of 3 ounces for tray or 10 ounces for tank. Rodinal, Azol, Citol, . OPP AN sy 0% Ges 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 VVQ_ VQ Q MO =2M.MS)''S> vs Pe Bootes, ce keys 30 41 55 73 94 125 165 210 for tray 9 I2 16 22 28 38 50 65 for tank Pyro-s0dia. ws ou. es 41 55 re 94 125 165 210 280 for tray 12 16 22 28 38 50 65 84 for tank Certinal, Rodinal, Azol, Victol..... 13 17 23 30 40 53 70 go for tray 4.3 5.75 york 10 13 18 24 30 for tank The above figures are cc. and are to be diluted to a total volume of ~ OD 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 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 23 334 PHOTOGRAPHY 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 27.0 3% 12 78 26.0 3% 13 76 24.5 334 14 74 23-5 4 15 72 22.5 4% 16 70 21 4% 17 68 20 5 18144 66 19 54 19 64 18 5% 21 62 17 644 221% 60 16 6% 24 58 14.5 7 26 56 13.5 7% 28 54 12.5 8 30 52 11.5 8% 32 50 10 9% a 4 9 1o 3 46 8 1034 40 44 7 1 43 42 6 1244 46 40 4.5 1344 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 VVOQ 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: 2. (oA Sas eae ee ee 500 cc. or I0 oz. Potassium carbonate (dry) sc...) ase ae 30 gm. or 280 er. sodium sulphite: (dry) 9 4.2.) sce ee -. 10 gm, or Qo gr. Glyein tes . of. . 3c cconvlas sane geen ee eee ee I5 gm. or 140 gr. ~—— os ee ae ee ee ee ne Se THE TECHNIQUE OF DEVELOPMENT 335 Cc. of Stock tobe Drams of Stock to be made up to 300cc. made up to 10 oz. for for Tank or 90 Tank or 3 oz. for Plate Glass j cc. for Tray Tray ON eg. ik san cn ind Sto Ve ee do 6 1% a eg xa a gap nk vada 7, 2 Or 8 Bie ae en aD 10% 2% Do oes ole hee as ss 13% 3% ee ceed c cane wne bet 18: 4% ee eds oa ci vac dev obec ds 24 6 re a 30 8 I a! 4 cus ons Ve otek. Skee 40% Io Temperature—Time Table Temp. Time Time Degrees Minutes Minutes and Seconds Fahr. Tank Tray oh pe irre 10 2M. s0 S. a a 11k 3 M. 10 S. Dre ce cee ee 123% 3 M. 30 S. ree ed ee cae 1314 3 M. 50 S. OS Ey ee 1434 AA O12 8S, CS 16 4 M..35 S. OO 17 5 M. I nk cv ere neces 185% 5 M. 20 S. as ae ge a 20 5 M. 36 S. OO on tS ee oe re 21% 6 M. Temperature coefficient, 2.2. The Efficiency of Time Development.—As has been stated previ- ously the time of development depends upon I. The maximum contrast of the plate (y.). 2. The velocity constant of development (). @. Lhe 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 (yo) 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 batchew: of 336 PHOTOGRAPHY the same plate. This is due largely to the rate at which the plates are 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 Rk) 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, 4 THE TECHNIQUE OF DEVELOPMENT GENERAL REFERENCE WorkKS BLecu—Stand-Entwicklung. Brown—Developers and Development. Hust—Entwicklung der photographische Bromsilbergelatineplatten. Luppo-CrRAMER—Negativ Entwicklung Bei Hellem Lichte. 10922. ReNGER-PatzscH—Die Stand-Entwicklung. 1920. SEYEWETZ—Le Negatif en Photographie. 1922. Watxkins—Watkins’ Manual. 10918. Watxkins—Photography—Its Principles and Applications. 1912. Modern Methods of Development. Photominiature, 1309. 1922. 337 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 1819.4 According to Abney and Meldola, the thiosulphates dissolve silver halide by uniting with it to form a compound of silver-disodium-thio- sulphate according to the reaction: ? 1. 2AgBr-+ NaS,O, =2NaBr + Ag,S,O, (silver-thiosulphate), 2. Ag,5.O; -+ Na,5,0, == Ag,S,0O,) Nano (silver-monosodium-thiosulphate), 3. Ag.5.0; * Na,S,O, + Na,S,O, = Ag,S,O, * 2Na,S,0, (silver-disodium-thiosulphate). When a very small quantity of sodium thiosulphate is brought into contact with a considerable excess of silver halide we have the result 1 Edinburgh Phil. Journ., vol. I, pp. 8, 396. 2In the terms of the reacting ion the equation may be written as follows (Sheppard, Elliot, Sweet, J. Frank. Inst., 1923, 195, 45): 1. Ag +S,0, = AgS,0, 2. AgS,O, + S,0, 2Ag(S,0,), 338 ae a ae ee, ee oe ie ee ee ee ae ee eS ee sa =. a Cer ee aN es a - . ' ee ee en ee ay ee eT eee ee ee ee ee ee ee ee ae ee ee a ee FIXING AND WASHING 339 indicated by equation (1). This silver thiosulphate salt (Ag,S,O,) is insoluble in water but soluble in sodium thiosulphate. Conse- quently, in the presence of an excess of sodium thiosulphate, it is immediately transformed into silver-monosodium-thiosulphate (Ag,- S,O, + Na,5S,O0,) which, like the first salt, is insoluble in water but soluble in sodium thiosulphate in which it is converted into silver- disodium-thiosulphate (Ag,S,O,:2Na,S,O0,). This double salt is soluble in water and may be removed from the film by washing in water. 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 ®* and by Warwick‘ 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 4 = a(I—k)", where a is the original amount of silver bromide in the negative; k, the fraction dissolved per unit time; +, the amount remaining after 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. The simplest explanation of the known facts is that the rate of fixation is determined primarily by the penetration of the sodium thio- 3 Investigations, Phot. J., 1906, 46, 235. 4 Amer. Phot., 1917, pp. 585, 639. 340 PHOTOGRAPHY 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 MINUTES IN TIME 40° 20° 0° 4o° so” 60° 10° TEMPERATURE OF FIXING BATHS (°CENTIGRADE) Fic. 180. 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.® | The results obtained for the effect of temperature on the time of fixing were plotted in the form of a series of curves (Fig. 180). 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 5 Brit. J. Phot., 1913, 60, 59. FIXING AND WASHING 341 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 20° 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 181, from Piper’s paper, gives the curves for the effect of varying concentrations at the same temperature, the time in minutes (eS eRe serzeeseeera/ge Pee er oe eel Tee | TENSE Ue NG ae FONSECA ae > (SR GRRRRRY Ae sie at SSS Ain Time in Minutes 10% 70%. 50% 40% 0% 60% 10% 80%, Strength of Hypo Solutions ( Per Cent) Fic. 181. 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° ee es, 0, 104, 122, 140, 158° F:). Influence of Ammonium Chloride on the Rapidity of Fixation.— Ammonium thiosulphate was recommended as a fixing agent in place 342 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 ® 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 4o 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 Lumiére and Seyewetz in 19087 and by C. Wel- borne Piper in 1914. . The results obtained by the latter investigator are shown in Fig. 182. 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. € Brit. J. Phot., 1914, 61, 193, 437, 458, 511. 7 Bull. Soc. franc. Phot., 1908, p. 217. e ee tee = Sa eee ee ee ee ’ Sr eee * BS ra at, se ee ee ka So ail ¥ eS eT ee, ee ee ee Ee eee Cee ey. eS COME he ee eT Se PU eee | Oe, eee nm uJ u FIXING AND WASHING 343 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”).............. to OF I50 gm. PeOVTy IOTICE. os dec es os cee tees ese 334 Oz. 32.6 gm. PENRO ee faye. hea ec ieee ees 100 0z. 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 iaa0eanerm gibi ats Ramen meme ene te | NJ i is ie i sl ty A ie a B macs e Ef Et a m4 ea Ey if UA te a az ie a Fic. 182. 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 “dichloric ” 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 7 ae ie 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 * 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 (87.5 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 | itimmediately 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 silver bromide which can be completely dissolved by a given amount of sodium thiosulphate. This matter, together with its practi- 8 Brit. J. Phot., 1024, 71, 172. : 9 This corresponds to approximately 240 square inches of plate surface. | - ree Ay 4 7 Pee ee eee ee & PI 5 es wee fs hia Wt = Rie ok — ae . ; Per Fe Pero Ss FIXING AND WASHING 345 cal consequences, was investigated by Lumiére and Seyewetz in 1907.1° The result obtained by adding various amounts of silver bromide to equal volumes of fixing bath of three different concentrations is set forth in the following table: I. 2. ay 4. 5. Rhee ae Maximum au ‘ : oO amount of AgBr weight Ratio between pao he leet silver which can be AgBr 2 and the hypo bromide dissolved in Ratio between | necessary to weight ens which can 100 cc. of bath T ends form the corresponding be dissolved without causing compound to the salt in 100 cc. a subsequent Na2S203 AgeaNa2Si0c of bath yellowing + AgeS203 5 per cent ee ett, 1.25 gm. 62 per cent 3.8 gm. 33 per cent I 46 sé 6.8 és 8 46 60 “6 ‘6 18 ie sé éé 66 fe ‘sh ““ 20.5 ‘“ EG “c 24 c ‘ Jae “6 i ‘i “ The effect of adding sodium bisulphite either alone or in combina- tion with chrome alum was also investigated. It was found that these additions reduce markedly the amount of silver bromide which is com- pletely dissolved by a given volume as indicated in the following table: 4. i Ratio between Weight of Calculated the maximum Composition Weight of AgBr which weight of weight of of the AgBr which can be AgBr AgBr not fixing can be dissolved Ratio between | necessary to giving rise to bath dissolved in in 100 cc. I and 2 form the stain and the 100 cc. of of fixing bath compound weight which fixing bath without NaeS203 corresponds to * staining the + AgeS203 the salt negative AgeNa2SaOc I5 per cent hypo, 1.5 per cent . ; y 1.65 gm. 2 r cent I1.4 gm. |I4.5 per cent Eadie 6.1 gm 5 gm 7 pe 4g 4.5 per c bisulphite, lye 15: per. cent hypo, 15 per cent sodium : : ; , 2,2 8 per cent 11.4 gm. 20 per cent bisulphite, 5-9 gm ou te 48 P - 0.5 per cent chrome alum _ These investigations indicate that, all conditions being the same, relatively dilute solutions are more economical in thiosulphate and that solutions acidified with sodium bisulphite may not be so completely ex- 10 Bull. Soc. franc. Phot., 1907, p. 104; Phot. J., 1907, 57, 120. 346 PHOTOGRAPHY hausted in practice as when bisulphite is absent. Assuming that the average 9/12 cm. plate contains 0.3 gram of silver bromide, 1 liter of 15 per cent fixing bath should fix approximately 100 plates. One liter of fixing bath containing 1.5 per cent sodium bisulphite should, under the same conditions, fix about 60 plates, while if 9.5 per cent alum is added about 75 plates should be fixed before the bath becomes ex- hausted. Using a 25 per cent solution of thiosulphate this would correspond to about 15,000 square centimeters of plate or film per liter. Approxi- mately this works out to about gooo square inches per gallon of 25 per cent thiosulphate, with bisulphite, and this figure has been confirmed by the Eastman Research Laboratory. Since thiosulphate is so cheap it is the height of false economy to overwork the fixing bath and owing to the fact that the above figures will vary to a certain extent with the type of plate, since some plates contain more silver bromide than others, it would perhaps be well if in practice the above limit be reduced somewhat, to say 7500 square inches per gallon. To ensure the fact that this amount is not exceeded accurate record should be kept of the number of square inches of plate surface fixed. This is most conveniently done by placing a small slate directly over the tank carrying the fixing bath and noting thereon the number of plates added each time. When calculation shows that the maximum number of plates permissible have been fixed, the bath is discarded and a new one made up. If all the plates are the same size calculation becomes very simple. While this may seem to be unneces- sary trouble, reflection will show that it is based on a sound scientific basis and that it is far better to go to a little trouble rather than to lose valuable negatives from incomplete fixation. The Fixation of Prints——Warwick in 1917‘ 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 11 Amer. Phot., 1917, 11, 639. ; 4 | P 4 § . a ? Oe — ae 2 ne Be ae ee |’. he > = thet il * uA ’ z . 5. ee 7 - a 5 ree — ai _— ain cette sh = —->. 2 2 7 m be Sve .) - Y ee ee aS eee ee ee re en ee ee ee eee ee, eee a ee a FIXING AND WASHING 347 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 _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 Lumiére 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 1 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 te a re eee é Dis +% oe oo . i a 4 yb f suey 348 PHOTOGRAPHY 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 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: [hiosulphate required % . for each 20 ounces of bath Stock solution Water 8 OZ eooeeeeeeee eevee ereee ee ereeeees ee eevee ee ees & 16 4 c: 6 e@eeseveevreevneeeveeeeevoeeeeeveveeevoeeeeeeeeeeereee eee e I Zz 8 . 3 SPA Se cs wag baw bee vile te ace he 10 10 E Bey wie ete vive 6 ivie bre: 6 pie bl oceig ie aiet nisl oust tal siete shane tenant 8 12 : 3 £2 unig ata Td whereas ip ith Rim cack kg F cee ei ee 6 14 a Bh Ae Ao cate, wih oii alaickdi aca ean is RAR 4 16 i 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, according to the equation H,S,0, 2 H,5S0, +S. FC ee 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 NaHSO,. The following is an excellent formula: | Se ae ee ee Tk ey ck ene ie eee Sodium. thiosulphate ( hypo ")’.,. <<. .5 «4s 5a 16 oz. 250 gm. Sodium bisuiphite so. 9 iis ene ko a SOx 47 gm. Water ‘to makes 7..y..04 ave Pha er ee 64 0z 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. FIXING AND WASHING 349 Potassium metabisulphite may be used in place of sodium bisulphite. The following formula is suitable: MPTP IQSHIMGLC vie ccs ccc ccevsct ess csceeees 6 oz. 300 gm. POtusseiuin metabisulphite..........0. 6.0 cece eee YZ oz. 25 gm. ETNIES 5. e x's « o.6 4 od leel ee sta cals ee ee eas 20 «02. 1000 cc. A formula using citric acid and sodium sulphite originated by Mr. E. J. Wall was strongly advised by Sir William Abney. The formula follows : a RS es co be wind fastens dceess VY oz. 15.6 gm. RR ETRTIEINICCULOLY) ics pacaccncdss cases esr 14 oz. 15.6 gm Mix in I oz. (30 cc.) of water and add to Sodium thiosulphate...............ceeseeeeeess 4 02. 250 gm. eta ION d kb sis cep Se nee dels wes woe’ 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: ?° Constituent Function Formula Sodium thiosulphate......... Fixing agent proper, dissolves Svar: halides it das fees woes s Na.S,O;- 5H.O Re eee ae Oak Se lesa es Clearing agent, promotes swell- ing and increases the speed of fixing, reduces stain and col- oration and regulates hardening ENE 0) o4 ots BAN a's is i oho eo H,SO, (Sulphurous or organic acid) Gress has hs piles veces Protects thiosulphate against decomposition by the acid. Anti-stain and anti-oxidant....Na.SO, (Sodium sulphite) USSG ie a rr Hardens gelatine, prevents . frilling and softening......... K,SO,- Al. (SO,)s, (Potash alum) Weel Cis 50,75 (Chrome alum) 12 The chemical theory of the acid fixing bath has been fully discussed in a paper by Sheppard, Elliott and Sweet of the Eastman Research Laboratory in the Journal of the Franklin Institute for July, 1923, 195, 45. 24 350 PHOTOGRAPHY The following formula using potassium alum is an excellent one for plates, films and papers: Sodium thiosulphate (“hypo”)... s4.05 Bee ee 16. Oz, 250 gm. Water to miakese. i005 os5 al ace ee 64 02. 1000 Cc. Dissolve separately and add to the above Powdered “alum. ss. 003 ia. «is 4s dees Ole i oz. 31 gm. Acetic acid ‘28 pér’ cent... 6 i scales eee 3 > OZ 186 cc. Sodium: sulphite (dry)... 5.5. 2e0s eee eee % oz. 31 gm. Water. to.maké.... .. 2i.5-205 0c saa ee tye gee 312 ce. In mixing the last solution, or hardener, it is best to use two separate solutions. Dissolve the alum and sulphite each in half the total 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 C.P. 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 Ritia. ; ; Se er FIXING AND WASHING 351 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”).............. ent OZ: 250 gm. DerueUIOOIG (OLY). oti eee ce ae ees sere y 50 gm. ANIMAS Ee os bs Caer dese ss news ae 2% fl. oz. 125 cc. TA cies co ce eon dns sce nesces 20) 02: 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. 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."* 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 13Lumiére and Seyewetz, Bull. Soc. franc. Phot., 1910. Warwick, Amer. Phot., 1917, p. 317; 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. 352 PHOTOGRAPHY 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 dM z—un | and therefore Then which may be written I initial concentration in film k = — lo Ss ae, eee i Ty ny ae oes ea : t concentration at time / The value of k is independent of the initial concentration and may be obtained from b Concentration at time ¢; = ———— log | =e tg — ti; -© \ Concentration at time hy From which the time required to reach any limiting concentration Cy is given by: | I Ct tr ; log C, + ta. 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 14 Hickman and Spencer, Ibid. ee a Se a — ee eee ee ee ee ey ee Se eae es ee ee ee ee FIXING AND WASHING 353 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 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. 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 of plates. The principle and construction of the apparatus are made clear in Fig. 183. 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 354 PHOTOGRAPHY 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. | JVEGATIVE WASHER WHITE MAPLE For Bx S Pur. Fic. 183. Windoe’s Washing Apparatus 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. 184) sends a thin spray of fresh water down both sides of the film which results in quick and effective elimination of hypo. A further development of the same principle devised by the writer for cut film is illustrated in Fig. 185. This apparatus is de- signed for use with the usual developing holders for cut films and is so arranged that a thin stream of water is applied to both sides of the film and after trickling down the surface of the film passes off by the drain, D. There is thus no admixture of fresh and hypo-con- taminated water while every part of the film is in constant contact with a moving stream of fresh water. A complete description of its construction will be found in American. Photography for 1926, p. 536. The Washing of Prints.—It has always been supposed that under similar conditions prints could be washed free from hypo in the same . DP 3 3 4 4 a a ‘ x a De PL ER ee ee OR ee a Ee Se ee i OE eS yee ee are ty Oe ere gee ey mes eet | i FIXING AND WASHING 350 -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.1* 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 f \ " TROX FILM WASHER | NTED 1,035,274 _George Murphy, Inc. WW AMIGA: SS | as Fic. 184. Trox Washer for Roll Film 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 15 Phot. J., 1925, 65, 443 356 PHOTOGRAPHY 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. Methods for Determining the Presence of Hypo.—The various sub- stances which indicate the presence of hypo by the formation of vari- —— SAWERAAARRARANAAANSN SSS NNNNNNSANNANANA NNNNNANANNAANA N NNNNANNAANANANANA NAN NNNA NNNNANANA AD \ NNNNNSSNANNAANA AARARAVBVBAL NNNNNSANNNNNANA Poe NNNNNNANNANANAAW NNNNNNNNANANNANANR NNNSNSNNANNAWANA N NNNNNNNANANANABNR NNNNNNANNANA NN NNNNANAAN N NN Fic. 185. Neblette’s Washer ous colored compounds are useful methods of determining to what extent the thiosulphate has been eliminated from a batch of negatives or prints. : Perhaps the most generally useful method is permanganate. Make up the following formula: . Potassium: carbonates, 2s. +s sane sce seek cee 5S yr; I gm. Potassium permanganate. .,c4.... on. B. Sodium sulphite (common salt)................ 75 gm. 24 oz. Sulphuric acid (Ce-P. 356 ene 15¢e: Y% Oz. Water to make... i... 6s sheye ay eos oka eo bh aleenete ek Rien ane For use take equal parts. ‘The stock solutions keep excellently but not when mixed and therefore the bleaching bath should be prepared immediately before use. No particles of undissolved potassium permanganate must be al- lowed to remain in solution A, otherwise there will be spots and blemishes on the negative. —— = os, = “eo Sie ee Se ee es ee ee DEFECTS IN NEGATIVES 367 The bleaching is complete in about three or four minutes after which the brown stain of manganese oxide is removed with a 5 per cent solution of bisulphite. Then rinse and develop in a strong light in a non-staining developer such as metol-hydrochinon.* Local yellow stains on prints or negatives may be removed by super- imposing a deep yellow filter over the negative and making a positive either by contact or in the camera and from this making a new nega- tive. A panchromatic plate must of course be used and the yellow filter must be a contrast and not an orthochromatic filter. Silver Stains—The use of an old and exhausted fixing bath con- taining an excess of silver in solution produces what is termed silver stain. A silver stain may also be produced by incomplete fixation of the negative in a fresh bath. In both cases the stain is due to the incomplete removal of the light-sensitive silver halide in the fixing bath. This undissolved silver halide is at first colorless but is grad- ually changed with time and exposure to a yellow stain. Hence the necessity for thorough fixing. In the event that it is decided to try one of the methods advised for removal of silver stain it is well to first make the best possible posi- tive from the stained negative using a deep yellow filter on a panchro- matic plate as previously described under developer stains, since there are no methods of removing silver stain chemically which are entirely successful. The following method advised by Mr. J. I. Crabtree is probably as good as any: Wash the negative thoroughly to remove all traces of hypo which may be present in the film and bathe the negative in a I per cent solution of potassium cyanide. (Cyanide is a deadly poison and must be handled with care.) The cyanide will dissolve any silver thiosulphate present and some silver sulphide but in time it will begin to dissolve the silver image at which stage the negative should be removed and thoroughly washed in order to prevent reduction. Immersion in a weak solution of potas- sium permanganate followed by washing and immersion in the cyanide solution will often be found of service in dealing with a very old stain. Miscellaneous Stains.—Stain sometimes occurs when ferricyanide reducer is used. ‘To remove this stain immerse the plate in Ee ORS AE ES gS A a A Sa a 30 gr. 6 gm. RCE ORR iting Gs os alae le ab od elalarw 30 gr. 6 gm. Re oe ow eave cig bh wae s dude eae ye IO Oz. 1000 cc. 5T am indebted to Mr. J. I. Crabtree for the above formula which is re- markably efficient. Brit. J. Phot., 1921, 68, 206. De 368 PHOTOGRAPHY Printing transferred to the gelatine owing to plates having been wrapped in printed matter is almost impossible to remove. Try the following : Hydrochloric “atid tociics «03s bs + 24¢ ka oe 5 drops 5.2 cc. Waters ce Si ed cas Sessa s 2s bo a5 ea eee eer T Of. 100° CC. A yellowish-white opalescence which causes negatives to appear as if made on opal or ground glass is caused by the presence of col- loidal sulphur due to the use of an improperly compounded fixing bath containing an excess of acid or to using the fixing bath at a very high temperature. In both cases there is a precipitation of sulphur which fixes itself in the film and produces a sulphur stain. To re- move a sulphur stain first harden the film in a 5 per cent solution of formaline, wash well and immerse in a 10 per cent solution of sodium sulphite at a temperature of 100 to 110° F. This is a risky pro- cedure but is the only means of removing such stains, Blue stains are most often due to iron in some form, although amidol produces a bluish stain which may be removed by dipping the plate in a 10 per cent solution of sodium carbonate. In addition to blue stains, iron salts may produce green or yellowish-brown spots and whenever these appear it is very likely that iron in the water used for mixing solutions, or in the water used for washing, is the source of the trouble. Stains and spots due to the presence of iron are gen- erally removable by means of the bleaching solution advised for the removal of developer stain. Other methods advised are the use of a 5 per cent solution of ammonia, or a 5 per cent solution of oxalic acid. A blue-green stain apparent after fixing occurs frequently when a chrome alum fixing bath is used at a high temperature. There is no known means of removing such stains. Prevention is the only cure. Transparent Spots.—Small microscopic spots irregular in shape and sometimes almost microscopic in size are due to dust. Keep the inside of the camera free from dust and clean plate holders now and then with a rag moistened with oil. Allow sufficient time for the oil to evaporate before again using the holders and do not use too much oil in the first place. The merest trace is sufficient. Dust all plates” carefully before placing in holders. Use a camel’s hair brush and do not brush too briskly, otherwise the glass will be electrified and at- tract dust thus making matters worse instead of better. The use DEFECTS IN NEGATIVES 369 of a stiff brush will produce friction marks and only a soft camel’s hair brush should be used and this but lightly. Small transparent spots, circular in shape, are due to air in the water used for diluting the developer. Distilled, or at least boiled, water is to be preferred for all solutions but should tap water be | used, it is necessary that it be allowed to stand until all the air has escaped. This is particularly necessary when high pressure water mains form the source of supply. Excessive agitation of the de- veloper is another source. A slow, steady, rocking motion is all that is required and is much better than an occasional vigorous rock. Small, circular, transparent spots with shaded edges are due to air bells adhering to the plate during development and protecting the emulsion from development. The diffuse edge is without doubt due to the slow encroachment of the developing solution. These are very apt to occur in tank development with closed tanks. Some workers find that immersing the plates in water before filling the tank with _ developer assists in preventing such pinholes, but undoubtedly the surest way is to use only water from which excess air has been ex- — pelled by boiling and to avoid carefully any undue agitation. Spots of irregular shape and about the same size as those formed by air bells are often found distributed along one side of the plate and less rarely over the whole surface. They are caused by a stale de- veloper. A spot of bare glass which is uncovered by gelatine is one of the few defects caused by faulty manufacture of sensitive materials and is seldom met with when using a reliable brand of plates or film. Opaque or Semi-Opaque Spots.—The most common cause of small irregularly shaped black or dark spots is the presence of undissolved particles of the developing agent on the plate during development. Care should be taken to thoroughly dissolve every chemical in com- pounding developing solutions, otherwise a few particles of the de- veloping agent or alkali may be left and. these when brought in con- tact with the sensitive material in development produce dark spots owing to the greater rapidity of development at such spots. A less common cause of such spots is the presence of iron in solu- tions or in the water used for washing. In this case, however, the spots are more likely to be colored than black. Brown or purple spots may be caused by dry particles of developing agents having settled upon the dry plate. Do not mix chemicals in 370 PHOTOGRAPHY the same room in which plates are developed if possible to use an- other room. Spots such as these may be removed occasionally by using one of the methods previously advised for developer stains. Touching the spots with nitric acid is sometimes effective but is rather risky. If the worker is familiar with the use of a knife. on the film they are best removed in this manner. Yellow spots, circular in shape, are due to air bells adhering to the plate in the fixing bath. If observed when removing the plate from the fixing bath they can be removed by swabbing the plate with ab- sorbent cotton and re-fixing. If of considerable age there is no means of removal other than those given under silver stain. Miscellaneous Troubles.—Streaks and blotches, resembling finger marks, brush marks, etc., are caused by old or incorrectly compounded developer. They are most common with hydrochinon or pyro and may be overcome by using a more concentrated solution. Cloudy or wavy appearance of the negative is due to the use of in- sufficient developer to cover the plate or by not rocking the tray often enough during development. A white crystalline deposit on the surface of the dry plate indicates very imperfect washing. Wash the plate again and make a thorough job of it. Immersion in a weak acetic acid bath may assist in remov- ing such deposits. Frilling or softening of the film occurs only in very hot weather or when there is a wide variation in the temperatures of the successive baths. If it is impossible to keep the developer cool, the plate may be immersed in formaline (10 per cent solution) before development, an acid fixing bath be used and care taken to keep the temperature of all the baths on about the same level. Acetone may with advantage replace the alkali in certain developers as it does not tend to soften the gelatine. Amidol which does not require an alkali is also very satis- factory. Frilling and blisters may also be. caused by using a fixing bath that is too strong. There is no necessity for using a fixing bath containing more than 30 per cent sodium thiosulphate and at such — concentrations there is but little danger of blisters or frilling except under abnormal conditions. . Negatives which are uneven in density due to having dried more . rapidly in some places than in others may frequently be improved by bleaching and redevelopment as already described. CHAPTER XVI REDUCTION AND INTENSIFICATION Part I. REDUCTION Reduction and the Three Classes of Reducers.—The operation by which some of the metallic silver forming the image is removed so as to secure less opacity is called reduction. All reducing agents are capable of converting the metallic silver into some salt which may be immediately dissolved away. The following table shows the different types of reducers and their characteristics: Name of Type Other Names Characteristics Examples Subtractive Surface All densities reduced by | Ferricyanide-hypo, Cutting equal amounts result- | Potassium perman- ing in greater contrast | ganate, Iodine- cyanide, Belitzski’s Proportional Progressive All densities reduced in} Neitz & Huse _per- same ratio, contrast manganate, per- unaltered sulphate formula Superpropor- Flattening The percentage reduc- | Ammonium persul- tional Persulphate tion is greater in the] phate, under normal thick parts than inthe} conditions thin. Results in less contrast The first comprehensive examination of a quantitative nature on the action of various reducers on the tones of a negative was made by Huse and Neitz of the Eastman Research Laboratory in 1916.1 Sensi- tometric strips were exposed, developed and reduced under accurately controlled conditions. The strips were measured before and after reduction in a Koenig Martens photometer, ordinary H. and D. meth- ods being applied to the data. The percentage of the original density removed by reduction from each step was then plotted against the log exposure for that particular density. In this way the curves of Fig. 186 were obtained. Curve 1 represents a reducer of the superproportional type, repre- sented by ammonium persulphate ; curves II and III represent reducers 1 Brit. J. Phot., 1916, 16, 7. 371 372 PHOTOGRAPHY of the subtractive type, curve II representing one division of this class of which potassium permanganate is typical and curve III another division of this class represented by Farmer’s ferricyanide-hypo re- ducer. It will be observed that this last attacks the lower densities more strongly than does the former. Curve IV represents a formula on Seed 23 plate Percentage of original density removed i 10 2 3 4 5B 6 7 8 9 Fic. 186. Sensitometric Action of Different Reducers. (Nietz and Huse) designed by H. C. Deck for proportional reduction and Curve V a modification of Deck’s formula worked out by Huse and Neitz. Farmer’s Reducer.—A typical reducer of the subtractive type and one in extensive use is known as “ Farmer’s” from its originator, Howard Farmer, but it is also called ferricyanide-hypo reducer. It consists of potassium ferricyanide and “hypo.” When applied to the plate the silver image is oxidized by the ferricyanide and silver ferro- cyanide is formed which is in turn dissolved by the “ hypo” according to the equation _ 2K,Fe,(CN),. + 4Ag = 3K,Fe(CN), + Ag,Fe(CN ),. Since a mixture of potassium ferricyanide and “hypo” rapidly de- composes, it is necessary to either prepare the reducer immediately before using or keep two separate solutions, one containing potassium ferricyanide and the other hypo. : The first may be a ten per cent solu- tion and the latter 20 per cent. Potassium ferricyanide will keep fairly well in water, provided it is protected from light by being kept in a dark cabinet or bottle of dark green glass. To reduce, sufficient hypo solution (one part hypo to four of water) is taken to cover the negative to be reduced, to which is added a few . REDUCTION AND INTENSIFICATION 373 drops of the potassium ferricyanide stock solution so the color of the solution is pale yellow—not green. Too little ferricyanide is better than too much, since in the latter case reduction proceeds so rapidly that the negative may be reduced further than desired before the action can be stopped. Where extreme reduction is desired the strength of the reducer may be increased. If at the end of five minutes reduction has not proceeded to the desired stage a fresh solution should be ap- plied. Farmer’s is a very satisfactory and convenient reducer but should be handled very carefully, since variations in the strength of the solution influence the character of the reduction—a strong solution tends to produce greater contrasts because it affects the shadows to a greater degree. : Belitzski’s Reducer.—This reducer is based upon the action of a mixture of the double oxalate of iron and potassium and “hypo” on the silver image. The iron salt yields its oxygen to the silver which forms silver oxide in a nascent condition which is at once dissolved by the “hypo.” The reducer keeps well in a dark place and may be used over and over until exhausted. In its action on the tones of the nega- tive it resembles Farmer’s very closely. The following is the formula : PeCeMBIER PONEIC COS aIALE 4 cin cc vids ccs cs swe dace ens 150 gr. 10 gm. NINE MTB oes we Ge bigs baw te ve ewes 125 gr. 8 gm. ee oe is sens pe cs nnn s seas bse ee 7 OZ. 200 Cc. After completely dissolved add: Jealicoacid...... NE eae yo hey isc os uke ee AALS 4O gr. 2.5 gm. and shake until the solution turns green. Then pour off the clear liquid and add: EEL CG iy Sh ce sce kes ved obaesewsebesddas 134 02z. 50 gm. Instead of the potassium ferric oxalate the following may be used: Ferric chloride (crystal)..............c0ceeeeees 100 gr. 6.5 gm. POO ART ORTIALO LS cy Glen su cs ciate ses ccrcwans 190 gr. 12.5 gm. Mercury and Cyanide Reducer (Eder’s).—The. following reducer is similar to Farmer’s, but reduces more slowly, is non-staining and intensely poisonous: TEE ARIES oof 5 vey i pe le ae 8a 20 gr. 5 gm. BRIE SEIOC cy 45s Ged ed hoa bdo Mowe nace es 10 gr. 2.5 gm. BePerIO MINCHIOLICG Ss: 6.) oe ki colina ek es 10 gr. 2.5 gm. EOE ile Ly fiaigin sji'eculd cock w tins MA bres na's.9 10 oz. 1000 cc. 374 PHOTOGRAPHY Dissolve the mercury, then the iodide and lastly the cyanide which will ° dissolve the red precipitate formed. On account of its intensely poi- sonous nature this reducer should be carefully handled and labeled poison. Iodine-Cyanide Reducer.—This is rather more energetic in its ac- tion on the shadows than Farmer’s and tends to clean out the lower densities to a greater degree without seriously affecting the higher densities. It is exceedingly poisonous and should be handled with care. It is non-staining and when used weak is a very useful reducer for over-developed bromide prints. Iodine (10 per cent sol. in potassium iodide sol.)... 30. min. 6 cc. Potassium cyanide (10 per cent sol. in water)...... 5 min. 1 €¢; Water to makesiy.. sc eee bea ek ee I oz. 100 Cc. Since iodine will not dissolve in water, but is readily soluble in potas- sium iodide, it is necessary to add about 150 grains of potassium iodide to just enough water to dissolve it, then add 45 grains of iodine and make up the solution to a total volume of one fluid ounce. Permanganate Reducers.—The introduction of permanganates as reducing agents is due to Professor Namias. The permanganates are strong oxidizing agents and if a solution of potassium permanganate containing a small amount of sulphuric acid is applied to a negative the silver is oxidized, forming silver sulphate, which is sufficiently soluble in water to be dissolved. The reaction is as follows (Namias) : 5Ag, + 2KMnO, -++ 8H,SO, = sAg,SO, + K,SO, 4 2MnSO, + 8H.,0O. Permanganate is similar in its action on the tones to Farmer’s and the other reducers which we have examined, but differs from them in being more nearly proportional in its action and not having quite the same “cutting’”’ effect on the lower densities. The difference in the two classes of reducers may be seen from the examination of Fig. 186, where curve II and curve III show the percentage reduction of the different densities for permanganate and Farmer’s respectively. I.: Potassium permanganate. } i)... Fale yen eee 24 gr. 50 gm. Water. to, make iis isis osceeee eeee i i eee 1000 cc. Hl - Sulphuric acid “G2 face. os an 24 min. 50 cc. Water :to. mare, ook si08 pean 2 ee eee t OZ. 1000 Cc. For use take 1 part of A, 2 parts of B, and 64 parts of water. When sufficiently reduced immerse in a plain hypo solution, fresh acid fixing REDUCTION AND INTENSIFICATION 375 bath, or 5 per cent solution of sodium bisulphite to remove the brown stain, after which wash well. Proportional Reducers.—-Reducers which act on all parts of the negative in proportion to the amount of silver present are variously known as proportional, true scale, and progressive, from which the first has been generally accepted of late as the most rational title. While under certain conditions ammonium persulphate may form a proportional reducer its action is uncertain and not to be depended upon but by the proper combination of potassium permanganate, which is a subtractive reducer, with the ammonium persulphate which is of the superproportional type (exactly opposite to the subtractive), a proportional reducer is obtained. The following formula is the one worked out by Huse and Neitz.” SoLUTION A POressim permanranate........0.....:...% 2.8. "gt. 0.25 gm. ReewereCent SUIpndric ACid....... 6. we aes TA Oz. re ee tee IPO tO, TAKES. . ccs cs oc ese ee eles 35 Oz. 1000 cc. Ammonium persulphate.................... 34 OZ. 26° ° - ern! 0 OE ye ae a 35 Oz. 1000 CC. For use take one part of 4 to three of B. Do not mix until ready for use. The time of reduction is from one to three minutes and should be followed by a one per cent solution of potassium metabisul- phite. Application of Proportional Reducers.—In practice the chief pur- pose for which a proportional reducer is used is to reduce over dense negatives which are due to over development. Since over develop- ment increases the silver deposits proportionately the effect of reduc- tion in a truly proportionate reducer is to lower the gamma or in effect is equal to developing for a shorter length of time. In Fig. 187 curve I shows the characteristic curve of a plate de- veloped to a certain gamma. Curve II represents a gamma of unity (1). Now, if the negative represented by curve I is reduced in a proportional reducer the result will be a negative possessing the gamma of curve IJ. A proportional reducer is therefore the only type which alters density without affecting gradation. It is thus the only reducer 2 Proportional reducers. Communication 39, Research Laboratory of East- man Kodak Co. British Journal of Photography, Oct. 27, 1916; Australasian Photo-Review, Dec. 1916. 376 PHOT@OGRAP EY which may be employed without falsifying to a certain extent the original gradation of the negative. Superproportional Reducers.—Superproportional reducers are necessary when it is desired to reduce the contrast of a negative in order to make it suitable for a particular printing medium. There is Density 2 Exposure steps Fic. 187. Action of a Proportional Reducer on the Plate Curve (Nietz and Huse) only one reducer having a definite superproportional action and that is ammonium persulphate. This must be used in an acid solution and is rather erratic in action, sometimes acting properly and at other times not. Much of its irregularity is due to the presence of small amounts of other substances, hence in purchasing one should secure only the C.P. salt and this should be kept in airtight containers as it decom- poses in contact with air. Theories of Superproportional Action.—Owing to its peculiar prop- erty of attacking the higher densities before the lower and to its erratic behavior, the chemical reaction of the persulphates with the silver image has been the subject of much speculation, but research has not yet been able to explain satisfactorily the reason for its unique property of superproportional action. | A. and L. Lumiére, to whom the introduction of persulphate as a reducer is due, developed the following theory of its reaction: * The action is regarded as proceeding from the back of the negative to the surface in exactly the reverse method as all other operations progress, thus the lower densities which lie nearer to the surface are the last to 8 Bull. Soc. franc. Phot., 1898, p. 395; Ibid., 1890, p. 226; Ibid., 1809, p. 390. REDUCTION AND INTENSIFICATION 377 be attacked.* Helain® and Lauder,’ however, proved that reduction does not take place from the back of the film by exposing plates through the glass and secured the same result, while microscopical in- vestigation by Pigg * and by Scheffer ® shows that the action is uniform on all of the grains of the film and not from the back to front as stated in the Lumiére theory. In 1906 Luppo-Cramer advanced what is known as the dispersoid theory.® In this the behavior of persulphate is supposed to be due to the fact that the silver deposit is not metallic silver, as commonly sup- posed, but a mixture of silver and silver bromide, there being more of the latter in the lower densities. The superproportional action is ex- plained by saying that metallic silver is more soluble in persulphate than silver bromide—a known fact. The action of certain substances which are solvents of silver bromide and render the action propor- tional is explained by saying that the solvent removes the halide so that it can be more readily attacked by the persulphate. The catalytic theory was developed by Schuller 1° and Stenger and _ Heller carried on a long series of experiments to prove it.‘ This theory declares that the cause of the superproportional action of per- sulphate is due to the catalytic effect of the silver ions formed during the reaction of the silver and the persulphate. Since the concentration of these ions increases more rapidly in the higher densities than in the lower the action is greater on the former. Further research will be required, however, to definitely explain the theory of persulphate re- duction. The Practice of Persulphate Reduction.—While reduction with per- sulphate cannot be said to be an absolutely safe and certain process 4 Resume Travaux Scientifiques, pp. 215, 216, 218; Brit. J. Phot., 1898 (45), D. 473. 5 Theory of Persulphate Reduction, Helain, Bull. Soc. franc. Phot., 1898, 15, 220. . 6 Persulphate of Ammonia, H. S. Lauder, Brit. J. Phot., 1890, 46, 725. 7“ Action of Ammonium Persulphate on the Photographic Image,” J. I. Pigg, Brit. J. Phot., 1903, p. 706. 8 “ Microscopical Researches on the Effect of Persulphate and Ferricyanide Reducers,’ Scheffer, Brit. J. Phot., 1906, 53, 964. 9“ Absorption Complexes in the Silver Grain as the Cause of the Persulphate Effect,” Phot. Korr., 1908, 45, 159. 10“ The Theory and Practice of Reduction,” A. Schuller, Phot. Rund., 1910, 24, 113. 11 Z, f. Reproductions technik, 1910, 12, 162, 178 and IgII, 13, 5, 20, 34, 50, 70, 84, 100; Zeit. wiss. Phot., 1911, 9, 73, 380. | Aad wy 378 PHOTOGRAPHY : even with the best of care, yet by the proper observance of several im- portant points serious irregularities in its action will be rare. Only the purest persulphate should be used. Much of the commercial per- sulphate contains traces of iron and as Sheppard has pointed out this has a catalytic action.1* The amount of iron necessary to affect its action is on the order of I part in 1,000,000 and the limit of tolerance permissible is about 2 parts to 1000 of Solid persulphate. A small amount of iron is not a disadvantage but it is essential that the limits are not overstepped and also that the chemical be uniform, or the varying iron content of different samples of persulphate will lead to error. The presence of soluble chlorides, bromides, sulphates, and nitrates in the water used for mixing is also a source of trouble and many of the difficulties would disappear if the precaution of using distilled water was adopted. Since the characteristic action of per- sulphate is vitally affected by the concentration of acid present, a certain amount of sulphuric acid is generally added. With distilled water the required acidity is secured by the addition of about one drop C.P. sulphuric acid to each ounce of solution when freshly — mixed. Stock solutions of persulphate are not advisable. The plate should be placed in the following solution which should be made up just before use and distilled water only should be used: Ammonium persulphate............ i ee 4 gr. 8.3 gm. Sulphuric» acid -C; Pits.) s 2k Gene I min. Pi Oe Water to. make. 26. 5.h.0) sb eeewe ae eee I Oz. 1000 cc. The action should be watched very carefully for it becomes more rapid with time and the negative may be reduced further than desired before the action can be stopped. ‘Therefore it is better to remove the negative from the solution just before the reduction has reached the desired stage, preferably using a plate lifter to avoid contamina- tion with the fingers, and place in a five per cent solution of sodium sulphite. While refixing is not necessary it is to be advised, since it leaves the film amenable to further treatment. Part II, INTENSIFICATION The function of intensifiers is to increase the density and contrast of a negative so as to obtain better printing quality. Intensification may be necessary for several reasons. The negative may be simply 12 Brit. J. Phot., 1918, 65, 314. Phot. J, America, 1918, 55, 299. he ae ee a, ee Te : REDUCTION AND INTENSIFICATION 379 under developed due to an error in the composition of the developer, or the time, or temperature of the same and in such cases the intensi- fier continues the action of the developer, building the negative up to a higher degree of contrast. Owing to over exposure, or lack of con- trast in the subject, the negative may lack the necessary contrast and intensification may be desirable to supply this deficiency. Intensification may be effected in several ways. The first and most common method consists in altering the metallic silver grains by treatment with substances which will unite with silver to produce greater opacity. The second method consists in altering the color of the deposit so that it is less actinic and offers greater resistance to the passage of chemically active light than the original deposit. The third method is similar to that formerly necessary in the wet process for building up sufficient density and consists in adding a new film of silver to the old, the increased amount of silver increasing the _ density. Intensification with Mercury.—After thorough fixing and washing, bleach the negative in: PUMPER PGT OTING si vei ye tice een esse beavcees I Oz. 62 gm. RU PS ef ale wc psc sees aseece vas 16 oz. 1000 cc. After cooling add hydrochloric acid............... 30 min. Ace: When the negative is completely bleached through to the back of the plate remove and wash well in running water; if possible for at least twenty minutes or by giving ten five-minute soakings if washed inatray. It is then blackened in one of the following: A. Sodium sulphite 10 per cent solution B. An ordinary developer as Amidol, Hydrochinon, Ortol, Glycin, Metol-Hydrochinon, etc. Pr eodiine suipnattimoniate...... 6. ce. eee es 200 gr. 20 gm. (Schlippe’s salt) OE Ee a ee eae ee 20 oz. 1000 sc. DTA OG) 5 an x oosin cee wcsiy vin ose'e a ae 20 min. 20 ~=min. a eg <6 cco vk oh dars vin = woes = I oz. 30. eA CG E. The following ferrous oxalate developer : A. Potass. oxalate (neutral)........... 5 oz. Beg ~ x try, REEUUAL CTs fo Fas sas Vitdaebrs vo bd es 20 Oz 1000 = cc When cool pour off clear liquid for use, MPIOICE OL IT OM.., esac swe cc at 5 oz. 250 gm. Pamunric acid .C.P iss owe. Te cea 30 min. sivo"ce, Reet ALTN Fase oe tint nwv aie ale'sissia 20 OZ. 1000 = cc 380 PHOTOGRAPHY For use take one part of B to three of A. Pour B into A and not vice versa. | The chemical reaction which takes place when the silver image is leached in mercuric chloride is represented by the following equa- tion: 2HgCl, + 2Ag — Hg,Cl, + 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: Hg,Cl, + NaSO, + H,O — 2Hg + Na,SO, + 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: He,Cl, + 2NH, — NH,Hg,Cl + NH,Cl, 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 AgHg, and consequently greater intensifica- tion. The reaction would therefore be as follows: Hg,Cl, + 2AgCl + 4FeC,O, + 2K,C,O, = 2Ag + 2Hg + 2Fe,(C,0,), + 4KCL* 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, J. S. C. I., 1893 vol. XII, p. 983. REDUCTION AND INTENSIFICATION 381 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 chloride as above and blackened in the following solution of potassium cyanide and silver: Pree OOTARSI CVATIOCS, - nn dses cc ceciccseseaeccce IO er. 23 gm. Ree Re as aia cee coc peseuvn cs eces 10 er. 23 gm. ee og cca kent isessacsnceeience t -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: Hg,Cl, + 2AgK(CN), > 2Ag + 2He(CN), + 2KCl. 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. ET PECMMIPEC TT CNIOTIO€S (6.55. c cece st cens sec eeanon 175 gr. 40 gm. eM ae gcc Gewa edn eot ewe es 10 Oz. 1000 cc. Pe PORATION. 5c wirs scleine ens es ced sens eee ees I Oz. 100 gm. ee a op holed an obs IO 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- 26 382 PHOTOGRAPHY ment in a non-staining developer such as amidol or metol-hydrochinon will render the negative less liable to ye!low in time. The chemistry of the reaction is as follows: ** 2Hgl, + 2Ag = Hg,I, + 2Agl, Hg,I, + 2(Na,SO,) = Hg + Hgl, - (Na,SQO,).. Silver Intensifiers—The following formula and method for silver intensification is that of J. B. B. Wellington and is the revised formula published in 1911. The film should first be hardened in the following bath: Formaline oo... ..cess+e0ecats i evnesn cute oon nn I part Water 22... ceca esd seo ue vere ce sce eq tm ee sb itt nana 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...............0eceeececeeees 20 gr. 2.3 gm. Potassium ‘bromide... ..4.4..2is.-ae0 05 san ee 20 gr. 2.3 gm. Water to makes... is. 26 ss eosin ned bensa eee 20 02. 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: titrate. 0. ssa > oan eas se ee 800 gr. 91.2 gm. Distilled water to imakesvc. 035.4 5ee eee 20 Oz. 1 liter B. Ammonium sulphocyanide.................. 1400 gr.. 160 gm. ELyp6 8. a ayes we © a eee Oe 1400 gr. (160 gm. Water to:makei¢. t5.0050 bo i 20 Oz. I liter (Both solutions keep well.) For use take an ounce of 4 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 Io per cent solution of ammonia. 14 Seyewetz, Le Negatif en Photographie. REDUCTION AND INTENSIFICATION 383 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 C. Welborne Piper and D. J. Carnegie.1* 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 CrO,. 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 not 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: Pee Reet PICU FOMALC. 5... ss ses verde de seen {-02. 50 gm. io se chy eek ne eon ne een e's 20 OZ. 1000 cc. Wemeerrecuioeie Berd CPs... cee ee cee rh, Mle-Oz. 100 Cc. BERIT C sie, oe. owns icp eb ences tegereres IO Oz. 1000 CC. Baths ready for use. A B C Degree of intensification De OO ON eee ie Ss 4 Oz. 8 oz. 8 oz. A—Maximum Pe SOEIT os Picea secs Sidr; 20z. 8oz. $B—Medium PE re es ea ee 16 oz. 10 Oz. 4 02. 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. V ROPE aR at 384 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 alkall. 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(UO,);[Fe(CN,) ], > 2Ag,Fe(CN), + 3(UO,),[Fe(CN,) Jo. 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 the density and con- trast of the negative. 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, Uraniumenittate.. 2ewees cess ere se 100 gr. 25 gm. Water to. make ..:9-5 tag he ea ee 10 oz. 1000 cc. S.. Potassium . ferricyanide...) uta ee eee 100 gr. 25 gm. Water, £0 ‘make... x0. <.9.0 re be recone eee IO 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,!® 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 I0 per cent solution of ammonium sulphocyanide or with Potassium citrate.2%..03..0-00u0 ue ee 5 gm. 38 er. Sodium sulphate o.oo ites eee oes 25 gm. 192 gr. Water to make. 2x. +. oc. esun 4 toe eae 1000 cc. 16 oz. If for any reason it should be desirable to remove the intensification 16 Phot. Ind., 1924, p. 234. ee a ee a eee wee ~ : | . | P ! . | REDUCTION AND INTENSIFICATION 385 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 : 2K,Fe,(CN),.-+ 4Ag + 6Pb(NO,), == Ag,Fe(CN), + 3Pb,Fe(CN), + 12K NO,. The following formula is recommended : RN Ne icc np ile ats os eee cath an 400 gr. 46 gm. MEME RET LICVATIOG, (wi. bs sc cc open sss ose ences 600 gr. 70 gm. ee ee a cease tke sv tenses 3 dr. 20 cc. Rae Pe ee ia ss cans save n one eo wreeews 20 02. 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 ee ETN ICG goin os ou cs ala ve de ois 0dbln ars mb dle’ ROA I Oz. 50 gm. RET INNO Ee Yip on Sd do ack om s vided ws e'ae nis ww bie eck 20 Oz. 1000 Cc. Intensification with Copper.—The copper intensifier is also only suited to line subjects. The reaction is as follows: CuSO, + 2KBr —> CuBr, + K,SO,, CuBr, + Ag— CuBr + AgBr. Applying AgNO,, CuBr + 2AgNO, — Cu(NO,), + Ag. Br. The following is a reliable formula: RIGOR IBLE 6 of sie 50's,0;0-4'4' + aie 5g 2 nwt lee be edie 100 gr. 230 gm. ey AE 6S nh bg coo is cov bole nee how das a cee I Oz. 1000 cc. Mremertmonitie OTOMHGE. vk. osc ce ech ave do deca ke 100 gr. 230 gm. BUMPER. a5 ees oa as a dete Rs Gee wa ed I Oz. 1000 cc. A and B are both dissolved in hot water. For use they are mixed 386 PHOTOGRAPHY and the negative bleached therein after which it is washed for a minute or two and blackened in Silver nitrate;s i. 2c... me PE Ee ee 45 gr. 100 gm. Water: (distilled) . oo .s.. ea poten ones ae I Oz. 1000 cc. 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. Clere in 1912,*" and more fully by Neitz and Huse.*® 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. REDUCTION AND INTENSIFICATION 387 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 : I. 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. 188 the percentage increase in density is plotted as the aphic Density Lae) S % Increase in Photogr 0 : A 6 8 1.0 12 Original Density Fig. 188. Sensitometry of Photographic Intensification. (Nietz and Huse) I. Mercuric chloride+ ammonia. VI. Chromium-+ amidol. VII. Mercuric bromide + amidol. X. Mercuric iodide + paramidopheno!l. 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- os 388 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. photographic gamma of intensified plate y ip Thus a7” RES (at Varna TRY PMC SE Say oe photographic gamma of original plate yy 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 Neitz: y ip Intensifier Blackener y op Mercuric : chloride... s0s0 cc vs ane ses 4k oe ammonia 1.15 ‘Potassium bichromate and HCl... s,s. s1e.5 sae amidol 1.45 Mercuric bromide........... ER Pere te PER amidol 1.15 Potassium ferricyanide and potassium bromide....... sodium sulphide 1.33 Cupric chiloridé..c ceo 0 nat os es nae eee ee sodium stannite 1.03 Permanganate and: HICl. 2... s.8 . na ee eee sodium stannite 2.05 Mercuric. iodide. <: 04 .< 0% ca vite «awk cults ee Schlippe’s salt — 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 eS : : : nee a ee eee. wee ee an Ee ale ee REDUCTION AND INTENSIFICATION 389 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. 189. 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. Fic. 189. 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 paraffine wax). GENERAL REFERENCE WORKS BENNETT—Intensification and Reduction, CHAPTER XVII PRINTING PROCESSES WITH SILVER SALTS I. PRINTING ON BROMIDE AND GASLIGHT PAPER ~ Development Papers.——The majority of present-day prints are made upon one of the many varieties of development papers. It is the process in general use at present and has practically pushed all others into the background except for particular purposes. The pictorialist may use one of the control processes, as gum-bichromate or oil, carbon or platinum, but the average amateur and professional uses develop- ment papers exclusively. Perhaps out of every thousand prints, only one is made by any other process. The reasons for the widespread use of development papers are not difficult to see. Since the process can be made more mechanical than any other, it is more economical and efficient for the production of prints in quantity. Owing to its speed, artificial light is used for exposing and we are, therefore, able to print at night or under any conditions of weather without being af- fected thereby. Commercial papers require no sensitizing or other operations prior to exposure, so that many of the unpleasant features and complexities of other processes are avoided. No other process is so well adapted to negatives of varying quality. Most papers are made in a wide variety of surfaces and in at least two colors, white and cream, and in at least two degrees of contrast, while generally three grades called hard, medium, and soft are supplied. There are two general divisions of development paper, bromide and chloro-bromide, commonly called “ gaslight.” The former is a fast emulsion similar to that of an ordinary dry plate, while the latter is a slower emulsion more suited to contact printing and having chloride of silver as a constituent. Characteristics of Development Papers.—The sensitometric char- acteristics of development papers were first systematically investigated by Mees, Jones, and Nutting of the Eastman Research Laboratories.* The methods of investigation are in general the same as in the sensi- tometry of photographic plates. Sheets of the paper to be examined 1“ Sensitometry of Photographic Papers,” Communication No, 21, Phot. J. (New Series) (19014), 54, 342. 390 a a le Ss eS 391 PRINTING PROCESSES WITH SILVER SALTS DAIJLSIN 9Y} 0} Jadeg Sulyulig ay} JO seIg 9y} Suljdepy ‘eo! ‘oI dq BIVOS“ONOT JO TIVOS |} ____—__——-aurzrom 40 FIVOS OL GaLdvdv Badvd HIVOG THOHS dO ZAUNO SAI LVOSN TIVOS eTHOHS Odd LNIUd NO TICVAIVLEO IVED ISHONOULS Badyd 1X MHA) OVI ye 392 PHOTOGRAPHY are exposed under either a sector wheel or a graduated wedge, de- veloped and measured in the photometer. The result when plotted in the usual manner gives the characteristic curve of the paper, which is in general form similar to the ordinary plate curve and shows practi- cally all of the valuable characteristics of the paper. The constants which serve to give a general idea of the quality of a paper are maxi- mum black, contrast and rendering power. These we will consider briefly, referring the reader for further information to the original paper. ; Maximum Black.—The maximum black is similar to the greatest density in the case of plates and refers to the deepest deposit which the paper will give. Papers which reflect above 8 per cent of the incident light are visibly gray-black. Papers reflecting from 6 to 3 per cent of the incident light have strong rich blacks while papers which reflect less than 3 per cent have very intense blacks.2 Other things being equal the strongest blacks are given by glossy papers since the matt surface always involves scattered light. The softer the paper the weaker the maximum black. | Contrast.—As in the case of plate sensitometry the contrast of a paper is measured by the slope of the straight line portion and is desig- nated as “gamma.” However, in the case of papers the velocity of development is high and development is, or should be, always carried to the maximum (gamma infinity). The contrast of a paper, there- fore, refers to the value of its maximum gamma. Total Scale-——Another matter also having to do with contrast is what is known as “ total scale ”’ which refers to the number of separate tones which the paper can render. It is quite possible for two papers to have the same total scale but different values of gamma. Rendering Power.—By this term is meant the capacity of a paper to reproduce a series of exposures by a series of densities having propor- tional values the same as the exposure scale. Thus a paper having perfect rendering power when used within its total scale would give densities proportional to the exposure. As in plate sensitometry this condition is realized only within the straight line portion of the curve and the length of the straight line portion measured along the exposure axis is called the “latitude” of the paper. When gamma equals 1.0, then exact reproduction will be obtained within the total scale while at other values of gamma proportional reproduction will be obtained. 2It is questionable if any paper reflects less than 3 per cent of the incident light. eee ollie 393 Pe ainG PROCESSES WITH SILVER SALTS DATIVSIN 9Y} 0} Jodeg surjulig sy} JO seas ayy sundepy “qo6l ‘dI] GAILYOUN 40 ATVOS TyLol— Uadvd AO ATVOS AIAVTIVAY: SENOL TICCIN tei i=) Q a oad Q a a ol Zz a) es) i= A ua, dvd LTH HOVTE 394 PHOTOGRAPHY Hard and Soft Papers.—In order to secure the best results from all classes of negatives, development papers are made in several grades, ranging from very contrasty to extreme soft. The range of contrasts available with bromide papers is somewhat limited as, generally speak- ing, only two grades, Soft and Contrasty, are made. With chloro- bromide or gaslight papers the range is wider and practically every paper of this type is supplied in at least three grades, Hard, Medium and Soft, while one or two have four grades. Now, on the relation between the scale of tones of the negative and the scale of tones of the printing medium depends faithful reproduction of the tones of the subject photographed. As we have said before, the negative is only a halfway step. It is the print for which we work, and it is this in which we wish to reproduce as accurately as possible the original subject. Therefore, since the relation between the scale of the nega- tive and the scale of the printing medium determines whether or not the print is to faithfully reproduce the tones of the subject, this rela- tion is a matter of importance to which we must give our attention. In Fig. 190, photograph a and its accompanying graph illustrate the condition resulting from the use of a long scale paper on a nega- tive 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 highlight 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 appearance lacking in contrast and vigor. In the same figure, b and its accompanying graph represent the 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. 191. 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 395 PRINTING PROCESSES WITH SILVER SALTS DAIWBBIN 2} 0} Jodeg Bunursg oy} Jo sess oy} Sundepy ‘161 ‘OI SATLVOAN dO FIVOS MTG J0 STVOS PIV TIVAV XOVIG asasmad 396 PHOTOGRAPHY 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 ¢ 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 ts lacking in con- trast, use a Hard, Contrast or Vigorous paper. 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 . PemiittNG PROCESSES WITH SILVER SALTS _ 397. lig. 192. Printing Machines for Amateur and for Professional Use > Lee ne ere me ae Woe) Wea | 398 PHOTOGRAPHY 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. 192. 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 * have been devised for 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 formule 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. 8 As, for instance, the Sanger-Shepherd Density Meter or Dawson’s Densi- tometer. | PRINTING PROCESSES WITH SILVER SALTS — 399 STANDARD Metot-HyprRocHINON DEVELOPER |) er a Bt. Pr ON) taaitas-ws. ~ es rete r, 75 gm. IMEI EUILO NC OTY ) icc obs. ca vceee ce tees oz. 13.824 fm: REVELL ATA Wilicsts bovis a Gd Gos ppd vce ess 60- opr. A A ei. reat POnate (CTY)... seeds ancdvcrv ness 14 Oz. 12.5 gm. Re is gE Gob tine vn Sus oe Ah OZ. 10003 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: A B G LT a ee 2.5 gm. 45 gr. LaWen sees Sodium sulphite (dry) 18.0 gm. 34 oz. 18.0 gm. % oz. Hydrochinon........ ae aoe: 10.0 gm. 180 gr. eae Ay ae Sod. carbonate (dry) . ga oe pies ee en 36.0 gm. 1% oz. js 1 ae eee 500.0 cc. 200z. | 500.0 cc. 2002. | 500.0 cc. 2002. A B Water 3 parts | I part I part 7 parts I part I part I part 3 parts I part 3 parts | 3 parts | 5 parts A PMormanceveroper. 2... .: ss. eee DoGmrreaerOeVOIODEr. 2.6 ee 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 AmiIDoL DEVELOPER FOR BROMIDE PAPER SET SES ag 325 gr. 20 gm. Pe ANUUOPNENO!) ... 2. ce ce ace ce ew nnn 50 gr. a heii, IEG G . 4c ne sca vas Save seadeecece 10 gr. 0.6 EMU, icc vs doc cvs ces cs tek neesas 20 02. 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. 400 PHOTOGRAPHY 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 0. 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.—Since practically all developing papers contain solu- ble bromide in the emulsion and bromide is absolutely necessary in the developer, there is an alteration in the characteristic curve with the time of development. Not only does the value of the inertia vary with development, but also the shape and character of the curve. Owing to the high velocity of development with papers, the maximum gamma of — development is reached very quickly, in the case of some papers within ten to twenty seconds, so that in practice developing papers are always developed to infinity. Owing to the fact that the maximum 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. 193 taken from the paper by Mees, Nut- ting and Jones of the Eastman Research Laboratory.* It will be observed that there is a vast difference between curves A and E. The latter is seriously distorted and the straight line por- ‘tion 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 irregularity. The golden rule in developing both gaslight and bromide papers is then: Develop to finality or as far 4“ The Sensitometry of Photographic Papers,” Communication No. 21, East- man Research Laboratory, Abridgments, vol. I, p. 68, Phot. J., 1914, 54, 342. . PRINTING PROCESSES WITH SILVER SALTS 401 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 [CSS a Bree ie) aN eer PT TT broke LY meals ah fe ae Ps =D OENSITY eel aeie =|) BEE & 6 li 16 18 20 2e 24 2 Fic. 193. Effect of Time of Development upon the ene Curve of Paper Emulsions bromide and professional chloro-bromide papers and by simple time methods in the case of the rapidly developing gaslight papers made for the use of the amateur. As in the case of plate development the fac- torial method takes care of the variation in temperature of the devel- oper, and it also affords an accurate indication of the rate of develop- ment. Since it is customary to develop several prints, one after an- other, in the same volume of developer which thus becomes weak- ened by use the time of development 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 ob- tained. Still another point in its favor is that it makes correct ex- posure absolutely necéssary as a print which is over exposed will be too dark when developed by the factorial system, while in develop- ment by inspection, the print would be removed from the developer when the proper depth had been reached, thus resulting in under de- velopment and loss of print quality. The Proper Factor.—The proper factor seems to be entirely a 402 PHOT OGRAPELY. matter of the developer and does not seem to be influenced particu- larly by the paper used. Thus, Kodak, Wellington, Barnet, and Illingsworth 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 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 = factor. Time of appearance The following is taken from tests conducted with the Eastman Amidol formula. Print No. I II III IV Vv Exposure (seconds) :s..ic02ly saicas A ie Seo eae II 15 18 27 38 Time of appearance (seconds)............... 15 14 14 13 9 Time of developments! 0 }> 409 2a a eee 300 | 210 | 168 | 104 54 Factor (nearest ))\ci4 ious 2 aes ee 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. Pe a q 4 PRINTING PROCESSES WITH SILVER SALTS 403 The Short Stop.—While prints may be rinsed in water immediately 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 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: REET Me eG ull kak vo Ps he paws eh vee vuleses 64 Oz. 1000 CC. rere cia 2h per Cent. (OOMM.) oi. cee. cece ce ees 4 OZ. 62.5) £0. 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- 404 PHOTOGRAPHY ing bath clean. There is more danger of overworking the fixing bath with prints than with negatives, since in the iatter 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 the number of prints fixed in a given volume of solution, in order KQ Fig. 194. Electrically Operated Print Washer. (Pako) 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- 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: Y Hypo? ey ee Sa as 16 oz. 250 gm. Waater. to makes. 6.58 vee eps wei eae 64 oz. 1000 cc. Dissolve and then add the following hardening solution, which may be made up in stock solution: Sodium sulphite dry... >. .. a ie [ c Ge eee eee ee PROJECTION PRINTING 431 Attention may perhaps be usefully called to the fact that with cer- tain lenses in which the front component has a strong condensing ‘6 action, so that the “inconstancy of aperture” (Chapter II, page 79) Fic. 212. Loss of Covering Power Owing to the Use of Short Focus Projecting Lens with Condensers. (Candy) 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 : ee 2a klsy sang oo ce da eV Vales 407 gr. 53 gm. RE RMEAMM tel Fo5 2c Kh, ade a SY aba He Vad 407 er. 53 gm. a Ni cg einige w Agia sow aig cig ¥ wih we ME 10%, 65 cc. Pee Ome) AGM se ee Pet ee we Che 8 er. I gm. Pc thy ec a) ea i a 16 oz. 1000 Cc. Any grade of cooking gelatine may be used. It is first soaked for 432 PHOTOGRAPHY Y% hour in about 34 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 50° 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 I oz. to each 100 square inches). The mixture sets in about 24 hours and the easel is then ready for use.* Perhaps an even more convenient method consists in using a large printing frame. Fig. 213 shows a commercial easel designed for use 3 Fic. 213. Ingento Enlarging Easel for Use with Printing Frame with printing frames and provided with guides in order that the frame 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,” ? illustrated in Fig. 214, 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. 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 weights sufficiently long to keep the sheet of paper flat during ex- 1 Amer. Phot., 1923, p. 717. } 2 Made by Westminster Photographic Exchange, Ltd., 61 Piccadilly, London, W. C., England. PROJECTION PRINTING 433 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 Fic. 214. Westminister Enlarging Easel 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.’ Seven years later Chapman Jones * investigated the scattering of light by the photographic plate and in 1909 Andre Callier in a paper before Puoss G. £., 1ngt, 10, 98: 4 Phot. J., 1808, p. 102. 434 PHOTOGRAPHY the Royal Photographic Society of Great Britain® 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. 215). The ray SN coming from the. Fic. 215. Scatter of Light by Negatives. (Callier) light source S is scattered in passing through the negative NV, 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 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 5 Phot. J., 1909, 49, 200; Zeit. wiss. Phot., 1909, 7, 257. PROJECTION PRINTING 435 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. 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 68). The following table, however, will show the conjugate distances for all ordinary de- grees of enlargement and for lenses of the usual focal lengths. By “degree of enlargement” is meant linear enlargement. Thus from 4 x 5 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 427 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, ges Bae, ae s = 436 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 | 5 Inches | 6 Inches | 7 Inches | 8 Inches 4 6 8 10 I2 14 i 2s cA Wh tae 4 3 2% 24% 27/5 244 2*/; 24 5 71% 10 12% | @I5 17% 20 22% PLS Spee b Selig ear 5 334 3% 3k 4 2/19 26/7 2/16 6 9 12 15 18 21 24 27 aR aap 6 4% 4 334 33/s 3% 35/7 3% “4 10% 4 17% 21 24% | 28 31% a Aa ee 7 5% 4% 434 41/5 4" ie 4 3°/10 8 12 16 20 24 28 3 3 6 Pee 8 6 5% 5 44/s 4% 44); 4% 9 13% 18 22% 27 31% 3 404 APS: Soto 9 6% 6 53/5 57/s 5% 51/1 5/16 10 15 20 5 30 30 4 45 Rios eras 10 714 6% 64 6 5°/s 55/2 5% II 164% | 22 27% 3 3844 | 44 49% 546... uae II 84 74% 64/s 61% 6/12 62/7 63/16 ra 18 24 3 42 48 54 O35 een: 12 9 8 7% 7/5 7 65/7 634 14 21 28 35 42 49 56 63 0 Nn ee 14 10% 9% 834 82/; 81/6 8 7% 16 24 3 40 48 56 64 a2 8) A ee 16 12 1024 | 10 93/5 9% 91/7 9 18 2 36 45 2 81 Renee ee 18 13% 12 1% 104/5 10% 10?/7 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 30-714. 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 437 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 431 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 projection 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 436. 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. All that is necessary to provide any enlarging lantern with an accurate focusing scale, by this method, is the precise determina- 6 Brit. J. Phot., 1924, 71, 171. 438 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+-(F/4). Eliminating F which occurs in both, the distance 3 to 4 is equal to %4—%, or 1% 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 2-14, Distance 3-114 = distance 1%4-1, Distance 6-8 distance 6-4. Accordingly the scale of positions on the baseboard of the camera will appear as in Fig. 216. Projection printing with a lantern thus fitted Fic. 216. 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 ‘* and Rev. F. C. Lambert * have described methods 7 King, Brit. J. Phot., 1906, 53, 188. 8 Lambert, Amat. Phot., 1921, p. 161. PROJECTION PRINTING 439 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 Ica 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. ...s ake suas ee 14 oz. 125 gm. Warm water to make..5.4. 02.02. ob: ae 4.0% 400 cc. Glycin ..... were t te ere ee tome 100 gm. Mix well and add gradually: Potassium carbonate... [alc cw.ctame vc one ste So Ge, 500 gm. Water to make... .... 05. 74 OZ. 750 cc. 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 Cea A. Hydrochinoti: 0.02. <4 vas +e eee eee ist 10 gm. Sodium euiphite: (dry) sia 22 ey ee 154 gr. 20 gm. Potassium: ferrocyanide;’,). 24s. 25a ee 922 gr. 120 gm. Water: to “make. « ox anapeee as: ae 16.07; © 100g sae B. Caustic sodas. iv cays naan «eecn eae ee 384 er. 50 gm. Water: to “makes... oc 05S 2 As Po eee 16 oz. 1000 cc. LANTERN SLIDES AND TRANSPARENCIES 451 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 305. 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 ne 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. 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 °4 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 0, should be employed; for the slower gaslight plates the Series 00, a bright-yellow, is safe. 452 PHOTOGRAPHY Fixing, Washing, Drying.—After development, rinse well in run- ning water and place in an acid fixing bath, the one 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 trans- parency 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, 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 tt should appear on the LANTERN SLIDES AND TRANSPARENCIES 453 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 3% 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 Monkhoven’s intensifier to 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. 454 PHOTOGRAPHY 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 Standard Slow, Paget Gravura, Welling- ton 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: 2 A, Metol-o, cscs es cigs: ation » pupae state, geet eee an 20 gr. 2. ‘gm: Hydrochinon: . $3 sc5s saeco oe 60 gr. 6 gm. Sodium sulphite {dry} .... 2: <3. 00. sues 3) 3gSOCats 35 gm. Sodium carbonate (dry)....55 70, es eee 350 er. 35 gm. Potassium. bromide.:. 4-4 (s405 sense 20 gr. 0.6 gm. Water to. makes 0. 2e.s as a 20 OZ. 1000 cc. B. Ammonium carbonate. 3145. ..5' ee eee I OZ. 100 «gm. Ammontum -bromidé..).5..... 4. sees eee eee { OZ. 100 gm. Water’ to ‘maker. oii... ae ee 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: Color Exposure x normal Developer Black 0 Fe i oe eee ee Normal At part, B part Warm-Black 9..0.4'vac vas Gee eee eee Normal x 1% A I part, B & part Browns. 5 eo) oA vial acelin eee ae Normal x 2 A 1 part, B &%& part VY AY TSO DIS © x5. sacs tooo wilco Normal x 3 A 1 part, B % part PUTO ious wecdinvaby . hoe Normal x 4 A I part, B &% part REG ae en oan bord. ee ee Normal x 6 A I 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. LANTERN SLIDES AND TRANSPARENCIES 455 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: RE eh, cles Vy es Pek e eden we eens go gr. 5 gm. Ce tO) Ey ee SE oe Se ea ae eae Qo gr. 5 gm. Oy ESS EE | Re eae Riper Ze eC. MER OR RO se ak ilies cys var sev aseececs 20 OZ. 500 cc. ECR Sc hc he xy a Ws ss cee seas ce te I Oz. IO gm. CS LS See a IO 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 ali 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 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- 456 PHOTOGRAPHY 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 the 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. So far, no definite information on the process can be given other than that which has been gained by methods of trial and error. Experi- ence alone can enable the worker to master the process. References to published work on the subject will be found in the bibliography at the end of the chapter. The Toning of Lantern Slides aiid: 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 slides 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. 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: LANTERN SLIDES AND TRANSPARENCIES 457 a he RA a 88 gr. 8.8 gm. RO SOU Py os ease sate pa ve kee eles TDZ, Soe. Srttie acti... 3. Oo SHAPIRO Ee Varn OR ae ea 176 gr. 17.6 gm. a aD GY uns Dinly hig dle-G.8 ovale e 6 20 OZ. 1000 cc. (ENCE Sa I Oz. 50 gm. Be eR PTE oo j 5.m osha nin wed emi he aoe ee 20 OZ. 1000 CC. For use take A, 24 parts; B, 1-2 parts; distilled water, 24 parts. The intensifying solution must be prepared fresh for each slide. The dry slide is immersed in the intensifier for one to one and a half minutes until the required degree of intensification is secured. If it is allowed to act longer than a minute and a half it begins to work un- evenly and produces a blue deposit. Stains on trays, fingers, etc., from the intensifying solution may be removed by acidified perman- ganate solution or strong hypo and ferricyanide. When intensifica- tion is complete the plate is washed in running water for one minute, then immersed in an acid fixing bath for five minutes and finally washed in running water for one hour. GENERAL REFERENCE WorKS FrRAPRIE—How to Make Lantern Slides. Harris—Practical Slide Making. LAMBERT—Lantern Slide Making. Mercator—Die Diapositiverfahren. CHAPTER THE TONING OF DEVELOPED SILVER IMAGES Introduction.—For many subjects a color other than that of the cold neutral black of the ordinary developing papers is desirable and, when the color is selected properly with reference to the nature of the subject, adds considerably to the artistic effect. While of late there Las come into being a class of developing papers made for the special purpose of producing warm-black and brown-black tones by direct de- velopment and while some few papers may be made by restrained de- velopment to produce brown and sepia tones, in general, recourse must be had to toning processes for other colors than the usual black and for warm-black. There are almost innumerable variations in a large num- ber of toning processes producing results of varying quality and differ- ing greatly in adaptability to various emulsions. With some methods of toning, the colors are only slightly inferior to the corresponding images of prints produced by a pigment process such as, for example, carbon. With others, however, the results are not always such as please, much depending upon the suitability of the emulsion and the character of the print, while in some cases the process of toning itself is not above objection. A work of this nature is not the place for a comprehensive review of all of the many toning processes and their modifications. Representative formule with manipulative details of the more generally useful methods, however, are included. Those in- terested in the subject to a greater extent than it is possible to give to it in these pages are referred to the bibliography at the end of the chapter where will be found a fairly complete list of the principal works on the subject published during the last twenty years. The Sulphur Toning Processes—The Print—The most widely used processes of toning are those in which the metallic silver of the black image is converted into a colloidal silver sulphide. ‘The colors obtained by such treatment range from purplish-brown, through sepia and various shades of brown, to a disagreeable yellowish-brown. There are a number of processes which fall into this class and these may, for convenience in treatment, be divided into two divisions: (1) the in- direct processes in which the metallic silver is first bleached and then 458 TONING OF DEVELOPED SILVER IMAGES 459 converted into silver sulphide by immersion in a bath of sodium, am- monium or barium sulphide; and (2) the direct method in which the conversion to silver sulphide is accomplished in a single solution. Certain differences exist in the nature and working of the two methods which may be more conveniently noticed when we consider the various processes separately. However, as the bearing of the black print on the final result is very nearly identical with all the processes of sulphur toning it is more convenient to consider this subject before proceeding to a discussion of the processes themselves. The color obtained upon toning depends to a certain extent upon the toning operation itself ; much more, however, on the exposure and development of the black print. If increasing times of development be taken and the exposure adjusted in each case so as to produce a print of approximately the same depth, upon toning it will be found that as the time of development is increased the color of the toned print becomes progressively colder in shade. ‘The student is advised to repeat this experiment in order that he may see for himself the exact effect of variations in exposure and development on the color of the toned image. From the series of prints so obtained it should not be difficult to select one which has the color desired. It will then serve as a guide for the development of future prints on the same paper which are to be toned. Owing, however, to differences in the temperature of various batches of the developing solution, the oxidation of the developing solution when in use, the reduction in its activity with use owing to the re- straining action of liberated bromides and to differences in the de- veloping speeds of various batches of the same paper, it is advisable to adopt the factorial system rather than to adhere to a straight timing method. The print to be used for sulphur toning should be developed to a rather high factor—the exact factor to be used depending some- what on the character of the emulsion and to a certain extent on the color desired. Once, however, the factor has been found which with a given emulsion produces a print which upon toning results in the de- sired color, duplicate prints of the same quality can be made at any future time. Where the total duration of development is so short as to make the factorial method inconvenient, as is the case with most of the developing papers of the gaslight type which are designed prima- rily for the use of the amateur, the adoption of a fixed time of de- velopment is perhaps the more satisfactory solution. In this case, however, care should be taken not to overwork the developing solution ‘eo |) ho eo 2 + ee Let ce: 460 PHOTOGRAPHY or uniform colors will not be obtained. Indeed for best results it is preferable to use fresh solution for each print, taking for this purpose only sufficient solution to cover the print. When development is conducted for a definite time, rather than by the factorial method, the amount of soluble bromide present in the developing solution has a very great influence on the resulting color. Therefore when for any reason time development is used the amount of soluble bromides added to the developing solution should be care- fully standardized in order that it may be possible to obtain the same tone in the future. Where development is by factor the amount of soluble bromide in the developing solution has but slight influence and the exact amount is therefore relatively unimportant. It is well, how- ever, in most cases not to use very much more than is Bia to pre- vent fog. The nature of the emulsion has considerable influence upon the re- sulting color. Asa rule, the faster the emulsion the colder is the tone obtained with normal treatment while the slower grades tend to pro- duce warm tones. These differences, however, may be, and often are, overshadowed by the exposure and development of the print. Thus, while it may be said that emulsions vary in their tendency to produce warm or cold tones, practically speaking, any desired tone within the range of the toning process used may be obtained when one has a. black print of the requisite character. In the case of sulphur toning by the indirect processes perfect fixa- tion is a matter of vital importance. The investigations of Lumiere and Seyewetz } have conclusively proved that the staining of the whites met with in sulphur toning by the indirect process is due solely to im- perfect fixation. They have shown that fixation is incomplete in a bath of hypo which contains more than 2 per cent of dissolved silver bromide and that prints fixed in such a bath will show a coloration in the whites on toning regardless of the time which they are left in the fixing bath. The authors therefore recommend the use of two fixing baths, one of which has been slightly used while the other is absolutely fresh, and the prints given ten to fifteen minutes’ fixation in each bath. It would be well as a matter of principle if workers would accustom themselves to the use of two fixing baths and discard the older of the two at regular intervals depending upon the number of prints fixed, its place being taken by the second bath which is in turn replaced by a fresh solution. 1 Brit. J. Phot., 1923, 70, 732. TONING OF DEVELOPED SILVER IMAGES 461 Sulphur toning processes such as hypo-alum and liver of sulphur (when used in a solution sufficiently strong) which contain a relatively large amount of hypo in their composition are without staining action on the whites of the print provided the fixation has been reasonably complete. Since these processes contain hypo as a constituent part there is no necessity for thorough washing in order to secure complete elimination of hypo. Thorough washing, however, is a matter of importance when the indirect processes are used since these are quite sensitive to its presence. The Hypo-Alum Process.—Of the several methods of direct sul- phur toning, the hypo-alum process is perhaps the most popular and is extensively used in American studios, practically to the exclusion of all other methods of sulphur toning. It is withal an excellent method for securing such colors as are within its range which may be said to extend from a slightly purplish-brown through various shades of brown to warm-chestnut-brown. It is regular and reliable in action produc- ing agreeable tones and free from the tendency towards extreme warmth of tone which makes it well adapted for many emulsions which with other processes produce disagreeable yellowish tones. The following is a reliable formula for the hypo-alum toning bath: ae tlh: 400 gm. ee ee ey. va as cence ke pes chee 80 fl. oz 2000 Cc aN E I ie ag asc pn ce kde ss eeree es 3% oz. go gm. Stir the solution well when adding the alum, then raise to the boiling point and boil for three minutes. Allow the mixture to cool and add the following silver solution, known technically as a ripener, which prevents the bleaching of the prints in the hot toning bath: eR er sy cone sis ks cbse ga din wa ewes 20 er. I gm. De ES SRS 5) Se 1 il, oz, 30 cc. Ammonia (.880) sufficient to redissolve the pre- cipitate first formed. The solution should be stirred vigorously while the ammonia is being added. It is then added to the hypo-alum mixture and the following solution of potassium iodide made up and added to the bath which is now complete and ready for use: Potassium iodide.......... DROW neers eae aE 40 gr. 3 gm. Pup Reem Mr RI Nn Si dee sly cs aces bv dace bn ca bele i fy oz: 30 CC. a 462 PHOTOGRAPH? The prints for toning, which should be slightly darker than required since there is a slight bleaching action in toning, should be fixed thor- oughly and rinsed in several changes of water, then immersed in the hypo-alum solution which should be heated to a temperature of about go° F. (32° C.). The prints are kept on the move while toning in order that there may be no danger of uneven toning from the over- lapping of the prints in the solution. At the same time the tempera- ture of the bath is gradually raised to 110 to 135° F. (43-57° C.). The temperature of the toning bath has a slight influence on the color of the toned print, the warmth of tone increasing with higher tem- perature. The temperature should therefore be regulated with respect to the degree of warmth desired in the finished print. The prints are allowed to remain in the hypo-alum mixture until there is no doubt that toning has proceeded as far as it will go. There is no danger of over toning as the action proceeds to completion and then stops. The time required for toning varies with the temperature of the bath and with the emulsion, varying from 15-30 minutes at temperatures from I10 to 130° F. (43-54° C.). When fully toned, the prints are removed and the surface swabbed with hot water by means of a tuft of absorbent cotton in order to re- move the precipitate of alum which forms and are then washed and dried in the usual way. The toning bath itself should not be thrown away but bottled up for future use as it improves with age. Several methods of accelerating the action of a hypo-alum bath have been advised. Thermit in the British Journal of Photography ? recom- mends that the prints after having been fixed in a plain hypo fixing bath be immersed in a 10 per cent solution of sulphuric acid for half a minute, then transferred to the regular toning bath where the action will proceed quite rapidly. W. E. A. Drinkwater in recommending a similar method adds to the sulphuric acid solution a small amount of hypo, as follows: Sulphuric acid. ..5 Vin se ae ee t fh.o2 6.5 cc Water cos xacc bs cece ee gheaece een ee 150 fl. oz. 1000 Cc. Hypo’ ca ..'s 6c 6 5 waibee 0 00F 4 Oz. 26.6 gm Prints transferred from this solution to the regular hypo-alum bath at a temperature of about 110° F. (43° C.) tone completely in a few seconds. . Zanoff’s Controlled Hypo-Alum Process.—With the exception of 2 1922, 69, 120. TONING OF DEVELOPED SILVER IMAGES 463 the slight control possible by varying the temperature of the bath, when the ordinary hypo-alum process is followed the tone of the finished print is determined once for all by the exposure and development of the print. Zanoff, however, has described a variation in the usual process by which there is greater control over the resulting tone in the operation of toning. The formule for the two toning solutions required are as follows: (ou) SI Si i a 20. OZ: 156.25 gm. RS ee cy eev vee eaes 2 OZ. 15.6 gm Bouline water (distilled). .o...05..... 128 oz 1000 = “tc Boil two minutes, allow to cool and then add: SRT DO OSOUATE.. 5. os te ete we eee 2.02. 15.6 gm. (OO SS re 60 gr. .o5 gm. esc es, ie ic x nis 8 $0 0'0'e'¥-9 8 I Oz. 7.8 gm. Wrermaertirhy- OTOMMGG. 0.6.65 ee we ew 180 gr 2.85 gm ea desc vgakeeis I Oz. 78 gm. Pour the bromide solution into the solution of silver nitrate and add precipitate and all to the cool hypo-alum bath. RR POT IG o dace y ec c eh ele be ees 15 gr: .24 gm. UL ae ee re Ate Ae haem 2 OF: 150. -Cc; a hak pene sekansiy ee 16 oz. 125 em. SS" aS RS a a 4.02. 31.25 gm. ey Sid ue hb aid eine os 128 oz. 1000 CC. Boil five minutes, then cool and add the following solution which has been prepared separately : To eosin dc kv aes ew want 30 er. .48 gm. Beoremetn BrOMHNGE. 6 occ ct eee 30 er. .48 om. WE 8 5 sc 5 eee ps 8 be we ¥ 02; 7 Oo CG: The prints are first immersed in the first bath for six or seven minutes, according to the warmth of tone required, then rinsed and immersed in the second bath until toning is complete. The longer prints are left in the first bath the colder is the final tone. Accord- ingly by regulating the time of immersion in this bath the tones may be regulated to meet the desires of the operator, so that the action of the bath is completely under control. The first solution is used at a lukewarm temperature; the second at the normal temperature of the ordinary hypo-alum bath.* 8 Abel’s Photographic Weekly, 1921, p. 224; Brit. J. Phot., 1921, 68, 680. 464 PHOTOGRAPHY Sulphur Toning with Acid Hypo.—When an acid is added to a solution of hypo, the latter is at once decomposed, one of the products being finely divided sulphur. A number of toning processes based upon the decomposition of hypo by an acid have been brought forward although none have come into extensive use. Processes of this nature were brought forward by Lumiére and Seyewetz and H. Soar in 1914, by G. S. Hoell in 1915 and by the Eastman Research Laboratory in 1922.4 In the method advised by the latter the prints are first im- mersed in a 5 per cent solution of sulphuric acid for ten minutes, then, after a brief rinse, in a 20 per cent solution of hypo saturated with borax. Toning with the Polysulphides.—A cheap and simple method of sulphur toning and one which produces acceptable results on many emulsions consists in the use of a polysulphide, usually in the form of the inexpensive “liver of sulphur,” a mixture of potassium poly- sulphide and potassium sulphate which usually contains certain im- purities in the form of potassium carbonate and potassium thiosul- phate. 3 The following formula is recommended : “Liver of sulphur’ :...0.. 4.3 eee YA oz 12.5 gm Hypo... 66 ot ioe ee 1 Oz. 25 gm. Water}: ci 06 bien eee elena Gane ee 20 OZ 1000 cc As the temperature of the toning bath must be about 80 to go° F. (27-32° C.) and since the “liver of sulphur ”’ itself has a softening action on gelatine it is well to harden the prints before toning in a solu- tion of chrome alum, unless an acid fixing bath containing alum has been used in which case the degree of hardening will probably be suffi- cient. As the toning solution itself contains hypo, only a brief wash- ing is required before toning. Toning requires from ten to fifteen minutes at the above temperature during which time but little outward change in the color of the print will be observed. As the subsequent washing proceeds, however, and the yellow discoloration disappears the final color of the print becomes apparent. Blue spots arise from the presence of iron, due either to the use of trays in which iron is exposed or to the use of impure “liver of sulphur.’ The remedy is, in either case, obvious. E. Underberg advises the use of ammonium polysulphide which he 4 Brit. J. Phot. Almanac, 1914, p. 66; Phot. Era, March 1915, p. 127; Brit. J. Phot., 1922, 69, 73. - TONING OF DEVELOPED SILVER IMAGES 465 prepares as follows:° A stock solution is prepared by dissolving pure sulphur in commercial ammonium sulphide until the point of satura- tion is reached and then decanting the clear solution which keeps well. For toning, from ten to fifteen drops (.3-.6 cc.) of this solution are added to ten ounces of water (284 cc.) which is heated to a tempera- ture of 85 to 95° F. (30-35° C.). Toning proceeds rapidly and is complete within 5-10 minutes. Underberg considers this method one of the best because of its simplicity and regularity, the warm tones produced, and because the action is progressive, so that the action can be stopped at any time, in this way securing an intermediate tone due to the admixture of the original black with the toned image. Lumiere and Seyewetz, who investigated the use of “liver of sul- phur ”’ as a toning agent very thoroughly,® are of the opinion that the action of liver of sulphur on the developed image is comparable to colloidal sulphur and that the course of the reaction is as follows: S+H,O=—H,S + 0, Ag, + O= Ag,O, Ag.O + H.S —Ag,S + H,O. As the actual toning agent in the above case is the polysulphide con- tained in the liver of sulphur it is plain that this might be used alone. The solution of potassium polysulphide may be prepared by the method described by Bullock.’ Dissolve one hundred grams of potassium hydroxide in water and make up the solution to a total of 1000 cc. (in English measures I ounce to a total volume of to fluid ounces). Saturate one half this solution with hydrogen sulphide and mix with the remainder. To this solution, which is substantially one of potassium sulphide, add 120 grams (1.2 ounces) of pure sulphur in powder, heat to the boiling point and boil for five minutes stirring rapidly all the time. The - potassium pentasulphide solution thus formed is then allowed to cool, filtered and kept in a rubber-stoppered bottle tightly closed. For use take 950 parts of water, 50 parts of the potassium penta- sulphide stock solution and 2.5 parts of a 20 per cent solution of am- monium sulphide. This bath as prepared will remain clear for about an hour after which the sulphur may begin to separate out. 5 Brit. J. Phot., 1924, 71, 50. © bru, J, Phot., 1023, 70, 733. 7 Brit. J. Phot., 1921, 68, 451. 466 PHOTOGRAPHY. The time of toning is from 15-25 minutes at ordinary temperatures but the action may be greatly accelerated by the addition of either potassium sulphocyanide or potassium selenocyanide. With the ad- dition of 2 per cent of sulphocyanide the rate of toning is approxi- mately doubled. Increasing the amount of sulphocyanide increases the rapidity of toning but leads to a more purplish tone, which, how- ever, may, in some cases, be an advantage. Single Solution Sulphide Toning Processes.—A process of sulphur toning in which a solution of sodium sulphide containing an oxidizing agent together with some body to take up the caustic soda formed was described by Milton B. Punnett * in 1908, and similar processes were described by Dr. F. Kropf * in 1910, by E. Blake-Smith ?° in 1911, and in a leading article in the British Journal of Photography the previous year, while Dr. W. Triepel ™ patented under British Patent No. 24,- 378 of 1910 a process of like character. The most successful of such methods, however, is that introduced by Mr. W. B. Shaw in 1923 in which the réle of the oxidizing agent is filled by nitro aromatic derivatives, such as, for example, nitro-ben- zene, sodium meta-nitro-benzene sulphonate and sodium 4-nitro- toluene 2-sulphonate. The toning solution is compounded from two stock solutions as follows: ™ | Saturated solution of barium sulphide: ....0..2. 150 .)see I5 parts 10 per cent solution sodium meta-nitro-benzene sulphonate............. I part Both solutions keep well if stored in tightly corked bottles. A fun- goid growth, however, will form in the nitro-benzene solution with time. This growth may be prevented by adding to the solution a small piece of thymol. Prints immersed in this solution tone to a good sepia or brown within three to five minutes at a temperature of 60° F. (16° C.). The tones vary considerably with the emulsion; the greatest variation be- ing observed with papers of the gaslight type. With bromide papers there is comparatively little variation in tone with different emulsions. The tones resemble very closely those obtained by other methods of sulphur toning and since the method is one which works rapidly, is 8 Brit. J. Phot. Almanac, 1908, p. 653; Brit. J. Phot., 1910, 57, 860. ® Phot. Rund., 1910, 21, 245; Brit. J. Phot., 1910, 57, 836. 10 Brit. J. Phot., 1911, 58, 140. 11 1910, 57, 835. 12 Brit. J. Phot., 1923, 70, 759. TONING OF DEVELOPED SILVER IMAGES 467 simple in composition, produces agreeable tones and does not require the application of heat it will undoubtedly develop into one of the most popular methods of sulphur toning. The Indirect Process of Sulphide Toning.—Of the many methods of producing sepia and warm-brown tones, the indirect process is one of the most reliable and is perhaps in wider use than any other method. The well-washed prints are first bleached in one of a num- ber of baths, the most prominent example of which consists of potas- sium ferricyanide and bromide, briefly washed and toned in a solu- tion of sodium, ammonium or barium sulphide. As the latter has a softening action on gelatine the use of an acid fixing and hardening bath is advisable particularly in summer or when high temperatures cannot be avoided. For bleaching the print any of the following mixtures may be used: A. Ferricyanide-bromide (B. J. Almanac Formula) PIRATE UCOTIIGE, 6.66 ec ve eve ete en eees 100 gr. il Sm. Prermeerery BOPeICVANICE, 60. kc ee ee dca es 300 gr. ace om, ey ce Re an ar ar aa 20 OZ. 1000 cc. B. Permanganate (T. H. Greenall) (a) Hydrochloric acid (31.8 per cent)....... 3 02. iSO: 7 ec. Dee PROS AIC 6 dine 6h. k is iss eue ees nies eee 20 Oz. 1000 CC. Coie ceeesicm permanganate... ... 1... sees 40 er. 4.5 gm. NR es nlc ek fee ite ce ees wees 20 OZ. T1000? =cC: For use take one part each of (a) and (0) to 6 of water. Both a and b keep well in tightly corked bottles. This bleacher has the advantage that any traces of hypo left in the print are destroyed. With the ferricyanide-bromide bleach the presence of hypo leads to reduction of the print since the two interact to form the well-known Farmer’s reducer. The stain which results from the use of perman- ganate is usually removed in the sulphide bath, however, should it re- main after sulphiding, the print is immersed in ee ea gt vig: si bneie se vain midgea dw ks Y Oz. 10 cc. ORE ee eye se ae isso 8 e's ea ge neler Od 50 oz. 1000 cc. SE OO ES ge SF i ne I 0z 20 gm until removed. The following phosphate-ferricyanide bleacher is recommended by Mr. T. H. Greenall as giving colder tones than the usual ferricyanide- 468 PHOTOGRAPHY bromide mixture. It may therefore be of value where a particular emulsion tends to produce an undesirable warmth of tone.*® Sodigm phosphates. 6s). cos ass eee 200 gr. 100 gm. Potassium “ferricyanide s.r ‘40 gr. 20 gm. Water CC. 17 A method of combined development and sulphiding in a single solution has been described by Mr. T. H. Greenall (Phot., 1912, p. 91; Brit. J. Phot. Almanac, 1913, p. 659), but has no advantages and some disadvantages over the method de- scribed. 18 Brit. J. Phot., 1921, 68, 25. 472 PHOTOGRAPHY The use of mercuric chloride has the effect of producing a certain amount of intensification which may be compensated for by reducing the amount of exposure or the time of development of the original print so as to produce lighter prints which will, when toned, be of the proper strength. The following table shows the composition of the lice wee solu- — tion for various tones and the relative exposures required for the original prints designed for toning: Color A B Water Rel. Exposure Normal sepia'>422 beng ee ae 40 parts | — parts 480 parts | 10 seconds Gool septa “- 7 Aone. gone pee 40 20. ons 480 Ougset Colder.sepia- os is eee RON ean 30 ae 480°" anaes Browneblackiin 20.6.5 ones eae a0 rite Pre heaeey 480 “ ose pes Engraving black ie a a erthee rey SNe 480.0% G. g2577 After having been bleached in a bleacher compounded as directed for the tone desired, the prints are washed briefly and passed through three successive baths of 1 per cent hydrochloric acid after which they are again washed and finally sulphided as usual. The engraving black tone, it may be mentioned, is not the cold neutral black of the ordinary developed print but is a purer, richer black, resembling very closely the tone of an etching or engraving. It has been the experience of the writer that the best results are se- cured on bromide papers and that these vary considerably in adapta- bility to the process. Toning with Copper. —_Dr. J. M. Eder ?® in conjunction with V. Toth claims to have been the first to show that silver prints could be toned to a reddish tint by treatment with cupric ferricyanide. Later Namias showed that copper salts mixed with potassium ferricyanide deposited red ammoniacal oxide of copper on silver prints.*° Eder on returning to the subject in 1900 advised the use of cupric sulphate, ammonium carbonate and potassium ferricyanide.2* The same year Mr. W. B. Ferguson, as the result of a long series of experiments, ad- vised the use of cupric sulphate potassium ferricyanide and a neutral citrate such as potassium citrate which he found far superior to am- monia or ammonium carbonate which had been advised by Namias and Leer: 19 Eder, Phot. Korr., 1876. 20 Namias, Phot. Korr., 1894, 327. 21 Phot. Korr., 1900, 36, 537. 22 Phot. J., 1900, 25, 133. TONING OF DEVELOPED SILVER IMAGES 473 Two solutions are required as follows: (British Journal Formula) mma RRA Og os cpg wads sae 'eicin'y boa 60 gr. 7 gm. Feeneemin citrate (neutral). .5....cs6..ac0c--s 240 er. 28 gm. Pee hig ns ik disk Cups wane eve ea 20 oz. IO0O cc. eee MEU FERTICVANICE, «6.6. c ces vee ee sees 50 er. 6 gm. Pere UIE CULT AIES ee kk ede es cava vaeas 240 er. 28 gm. Pete sd hoe ee oka Ue dae evi oa Sa dawld’ 20 Oz. 1000 Cc. For use take equal parts. Should the prints appear purplish in the highlights increase the amount of potassium citrate in either A or B. The range of colors obtainable with copper toning extend from warm-black through varying shades of brown on to red chalk, the ac- tion being progressive so that the various tones follow one another in a definite order as the action proceeds. There is no intensification as with uranium and the results are quite permanent. According to Namias the tone is much improved if the prints after toning are immersed in the following bath for fifteen minutes: RNR ys Gh ca ub vA vee nes des ev ce 154 gr. 20 gm. PA MRIMPII SIT OMLOTICC Gs csi. oslo vc bala as dee va eases akc of) 50 gm. DE NOES CMY OS ok pia konpa So Mlei's needs aoe « 77 min. IO cc. eM IA vince cadre ace waded 16 Oz. 1000 Cc. This bath may be used repeatedly and keeps quite well. As some silver chloride is formed, refixing is necessary, but since an acid bath would have a reducing action on the image a plain hypo bath with a concen- tration of about 5 per cent should be used. Namias has lately recommended another method of copper toning.?° In this the prints are first bleached in MMe PAE leet ive dials wou ce ee eck odd ey wale 614 gr. 80 gm. PUeGte eT CUTALCY 6 cic en vcd cance nc we wane 8I er. 10.5 gm. EE OS a a ee er 16 Oz. 1000 CC. Pee eT OVA N IC seks oe ce a aeeine ene + oe 73 oF. 9.5 gm. After bleaching the prints are washed well and redeveloped in a metol- hydrochinon developer containing 0.2 per cent potassium bromide. If development is carried out in daylight the silver ferrocyanide is reduced while the copper ferrocyanide is unaffected. As the process has some intensifying action the prints should be somewhat lighter than actually required. Toning with Uranium.—The range of tones obtainable by toning 221i. Prog. Fot., 1915, p, 347; Bull. Soc. franc. Phot., 1922, 64, 26. 474 PHOTOGRAEIES with uranium extends from warm-black, through various shades of brown to plum colors and various shades of red, terminating in a bright brick-red. The toning action is progressive, the various colors appearing in a definite order as the action is allowed to proceed. Ow- ing to its intensifying action, uranium toning is not a process for dark, fully developed prints and prints which are to be toned with uranium should be made somewhat lighter than is required of the finished result. ) if As regards the permanency of prints toned with uranium there is some question. While in many cases the results are reasonably perma- nent, except for a slight metallic luster which forms around the edges, in other cases the toned image does not appear at all stable. While much is no doubt due to improper manipulation during and after ton- ing, when all is said and done, prints toned with uranium cannot be said to be very reliable. There are numerous methods of toning with uranium. We give Sedlaczek’s method, which, if not the best, is one of the best, having behind it the experience of a man who has devoted much time to the subject. : As a result of theoretical reasoning and considerable research, Dr. Sedlaczek recommends the following formula: *4 Uranyl nitrate...) cv pascasica ae an ee 38 er. 5 gm. Potassium citrate. s..5.-.9 <4 5 oe te ee 38 er. 5 gm. Potassium ferricyanide. ...\..4 7.50 se 15 gr. 2 gm. Ammonia ‘alums. 5.5 Ooi ee ns ee ee 77 gr. 10 gm. Pure hydrochloric acid..¢ 5a, eee 2 min DLS: ec. Water. (oi) bisa Beaded cee ae 16 oz. 1000. cc. It is a matter of considerable importance that the print be thoroughly washed as the uranium bath is decomposed by hypo, producing stains which cannot be removed. A print immersed in the above solution shows virtually no change in the first half-minute, after which a slight brownish coloration be- comes apparent which finally deepens into a reddish-brown. The colors produced with a bath of the above composition are far superior to those produced by the older methods, being darker and richer owing to the presence of some of the black silver image. The removal of the yellowish coloration after toning is greatly facilitated by the use of the following bath: 24 Phot. Ind., 1924, p. 234; Amer. Phot., 1925, p. 8. TONING OF DEVELOPED SILVER IMAGES 475 STP uni S252 a) SO a ae a 38 er. 5 gm. Sodium sulphate (not sulphite)..........0. ceca 192 gr. 25 gm. Paes Orr has 4 4 5 See ge hie Re EY Bogert eg Ra 16 oz. 1000 cc. _ Three or four such baths may be required to remove the yellowish coloration entirely. The print should then be washed in running water free from alkalis for a minute or so, then fixed for five minutes in a 0.5 per cent solution of hypo followed by washing in running water made acid by the addition of 0.1 per cent of glacial acetic acid. The impermanence of uranium-toned prints, of which so much 1s heard, is due, according to Dr. Sedlaczek, to the omission of after fix- ing, or to the omission of treatment with hydrogen sulphide. Uranium- toned prints thus treated may be considered reasonably permanent. For this latter treatment either of the following sulphiding baths may be employed : MP ICG yh Silden bi cea cd ese we nes IQ gr. 2.5 gm. bo 21 Sn 8 min. i tec DC ee ki a keh ests Se eat souse 16 Gz... {| 1000. te. or DR RRC ies gee ea's. is ad od ayes he na 8s wT Se: 10 gm. PRC ACH hy es kee ck keer seeks wee 23 min. 2 CG: Bye ety Ta gro oe a 16 oz. 1000 cc. Combining the fixing bath with cobalt produces colder tones tending to violet as the amount of cobalt is increased. For this purpose the following formula may be suggested as a beginning: PN eS herd ba ek ewe be Geen 35 gr. 5 gm. Peer tt ee a dh gals ass Cc wale ba ieae wit 8 er. I gm. CE ky ees ovens saves. PrN ie Bi veer 8 gr. I gm. Rees Mea ee i ed, ya) Bleck ek wacee eee bela 16 oz. 1000 cc. Iron Toning Processes.—The use of toning processes employing salts of iron is rather limited, being confined principally to blue ton- ing, although by combination with uranium or by sulphiding green tones may be obtained. | For a blue toner the following formula is recommended : (B. J. Almanac Formula) Ferric ammonium citrate (10 per cent solution)...... 2 G2: 10" ce, Potassium ferricyanide (10 per cent solution)........ 2 Oz. IO cc, Preece acin, 110 per cent solution) ......04 04s es acco ws 20 Oz. 100 CC. 476 PHOTOGRAPHY The well-washed prints are immersed in this solution until the re- quired tone is reached, then washed in running water until the whites are clear. Green tones may be obtained by toning with iron and sulphiding. The green tone is due to the combination of the blue image (produced by toning with iron) and the yellow silver sulphide produced by sul- phiding. Three stock solutions are required: = A. Potassium ferricyanide. 20), poe, eee eee 5 gm fee tge. ¢ Ammoniaie A. ts on. 58 ee eee 5 drops Water to amakedic, spescde ieee 4 rede ae 100 cc 3% oz B. Ferric. ammonium, citrate..5s. 12. see 2 gm. 3327 Hydrochloric: acidi(conc.) o.).2 ste eee 5 cc. 80 min. Water to make. .o. 24 2s oh sine ee 100 cc. 3% oz. C.. Sodtum sulphuie cease te - cae a got eee I gm. is )gr, Watet sgs.nd. Wak ie ce oe ee 100 cc. 3% oz. Hydrochloric atid: ((cont.). 9424.0 pa eee 5 ce. 80 min. The well-washed print is placed in A until completely bleached, then washed free from stain, placed in B for four or five minutes, rinsed two or three times in plain cold water and finally transferred to C for 5 minutes. A short washing in running water completes the process. ‘ The purity of the whites of the print depends upon the washing fol- lowing the bleacher and it is therefore necessary that this operation be thorough and complete. The pale blue of the highlights which is so observable on the wet print generally disappears on drying. Toning with Vanadium.—Apparently the first description of va- nadium as a toner was made by Prof. R. Namias in 1go1.7° The method adopted by him was to immerse the print in a solution of a ferricyanide and then into a solution containing a vanadium salt. The normal color of the silver image toned with vanadium is yellow and in 1903 Namias introduced the following formula for obtaining green tones—the green tone being due to the presence of a blue ferriferro- cyanide image and the yellow vanadium ferrocyanide : 7° Ferric chlortdé: |; 2 i nck es sa 4.8 gm. 23. EP, Vanadium chloride: olcs os eon eee ee 4 gm. 17.5 gr. Ammonium chloride...) ee ee 10 gm. 43.8 gr. Hydrochloric: dad (cones)... ina ee eee 10° ec 48 min, Waters. cc0. oo) ieee a ee 1000 CC. 10 OZ, .' The objection to this, as well as all early methods of vanadium toning, 25 Eder’s Jahrbuch, 1901, p. 171. 26 Eder’s Jahrbuch, 1903, p. 158. a TONING OF DEVELOPED SILVER IMAGES 477 is that the solutions used contain a chloride and hydrochloric acid so that there must be some silver chloride formed and this has the effect of reducing the transparency of the image and hence its brilliancy. Mr. E. J. Wall?’ has worked out and described a method in which this objection is overcome by the use of either the oxalate or sulphate of vanadium. | Either of these salts can be made quite easily from ammonium meta- vanadate which is a comparatively inexpensive salt. To make the oxalate place 100 grams (3 0z. 230 grains) of ammonium metavanadate in a beaker or evaporating dish and add 460 grams pure oxalic acid. To this add 500 cc. (17 oz. 287 minims) distilled water stirring con- stantly all the while and then heat the mixture. As the temperature rises it forms at first a thick paste which becomes more fluid as the temperature rises while the color changes from white to orange-red and finally to a dirty gray-green. More water may be added and heating continued until a perfect solution is obtained. The color will then change to a brilliant blue and the total bulk of the solution can be made up to 1477 cc. (52 oz.) when we have a.20 per cent solution of vanadium oxalate containing a slight excess of oxalic acid. The actual toning solution is compounded as follows: Vanadium oxalate solution (20 per cent)... 50 CC. Y% fl. oz. (xalic acia (Sattirated solution)........... 50 cc. Y fl. oz. Ammonia alum (saturated solution)....... 50 cc. Y% fl. oz Ferric Oxalate solation. . 0.2.6.0... core. eee quant, suff. CO oer ny ee 50 cc. 1% fl. oz. Potassium ferricyanide (10 per cent solu- CMRI AEG eI arg odo ls is “ba dno 6 v9 ms 10 cc: 48 min. WA Sa ip les ie Ge Gl ge ge ea 1000 cc 10 oz To prepare this solution add the oxalic acid to the vanadium and add half the water, then add the alum solution and then the ferric oxalate. The only means of determining the exact quantity of this is by trial. The more used the bluer the toned result. The ferricyanide should be mixed with the glycerine and the other half of the water, then added to the remainder of the solution. This will result in a bright, clear, green solution Which will not precipitate while toning. As it is sensi- tive to light, however, it is best to use it by artificial light. The alum is added for the purpose of keeping the highlights clear and the acid helps to keep the solution while in use. The glycerine is not absolutely necessary and may be omitted if desired but the bath is then more likely to produce a deposit. 27 Phot. J. Amer., 1921, 57, 96; B. J. Almanac, 1922, p. 305. 32 it fi 2 eee 478 PHOTOGRAPHY Toning requires from ten to fifteen minutes, after which the prints are to be immersed in a Io per cent solution of sodium sulphate for five minutes, washed briefly and dried. Fixing is unnecessary. Minor Toning Processes.—There are a number of minor processes of chemical toning of limited application owing to the unsatisfactory character of the result or to the difficulty of securing consistent results. Recent investigations of some of these processes have shown that they are capable of considerable improvement and it appears quite likely that in the future some of them, at least, may be more widely employed than at the present time. This-is particularly true of toning processes involving the use of stannous and cobaltic compounds. In both of these fields considerable development has taken place in recent years, largely as a result of the work of Formstecher and of Druce in the case of the processes with stannous salts and of P. Strauss with cobalt processes. The reader is referred to the published papers of these workers (a list of which will be found in the bibliography following this chapter) for further information on these processes. © There has likewise been a renewal of interest in processes of sele- nium toning and a number of patents have been taken out, and several papers published on toning processes involving the use of selenium. It has not as yet, however, come into general use, except in a limited way with certain products for which it has proved especially suitable. The same is true of several processes employing hydrosulphite as worked out by A. Steigmann and for colloidal silver processes de- scribed by Lumiére and Seyewetz, Rawling, Formstecher, Shelberg and others. | In a work on the toning of photographic images these minor proc- esses would of necessity assume considerable importance. In a gen- eral work, such as this, lack of space prevents a lengthy treatment of such processes as are not in general use. Books Braxe-SmitH—Toning Bromides and Lantern Slides, 1904. FRAPRIE—How to Make Prints in Colors. Meses—Der Bromsilber und Gaslicht Papier Druck, 1913. e SEDLACZEK—Die Tonungsverfahren von Entwicklungspapieren, 1906. STENGER—Die Kopierverfahren, 1926. : CHAPTER XXI PRINTING WITH SALTS OF IRON AND PLATINUM THE PLATINOTYPE PROCESS Introduction.—Platinum is one of the most stable of metals. It is affected very little by the strongest alkalis and not at all by sul- phuric, hydrochloric or nitric acids nor any substance found in the atmosphere. It follows, therefore, that prints, the image of which consists of pure metallic platinum, are as stable as the paper on which they are made. Not only are platinum prints permanent but they also have a certain intrinsic quality that is not possessed by any other process. Perhaps no printing process can reproduce all the original gradations of a good negative so faithfully, while many shades of sepia, warm and engraving black of unsurpassed purity may be easily obtained. Platinotype is also one of the simplest processes to manipu- late. The sensitiveness of certain salts of platinum to light was observed by Sir John Herschel in 1832 and by Hunt in 1844, but the develop- ment of the process is due to W. Willis, an Englishman, who took out a patent for the first practical method in 1873; a second followed in 1878, and a third in 1880. The work of two Austrian investigators, Pizzighelli and Hubl, also deserves mention more particularly for their work on the direct printing-out method. Their little book + is a com- plete treatise on the subject and despite its age is still one of the best textbooks on the process. The Theory of the Process.—Although platinum salts in the plati- nous state are sensitive to light, particularly in the presence of organic matter, the present platinotype process is an indirect one depending upon the reduction of a ferric salt to the ferrous state upon exposure to light and on the fact that this latter when dissolved in a solution of potassium oxalate is capable of reducing a platinum salt to the metallic state. ; . Paper is coated with potassium chloroplatinite (K,PtCl,) and ° 1 Platinolype, translated by Abney—published by Harrisons, London, at 2 shillings. (Now out of print.) 479 480 PHOTOGRAPILY ferric oxalate (Fe,(C,O,),) and dried. On exposure to light the ferric salt is reduced to the ferrous state in proportion to the amount of light action and upon immersion in potassium oxalate solution the ferrous salt is dissolved and the platinum salt with which it is in con- tact is reduced to the metallic state. Berkeley’s formula, which is generally accepted, is as follows: 6Fe(C,0,) + 3PtKeCh, a 2Fee(C20x4)3 -f Fe.Cle “bo 6KCI oxalate + chloride + (Platinum chloride) The remaining salts are dissolved in baths of dilute hydrochloric acid, leaving an image consisting of metallic platinum. Commercial Papers and their Treatment.—Platinum paper is sup- plied in a wide variety of surfaces and in black and sepia. The paper is extremely sensitive to moisture and therefore it is sent out: in sealed metal cans which contain a small quantity of-a moisture- absorbing salt so as to keep the paper dry and in good condition. The can should not be opened before the paper is to be used and then only in a dry place. When opened the can should not be allowed to lay around open in the workroom but the paper which is needed for immediate use should be removed and the remainder again placed in the can and the latter sealed. If it is necessary to remove a few sheets at irregular intervals, it is advisable to insert a freshly dried piece of calcium chloride each time in order to take up any moisture which the paper might have absorbed while the can was open. Too much care cannot be taken in keeping the paper dry, for if kept dry it wih remain in good condition almost indefinitely, while if allowed to ab- sorb moisture from the atmosphere, it will spoil very quickly and yield flat and lifeless prints. For best results the negative should have a little more contrast than is necessary for soft gaslight paper although the contrast can, to a certain extent, be controlled in development. For a full scale print the extreme shadows of the negative should be free from fog and the highlights plucky yet not blocked up. A little experience will quickly show the proper kind of negative. Thin, under exposed nega- tives are not suitable as the process reproduces all the faults as well as all the beauties of a negative. For some reason, a moderately thin negative is better than a dense one and care should, therefore, be taken when making the negative not to overtime. SALTS OF IRON AND PLATINUM 481 Exposure.—The paper is very sensitive to light and should be handled only in artificial or exceedingly subdued daylight as it is from three to four times as fast as the silver print-out papers which we have been considering. Very bright daylight should not be allowed to reach the paper nor should it be exposed to the direct rays of a strong artificial light for any length of time. The negative, which should be thoroughly dry, is placed in the frame with the emulsion side up and the paper placed with its sensi- tive side in contact with the negative in the same manner as has been described in connection with other print-out papers. One essential _ difference, however, consists in the use of a sheet of waxed paper or vulcanized rubber over the paper to prevent the access of mois- ture from the atmosphere while exposing. This is especially im- portant on damp or dull days when the exposure is prolonged but is always to be advised. The progress of printing is examined in exactly the same way as with gelatine P-O-P, greater caution, how- ever, being taken not to expose the paper to strong light while the examination is being made. It is rather difficult to describe the ap- pearance of the paper when the exposure has been sufficient but the precise moment at which the exposure should terminate is easily gained with a little experience. For the beginner the best guide that the author can give is the following: When the image is fully seen in brownish gray against the yellow surface of the paper and full detail can be seen in the shadows the exposure 1s sufficient. In the majority of cases, however, especially with papers that are old or those that are home-made, a test should be made. Over exposure will, of course, give a dark print while under exposure will give a light print lacking detail in the highlights. Development.—The chemicals necessary for developing may be obtained from the American agents Willis and Clements of Philadel- phia in %4 Ib. packages or the following formula may be used for all grades of black papers: SE G00 5 oz. 33-5 gm. ee 15 Oz. 1000 Cc. Hot water is required in making up the solution but the developer should be allowed to cool before using. It keeps indefinitely and may be used over and over, sufficient fresh solution being added to it from time to time in order to keep up the required volume. 482 PHOTOGRAPHY Since the image appears almost immediately, the paper must be immersed in the solution in such a manner that it is evenly and quickly covered in one sweep. Any air bubbles which appear should be carefully removed with a soft brush or the tip of a finger. With cor- rect exposure, there is no fear of over development and after a full minute’s immersion the print may be removed and immersed in a clearing bath composed of Waterirecs cc. cas Sieh eieeee eh ea oe 60 oz. 1000 cc. Hydrochloric acid ‘C.P.es. 2. ae I Oz. 16.6 cc. After five minutes’ immersion in this bath the print should be trans- ferred to a second bath of similar composition for five minutes and then to a third for another five minutes after which it is washed for fifteen to twenty minutes in running water and dried. Prints dry better when hung by the corners on a line than when placed between blotters. Variations in Contrast.—For softer prints one of the following modifications must be made: a. Heat the developer and print slightly less. On no account, how- ever, should the temperature of the developer exceed 180° F. b. The addition of a small amount of hydrochloric acid, say 1 drop C.P. to each ounce of developer. . Old paper gives less contrast than fresh. | . Some authorities recommend that printing be conducted under signal green glass but in the writer’s experience the increase in contrast secured in this manner is almost insignificant. SS For greater contrast: a. The developer may be diluted and greater time allowed for its action. It should not, however, be diluted further than one part of the normal solution already given to 4 parts of water. b. The addition of small quantities of potassium bichromate to the developer. The amount for a given negative can be deter- mined only by experience, but only a small quantity is needed - and the addition of three or four drops Io per cent solution to each 16 of developer has a considerable effect. In no case should there be more than I grain to Io ounces of developer used. - c. Diluting the developer with an equal part of glycerine and clear- ing in a strong acid bath. When using the last named method the image requires to be some- what darker than usual. Development is conducted in the usual man- fae OF TRON AND PLATINUM 483 ner but owing to the restraint exercised by the glycerine the action is slow and the shadows develop more rapidly than the highlights. As soon as the desired depth is reached, the print is removed from the developer and immersed in a strong acid bath to arrest further de- velopment. The bath for this purpose should be composed of SMa tg hee at TOG oe a wey acdsee 30 Oz. 1000 cc. Pee ACI GE ys scifi ks wis v heo ee Sede ences I OZ. 24.4 OC; Owing to the difficulties of adjusting exposure and avoiding streaks in development, the use of potassium bichromate is preferable for the purpose of securing increased contrast. However, many workers employ the glycerine method in order to secure the peculiar velvet effect which it gives because the image is held upon the surface in- stead of sinking within the pores of the paper. Variations in Color.—A special “sepia”’ paper is supplied by the Platinotype Company, but sepias and various shades of warm-black may be secured on the black papers by the addition of mercury to the developer. | In general the “sepia” paper is handled in the same manner as the “black” but the following points require separate mention. The paper is rather more sensitive to light than the black papers and, there- fore, prints faster and requires to be protected from the light with greater care. The following developer is recommended: PCM AC OCVOIOUET . . 4... s oe ieee eke swede tees IO parts—20 oz. AeanCumeraneatubated SOliutiON....5.. 0.) 00 cokes ee yo I part — 2 oz. or the special sepia developing salts sold by the Platinotype Company. For the best results, the developer should be used at a temperature of 150° to 160° F.; but very good results, particularly with certain nega- tives, may be secured in a cool developer. Trays which are used for the development and clearing of sepia prints should be set aside for that purpose only and not used for black _ papers. Neither should the two papers be cleared in the same solu- _ tion nor should they be washed together. Very fine sepia tones may be secured on black paper by the addition of mercury. The use of mercury alone will degrade the highlights so glycerine must be added to retard its action. The following developer is advised by F. J. Mortimer: Pile tee At Dlacke GEVELOPEL. oui 3% sacca nis cde vurlee ewe’ I part—1I0 oz. RE RT Rae ha ats Shiga us gia eipmey 9% oun I part—I0 oz. B. to per cent solution of mercuric chloride in alcohol. 484 PHOTOGRAPHY For use, A and B are mixed according to the tone desired—the larger the proportion of B the warmer the color. The following proportions are suggested : A 40 parts, B I part (a) A 30 parts, B 1 part (0) A 20 parts, B I part (c) A 20 parts, B 2 parts (d) A 20 parts, B 3 parts (e) A 20 parts, B 4 parts (f) (a) gives a warm-black; (b) brown-black; and (f) a warm-sepia ; this last is the maximum amount of B which it is permissible to use. For a given depth of printing (a), (b), and (c) give darker prints while (d), (e), and (f) give lighter prints, so it is necessary that an allow- ance be made in printing in order to secure prints of the desired depth. Another formula due to C. F. Inston is as follows: A, Potassium oxalate. 4... +5 sone ee 2 02; 142.8 gm. Water ani cexk ante nae . o's Ue on de 14 02. 1000 CC. B. Potassium ‘crtrate.: 3.0. ss ee 2 150 er. 21.5 gm. Citric -acid® st oe Se eee 240 gr. 34.3 gm. Mercuric chloride: 930.6 O34 (Ree go gr. 12.8 gm. Water snc UR a ee 14 Oz. 1000 cc. For use take equal parts of A and B and use at a temperature of about 100° F. Mercury-toned prints should be cleared in a bath of about one third to one fourth the usual strength, say: Water oc coin JO, ee Cee a ee 200 Oz. 1000 cc. Care should be taken not to overwork this weak acid- ‘path or the prints will not be permanent. Silver-Platinum Papers.—Owing to the very high price of platinum in 1913, the Platinotype Company introduced a silver-platinum paper under the trade name Satista. The prints on this paper are excellent and practically indistinguishable from true platinotypes. They are — Juminous and full of atmosphere and the shadows rich and transparent. Moreover, the prints are reasonably permanent and the manipulation of the paper is very simple. The paper is very sensitive to moisture and must be kept in airtight tubes like platinotype. Exposure is con- ducted in the same manner as platinotype but as the paper is faster, SALTS OF IRON AND PLATINUM 485 only about one fourth of the time is required in printing. The de- veloping solution consists of oxalic acid and potassium oxalate, with the addition of a small amount of ammonium chloride in the case of weak negatives to increase the contrast. After development, prints are cleared in a solution of binoxalate of potassium, washed for eight minutes in running water, fixed in a bath of sodium thiosulphate (“hypo”) and finally washed for thirty minutes in running water to eliminate all traces of the latter salt. The cost of the paper is about one third of platinotype and full supplies may be obtained from the agents, Willis and Clements, Philadelphia, Pa. Formulas for the preparation of similar papers have been published by Thomson.?. The following is the sensitizer advised : ee RI 0g ae ae 20 gr. 41.6 gm. Iron and ammonium citrate (green)........ SOM Tice? 41.6 gm. ee te en a a 20 gr. 41.6 gm. RAEN MHI cc cs leks ere ee dae 10 min. 20.8 cc. Potassium bichromate solution......... from 3-I0 min. 5.5-20.8 cc Oy TS COP 8 Ss Sa eS a IO gr. 20.8 gm Mix in above order and allow to stand for twenty-four hours. The paper may be sensitized either by floating or by brush and is dried in a moderately warm room. When dry, it is ready for exposure which is conducted in the same manner as platiriotype. The stock developing solution consists of : PMI RARE ee. iano ee dais Bebe OS 1.02; 1000 ~—- CC. ROMER ae ese educa we bdecsaeaeeds 4O er. 85 gm. Oe Sy Say Coie at HL a eae 10 gr. 21.25 gm. WIR AIG AEG ay cc ee nn vce vce wees Se eis IO gr. 21.25 gm. Filter and use clear solution, diluting for use with seven parts of water. ‘To secure pure black tones, the ferric oxalate must be ab- solutely fresh. If the image lacks strength, use a strong developer. Prints blacken immediately and after development are remoyed to a bath of hypo, 10 grains in six ounces of water (3.5 gm. to 1000 cc.), for ten minutes. Contrast may be regulated by the proportion of potassium bichromate (5 per cent solution), using from one to ten drops to each ounce of sensitizer (2-20 cc. to each 1000 cc.) accord- ing to the degree of contrast desired. The platinum solution named in the above formula is as follows: 2 American Photography, 1915, Nov., p. 632. | 486 PHOTOGRAPHY Potassium chloroplatinite; 6.0. Sate ee 15 gr. 15.6 gm. Phosphoric 4aGid pumiond cy fos Stes eee a aa 2 de: 150 ce: Distilled ‘waters sich Bes au.5 aieses Suivi: Bad ee I oz. 500 cc. When dissolved add water to make a total of two ounces (1000 cc.). The Kallitype Process.—Kallitype was the name given by W. W. Nicol to a ferric printing process in which ferric salts are reduced by exposure to light to the ferrous state and in this condition are able to convert a silver salt into the metallic state. The process is, therefore, similar to platinum excepting in the use of silver in place of platinum. Suitable paper is sized in a solution of arrowroot: Bermuda arrowroot... 2. s:cs.+ 5 sees ete go gr. 18.7 gm. Water’... (shea bes Se eee ee 10 Oz. 1000 CC. Using a little of the water make a thin cream of the arrowroot. Then heat the remainder of the water to the boiling point and add to the arrowroot mixture. As the solution does not keep it must be made up fresh for each batch of paper sized. The sizing solution is best applied with a Blanchard brush. When dry the paper is sensitized with: Ferric oxalate...c.diveccdsbunx Ss eames fee ae 75 gr. 15 gm. Oxalic acid. ou. 6d iis ache ele ou 5 er. I gm. Silver’ nitrate: Poco P se, ue Peaits oe ae ee 30 gr. 6 gm. Distilled iwater, cao one ee ae a Vo ake a eee I Oz. 100 cc. The ferric oxalate must be dissolved with the oxalic acid in warm water, then filtered and the silver nitrate added. The operations of sensitizing, drying, and exposing are as with platinotype. Both black and sepia tones may be had, depending on the developer used. For black tones the following is recommended : Borax > is Fics sike eae's ek ee ee eee I oz. 100 gm. Rochelle salt (oT po ae : Y% Oz. 75 gm. Distilled .watér. a: ? eye" ee ae. a a Fo we | | 4 | : THE GUM-BICHROMATE PROCESS 513 Dissolve the mercuric chloride in a small amount of water and then add the arrowroot, stirring the same until a thin cream is obtained. Then add the remaining water and the gum-arabic. The latter will dissolve more rapidly if suspended in the solution by means of a cheese- cloth bag. From sixteen to twenty-four hours will be required for the latter to completely dissolve. SENSITIZING SOLUTION The stock solution of sensitizer consists of a solution of potassium bichromate : SL etl es le aa a a 1S oz. 1000 Cc. BOONE MIEN POMIALE 06. 6. cals ands vee ee en es 720 er: 96 gm. Both of these solutions keep well. The actual mixture used for coating varies with the paper and the negative and also with the effect desired. Practically every worker develops a different formula after practice and while there may be little difference, yet it is better adapted to his own personal methods of working. | However, the following formulas are given for the benefit of the beginner : SHADOW COATING (sum- solution... ...«. ae An Sek ba dy Y oz. 15 gm. eee TM i a DG bee wala weno 1 Oz. 15 gm. EEO) AINE. oc oc ih vs cans saa dio eve tee es atta 4 in If the negative has a short scale of gradation it may be possible to use the above for all of the printings; if, however, this is not the case and the negative has a long scale of gradation and prints well with bromide or platinum, then it will be necessary to vary the coating mix- ture so as to secure a longer scale. It is generally necessary to make three printings: one for the shadows, another for the halftones and finally one for the highlights. ‘The following are advised for the half- tone and highlight coating mixtures: HALFTONE CoATING MIXTURE emer AP SL ek Cha ce ga eb he ee © TY oz. ie) em, eR eE ee isl dice. ie aulee We Ca « 1% oz. 1S) ori: POE eC A POT TUDE fo ob na Sa es es wiv owes 260% 18: verte se HIGHLIGHT CoatiInGc M1IxTuRE SMES ok Falk oe ae hile an hoa wow ea dances ue oz. 1s gm. Sensitiger .i 6. ::.. sald 8 ae WERT SOARS gO ge CA Ge ar, | ae By a oo POPNOW? TUDE. ie ca ge de rec cee ede cee® ay ce ode 3 514 ? PHOTOGRAPHY Effect of Varying Proportions of Coating Mixture.—Although the above may be regarded as an average formula, considerable variation is possible, but the beginner will do well to stick by the above until he is familiar with the process and knows what steps to take in order to secure the desired result. Increasing the amount of pigment gives a longer range of tones but the whites are stained and lack purity. An excess of gum makes the coating hard to blend smoothly and produces a thick film which may chip off in development. A moderate increase in the amount of gum solution gives greater contrast and the high- lights may be blocked. An excess of sensitizer gives a coating which is difficult to spread and one which gives flat, lifeless prints. A rough paper will take a thicker coating than a smooth one and is also more easily coated so that the latter is not to be recommended for the beginner. Figure 223 gives schematic curves for gum pigment with varying amounts of gum, pigment and sensitizer. Examination of the same will show that (1) has a long scale of tones with little contrast, (2) shows a shorter scale of tones with greater contrast, while (3) shows a short scale of tones with high contrast. ‘This gives an idea of the a) Maaazrez2 9 67 Ee eG OTE TF HDR Fic, 223. Curves Showing the Influence on Contrast of Variations in Propor- tions of Gum, Pigment and Sensitizer in the Gum-Bichromate Process (Anderson) immense variation which may be produced in a single printing by alterations in the composition of the coating mixture. Further varia- tions may be made by using different coating mixtures for the different printings and by local control, so for these reasons it may be readily seen that gum is justly regarded as one of the most flexible processes which we have for the production of positive prints. Coating.—The coating mixture having been prepared, the sensitiz- ing of the paper may proceed. Rough papers are the easiest to coat and the beginner is therefore advised to start with a rough paper, as et ee F ph oats i ar oe he ae Eee Ey ei en i ee a THE GUM-BICHROMATE PROCESS 515 Whatman’s. The sheet should be larger than the negative as it is next to impossible to secure a perfectly even coating to the very edge of the paper. A sheet 11x14 will be suitable for either 8x Io or 10 x 12, while an 8x Io sheet is sufficiently large fora 5x7. Attach the paper to the drawing board with push pins and pour the coating mixture in the center. The paper will be more easily coated if it is immersed in water and blotted before being placed on the board. For the first 11x 14 sheet of paper, about one half ounce of the coating mixture will be required, while succeeding sheets will require some- what less, owing to the brushes becoming charged with the gum. The rubberset bristle brush is used to spread the coating over the paper so_ that every part is covered with the coating mixture. When this has been done, the blender comes into play. There are two brushes which may be used for blending (one made of fitch and the other of badger) and the details of the operation depend somewhat upon which is being used. With the former the brush is held nearly vertical and drawn slowly and regularly across the paper—always in the same direction. When the sheet has been covered in one direction, it is again gone over in the opposite direction, to secure a perfectly even coating. With the latter, the action may be “ whippy,” the vigorous handling of the brush lessening as the operation proceeds. The exact manner of handling the brush and the time to stop blending will come with a little experience. In general, it may be said that when the point is reached where the tendency of the gum solution is to run into small puddles, the operation should be stopped whether the surface ap- pears completely even or not. Any irregularity will disappear in dry- ing or development or will be covered by subsequent printings. As soon as the operations are complete the utensils should be washed free from the gum solution, as it is very difficult to remove when dry. Drying.—The paper may be dried in a dim light, provided it is to be used as soonas dry. ‘The paper is insensitive when wet but becomes sensitive when dry. However, if the paper is to be stored for any length of time, it should be dried in the dark as the action of light con- tinues, and the gum will become completely insoluble unless it is dried ina dark place. The time of drying will depend altogether on the tem- perature of the room, but with an ordinary room temperature of 65 to 75° F. (18-24° C.), the time required should not be over an hour or so. If not for immediate use, it should be placed in an airtight con- tainer, similar to platinotype, for storage. The paper is at its best when fresh.} Exposure.—As the image is invisible a photometer must be used to gauge the time of exposure. The photometers illustrated and de- scribed in the former chapter on carbon printing are suitable for the purpose and the serious worker should secure one of these. When the light is steady, the frame may be loaded with proof paper and the time required for reaching the desired depth noted and the gum paper exposed directly afterward for the same length of time. Since, how- ever, it is rare that the light is uniform, the use of an actinometer is to — be advised. There is a good deal of latitude in exposure but correct exposure will greatly simplify development and give the best results. | Over exposure is preferable to under exposure, as the development of the former may be forced by the use of hot water or an alkali, while the latter once in the developer is useless. If it is known that the print is under exposed, it may be laid away for several hours before development. The action of the light continues in the same manner as — in carbon printing and this method may, therefore, be used with ad- vantage when the light is dull and long times of exposure are required. But since it is difficult to determine the exact rate at which the action proceeds, it is preferable to expose fully and develop in the normal manner. Development.—One of the “talking points” for the gum-bichro- mate process when first introduced was the ease with which local values might be altered by the use of a brush or sawdust, etc., in develop- ment. While, to a certain extent, local work of this nature is now done, most workers now content themselves with automatic develop- ment. The exposed print is immersed face up in a large tray of cold water and as soon as limp turned face down, care being taken that no air bells are imprisoned beneath it, where it is left with an occasional examination, for one half to one hour. If the image is completely developed within ten or fifteen minutes the print is under exposed and may as well be thrown away. If the image does not appear within j one half to three quarters of an hour, the print has been over exposed __ and the temperature of the water may be raised slightly. The use of an alkali is not to be advised when multiple prints must be made, nor should the temperature of the developer be raised over go degrees. It is better to prolong the time than to raise the temperature or resort to 516 | PHOTOGRAPHY 1 Heat may be used for drying but not at a higher temperature than 180° F. GS +n THE GUM-BICHROMATE PROCESS 517 an alkali in such circumstances. When the solution which drains from the print, when removed from the water, is practically clear, develop- ment may be considered complete and the print placed in a horizontal position to dry, care being taken that nothing comes in contact with its surface until dry. When dry it is ready for the sensitizing, exposure, etc., for the second printing. To lighten any local portions the print may be held under water and a stream of water from the tap-allowed to fall upon the desired por- tion, or an atomizer used. Greater emphasis may be secured by using hot water or by holding the print so that the stream of water falls di- rectly upon the surface. Local values may also be lightened by the _use of a very soft brush. ‘There is a tendency for brush work to show graininess and for that reason it should be avoided whenever possible. If it is desired to darken any part, the coating mixture may be pre- pared and applied by means of a brush to the desired portions and the whole exposed to light when dry. It is then washed in water for about half an hour and dried in the regular way. It is necessary to include the sensitizer in order to secure the same tone as the original deposit. Registration.—When making multiple prints it is necessary to em- ploy an accurate method of registering the separate printings so that they fall exactly over each other. Many methods have been devised GUM PRINTING FRAME SPR Sp 5 oN 5 CORNER DETAIL iy sy Fic. 224. Owens’ Frame for Multiple Printing for this purpose and a few of the most generally useful will be de- scribed here. A very accurate and convenient method is that devised several years ago by Horsely Hinton for making combination prints. The sensitive paper, which must be larger than the negative, is placed face up on a smooth drawing board and the negative placed with the 518 PHOTOGRAPHY emulsion side in contact, when necessary a thick piece of plain glass being placed over the whole to secure perfect contact. On one side of the negative two stout pins are fixed firmly in the board and two similar pins placed against the negative on the contiguous shorter dimension. Registration is secured by replacing the pins in the holes after each printing and forcing the negative up against them. James Owen in American Photography, 1923, p. 416, describes a printing frame designed by him especially for multiple printing. The construction is simple and obvious for the purpose in view, which is to provide a backboard on which to lay the sensitized paper either for the first printing or after one or more previous printings; dn the paper is laid the negative, and the glass panel is then clamped down by means of two simple wooden buttons, with minimum chances of disturbing the registered relation between negative and paper. Registration is accomplished by using the corners of the negative as reference points. When the first printing has been made, clean impressions of the edges of the negative are usually left. With a hard pencil the edge lines are prolonged as shown in the accompanying diagram, before applying the second coating. Whatever shrinkage the print has undergone its first or succeeding development is readily distributed as shown in the diagram. With some hard finished papers there is little or no shrinkage but the pencil lines serve for registering the corners of the negative exactly with the corners of the printed image. The essential point is that a frame of this type is a simple device in which print and negative may be quickly registered and clamped for printing without slipping out of register. The size of the frame naturally depends upon the largest size of negative to be printed from. It should be several inches larger, all round, however, than the largest negative to be used. Figure 224 will give a general idea of the principle and the details of construction. Mr. William H. Zerbe uses a frame several sizes larger than the negative with a sheet of plain glass, to this glass he attaches in one corner two strips of glass to form a true square. On the sides of these strips, about where the center of the negative will come, a piece of gummed paper is fixed. Lines are then drawn across in the exact center from side to side and top to bottom as shown in Fig. 225. The back of the paper is worked off with a T square, either before or after coating, making a line about 34 to 1 inch at the edges in the center of sides, top and bottom. In this way the marks are square although the paper may not be. THE GUM-BICHROMATE PROCESS 519 The negative is then placed in position, one corner being forced into the frame made by the two strips of glass. The paper is then placed over the negative and the marks on the paper and the strips gummed on the glass made to coincide. After the first print is developed and Gi Iw aE TZ had t eee tada j | 7S NN Fic. 225. Zerbe’s Method of Registration ES LS LAY ADEA LY Tle TE S OES ASS s 5 IN A iN ix HS i aN any AN % BY in N Kew Ww. a a\ See ee ee SQq9y 2 SQA S7IPFF 0 $3 1 \ Si NH . S pare Zh Ye WH recoated, all that is necessary for registering is to make the registra- tion marks coincide again. Should there be any stretching of the paper it will be distributed four ways and minimized.? Gum-Bromide and Gum-Platinum.—To avoid the difficulties of multiple printing (which is necessary in the ordinary method of gum- bichromate in order to secure an image having the proper depth and gradation) while at the same time preserving the quality of the gum print, many workers have combined the process. with bromide or platinum, using a print made by one of the latter as a foundation print | to supply the needed depth to the shadows and half tones, over which a gum-pigment image is made to secure richness and quality of image which it alone will yield. Since the character of the finished print is greatly affected by the depth and contrast of the foundation print, the ultimate end should be carried in mind when making the bromide or platinum print. Gen- erally speaking, the contrast of the foundation print should be rather stronger than usual. The shadows, however, must not be as dark as <- | 2 American Annual of Photography, 1923. 520 PHOTOGRAPHY desired in the finished result, for it must be remembered that their intensity will be increased somewhat by the layer of pigment superim- posed over them. , The proper depth varies in exactly opposite ratio to the amount of pigment used in the gum coating :—the stronger the gum-pigment coat- ing, the lighter should be the shadows of the foundation print—and vice versa. If the intention is to use a gum coating weak in pigment, » so as to obtain just a slight glaze of pigmented gum, the foundation print must be almost as dark as the finished result is required, but, on the other hand, a strong pigment image only requires a weak founda- tion print sufficient to give the additional intensity required by the darker tones of the subject. It.is therefore necessary that one have definitely in mind the effect which is desired and proceed to make the foundation print accordingly. Before exposure the bromide or platinum paper, as the case may be, is placed upon the negative and its position registered by any of the ‘means already considered. It is then exposed and developed in the usual way, to produce an image of the required intensity. When dry the print is ready for the gum-pigment coating. No definite rule can be given for the coating mixture as so much depends upon the strength of the foundation print and on the effect desired. The only way to determine the quantity of pigment to employ in any particular case is to spread some of the mixture upon a waste print of approximately the same character and note the effects. The operations of coating, exposure and development of the gum-pigment image are practically identical with multiple gum-bichromate and need not be repeated. The Powder Processes.—There is still another series of non-trans- fer pigment processes based upon the action of light on bichromated Fe ee ee eee ae Ve ae ee a EE ————— es as eee colloids. This series comprises those processes which are collectively termed the powder processes. In these an image is first formed in bichromated gelatine after which pigment in powder form is dusted over it. The pigment adheres to those portions of the image repre- senting the shadows and which consist of a soluble colloid, while it adheres only with difficulty, or not at all, to those portions which con- sist of a less soluble or insoluable colloid, and in this way the image ~ is produced. Of the large number of processes of this nature de- scribed in the older works, such as Abney’s /nstruction in Photog-— raphy, practically none have survived. Mr. E. J. Wall, however, has THE GUM-BICHROMATE PROCESS 521 recently described a powder process which would appear to have a more promising future.* | The bromide print, which should have been fixed in plain hypo and not in an acid fixing and hardening bath, is transferred directly from the last wash water to the following solution : MRT IUACE Ye peg os ik eon he ee oe a ee ee ws a 5 5 gm. eG MOG i vhs se ee he ea a eee aes 45) mini” Gir cee ce al ee ee: 0.5 gm. TCE OEE ONICG. gcd ihe ve oven sc oe ene aaes 196 f..0z., 100.cc REF 2 se eve a vials lanl a's wieeee ae 165.07. 1000 cc. which should be used at a temperature of 70° F. (21° C.). The duration of the action ranges from 5-20 minutes, being dependent probably on the emulsion. For the powdering of the image any inert pigment, black or colored, may be used, but it should be as finely ground as possible, or the re- sults may show an undesirable amount of grain. The powder may be applied either with a very soft brush or by means of a little sieve hav- ing a bottom of the finest muslin. Some of the powder is placed in the sieve which is held over the image and tapped with the finger. This method is perhaps preferable when using a pigment containing rather coarse particles. Resinopigmentype.—Resinopigmentype, a method worked out by Prof. Rudolph Namias, belongs to the same class of printing processes which we have just discussed. It has attracted considerable attention among pictorialists on the continent and Mr. Joseph Petrocelli of New York has produced some very beautiful work by the process. It is especially adapted to subjects which do not have a high degree of contrast, as, for instance, winter scenes, the effects of fog, rain, etc. On the other hand, it is ill adapted to images requiring vigor and con- trast, for it is impossible to obtain absolutely pure whites. | The point of departure of the Resinopigmentype process is in the use of a positive transparency, which may be on glass, film or paper. This may be made from the negative either by contact or enlargement and must not be excessively contrasty. The paper supplied by Professor Namias is sensitized by immer- sion of the sheet of paper for three minutes in a 5 per cent solution of potassium or ammonium bichromate and drying in absolute darkness. It is preferable to sensitize in the evening, then the paper will be ready 3 Amer. Phot., 1924, p. 428. 522 PHOTOGRAPHY for use the next day. Nevertheless, the positive paper may be kept for one week in winter, or two or three days in summer, but the best results are obtained with the freshly sensitized paper. The paper is printed behind the positive in the manner usual with daylight printing papers, and the exposure is continued until there is a faint brown coloration under the transparencies of the positive, with the details of the half-tones lightly visible. Over exposure is to be avoided. | The most simple and effectual method of raising the relief is to leave the print film side down in a bath of cold water for several hours to eliminate the excess of bichromate. After soaking, the sheet of paper is placed in water at 50° C. (122° F.) for from two to five minutes, which produces a distinct image in relief. If time presses, the print may be swelled quite rapidly by plunging direct in water at 37° C. (98.6° F.), to which % per cent of ammonia has been added. After rinsing in cold water the print is ready for powdering. The excess of moisture on the swollen surface is removed with blotting paper or chiffon, but not with shaggy cotton or wool. One is then ready to begin powdering by means of a soft brush of polecat hair of medium size, dipped in the powder especially prepared for the process. On continually passing the brush over the paper, the image ~ appears and this operation is continued until the image is sufficiently vigorous. When necessary to remove any excess of powder, use a fresh brush. If the image obtained is deelents in contrast, it indicates that insuf- ficient powder has been applied, and in this event the proof is placed in a tray of cold water to detach all the powder and a higher relief if produced by ammonia water as previously recommended. The soaking in water advised is often insufficient to remove the last traces of bichromate, especially if the rapid method of swelling al- ready indicated is employed. The yellow stain is easily removed by immersing the print, before swelling, in a 10 per cent solution of sodium bisulphite or a § per cent solution of potassium metabisulphite. It is necessary to do this before powdering, because it removes the same when applied to the print. THE GUM- BICHROMATE PROCESS | 523 GENERAL REFERENCE WorKS Brexnrens—Der Gummidruck, 1912. DemacHy AnD MaskrLtt—Photo-Aquatint or the Gum Bichromate Process. DEMACHY AND MAsxkeL_~t—Le Procede a la Gomme-Bichromatee ou Photo- Aquatinte, 1905. Eper—Das Pigmentverfahren, der Gummi-, Oel-, und Bromol-druck und ver- wandte photographische Kopierverfahren mit Chromalzen, 1920. HaNNEKE—Das Pigmentverfahren, 1912. von HormMerIsteR—Der Gummidruck, 1907. KosEt—Der Gummidruck, I9oo. Koset—Die Technik des Kombinations Gummidruckes un des Driefarben- Gummidruckes, 1906. Kosters—Der Gummidruck, 1904. Mayver—Der Gummidruck. QuEDENFELDT—Die Praxis des Geninidruck- verfahrens, I9QIo. RicHArps—The Gum-Bichromate Process, 1905. ’ STENGER—Neuzeitliche photographische Kopierverfahren. STENGER—Die Kopierverfahren, 1926. WarreEN—A Handbook to the Gum-Bichromate Process, 1808. ZIMMERMANN—Zimmermann’s Method of Gum-Bichromate. Photominiature, 113. CHAPTER XXIV THE OIL PROCESSES Introduction.—Oil and its companion, bromoil, are now two of the most widely used of all pictorial printing mediums. ‘This is due, with- out doubt, to the enormous flexibility of the process and to the ease with which the artist can alter values of the original negative to secure the particular effect he desires. The photographer has absolute con- trol over his result as any part of the picture may be darkened, light- ened, or even omitted at will. No other process offers the same facility to quite the same extent, although gum is a serious rival. | In the oil process, paper is coated with gelatine, sensitized with po- tassium or ammonium bichromate and allowed to dry in the dark. When dry it is sensitive to light and is exposed under the negative in the’ same manner as platinotype. When exposure is complete the print is placed in a bath of water in order to eliminate the bichromate stain and to allow the image to swell. In the bath the print gradually takes on a relief which is more pronounced in the strong highlights, since these have been more completely protected from the light and are, therefore, more soluble in water. The print is then removed and inked up with pigment applied by a brush. The shadows, owing to the fact that they have absorbed little or no water, readily take up the ink from the brush, while the highlights only take the ink with diffi- culty. Thus the image appears under the action of the brush and is gradually worked up to the desired depth by the application of ad- ditional pigment. The use of a hard pigment increases contrast, while thinning down the ink with medium causes the ink to adhere more easily and reduces contrast. The effect is also dependent upon the manner in which the brush is handled and this places an added means of control in the hands of the worker. Materials for the Oil Process.——The materials for the oil process are few in number and comparatively inexpensive. A good negative, one that has been properly exposed and has sufficient contrast to make a good bromide print, should be selected for the first attempts. While an experienced oil printer can secure a fair print from any reasonable negative, the beginner is advised to select a first rate negative as pig- 524 a THE OIL PROCESSES 525 menting will then be easier and the result more likely to be successful. Aside from the negative and the paper, brushes and inks, which will be discussed at some length subsequently, the worker will need a solu- tion of potassium or ammonium bichromate for sensitizing, a supply of blotting paper, pallette knife, several pieces of glass about 5 x 7 and megilip or medium for thinning the inks. Papers for the Oil Process.—There is. no doubt of the fact that a great deal depends upon the selection of a suitable paper. While there are no important differences in any of the papers that are suit- able, some workers have better success with some papers than others, owing no doubt to a personal difference in manner of inking and the effect desired. for the beginner, the best advice that can be given is to select one of the papers named and stick to it until he is sure of himself. Then he may try other papers and experiment until he finds if any other suits him better. | The original Rawlings paper is supplied by Messrs. Griffin, of _ Kingsway (Kemble Street Corner), London, England. It is an ad- mirable paper in every respect and is one of the best papers that the beginner can use. It is made in smooth and rough and in sizes from 3% x 4% to 16/18. As compared with other papers, the price is rather high. , The Autotype Company of London also issue two papers, No. 1 and No. 2, for the oil process. No. 1 is a smooth white paper; No. 2a toned paper with a fine grain. It pigments easily and stands vigorous brush work well. The final carbon support for double transfer is also used by some. Double transfer papers which can be advised are the Autotype papers Nos. 76, 77, and 9o. Several double transfer papers manufactured by Illingworth have been recommended by several workers, notably Demachy and Namias. Nos. 125 “ Thick Smooth” and 117 “ Thick Rough” can be recom- mended as suitable.* Brushes.—The brushes employed are especially made for the proc- ess. They are made from fitch hair and were formerly made only in France but are now also made in England. They are made of short spring hair and the end is cut at an angle. The quality of the brushes employed has a direct bearing on the finished result and only brushes of the best quality should be purchased even in the beginning. It is useless to try to get along with brushes 1 Instructions for coating paper with gelatine will be found on page 21 of The Oil and Bromoil Processes by Mortimer and Coulthurst. 35 ME at We Fe he ot 526 PHOTOGRAPHY made for other purposes and, while good brushes are rather expensive, they last a long time if kept in good condition and their purchase is a distinct economy. To begin with, three of these brushes will serve. These three may be Nos. 14, 10 and a small one for detail work. A larger “ Prima” brush, which owing to its being made of hog hair is cheaper, may be used for preliminary pigmenting and will be well worth its cost. As the worker progresses, it will be well to purchase additional brushes in order that he may lay down a charged brush and take up a new one when it is desired to apply ink of different con- sistency to any part of the print. Mr. F. J. Mortimer, who is one of the best authorities on the process, states that the following brushes cover all the requirements of the most advanced worker: © 2 No. 14 Stag-foot Fitch brushes 2 No. 10 Stag-foot Fitch brushes 2 No. 7 Stag-foot Fitch brushes 1 No. 5 Stag-foot Fitch brush t No. 10 Straight top brush 1 No. 5 Straight top brush Brushes should be kept in good condition, not only because they are expensive, but because the quality of the work depends, to a large ex- tent, upon their condition. When pigmenting is complete the brushes should be completely cleaned and not allowed to become dry; when this occurs it will be practically impossible to remove the hardened pigment without destroying the good qualities-of the brush. Soak a clean rag in gasoline and rub the end of the brush on the rag. This will com- pletely remove any pigment adhering to the ends of the hairs but if the brush has been allowed to become clogged and the pigment is spread up within the brush, it will be necessary to soak the same in gasoline and finally wash out in soap and water until the brush is absolutely {ree from both pigment and gasoline. Take care not to get the brushes out of shape while cleaning and when they are thoroughly cleaned wrap them in a piece of white paper and place a rubber band around ~ the handle in order to keep the brush in its proper shape. 4 Pigments.—Pigments are made especially for the process and are almost entirely of English and French manufacture. They are thick, — stiff, greasy inks very similar to those used for lithography, and the — latter may be used for the oil processes, but on the whole it is better to purchase that made especially for the process. There are several — excellent pigments on the market. The “ Rawlings” pigments are ~ ee a Tee EE ROGESSES 527 excellent and Sinclair’s “ Permanent ” inks, as well as Roberson’s, may also be recommended. The Ault and Wilborg Company of Cincinnati will make ink for the process upon special order, which costs about half of the foreign product. A large variety of different colors can be obtained but black is recom- mended for the beginner, as it suits almost any subject. Sensitizing.—There is no necessity of varying the concentration of the sensitizing solution of bichromate for different classes of negatives, as is the case in carbon printing, for the final result is under complete control in the operation of pigmenting. Therefore, it is better to select a reliable sensitizing formula and make it the standard. Weak or thin negatives will be difficult to handle no matter what sensitizer is employed. The following formula is recommended by Mr. F. J. Mortimer : ya vu stad ae vet ceblwcavsasubwan 10 oz. For use take one part of the bichromate stock solution and two parts of methylated spirit. Care should be taken to thoroughly mix the two. The potassium salt cannot be used with methylated spirit as the latter precipitates the salt. Potassium bichromate, however, may be used with acetone in place of methylated spirit in order to secure a quick drying sensitizer and the following formula can be recommended : I part of acetone to each part of potassium bichromate (saturated solution). Many other formule are recommended by different workers but there is no particular advantage over those which have been given ex- cept that some of them keep longer when mixed ready for use. The paper may be sensitized either by immersion or brushing. The latter method is better since it is quicker and cleaner but paper so pre- pared does not keep so well as that coated by immersion and is best used very soon after it is dry, although it will remain in fair condition for twenty-four hours. Paper that has been sensitized by immersion will keep three or four days. Sensitizing may be conducted either in ordinary artificial light or weak daylight but should be removed to a perfectly dark place to dry. No gas should be burned in the rooms used for drying the paper. The paper must be thoroughly dry before use. To sensitize by immersion, sufficient sensitizing solution is poured in MS er x 528 PHOTOGRAPHY a tray to cover the bottom to a depth of half an inch. The paper is placed with the gelatine side upon the surface of the solution and allowed to remain two or three minutes, removing it at intervals in order to break air bubbles. Drain the paper and hang up to dry. A Blanchard brush is the most convenient and practical brush for sensi- tizing. This consists of a piece of glass with fluffless flannelette wrapped over one end and secured with a rubber band. Using this brush the paper may be pinned to a board and sensitized in much the same manner as gum-bichromate paper ; the operation is much simpler, however. | Exposing.—The operation of printing is similar to any other print- ing process as P-O-P or Platinotype. The paper is about four times as fast as the former and slightly faster than the latter and care must be taken not to allow actinic light to reach the same in loading the frame or while examining the progress of printing. The image is semi-visible, in this respect closely resembling Platinotype. Printing is continued until detail is visible in the highlights. A few trials will show the proper stage to print. A photometer is not really necessary but may be a help where it is desired to make several prints as nearly alike as possible. It is advisable that the sheet of sensitized paper be at least an inch larger than the negative, in order that the print may be inked to the edge without danger of getting moisture from the inking pad on the brush. There is a slight continuing action after exposure and where several prints have to be exposed before pigmenting the prints should be im- mediately placed in a tray of water in order to stop the continuing action. | After exposure the print is immersed in water to eliminate the bi- chromate stain and to produce the necessary relief. Paper that has been sensitized by brushing will not take so long to become free of the bichromate as that sensitized by immersion. The water should be changed frequently or running water may be used. From one to two — hours will be required to produce the degree of relief necessary for pigmenting. The exact time will depend upon the climatic conditions and the temperature of the water. Warm water will produce a high © degree of relief very quickly but there is a danger of its affecting the © gelatine and causing the half-tones to be lost. Moderately warm — water, say about 75 or 80° F., however, may be used. Demachy has — suggested that a very small amount of sodium bisulphite be added to — J ee eo oer, a | | ee eee a Peo PROC SSES | 529 the first few washing waters in order to facilitate the quick and thor- ough removal of the bichromate. After washing the print may be dried and pigmented at some future time or it may be placed on the inking pad and the operation of pig- menting begun at once. In the former case, it will be necessary to soak the print in water for about an hour in order to raise the relief and get the print in a suitable condition for pigmenting. If it is de- sired to pigment at once, the print is laid upon the wet pad to be de- scribed and the surplus moisture taken off with a blotter or, better, with a ball of silk or flannel. : Pigmenting.— This is the most important stage of the process. Un- fortunately, it is very difficult to give any precise information upon this point since it varies for every worker and methods which may be per- fectly adapted to one individual may be utterly useless with another. It is an operation in which the worker must develop his own methods and where his own skill and individuality must find the way. Never- theless, it is hoped that the few particulars which follow will be of assistance to the beginner. It is necessary to keep the print wet from underneath during pig- menting and for this a pigmenting pad is used. Pads are a commercial article and may be obtained from any of the dealers carrying oil ma- terials or one may be improvised from a sheet of glass and four or five sheets of blotting paper. Soak each sheet of blotting paper in water until thoroughly wet and then place on top of one another upon the glass plate. Upon this spread one or two thicknesses of muslin or cheesecloth. Then remove the print from the wash water and place on top of the pad. With a blotter or piece of silk take off the excess moisture, being careful not to make the surface wholly dry. Only take off the excess water. It will be noticed that the image stands out in relief and the appearance of the print at this stage is a good indication of the way it will take the pigment. The greater the relief the more readily the ink will take. Squeeze out a very small quantity of pigment on the palette. Only a small quantity is needed, as a piece the size of a pea will do for sev- eral8x10prints. Instead of a palette the glass side of an old negative may be used. Spread the pigment out in a thin layer with the palette knife and tap the brush on the same so as to take up the pigment on the end of the hairs. When the brush has become charged with the pigment, work it around on a clear portion of the glass in order to distribute the pigment evenly. It is well to prepare pigments of two 530 | PHOTOGRAPHY consistencies at the beginning, since it is rare that the same pigment can be used throughout the operation owing to the fact that some parts of the image require a softer pigment in order to make the pigment adhere. Either linseed oil, Robertson’s medium or megilip may be used to soften the hardink. Only a trace of any one of these is neces- sary to completely change the ink and, therefore, they should be added with caution. As the image is only faintly visible, a straight gaslight print will be of service in indicating what should be done and if this print has been “worked up” in the same manner as the oil print, then the worker will have a clear and definite idea of the alterations to make in order to secure the results desired. The proper method of holding the brush is illustrated in Fig. 226. While the manner varies somewhat with the worker, the above may be Fic. 226. Proper Position of the Brush in Pigmenting. (Mortimer and Coulthurst, The Oil and Bromoil Processes) ‘ taken as a safe position to use. The brush must be held lightly and not gripped. Nor should it be held close to the hair. The position is very similar to that adopted by the painter and if one has a friend who is a painter, he can no doubt secure some advice from him on this point. The method in which the brush is handled to apply the pigment also varies greatly as almost every worker has developed individual methods in practice. Some have a pressing, smudging action while others simply dab the brush on the surface. It is difficult to give any precise directions on this point and the worker will develop a dis- tinctive method of his own with practice. It is well to begin pigmenting with the ink as it comes from the tube : : and continue until the image is distinct and the shadows well defined, when it may be advisable to change to a softer pigment. It is easy to ee ke Pee: THE OIL PROCESSES 531 tell when more pigment is needed for the brush will begin to pick up the pigment instead of depositing it. Use the hard ink as long as pos- sible for it is possible to apply softer ink over the hard if the latter refuses to adhere but hard ink cannot be applied over soft ink. To lighten any portions which have been over-pigmented the opera- tion known as “ hopping ” is used. In this the brush is held vertically above the surface and allowed to fall upon the paper. The brush is never dropped from a greater height than about two inches or the gelatine might be punctured and the print destroyed. Wire holders are supplied to hold the brush so that hopping may be carried on with- out fatigue. The operation should be looked upon as a corrective only and not used unless necessary as far better results are obtained by straight pigmenting to the point desired, but there are cases where it will be necessary to ink over small details and use the hopping action afterward to clear out the highlights so the worker should become — familiar with the operation. It is necessary that the print be kept in a moist condition through- out pigmenting or the pigment will adhere all over. If the side of the print underneath feels dry to the touch the pad should be re- wetted and the print may also be placed upon the surface of a tray of water for a few minutes and, finally, again removed to the pad, and pigmenting begun. Success in oil printing is dependent on two things—understanding and practice. The worker must not be discouraged if his results at first fail to satisfy but must stick it out and he will gradually note im- provement in the results. His first endeavors should be directed towards securing a smooth, even application of the pigment and be- coming familiar with the results secured with different inks and man- ner of handling the brush. After he feels that he has mastered the technical principles involved and can make a good straight print with certainty, he may then attempt to make alterations as his artistic taste may direct. Local values, however, should be studied very carefully and alterations should not be made until the worker has satisfied him- self that they are advisable. It is not exceedingly hard to master the oil process technically but to master it artistically is an achievement and one deserving of the reward which such masters as Demachy, Mortimer, Job and Puyo have received. Incorrect Exposure.—In the case of over exposure, the print takes up the pigment too easily and the shadows soon block up and lose 532 PHOTOGRAPH. their details while the half-tones appear smudged and the highlights take the pigment also and rapidly darken. Ii the print is much over exposed, it may as well be thrown away, but if only slightly over ex- posed, the use of a hard ink and hopping may produce a passable re- sult. It is better for the beginner to throw away a print of this na- ture and make another as all the skill of an advanced worker is re- quired to get a passable print from one which has been incorrectly exposed. : With correct exposure, the shadows take the pigment gradually and the half-tones and highlights keep their proper relations. ‘This shows that the print has been suitably exposed for the pigment in use and all that is necessary is to keep on applying the ink until the desired result has been secured. Success in pigmenting depends greatly upon the exposure and every care should be taken to secure correct ex- posure in order that pigmenting may be a straightforward and certain operation. When the print has been under exposed there is difficulty in making the pigment adhere even in the shadows. Prolonged brush action causes the deposit to become granular and thin. This granular ap- pearance is always an indication of either under exposure or the use of too hard anink. The addition of a small amount of megilip or oil may be sufficient to soften the pigment so that it will adhere. If the first addition is not sufficient more may be added but it is better to use a hard ink than a soft one as the latter does not preserve the con- trasts properly but tends to produce the effects of over exposure. Only a very small quantity of oil or megilip is necessary to soften - the ink and care should be taken not to make it too soft. As in the case of over exposure, a print which has been very much under ex- posed is unsuitable and should be thrown away. - Drying and Mounting.—When pigmenting is finished the print may be hung on a line in a dust-free room to dry. It may be placed in a horizontal position for drying but in this position it is more likely to collect dust. The paper and gelatine may require two or three hours to dry but the pigment takes quite a time and it is well to allow at least twenty-four to thirty-six hours for drying. For mounting, it is best to use the dry process but the pigment should be thoroughly dry before mounting or it will come off while in the press. If glue or paste is used, it is best to only tip the corners so that they stick to the mount and not to try and mount the print flat. The print may be _ ; | ; ey en en eee Se ee ee ee ee ee Cae ae eee a ee a ee ee Pe a ee oS ee AT fie ho 9 aii aa See ea oe 2 ees | ey pe OR THE OIL PROCESSES 533 rubbed with a soft cloth after the pigment is dry to remove any par- ticles of dust adhering, but so far as possible these should be avoided by drying in a perfectly clean and dustless place. The point of a sharp knife may be used to remove loose hairs, etc., which are em- bedded in the pigment. Duvivier’s Process.—Monsieur Duvivier in his work Le Procédé a ’Huile en Photographie describes a new process of oil printing in which the usual gelatine paper is replaced by one with starch. A thick, unsized paper is coated with starch, sensitized and exposed in the same general way as usual in oil printing. After exposure and de- velopment, the print is dried. It is then placed face up upon a pad of wet blotting paper. The paper is able to absorb water from the back, but those portions which represent the shadows and half-tones of the image are protected to varying degrees by the bichromated starch coating and remain dry while the highlights and lighter tones take up water in varying proportions. The highlights thus become moist enough to repel the ink while the shadows being dry take up the ink readily and the print is thus in a similar condition to the swol- len gelatine used in the usual oil process. In the case of the starch process, however, the difficulties of pigmenting are lessened owing to independence of the variations in the degree of swelling and conse- quently the adjustment of the pigments does not require to be as fine, so that the process is much simpler than oil. For full details the original work should be consulted. The Bromoil Process.—Very similar to the oil process is Bromoil. The bromoil process, in brief, consists in the making of a good bro- mide print in the ordinary way and bleaching this in a solution which produces an image in insoluble gelatine having the property of taking up pigment from a brush in just the same way as oil printing. Ow- ing to the fact that an ordinary bromide print is used, no daylight is necessary at any stage and as enlarged negatives are not required when an oil print larger than the original negative is desired, the bromoil process is a very popular one among pictorial werkers and bids fair to entirely supplant the older oil process. The Choice of the Paper for the Bromide Print.—While theoreti- cally any bromide paper should be suitable for bromoil, in practice such is not the case. There are considerable differences amicng va- rious papers in respect to adaptability to bromoil, while there are some few papers which can be used only with difficulty. The qualities of A ae 534 Si PAOPTOURAT Eas a bromide paper adapted for bromoil as indicated by Professor Namias are: : 1. Hard, durable and well-sized paper base. 2. Emulsion rich in silver and gelatine and thickly coated. 3. No hardening substances to be added in manufacture. It is not possible to use papers the swelling power of which has been lessened by hardening with alum, or other means, in the process of manufacture. The principle of the bromoil process is that a tanning of the gelatine shall take place differentially in exact proportion to the opacity of the original silver deposit, so that we get a tanned image in a bichromated colloid. -If, however, the emulsion has been hard- ened in manufacture the gelatine is already tanned and has lost most of its swelling power, so that it is impossik‘te to get the degree of re- lief necessary for proper pigmenting. | To determine whether a particular brand of bromide paper is suit- able for bromoil, Dr. Emil Mayer, in his Bromoil Printing and Trans- fer, suggests that an unexposed sheet of the paper be dipped in water at a temperature of 86° F. (30° C.) and the behavior of the gelatine film observed. If this swells up considerably and becomes slippery and shiny, the paper has the necessary swelling power and can be used for bromoil. A smooth matt paper is the best adapted for bromoil. Glossy papers are unsuitable, and there is, in many cases, difficulty with the rough surfaces of certain brands of paper. While most of the reputable brands of bromide paper may be used successfully, several manu- facturers now supply papers made especially for the bromoil process. These withstand rough treatment better, and being more thickly coated and unhardened, give more relief than ordinary bromide papers (the © a emulsion of which is nearly always partially hardened in manufac- ture), and are consequently more easily pigmented. Among such papers available at present, the following may be mentioned: Welling- ton Bromoil, Vitegas for Bromoil, and Gevaert for Bromoil. The Production of the Bromide Print.—It cannot be too strongly emphasized that the production of a bromide print suitable for brom- oil is a matter of great importance and one on which the success, or otherwise, of later operations largely depends. In fact, one should not attempt bromoil until he has complete mastery over bromide print- _ 3 ) ing and can make it responsive to his demands. The bromide print ‘T=? = > ———— Ee ae —— ee ee THE OIL PROCESSES | 535 to be used for bromoil should be the best which that particular nega- tive will produce. The best prints for bromoil are the result of cor- rect exposure and development for a period slightly less than that re- quired for full depth. To this end the factorial method of develop- ment may be used as indicated in the chapter dealing with bromide printing. A lower factor should be used, however, and Dr. J. B. T. Glover recommends the use of a factor of 5 with the following amidol developer, which is the standard formula of the Kodak Company : ° Penn oy Oe ow vase ne eee ces 26.2 gr. .6 gm. Benmore nnitestdry) osc. eed es el TIO. gr. 25 gm. Potassium bromide (1 per cent solution)....... 7. eatin. 15 cc. em ie RSP, ls oy apn die cs oi are WA Riess 10). OZ, 1000 cc. Practically all developing agents in general use have a more or less pronounced tanning action on gelatine. The use of such developing _ agents, therefore, has the effect of producing an additional tanning action, not only on the shadows, where it might be desirable in certain cases, but also on the highlights where tanning of any kind is ob- jectionable. ‘The use of a developer with a pronounced tanning action has, in fact, the same effect as general fog in negative making. With emulsions which have been hardened in manufacture the use of tan- ning developing agents obviously is even more objectionable than in other cases. Accordingly the use of a developing agent without tan- ning action on the film is desirable. Such agents are amidol (Diamido- phenol), glycin and the iron developer. While the last named is, with certain precautions, excellently adapted to the development of bro- mide papers for ordinary purposes it is unsuitable for prints to be used for the bromoil process. Glycin is not especially well adapted to the development of bromide papers as it is slow in action and of the three, amidol is indisputably the best. There is no especial virtue in formulas and that given above will answer any requirement. The worker, however, may use that advised by the manufacturer provided development is regulated properly. The fixing which follows development is an important operation. A plain bath of 20 per cent hypo should be used. This must be made fresh for each bath of prints and discarded after use. The use of an acid fixing and hardening bath is to be avoided as, owing to its action on gelatine, considerable difficulty is experienced in bleaching and in securing the necessary relief for pigmenting. a, J.-F ., 1021, 68, 87. 536 PHOTOGRAPHY Washing should be thorough, as the slightest trace of hypo left in the print will cause trouble in bleaching. At this point, before leaving the subject of the ieonikde print, it is well to remark that the print should have a plain white margin of at least half an inch. Bleaching of the Bromide Print.—As soon as washing is complete the bromide print may be bleached, or it may be dried and kept for bleaching and pigmenting at some future time. It is perhaps prefer- able, however, to allow the print to dry at this stage. Then when ready for bleaching it can be immersed in water for a few minutes until thoroughly limp. The functions of the bleaching solution are two in number: (1) It removes the visible silver image and (2) it causes a tanning of the gelatine film corresponding to the silver image that disappears. In place of the original image of metallic silver, there then exists an invisible one of differentially tanned gelatine. Ordinary reducers are — therefore unsuitable. They dissolve the silver image but do not pos- sess the property of tanning the gelatine film in the required manner. Dividing bleaching and tanning agents for the bromoil process into the substances they contain we have: a. Copper sulphate, potassium bromide, potassium bichromate. b. Copper chloride or sulphate, potassium bromide, chromic acid. c. Copper’ chloride, sodium chloride, potassium bichromate. No reliable methods of testing being available, it is impossible to say that any one of these is better than the other. Bleachers of all three types are used by various noteworthy exponents of the process and so much depends upon a knowledge of the bleacher and its action, and the mode of pigmenting, that it is in manipulation rather than in the — q type of bleaching solution that the causes. of failure should be sought. Dr. Mayer, the celebrated Austrian expert, gives the following formula fur a bleaching solution: . : A) Copper sulphate aio... 2) 0 eee ee ee % Oz. 20 gm. Waterinc: b-awa. Laois tw HE Qe eee 3% 02. 100 cc. B.. Potassium bromidé..):.cincutactetes a eee ¥4 OZ. 20 gm. Weiter oe sous was aga Maman igi eae haan 314 oz. 100 cc. C. Potassium: bichromiate,... cy cee Oo ee 150 “gr. — Io gm. Water iets sse male ee oe ae 34 oz. 100 cc. es on ars PROCESSES ~ DOL For use take: TEM iS swale halts sp bada os Dra, Cre: 60 cc. I OE ann ae ee O23 60 cc. RE AO nfo. bids, 2082 e hsb, vib o's es ov ales *% OZ. 20.CC. Te ny ain un & aston 4.slapieiais'n « «ne ve i See a 450 cc. Tepe AU | (COME... ce ees ay cess 15 drops Raymond E. Crowther advises the following bleaching bath which he claims is entirely without action on plain gelatine but exerts a power- ful tanning action in conjunction with the silver image: Copper sulphate (crystal) 10 per cent solution 170 min. 9.6 cc. Potassium bromide Io per cent solution........ 130. = min. ps an Chromic acid 1 per cent solution.............. 45 min, 27 OC, een es. cee cease eevee s 3% oz. 100 CC. This bath should bleach the image in 3 minutes at 60° F. When the temperature is abnormally low, say 40° F., the bath may be used double strength. If the print has not been completely fixed and washed, the bleaching operation will not be successful; it therefore affords a means of indicating the thoroughness of these operations.® Writing in the British Journal of Photography on the Bleaching of the Bromoil Print (1924, p. 427) H. J. P. Venn, B.Sc., strongly ad- vises the use of two separate baths for bleaching and tanning. Before bleaching the print is soaked in water for 5 minutes and then drained and transferred to the bleaching bath (No. 1) composed as follows: Copper sulphate (10 per cent eee eee er eee ere re 95 parts Ree GIFU Ge ici eisai = 5.5 nce wa oda a Gree 4a Me ay Moe ae Oe 5 parts It is then drained and transferred without rinsing to the tanning bath (No. 2) which consists of isoguer sulpiate, (10 per cent solution): i200)... ees es eae 90 parts Potassium bichromate {1 per cent solution) ........2..-)..00.- 10 parts It is allowed to remain in this bath for 4 minutes, then washed in several changes of water, each of five minutes’ duration, and fixed in a 10 per cent solution of hypo for two minutes. After about 15 minutes’ washing the print is dried, being again soaked in water before inking up until the desired relief has been reached. This time will vary with the grade of paper. The times of soaking for a few of the more common grades-are as follows: 3 A. P., 1921, p. 446; 1922, De 2 538 PHOTOGRAPHY Kodak Rovaree ion a. geciner |) ae eee 45 min. at 75° F., (ear Barnet: (Geter eres ooo he egy a 30 min. at 65° F. Corie" Wellington Bromo cco eo. isk a ee 30 min. at 65° F. Cae" Somewhat longer periods of soaking will do no harm but the ink used will then require to be slightly softer. To secure a print suitable for pigmenting from a contrasty negative increase the amount of potassium bichromate by using a 5 per cent solution. When used in the two-bath process this does not complicate the process of making. Chemical Theory of the Bleaching Operation.—Not a great amount of work has been done on the chemical theory of bromoil and little is known of the exact nature of the chemical reactions which form the basis of the process. The reactions which take place in the bleaching and tanning of the bromide print are, according to Mr. H. J. P. Venn, as follows: Bleaching : (1) Ag + CuSO, + 2KBr = Ag + CuBr + K,50,. Production of tanning agent: oe (2) 6CuBr + 6K,Cr,O, = 6CuCrO, + 6KBr + 3K,CrO, + Cr,O, -CrO, (chromic chromate. ) Tanning : (3) Cr,O,-CrO + gelatine = chromated or tanned gelatine.* The exact chemical composition of gelatine tanned by the chromates or the reaction between the two substances which results in the in- soluble condition has never been definitely ascertained although many authorities, notably Lumiere and Seyewetz, have done much work on the subject.® Fixing.—In the process of bleaching and tanning a secondary image of silver bromide is formed. ‘This image is light sensitive and, while not visible at the time, will appear upon exposure to light. It is there- 4 As a result of later investigation on the theory of the bleaching bath in the bromoil process, H. J. P. Venn suggests the following equation as more nearly representing the course of the reaction in the second stage than that given above: 3Cu.Cl, -- K,Cr.O, + 7H,O = 2KCl + 2CuCl, + 4Cu(OH,) + 2Cr(OH),. 5 Lumiére and Seyewetz, Bull. Soc. franc. Phot., 1904, p. 73; 1905, p. 440; 1905, p. 461; 1905, p. 541. 3 ee ee ee a ee eS ee ee ee ee ee a ee Pe ee eee ee ee ee ee ee ee eee ee Ss a ee oe Orne ee a ee ae ae Pe ay THE OILePROCESSES 539 fore necessary to fix a second time, in order to remove this silver bro- mide. ‘The fixing bath for this purpose consists of a plain 10 per cent solution of hypo. The usual thorough washing should follow the fix- ing operation. The print must then be allowed to dry normally.® Producing the Relief—When ready to begin pigmenting, the print is immersed in water and the gelatine allowed to swell. The degree of swelling is controlled principally by the temperature of the water and, to a lesser extent, by the time of immersion. The higher the tem- perature of the water in which the print is soaked, the greater the swelling and the more pronounced the relief. With insufficient im- mersion, or the use of cold water, the degree of swelling will be in- sufficient and such prints will, when pigmented, have a short scale of gradation with poor tones. The use of excessively warm water, on the other hand, will produce a pronounced relief which, when inked up, may produce a result having greater contrast than is desirable. Be- tween these two extremes of temperature lies an entire series of inter- mediate stages, which may be employed as occasion demands. Different papers vary as regards the temperature necessary for pro- ducing the best relief. Some are ready for pigmenting after soaking for several minutes in water at ordinary room temperature. Others require as high as go° F. (32° C.) or more; the general average being about 75-80° F. (24-27° C.). The worker must learn by experience the temperature to use for his particular brand of paper and manner of working, always remembering that it is best to start with a rather low degree of relief, which may be raised quite easily, 1f required, by soaking iri warmer water, while a relief.once too high can be reduced only with difficulty. Should soaking in warm water at a temperature of 95° F. (35° C.) be insufficient to produce the desired degree of relief, the print may be immersed in a I per cent solution of sodium carbonate as recommended by E. Guttmann. As a rule, however, this _ method should be used only as the last resort. Pigmenting.—The relief having been raised to the required stage the print is placed upon the wet pad of blotting paper and the surface moisture carefully removed with a clean, dry, lintless blotter. It is necessary that all the moisture on the surface be removed or the pig- ment will not adhere evenly. Pigmenting is conducted in practically the same manner as with the 6 According to one method the prints are bleached after development and before fixing. This removes the necessity of the second fixing, but is not so reliable as the method we have described. 540 od OD OG AOU dl on Bs oil process, but there are a few points which might be mentioned. It is best to begin with a stiff pigment in all cases and only apply the soft ink towards the end when it is desired to finish off the roughness of the gradations. Always have a margin on the original bromide print, otherwise there is a danger of getting the brush wet when attempting to pigment the edges. When the wet brush is transferred to the print it immediately begins to remove the pigment and the work will have to be done over again. Do not be ina great hurry. Work quickly but with care and do not treat the delicate gelatine surface roughly or it may be destroyed. Do not be afraid to apply plenty of pigment but do not try to put it all on at once. Smooth it down to an even tint on the palette and take up a little on the brush at the time. When this is exhausted take up more. A smooth, even tone will result if plenty of pigment is used and it is thoroughly worked into the surface. If the pigment is not well worked in, the print will be weak and “ gritty ” and the tone will be impure. | Beginners usually make the mistake of jumping about from one portion of the print to another. Do not do this. It only makes it more difficult to get an even, smooth result. Work systematically, starting on one side and covering the entire print as you go. All detail which is to appear in the finished print should be apparent after the first inking. If parts of the image are inked strongly before the desired details appear, it is difficult to ink these later. Particular care is necessary, especially in the case of large prints, or where a long time is required for inking, to keep the paper stock thor- oughly wet. To this end it is well to soak the print in water fre- quently during pigmenting. A partially dry surface is responsible for many of the troubles met with in pigmenting and if the print is re- soaked for 5—10 minutes in water whenever any difficulty is met with in pigmenting much better results will be secured and many of the supposed difficulties of the process will disappear. Mr. Chas. H. Partington in the American Annual of Photography for 1922 adopts what is probably the most satisfactory method of indicating to the beginner in bromoil printing precisely what brush — work will accomplish. By the courtesy of Mr. Partington I am able to reproduce the print and accompanying data, which serve to show graphically the effect of variations in pigmenting on the appearance of the image. * In Fig. 227, the section, A, has been pigmented with a heavily Seha-wies Pr wee Ver OCR S SES 541 charged brush and no attempt has been made to change the result. This blocks the shadows and increases the gloss of the highlights pro- ducing a soot and whitewash effect. A better result would have been obtained by using less ink and a quicker action of the brush. Fic. 227. Results in Pigmenting. (Partington) At C is shown the effect obtained by not having enough ink on the brush. This gives a very soft, flat result. At D ink was applied as at 4 but “ hopping ”’ was resorted to to re- move the excess ink. The trees in E were “hopped” in order to lighten the tones and give the effect of distance. The portion at / shows the surface when first inked and is included for the purpose of showing the effect when the ink is first applied. 36 : 542 PHOTOGRAPHY At G, the print has been inked with a properly charged brush and slightly “ hopped ”’ to give additional contract. Namias Method of Pigmenting.—A very ingenious method of pig- menting is advised by Namias.’ A small portion of hard ink is carefully mixed with ten times its weight of finely rectified turpentine. The brush is at once charged with this mxiture and the operation of pigmenting begun as usual. The surface of the print quickly becomes covered all over with a fine coating of ink. Continue the dabbing action of the brush. Gradually the shadows appear to take on ink and gain intensity, while the details and half-tones become clearly separated from the highlights. As the turpentine evaporates, both on the print and on the brush, the ink becomes thicker and thicker and as it increases in consistency it begins to pass the print to the brush in the highlights, and from the brush to the print in the shadows. Thus in a very short time the image be- comes built up to a surprising extent. The first part of pigmenting complete, a thicker ink is used. Three parts of hard ink and two parts of soft ink are dissolved in 4-5 times its weight of Rectified ‘turpentinék.. salsa ee Jn sce Oe aes ‘-%, part Gasoline (pure)... \.c0 cea ys wea wan te Pace «9c 2 parts This second ink is applied in much the same way as the first but is intended for the lighter tones and must be used considerably harder than the first, or the former will be removed. | Should the final result be unsatisfactory the print may be swabbed with absorbent cotton saturated with benzol. This will remove every trace of the pigment. : The treatment of the finished bromoil after pigmenting is identical with the oil print already described. Defatting the Firtished Bromoil.—After the print is dry it is well to remove the oil which is included in the ink, and which has the effect of giving a slight gloss to the print. The sheen is greater in the shadows than in the highlights especially if a soft ink has-been used, as soft inks contain a larger percentage of oil. To many this gloss constitutes an objection, while there is in addition the danger that the ink may in course of time, through oxidation, give rise to colored stains. For removing this oil some solvent such a benzol, carbon tetrachlo- 7 Brit. J. Phot., 1914, 61, 626. nt ~ A iS Tes - = THEO PROCESSES | 543 ride, etc., should be used. Owing to the fact that soft ink may be dis- solved by carbon tetrachloride, which is a more energetic solvent than benzol, the latter is preferable. It is poured in a dish and the per- fectly dry print immersed in the liquid for 5-10 minutes. Bromoil Transfer.—Oil transfer, first introduced by Robert Dem- achy about 1906, is now one of the most popular printing mediums among advanced pictorialists. Bromoil transfer, a natural develop- ment of oil transfer, consists, as its name indicates, in transferring the pigment from the original bromoil to a-sheet of plain paper which may be of almost any surface, texture or color. As the greasy pigment on the bromoil lies on top of a more or less tanned and swollen gelatine film, when brought into contact with any uncoated paper and passed between rolls under pressure, it will leave the bromoil print and adhere to the plain paper. The image in this case, then, consists of pigment on a plain paper base. ‘Transfers, accordingly, have a distinctive ap- pearance entirely unlike that of any other printing process, with the exception of photogravure, since in all other processes the image is imbedded in gelatine or in some other colloid, while in these two proc- esses the image lies on a plain, uncoated paper. Added to this is the advantage of being able, by combination transfer, to extend the scale of gradation, and exercise over the finished result a degree of control which is beyond the limits of even the bromoil process, as flexible as this may be in the hands of the expert. Making a really good transfer is not as simple as might be assumed from an outline of the operation. Familiarity with the bromoil proc- ess, even, does not assure the worker of being able to produce ac- ceptable transfers at the start; only by constant experiment and study can one hope to master the process. But the results are such as to amply repay one for the labor involved in mastering the process and one who has become thoroughly familiar with oil or bromoil should not rest satisfied until he has also attempted transfer. The Bromide Print.—In general bromide papers which are suit- able for bromoil are also adapted to bromoil transfer. According to Mr. C. J. Symes, super-coated bromide papers (i.e. papers which have received a double coating in order to render them non-abrasive) yield particularly fine transfers under certain conditions, namely : (1) The image must be rather stronger than for bromoil; there must be a distinct veiling of the highlights and the print, as a whole, must be a shade darker than if the straight print were intended for exhibition. : 544 PHOTOGRAPHY (2) Development of the bromide print must be full. If the kodak amidol formula is used the print must be developed to a Watkins fac- tor of at least 16. (3) The print must be swabbed with cotton wae before inking, if a bleacher of copper chloride, hydrochloric acid and bichromate is used. | (4) Each batch of paper must be tested for the time of soaking, owing to possible variations in the super-coat.® Preparation of the Bromoil.—To obtain a transfer of peda aaatiey soft ink must be used in pigmenting the bromoil as it is impossible to transfer hard ink with certainty, owing to the tenacity with which it adheres to the original bromoil. Soft ink, however, cannot be used unless a high relief is obtained or the ink will adhere to the highlights of the bromoil and a print of the proper gradation cannot be obtained. Consequently it is necessary to start with a rather high relief; this fact must be borne in mind when the print is being made ready for pig- menting and the temperature of the water in which the swelling takes place regulated accordingly. The use of a high temperature, however, may cause the gelatine in the highlights to soften to such an extent that it pulls off in pigmenting or in transfer. When this occurs it is well to make use of ammonia as previously described. As the ink is more easily transferred from the highlights than from the | shadows, in consequence of the greater relief of the former and the fact that owing to the tanning of the* gelatine the pigment in the shadows is more strongly retained than in the highlights, the contrast of the transfer is usually much less than that of the bromoil. In pig- menting, therefore, the bromoil is made considerably more contrasty — than would be required were it to be left as it is. Owing to the fact that the transfer of ink in the shadows may not be complete, it is the practice of many workers to considerably over ink such portions in order that the transfer may have the proper depth in the shadows. © To reach the same end other workers have recourse to multiple trans- fer; the first bromoil. being inked normally and transferred, then reinked, paying especial attention to the shadows. Dr. Emil Mayer has found that the difficulty of transferring a bromoil to transfer paper without loss of depth in the shadows, due to an incomplete transfer of ink, may be overcome by first passing the transfer through the press with comparatively light pressure, then 8 Brit. J. Phot., 1923, 70, 103. THE Oll. PROCESSES 545 separating the bromoil and the transfer paper (without shifting their relative position) so as to expose both surfaces to air. Then place the two in contact and run through the press the second time with in- creased pressure. With this procedure the transfer of ink is almost complete and there is no necessity for over pigmenting of the shadows, or for a second inking. He also finds that there is no advantage in passing the transfer through the press repeatedly with increased pres- sure, when this procedure is followed, as the transfer of ink takes place immediately and increased pressure only serves to produce an un- necessary strain on the gelatine.° The Transfer Paper.—Theoretically any paper should be suitable for the transfer but in practice there are some marked limitations. Without going into a detailed discussion of the adaptability of various makes of papers, it may be said that only pure rag paper is suitable. The commercial water-color and drawing papers of reliable makers are, as a rule, suitable for the transfer, as are the Japanese and Chinese papers, but the very best paper is that manufactured especially for copper-plate printing. In general, however, the worker will not have much difficulty in using reliable makes of drawing papers, such as, for example, the Strathmore papers of the Mittineague Paper Co. which are obtainable from most dealers in art goods. With very absorbent papers sizing may be necessary, as the pig- ment sinks into the pores of the paper and the picture has a flat, “sunken in” appearance. For this purpose make up the following solutions : eee eR hea cs ces oh dees eadyes- LOO CC 2 OZ: The arrowroot should be rubbed up with a small quantity of water and added with constant stirring to sufficient boiling water to make a total volume of approximately 100 cc. (3 0z.). This is applied with a Blanchard brush or tuft of absorbent cotton and the paper allowed to dry without heat when it is ready for use. As a general rule the transfer paper should be used dry. There are some few papers, however, which require to be slightly dampened. For this purpose it is sufficient to thoroughly and evenly dampen two sheets of blotting paper and place the sheet of transfer paper between them and under slight pressure for several minutes. As different ® Amer. Phot., 1924 (July), p. 410; Brit. J. Phot., 1924, 71, 412. 546 PHOTOGRAPES 2 i papers act differently in transferring, the beginner should stick to ene make and surface of paper until he is thoroughly familiar with it. — Then he may, if he desires, experiment with other makes and surfaces. | The Transfer Press.—Special presses for transfer are supplied by _ the Autotype Co. of London and by Sinclair, also of London. These . i Fic. 228. Transfer Presses are similar to the presses used by copper plate printers and are ex- pensive. Very good work can be done, however, with one of the old burnishers as used in past days for the glazing of prints or with the better grades of domestic wringers. Whatever the type of press it should satisfactorily fulfill two requirements: (1) the pressure on the rolls must be absolutely even and capable of regulation by the worker and (2) one must be able to examine the condition of the transfer at any time without danger of shifting the position of the bromoil or the transfer. We illustrate in Fig. 228 the Autotype and Sinclair presses. Perhaps one of the best forms of press for bromoil transfer is that THE OIL PROCESSES 547 described by K. Prett.*° This press, which is shown in Fig. 229, is similar to that used by collotype printers. The bromoil in contact with its transfer paper is placed between the two plates of the pressure pad, P. ‘The pressure is then adjusted as required by means of the pres- _ sure bar, C, and the movable pressure pad, P, is drawn underneath the pressure bar, C, by the windlass, W. The whole affair may be made Fic. 229. Prett’s Transfer Press of wood; the two plates composing the movable pressure plate, P, being lubricated with a little talc to make them slide regularly. The pressure which can be attained exceeds that of the roller type of press while there is no danger whatever of a displacement of the bromoil _and there is less wear on the original bromoil than possible with any other type of press. Transferring the Pigment.—As soon as pigmenting is complete the bromoil is ready for the transfer. For this purpose we require, in addition to a suitable press, two sheets of blotting paper, three sheets of thick, hard, glazed pasteboard and a pad of felt—all of which should be at least double the length of the bromoil print. On one of the sheets of pasteboard is placed one of the sheets of blotting paper and on this the pigmented bromoil, face up. Over this is placed the transfer paper, and over this another sheet of blotting paper. These two sheets of blotting paper serve the purpose of absorbing the mois- ture squeezed from the bromoil print which might otherwise cause trouble. Finally a sheet of pasteboard is placed over the blotting paper, then over this the felt pad and lastly another sheet of paste- board. 10 Phot. Rund., 1923, 1,5; B. J. P., 1923, 70, 300. ae © ee 548 PHOTOGRAPHY The entire pack is now inserted between the rollers and carried through once with a uniform motion and with but slight pressure. , The pressure is then increased slightly and the pack carried back through the press in the opposite direction. Then the top of the press pack is removed, the cover of the transfer paper raised and the ap- pearance of the transfer examined. If the transfer of ink is only slight, the press pack is replaced and carried through the press again with increased pressure. Then if the shadows still lack intensity reg- istration marks should be made, the bromoil print removed, resoaked in water and the shadows pigmented after which the bromoil is placed on the transfer paper, its position registered, and again passed through the press. The pressure should not in any case be so great that the rolls can be started only by a decided effort; they must always move easily and smoothly. -“ Repeated slow passage of the press pack through moderately tightened rollers is always more advanta-. geous than a single passage under very heavy pressure.” ** With heavy pressure there is likewise the danger of destroying the bromoil, as the gelatine film in its swollen condition may adhere to the trans- fer paper. This trouble, however, is occasionally met with when using some papers with only a moderate amount of pressure. To prevent this, Dr. Mayer suggests that the transfer paper be sprayed with oil of turpentine by means of an atomizer.” After spraying the sheet is allowed to stand for fifteen or twenty minutes in order that the turpentine may evaporate. ‘This is a cer- tain preventative of sticking, but sufficient time must be allowed for the turpentine to evaporate, or muddy, uneven transfers will result. — Zaepernick’s Chemical Transfer Method.—In American Photog- raphy, 1924, p. 732, Hans Zaepernick describes a method of bromoil transfer which he terms chemical transfer. He says: The chemical transfer in its simplest form consists of dampening the paper on to which the bromoil, prepared in the usual way, is to be transferred, not with water, but with a solvent of the ink. The solvents for the greasy inks are: petroleum ether, benzine, benzol, and oil of turpentine. The transfer of the ink from the bromoil to the new surface is effected after solution has taken place by the absorption and adhesive power of the transfer paper. For perfect trans- fer of the ink, light pressing together of the two surfaces is essential. Even the light pressure obtainable in a printing frame or light rolling with a roller squeegee is enough. If this method of working is adopted, the bromoil should only be lightly inked. 11 Guttman, Bromoil Printing and Transfer, p. 164. 12 Brit. J. Phot., 1924, 71, 412; Amer. Phot., 1924 (July), p. 410. THE OIL PROCESSES 549 If the inking has been too heavy, the transfer will, as a rule, be too plucky as all the ink goes on to the transfer paper. The degree of hardness or consistency plays but a subordinate part in this process. If oil of turpentine is used for dampening the paper, black inks show a brown- ish tinge. With benzine this does not. occur. The advantages of chemical transfer are that since but little pressure is re- quired, it is not necessary to invest in an expensive press and that any kind of paper, even the extremely thin Japanese tissue, may be used. Rowatt’s Process.—In the Club Photographer for February 1922, 157, Mr. J. Rowatt describes a method of offset bromoil transfer which he claims removes most of the difficulties of the ordinary bromoil transfer. The pigmented image of the bromoil is transferred to a rubber blanket and from the latter to the final support. The bromoil print accordingly does not require to be reversed as in ordi- nary bromoil transfer, when unreversed prints are required. For more complete details we must refer the reader to the original. Multiple Transfer.—Multiple transfer is employed in the same gen- eral way and for the same purpose as in gum-bichromate printing— namely, to lengthen the scale of gradation in order that every pos- sible tonal value contained in the negative may be properly rendered. The multiple transfer may be made from one or more bromoils. If only one bromoil print is used, it is first inked up with hard ink, so adjusted to the relief of the print that the shadows alone absorb any considerable quantity of ink, the lighter half-tones and highlights re- maining untouched. This corresponds to the shadow coating in gum- bichromate. The pigmented image having been transferred to the transfer paper and means of registration provided in order that it may be placed again in identically the same position, the bromoil is again pigmented, but this time with a soft ink so as to produce a thin, smooth film of ink which reproduces the highlights and half-tones while adding but little, or not at all, to the shadows. This transfer obviously corresponds to the highlight coating in gum-bichrornate. Instead of using the same bromoil print for both transfers, two separate bromoils may be used. This method has the added advantage that different papers may be used for the two bromoil prints, and that the degree of relief of the two prints may be regulated so as to more easily obtain the effect desired in pigmenting. 550 PHOTOGRAPHY GENERAL REFERENCE WorRKS DEMACHY AND Puyo—Les Procedes D’Art en Photographie. Duvivier—Le Procede a L’Huile en Photographie. Eper—Das Pigmentverfahren, der Gummi-, Oel-, und Bromol-druck und ver- wandte photographische Kopierfahren mit Chromalzen. FuHRMANN—Der Oeldruck. GutrMAN—Die Se‘bstbereitung der Bromoldruckfarben. GutrMAN—Der Umdruck in Bromoldruckverfahren. KuHN—Technik der Lichtbilderei. LAMBERT—Oil and Bromoil. Mayer—Das Bromoldruckverfahren. Mayer—Bromoil Printing and Transfer. English translation by Fraprie. Meses—Der Bromoldruck. Puyo—Die Oilfarben-kopierprozess. Puyo—Les Procedes aux encres Grasses. SINCLAIR—How to make Oil and Bromoil Prints. TILNEY AND Cox—The Art of Pigmenting. TILNEY AND JupGE—Oil and Bromoil Transfer. MortTIMER AND CouLTHuURST—The Oil and Bromoil Processes. . STENGER—Neuzeitliche photographische Kopierverfahren. (Ozobrom, Brom- silber, Pigmentpaper, Oldruck, Bromoldruck.) STENGER—Die Kopierverfahren, 1926. Photo-Miniature No. 106—The Oil and Bromoil Processes. Photo-Miniature No. 186—Bromoil Prints and Transfers. , : J CHAPTER AXV COPYING Introduction.—Copying is a branch of photography in which many do not succeed, not because of any inherent difficulties the work pre- sents, but because the essentials of the subject which are necessary to success are not thoroughly understood. With the proper apparatus and materials and an understanding of the factors involved, copying is in no ways more difficult than other photographic work and provided the worker knows what he is about he should meet with but little diffi- culty. In discussing the subject we will consider first the apparatus ad- visable, then the optical principles involved and the proper treatment for different classes of copies and finally the photographing of small objects in the studio. Apparatus for Copying.—In hardly any branch of ordinary photo- graphic work is apparatus so important as in copying and for this reason the question of equipment should be settled before the work is begun. In the first place it is essential that some means be pro- vided whereby the camera may be moved to or from the subject with- =— 2s —— = —_ - — Fic. 230. Copying Stand Fic. 231. Book Holder for Copying out destroying the parallelism necessary to prevent distortion. Stands for this purpose are made by several firms or.a simple arrangement may be made at home by anyone familiar with tools. Figure 230 shows a simple fixture which fills all ordinary requirements and is of simple — 551 002 3 PHOTOGRAPHY construction. The essential parts are the tracks AA on which the camera moves to and from the easel C which is rigidly fixed at right angles to the base B provided for the camera. In cases where an apparatus like this cannot be used, as when photographing a large map or oil painting, a small celluloid T square should be used to determine if the image of the subject on the ground-glass is free from distortion. For copying from books a holder such as illustrated in Fig. 231 is very convenient. On the whole, however, it is much simpler to use a vertical stand, as it is much easier to keep the page flat when the book is in this position. In fact, a-vertical stand is more convenient for nearly all general copying as there is no trouble in attaching the print to the easel and, if daylight be used for illumination, it is easier to secure uniform illumination with the print in this position. Many of the stands on the market may be used vertically as well as horizontally and, as we will see later, the possibility of using the stand in a vertical position is particularly advantageous in another form of copying. Methods of Illuminating the Print.—The light which illuminates the print to be copied should not only be evenly distributed over the whole print but it should also come from more than one source. The reason for this will be all the more apparent when we have to deal with papers of coarse and irregular texture such as used for drawing pur- poses. A side lighting from a single concentrated source accentuates the graininess of surface by causing the innumerable projections to cast shadows on the side away from the light. At the same time the projections themselves receive the direct illumination on one side and © therefore we have a highly lighted spot in immediate contact with a deep shadow so that the irregularity in the surface of the paper is made far more noticeable than is actually the case and the copy shows a “ graininess ’ which is almost inconceivable when the original is ex- amined visually in a good light. When copying by daylight it is very difficult to secure uniform illumination and prevent the appearance of an undesirable amount of “eraininess.” All papers which do not have a glazed surface may be copied in the position shown in a of Fig, 232. For glazed prints this position is unsuitable, as the highly glazed surface reflects light into the camera and obscures the image. In such cases, and also in the case of some matt papers which have a “ velvet ” or enamelled surface, the relation between the print and the light source should be that shown in b of the same figure. The presence of reflections can usually be determined from the ground-glass but an infallible rule is to remove ——— COPYING 553 the ground-glass and the lens and examine the print from the back of the camera at various angles. The constant fluctuation in the strength of daylight and the difficulty of securing even illumination make artificial illumination especially desirable. For several years the writer used with complete success the arrangement illustrated in Fig. 233. The interior of the box which Fic. 232. Illumination of the Copy Using Daylight encloses the mazda lamps is painted with white enamel to increase the reflecting power and the bulbs are placed back from the circular open- ing so that no stray light can reach the camera even when very close to the copy. Frosted light bulbs were found to give better illumina- tion with less tendency to reflection and glare than plain bulbs. While BiG. 29%: Copying Apparatus for Artificial Light (Rose, The Commercial Photographer) somewhat elaborate this outfit is easily constructed and fully repays its expense where a considerable amount of copying must be done, for the circular system of lighting is the most effectual way of avoiding _“ graininess” that the writer has been able to find. Two mercury vapor tubes, one on each side of the copy, make a satisfactory light but the initial expense is higher. Two large mazda lights, one on each side of the copy, are sufficient when dealing with small copies but fail with very large originals and it is also difficult at times to avoid reflec- tions. . 554 PHOTOGRAPHY Copying Cameras.—For copying the worker has the choice of the instruments made especially for the purpose or the use of view or other types of plate cameras. Regular copying cameras have a long bellows, a central compartment for lenses and quite often a frame of kits at one end for negatives which are to be reduced to lantern slides. When necessary the lens board may be moved from the central par- tition, where it is placed when making lantern slides, and substituted for the frame of kits in order to obtain greater bellows capacity. Cameras of this type are made by a number of firms and reference to the catalogs of large dealers will show what may be expected in a camera of this type. Long bellows view cameras which focus from the rear are very satisfactory for copying: in fact in one respect they are actually more convenient than those made especially for the purpose. Copying cameras are seldom fitted with a rising and falling front, but this feature is very convenient at times as it enables the image to be prop- erly adjusted on the ground-glass without the rather laborious operation of removing the print from the easel and replacing it in what is esti- mated to be the correct position. The swing back fitted to view cameras is also useful at times, enabling distortion in the copy to be corrected. At all other times it should be securely locked in the perpendicular position so that it is parallel with the easel. Copying with hand cameras having short bellows is possible only when supplementary lenses are used. These, while useful in such cases, cannot be recommended as they affect the definition of the ob- jective to which they are applied. The Objective for Copying.—Difficult copying demands high-grade objectives. While very good copies can be made with rapid rectilinear lenses these have a falling off in definition towards the margins in addition to astigmatism, both of which are only partially remedied by stopping down. ‘The anastigmat with its flat field and its high free- dom from all kinds of aberration gives critical definition and needs but little stopping down, so that for all work which demands utmost sharpness they are far superior to other types of lenses. Lenses well corrected for astigmatism and curvature of field may still have zonal errors or residual spherical aberration. Zonal aberrations detract from the crispness of definition and reduce the limit of sharpness to which one can work. Coma is also a serious disadvantage to a lens used for copying as it gives negatives having a flat, fogged appearance which is sometimes mistaken for errors in exposure or development. COPYING 555 Lenses of medium aperture are superior in these respects to those of _large aperture, even though both be used at the same aperture, owing to superior correction for zonal aberration. Unquestionably the best lens for all classes of copying is the process anastigmat, such as the Cooke Series V, //8, Goerz Gotar, F/8, Gundlach process F/9, Velo- stigmat process /’/8, etc., but anastigmats of the type represented by the Dagor, Protar, Turner-Reich, and Tessar 11B, /’/6.3, are satis- factory for all but the most critical line work. While the use of a short focus lens means a saving of bellows ex- tension and allows the camera to be closer. to the copy for a given degree of reduction or enlargement, it has the disadvantage that the front of the camera may, in certain cases, interfere with the lighting of the subject while at the same time the danger from reflections is greater owing to the larger angle subtended. In general it is well to -choose a lens having a focal length equal to, or slightly greater than, the diagonal of the largest plate. Focusing.—For accurate focusing a fine-grained screen is needed. Much may be done to improve matters by simply applying vaseline to the ground-glass already in the camera but a much better result can be secured by replacing the ground-glass with a specially made screen. A very suitable grainless screen can be made at home at a very small expense. Take a fast plate (unexposed) and develop from fifteen to twenty minutes in a non-staining developer such as amidol or M—O without a restrainer so as to secure a slight general fog. Rinse and transfer to the following solution: PSE 0 IO gr. 25 gm. ee AiG AE CONC. ke es ee ee ieee es 10 min. eS CC, RR ee, Vy cy ey ne Sine cime ne ctos I Oz. 1000 cc. After several minutes’ immersion in this, remove and rinse briefly in running water, then fix, wash and dry in the ordinary way. A screen prepared in this manner is denser than one of ordinary ground-glass but shows far more detail owing to its freedom from coarse grain. When dry it is well to rule the screen with vertical and horizontal lines 14 inch apart to assist in determining the size of the copy directly without measurement and to indicate the presence of distortion. When this is done, the screen may be coated with negative varnish to protect it from atmospheric action. For obtaining critical focus a PAnenineE must be used. The parallax focusing method, or the use of the Le Clerc diaphragm, in conjunc- 556 PHOTOGRAPHY tion with a focusing magnifier affords the simplest and most satisfac- tory method of obtaining the exact focus. To use the former method proceed as follows: Remove the gelatine coating of the prepared focusing screen from a small portion about an inch in diameter at the center of the screen. On this clear space glue a piece of tinfoil with a sharp edge. A magnifier is adjusted to sharp focus over the tinfoil and may be permanently affixed in this position. As the eye moves sideways in observing the image an apparent displacement occurs. When critical focus is secured there is no apparent displacement and the image and the sharp edge of tinfoil lie in the same plane. Clerc’s method may be used only when the lens is fitted with re- movable diaphragms, generally termed Waterhouse stops. As practi- cally all process anastigmats are fitted with removable diaphragms this method becomes very convenient when such lenses are used. ‘To pro-. duce the Clerc focusing diaphragm lay off on thin metal a circle equal to the diameter of the inside of your lens barrel. Inside of this circle, lay off a concentric circle equal to the diameter of the largest diaphragm of the lens. Draw a diameter of the inside circle and divide into four equal parts, and at the two points between the center and the circum- ference of the circle draw perpendiculars to the diaphragm until they cut the circumference of the inner circle. Then cut out the segments and blacken the metal with dead black, matt paint. When focusing with the diaphragm in place there will be a double image but when critical focus is obtained the images unite and form a single distinct image. Remove, insert proper stop, and expose. Copying to Scale—Assuming that the exact focal length and the position of the nodal points are known, the worker can enlarge or reduce to scale simply by a graduated scale applied to the camera and the stand. The conjugate distances for various degrees of enlarging or reducing and for lenses the focal length of which varies from 3 to 12 inches are given in the following table. When copying on an en- larged scale the distance from the subject to the lens is less than that from the lens to the plate while when copying on a reduced scale the reverse is the case. Where the positions of the nodal planes are unknown, the following method worked out by Mr. D. Charles? may be employed: The first requirement is that the ground-glass focusing screen should allow of horizontal movement in its frame over a small distance 1 Brit. J. Phot., 1919, 66, 736. COPYING 557 DISTANCES WHEN ENLARGING AND REDUCING Times of Enlargement and Reduction Focus of Lensg,. . Inches I Inch 2 Inches | 3 Inches | 4 Inches | 5 Inches | 6 Inches | 7 Inches | 8 Inches <%, 6 9 12 15 18 21 4 27 6 4s 4 3/4 3°/s 31/2 3°/r 3*/s 3/2 7 10!/s 14 17'/, | 21 24/2 | 28 31/2 7 51/1 4?/s 4°/s 41/5 44ie 4 3 /i6 4 8 12 16 20 24 28 3 36 8 6 5}/s 5 44/5 4?/3 44/7 4*/2 41/2 9 134, | 18 22\/ | 27 30/2 | 36 40'/s 9 63/4 6 5°/s 57/5 5'/4 51/1 51/16 5 10 15 20 25 30 a 10 7'/s 67/3 61/4 5°/6 5°/1 5°/s 51/2 II 16'/2 | 22 27'l2 | 33 38'/ 44 49'/2 II 81/4 7/s 67/s 63/5 65/19 62/7 63/16 6 12 18 24 30 36 42 48 12 9 8 7/2 71/5 7 68/1 63/4 7 14 21 28 35 42 49 56 63 14 10/5 9/3 83/4 8?/; 81/6 8 7'Is 8 16 24 32 40 48 56 64 72 16 12 102/s 10 93/5 9!/s 91/7 9 9 18 2 36 45 4 63 7a 81 18 13!/o 12 11!/4 104/; 10!/ 102/7 10!/s 10 20 30 40 50 60 70 80 20 I5 13!/3 I21/p 12 112/3 114/ Tafa II 22 33 44 55 66 77 88 - 99 7 16!/» 14?/3 13/4 1345 125/¢ 124/7 123/s 12 24 36 48 60 no 84 96 108 24 18 16 15 14?/5 14 13°/7 13}'/2 The table is used as follows: Knowing the focal length of the lens to be used and the degree of (linear) enlargement or reduction, look up the figure for enlargement or reduction in the upper horizontal row, and carry the eye down the column below it until it reaches the horizontal line of figures op- posite the focal length of lens in the left-hand column. When enlarging, the greater of the two distances where the two lines join is the distance from lens to the sensitive paper or plate. The lesser is the dis- tance from lens to negative, or picture being enlarged direct in camera. When reducing, the distances are vice-versa: the greater is the distance from lens to original, the smaller from lens to sensitive plate. (British Journal of Photography.) 37 558 PHOTOGRAPHY of 14 to % inch, as may easily be done by cutting a strip of this width off one end of the focusing screen. A pair of fine lines is then drawn on the focusing screen exactly vertical and two inches apart. The left line should be in the center of the ground-glass and the other two inches to the right of it. On the copying easel an accurately graduated scale is fixed: a paper scale may be glued to the surface of the easel or a wooden or metal scale set in flush with the surface. A vertical line is drawn on the easel close to the center, so that its image will coincide with the central line on the focusing screen. ‘The scale should be fixed about halfway up the easel at right angles to the central line, with its zero on the line and the graduations lying to the left and upside down. It is a very simple and rapid operation to slide the ground-glass so that the left hand falls on the zero of the image scale and to note the figure cut by the right-hand line. Thus it is possible to measure in- stantly the image of the rule by the two-inch column on the ground- glass and by focusing and movement of the camera get any desired degree of reduction or enlargement. Where it is required to copy subjects to exact size, or to a certain degree of reduction, at frequent intervals it is convenient to mark on the camera and the stand the positions occupied so that focusing may be avoided in the future. Exposures in Copying.—Five things determine the time of ex- posure in copying: . The strength of the light illuminating the copy. . The character of the original to be copied. . The speed of the plate used. . The actual aperture of the lens. . The effective aperture of the lens for the degree of reduction or en- largement being made. wm BW NN & The strength of the light illuminating the copy is constant when . artificial light is used and may be determined with sufficient accuracy for all practical purposes by a few trial exposures. When copying by daylight an actinometer should be used. The second factor is the one giving the most trouble since it follows no definite law and does not permit of measurement conveniently. Only experience can show what allowances must be made for different types of originals although the following table may be of some as- sistance in this respect. a a ee COPYING 559 Fraction of the total Wat- kins meter Original Relative time time Matt or setui-matt bromide prints, platinums, pencil or ink sketches, steel or wood engraving........ : COMES, PR! Cire ae aare 14 Glossy purple P-O-P contrasty bromide prints, black carbon, black photogravure...... ab aie SRE TS 3 TU hh Saath eae Ve Eichings in brown, sepia-toned bromides, red or green SO ee RAs a re 11 Bo ae ae ee ee ae 1% Contrasty sepia and red prints, gum bromoil and ee iv ah wclse Gs le a ae a ae ae A The type of plate used depends to a certain extent upon the class of subject: thus for line work in pure black and white a process plate must be used; for colored subjects an orthochromatic or panchro- matic plate is required while medium speed, non-color-sensitive plates are satisfactory for photographs and like subjects in monochrome. This matter will be discussed more fully when we come to deal with the handling of these various classes of subjects. With a suitable lens there is no necessity for the use of a very small diaphragm provided focusing has been properly done. Larger aper- tures tend to produce more brilliant negatives and lessen the danger of unsharpness due to vibration during exposure. If the lens is at all suited to the purpose there should be no need whatsoever for the use of a smaller diaphragm than F'/16. The values of the various diaphragms, however, are not constant as in general work but vary considerably with the degree of reduction or enlargement. Thus when copying full size the distance from the nodal plane to the plate is twice as great as the focal length of the _ lens: hence under these conditions the actual value of the stop has in- creased four times so that f/8 has become F/16, F/11.3 has become F’/22, etc. When copying on an enlarged scale the increase is much greater. By working with a certain definite diaphragm the relative exposure for copying or reducing may be calculated from the follow- ing table provided the correct exposure for the same class of subject and the same plate is known for a given degree of reduction. Provided it is possible to illuminate the copying easel with lamps of sufficient brilliance to enable them to be retained at a fixed distance for all originals regardless of size, exposures in copying may be cal- culated quite simply by the method described by Mr. D. Charles in the British Journal of Photography.’ 2 Brit. J. Phot., 1922, 69, 709. 560 PHOTOGRAP ES RELATIVE EXPOSURES WHEN COPYING OR REDUCING New Scale of Reduction for which Exposure is Known Scales of Reduction I 3/4 2/3 1/o 1/g 1/4 1/5 1/6 L/g 1/10 1 /o9 1/39 I I I/, | to | 18/4 | 24a | 242 | 3.1 Be SIS inane eee 3/4 3/4 I 11/10 13/4 13/4 2 2 21/4 2!/» 21/2 3 */s Ye} He |r | Uf | fe | i | 2 | 2 ae ee ee 1/y 3/5 3/4 4/s I 1}/, 11/9 1}/, 13/, 2 2 2 2 1/5 ah 3/5 2/5 gia Ye | U/s | B/s | fe | Je | fe | 15/4 1/4 gL meek ee oe ge NP I Me | U/s | U/s | Uf2 | U/e ‘/s Ss | Me | Mattei leat I fal) | Ti/eaaVe arrears 1/¢ eRe) Af ale NP aaa I I Ye | U/s | 1/4 Ys Me | "ls | "es "Js | "se | Sees I I I/io| /s Io Ys) 2s}. Aled Me) a) Shee I I 1'/g log an hed od sa FP oe 2/5 ah, eh ad I I { To use this table find in the top horizontal line the scale of reduction for which exposure is known. Under this scale the relative time of exposure for different degrees of reduction will be found opposite the new scales of reduction marked in first vertical column. The power and position of the lamps having been standardized at the start, a lens is placed on the camera and the diaphragm adjusted so as to be exactly one inch in diameter. This for an eight-inch lens would be F/8, fora lens of eleven inches focal length F/11, ete. The camera is then extended so that the distance from the stop to the plate is sixteen inches and the whole camera moved back and forth until some matter on the easel becomes critically sharp. A print is then pinned up and a plate exposed in steps, developed and the correct ex- posure noted. A sheet of paper is then inscribed with the ordinary apertures from 7 4 F 4 F/6 to F/45 inacolumn. Opposite F/16 is written the exposure ar- - rived at by the test just described. (It will be evident upon considera- tion of the conditions under which the test was made that since the lens opening of one inch is one sixteenth of the bellows extension, the effective aperture, or the actual working speed, of the lens is F/16 regardless of what the aperture marked on the lens mount may be.) Opposite each other diaphragm is written the proportionate exposure following the usual rule. COPYING 561 This procedure is then repeated for a line subject using a process plate.and for any other particular class of work which requires dis- tinct treatment, and the corresponding exposures marked against each diaphragm. A scale is then affixed to the camera so that zero point coincides with the diaphragm of the lens. Obviously, the actual extension of the camera can be determined by observing the figure against which the ground-glass stands. From what has been said before it will now be evident that the extension indicates the value of the stop without any calculation whatsoever. ‘Thus if the extension is eight inches the exposure would be read off opposite F/8, if 22 inches opposite F/22, etc. If for any reason it is necessary to use a smaller diaphragm the proper exposure may be determined by the usual rules governing the exposures of different diaphragms. The Copying of Subjects in Pure Black and White.—Having be- come familiar with the fundamental principles underlying all copying and applicable to subjects of all classes we will consider in some de- tail the proper methods of handling each class of copy in order to obtain the best results. Subjects in black and white embrace an extensive and varied field which includes charts, graphs, maps, pen and ink and pencil sketches, wood and steel engravings, etchings and half-tone reproductions. The photography of such subjects, while quite simple in itself, de- mands precise painstaking attention to every detail if the best results are to be obtained. While the subject of lenses has already received attention, it may be well to remark at this point that in dealing with line work the best corrected objective is none too good; especially is this the case when dealing with subjects containing very fine detail or when the degree of reduction is considerable. In such cases the absence of zonal ab- berations and coma is particularly desirable and the process anastig- mat is well worth its additional cost where work of this type must be done. The plates required for handling this class of copy are known as Contrast, Process or Photo-Mechanical plates and are made to give a very high degree of contrast. Wet collodion is still unsurpassed for line work but its use is beyond the capabilities of most workers, but with care all that can be done with collodion can also be done with eelatine although it must be admitted that a satisfactory result from 062 PHOTOGRAPHY difficult originals is more difficult to secure with gelatine than with collodion. Rapid plates as used for general work cannot be used for this purpose as they do not have a sufficiently fine grain and are un- able to give the great density combined with absolutely clear lines which is required for this class of copy. Plates of the process type suitable for line work are all comparatively slow and range in speed from about Watkins 15 to Watkins 45 but work very free from fog and readily give great contrast and density. Typical plates of this class are Cramer Contrast, Seed Process, Eastman Process film, Im- perial Process, Wellington Ortho Process, Ilford Process and Half- Tone, Barnet Process and Gevaert Process. Development of Process Plates.—lhe development of process plates is best conducted by inspection using a concentrated hydro- chinon or glycin developer. The following is considered the best formula for obtaining the maximum contrast: A. Sodium’ bisulphite.. $5 62 cen 2. 5 on ee 375 gr. 25 gm. Hydrochinon 0. hei ee ods os 0 eas 25 gm. Potassium: bromides)... 2.303 (2 ee 375—s grr. 25 gm. Water: to: make. aoe. Shae ‘92> :RiCs 1000 cc. B. Caustic® sodas iici.02 4a oe oi eee ee 1% oz. 45 gm. Water to make. ...5..00:.5 25 chico ee 32° og, 1000 cc. For use take equal parts of A and B. The developer will not keep when mixed and a separate batch should be used for each plate. An- other formula which has good keeping qualities and gives good con- trast is as follows: , Hydrochinon 2.43.2) <2. Sea's fas ae 130 gr. 15 gm. Sodium ‘sulphite: (dry) 0.0.4.5), Ue oe 3 OZ. 150 gm, Formialite -* s,s 44 sq: adbeast cminpn wl’ 0 gee pale ae ae 20 cc. Water -to make... 7.0.5.5 en te 20 OZ. 1000 cc. This is one of the few cases in photography in which the author prefers development by inspection to the time and temperature or fac- torial methods. For one thing, a comparatively bright light may be used with safety so that there is no difficulty in judging the appear- ance of the negative. As process plates fix back considerably, de- velopment must be carried just as far as possible without causing the delicate lines to veil over. A slight veiling, noticeable towards the close of development, may be disregarded, as it will disappear in the fixing bath. If development is carried as far as possible without pro- ducing fog, the density will be all that is desired, unless exposure has a COPYING 563 been insufficient. If the exposure has been insufficient the negative will lack density when removed from the fixing bath, even though the density appeared to be sufficient when development was concluded. On the other hand should the lines begin to veil over in the early stages of development before the requisite density is obtained, over ex- posure is indicated. In fact, exposure to suit the original is the key to the whole problem, provided the proper plate and developer are used and development is carried to the limit. Iexcept with weak originals, or through faulty exposure or develop- ment, intensification will not be required. When intensification is necessary Monkhoven’s silver-cyanide method, lead or copper are suitable for the purpose. For the print glossy papers are generally used, especially if there is an abundance of small detail. It may be noted that a hard, vig- orous paper gives a cleaner-cut black line than the normal or soft varieties. Copying Photographs or Like Subjects in Monochrome.—Here the object is to reproduce the various tones of the original as correctly as possible. A slight loss is inevitable, particularly at the ends of the scale of gradation, but if the copy is well made the loss should be small and practically indistinguishable. Especial care must be taken to minimize the grain of the original, particularly if the surface is matt or rough. The apparatus described earlier in this chapter will be of great assistance in this respect. Under exposure and forced develop- ment, or the use of a contrast working plate, accentuate any tendency to “graininess”? and in such cases it is well to expose fully and shorten the time of development somewhat. When very contrasty originals must be copied the use of an ultra rapid plate will assist ma- terially in toning down the extremes of contrast, but for general work high speed plates are not to be advised and better results will be secured by the use of comparatively slow plates ranging from Watkins 50 to Watkins 150. Plates of this character, made especially for this class of copying, are made by practically all manufacturers. i =p 940/d > Fic. 236. Butler’s One-Exposure, Three-Color Camera Mr. E. T. Butler. Part of the light entering the lens on the right is reflected by the first mirror and after passing through the red filter forms the red-sensation negative. The light passing through the first reflector strikes the second reflector which reflects a portion of it to —— se oa, pe / i : ‘. 4 r 7 ] a ae NATURAL COLOR PHOTOGRAPHY 573 form the blue negative while the light which passes through this re- flector forms the green-sensation negative. It is necessary that all three negatives be of the same size and sharpness, hence the distance traversed by the light rays must be the same for all three images. Furthermore it is essential that the reflectors do not produce a double image to overcome which it is necessary to cover the back of the glass with colored gelatine. This color must be the minus color of the taking screen and since the first reflected image forms the red sensa- tion the gelatine coating on the back of this reflector must be minus red or blue-green while that of the second reflector must be minus blue or yellow. In order to avoid the necessity for a one-exposure, three-color camera, Louis Ducos du Hauron suggested a tri-pack, the three plates or films being bound up together with their respective filters in be- tween. Critical sharpness, however, is impossible with such an ar- rangement as it is impossible to bring the three emulsions sufficiently close together. This alone might not be a grave objection in certain cases and might be disregarded but for another more serious difficulty. The light which passes through the first plate is diffused by the particles of silver salt making it impossible to secure a halation-free image on the second plate. Halation is of course even more pronounced on the third plate since in this case the light has been scattered by two emulsions. In addition there is the difficulty of adjusting the speeds of the three plates so that each will be properly exposed in the same time. In practice, therefore, attempts to develop such methods have not been very successful. Additive and Subtractive Three-Color Photography.—The color- sensation negative records by density the presence of that particular color in the subject; i.e. the red-sensation negative records the red of the subject in terms of greater or lesser density according to the amount of red present in the various portions of the subject. A positive transparency from this negative will reproduce the red sen- sation by means of its clearer parts. The parts of the subject con- taining the purest red will be represented by clear glass, those parts with some red by a medium density while those parts containing no red whatever will be of maximum density. Now if this transparency is viewed in red light it will reproduce the red sensation of the original subject. In like manner the blue and green transparencies will, when viewed in blue and green light, reproduce the respective color sensa- tions of the original subject. 38 574 } PHOTOGRAPHY The three records may now be combined and the natural colors of the subject reconstructed by placing each transparency with its proper filter in a viewing instrument constructed like the one-exposure, three- color cameras already considered. This procedure, like most others in three-color photography, was first developed by Louis Ducos du Hauron. It reached its highest development in the hands of Mr. F. E. Ives whose Kromskop has never been surpassed for absolute fidelity in color reproduction. It is to be noted that in this case colored light is added to colored light. We start with colored light from which we produce white by addition. Hence such processes are termed additive processes. Red. Green SWhite Bites To recombine the three-color sensations on paper or in a single transparency it is necessary to superimpose three separate images of the proper colors. The white paper on which we place our colored images reflects all three primary colors, red, green and blue, which as we know, form white. Now when we print from the red-sensa- tion negative we are printing from the thinner parts, or those parts which represent the absence of red in the subject. Hence the red- sensation negative must be printed, not in red, but in a color which completely absorbs all red. But while red is absent either one or both of the two other primary colors may have been present in this portion of the subject. The color of the image, then, must be such that it not only absorbs red but reflects green and blue. It will, therefore, be a. minus red or blue-green. The red-sensation negative is thus printed in minus red or blue-green; the green negative in minus green or magenta which absorbs green and reflects blue and red while the blue- sensation negative is printed in minus blue or yellow which reflects red and green but absorbs blue. ) Superimposed in full strength these colors absorb all color and the result is either black or gray according to the amount of light reflected. Intermediate colors are produced by the mixture in various propor- tions of the three fundamental colors while the total absence of color will produce white, since this is the color of the paper base. It will be observed that in this case we start with white light from , P 3 nel oe ae ee ee 4 NATURAL COLOR PHOTOGRAPHY 575 / which we produce color by subtracting various colors, hence such processes are known as subtractive methods. /Red—Minus red or blue-green eee vinus green or magenta Blue——Minus blue or yellow Subtractive Printing Processes.—The principle of the subtractive method has been developed in a wide variety of processes. But few of these, however, are generally employed and these now only in cer- tain quarters, the development of the screen-plate processes having largely killed the interest which was shown in such methods a few years ago. ‘The three-color images for the subtractive processes have been produced by means of trichromatic carbon tissues and lately by three-color carbro; by the production of dye images by mordanting methods, by the transference of dyes or the relief or imbibition process represented by the Pinatype method; by three-color gum- bichromate and by the toning of silver images. While prints by these | methods are often quite pleasing from the artistic standpoint there is a tendency, more noticeable in some processes than cthers, to dull and imperfect colors lacking in brilliancy and transparency owing to the depth of the three superimposed pigment or dye images. This together with the very practical difficulties involved in producing the three images and in properly superimposing them, the complexity of the process and the care and delicacy demanded at every stage, places the process beyond the possibilities of the average worker, hence such methods have failed to make much headway. Multi-Color Screen Plates.—In just the same way that a painter may secure a certain color by the juxtaposition of ‘dabs of pigment of two colors which when viewed at a distance merge to form a single color, so it is possible to secure on a single plate all three color-sen- sation records by eniploying in place of the usual solid color filter a multi-color screen composed of a large number of small color screens evenly distributed and so small as to be practically invisible. The multi-color screen was the conception of Louis Ducos du Hauron whose patent of 1868 suggested that a sensitive plate be ex- posed behind a screen composed of fine parallel lines, red, green and blue. The red lines collectively record the red sensation of the sub- ject, while in like manner the green and blue lines collectively record the green and blue sensations respectively, so that all three funda- mental color records are secured on a single plate. Consequently 576 PHOTOGRAPHY when a positive from the original negative is placed in contact with the multi-color screen in the position occupied by the negative, so that the lines in the positive recording the red sensation are behind the red lines of the multi-color screen, the colors of the subject become visible, the same principle being brought into play as in the viewing camera. The multi-color screen plate process of color photography is thus an additive method. While Louis Ducos du Hauron was the first to develop the idea of a multi-color screen, the practical development of the method is largely due the work of Professor Joly of Dublin and James Mc- Donough of Chicago. The former was granted a patent (B. P. 7743 of 1893, 13,196 of 1894) for a screen plate with parallel red, green and blue lines having a width of about 0.12 mm. (1/200 inch).. His patents together with those of an American inventor James Mc- Donough of Chicago, who had devised a similar screen plate but with finer lines, were acquired by a syndicate which placed the process on the market but owing to the difficulties met with in manufacturing the screen plates economically it soon ceased to exist. In succeeding years a large number of patents have been taken out for multi-color screens employing not only ruled lines but various geometrical shapes such as squares, rectangles, circles, etc. As these, with one exception, are no longer on the market we will not linger to consider them but pass directly to the second type of screen plate in which the color screens are distributed at random and do not form a definite geometrical pat- tern as in the two examples just quoted. The two most conspicuous examples of this type of screen plate are the Lumiére Autochrome in- troduced by A. and L. Lumiére of Lyons in 1907 and the Agfa Color Plate. With a multi-color screen of a definite geometrical pattern the screen may be separate from the sensitive plate; the positive trans- parency from the negative made behind such a screen being placed in register with another similar screen for viewing purposes. With the second type of multi-color screen, known as the mosaic screen plate, this is impossible and the negative image obtained by develop- ment must be chemically reversed. The separate screen-plate method permits of unlimited duplication as one need only make as many posi- tive transparencies as required. With the mosaic screen plate, how- ever, duplicates can only be made by rephotographing the original and at the expense of some loss of brilliancy of coloring. The duplicating NATURAL COLOR PHOTOGRAPHY 577 method is perhaps the simplest for the beginner in color photography but both processes are well within the possibilities of the amateur who is already conversant with the principles of ordinary photography. As regards the faithfulness of color reproduction there is little difference between the two methods; the colors as reproduced by the mosaic screen plate, however, are supposed to be somewhat softer and with less tendency towards glaring color than the duplicating method. But this difference is so slight as to be of little if any importance. The Autochrome Plate—TJhe Autochrome multi-color screen is an example of the mosaic screen and was the first of such to meet with success. The method of preparation is most ingenious. The colored screens are composed of a particular form of starch grains ranging in size from 10/1000 to 15/1000 of a millimeter (.0024 inch). Separate lots of these grains are dyed orange, red, green and blue-violet. These are then mixed in such proportions that the result shows no predominating color and this mixture is spread over the glass plates. The gaps between the grains are filled in by means of extremely fine charcoal dust, after which a layer of waterproof varnish is applied so as to separate the screen from the emulsion which is coated on top of it. As these starch grains number 6000 to the square millimeter (about 4% million to the square inch) they are invisible to the eye. When observed with the microscope at a magnification of about 125 times, the appearance of the screen is illustrated in Fig. 237 in which the darker circles represent the blue-colored grains, the half- tone circles the red grains and the lightest circles the green grains. From this it is evident that the grains of any given color are very evenly distributed throughout the screen. This is of course neces- sary for the opposite state of affairs would result in color patches which would render proper color reproduction impossible. Over this multi-color screen is coated a thin, highly color-sensitive emulsion. As it is impossible to make this emulsion equally sensitive to all three colors, it is necessary to compensate for this deficiency by means of a filter applied to the lens. As the absorption of light by the multi-color screen is considerable (the Autochrome screen ab- ~sorbs about 92.5 per cent of the incident light, which amount is still further increased by the compensating filter which must be employed), the working speed of the plate is much less than the ordinary plate or film and is about 4 Watkins or 2.4 H. and D. Very rapid exposures 578 PHOTOGRAPHY are, therefore, impossible-unless the plates are hypersensitized or flash- light is employed. The former operation is not one which should be attempted by the novice. The Compensating Filter.—The filter supplied by the Lumieres is calculated for use with average daylight. As the spectral com- position of daylight is never constant, however, and moreover varies greatly in different localities, it is obvious that any single filter is at Fig. 237. Autochrome Screen. 125 best a compromise. With the vast majority of subjects, however, and in the temperate zones the filter supplied by the manufacturers is entirely satisfactory but certain subjects which are unusually strong in blue and violet rays require a deeper filter. Thus in early morning or late afternoon when the light is rich in color, subjects including far distances show marked blueness in these portions. Likewise in ma- rine photography or with subjects having wet surfaces, snow scenes, etc., excessive blueness of tone is often observed. Achille Carrara has found that the intense blue of the Italian skies and lakes leads to excessive blueness in the finished result. In such cases it is necessary to employ a filter absorbing a greater amount of ultra violet than the standard filter. For this purpose an additional screen of esculine or Filter Yellow K may be employed. Or the usual filter may be supplemented with a Wratten K1 filter for a part of the exposure, or, in extreme cases, for the entire exposure. ? NATURAL COLOR PHOTOGRAPHY 579 The use of filters which absorb too much ultra violet leads to a prevailing yellow tint in the completed transparency. Special filters are required for artificial light sources. These may be obtained on special order from the manufacturers. Handling and Exposure of the Autochrome Plate—The Auto- chrome emulsion being sensitive to all colors must be handled either in total darkness or by a safelight formed of the Virida papers of the makers. As it is not a difficult matter to load plate-holders in total darkness when one has become familiar with the operation, it is ad- visable to place the plates into the holders in total darkness. A gen- eral greenish tint in the finished positive may often be traced to the ise of an unsafe light or to excessive exposure of the plate to the Virida light when loading. Since the multi-color screen must be in front of the sensitive emul- sion during exposure, the glass side of the plate is placed towards the lens. The sensitive film being very delicate it is protected by a piece of cardboard which should not be separated from the plate until the moment of development. Otherwise the delicate film may be damaged and the plate soon develops fog. Before inserting the slide it is well to brush off any adhering par- ticles of dust or other substances which may be adhering to the glass side of the plate in order that such may not produce a plentiful crop of black spots in the finished result. As the plate is exposed through the glass a correction is necessary when focusing. If the filter is placed behind the lens this correction is made automatically and this is the proper method to employ with a fixed focus camera or those focusing by scale. If the filter is placed before the lens the ground-glass may be reversed so that the ground side is on the outside. One may move the lens back a distance equal to the thickness of the Autochrome plate (1.8 mm. or %4 inch) or em- ploy a Zeiss Ducar filter which automatically compensates for the thickness of the plate and allows the same camera to be used for either ordinary or color work without any inconvenience whatsoever. Exposure.—As in ordinary photography, and to an even greater de- eree, success in color photography with screen plates is dependent upon correct exposure. While ordinary plates and films have considerable latitude in exposure, so that one or two times more will still produce a usable negative, the margin of error is very small in color photog- raphy, only a few per cent at the most, and correct color rendering 580 PHOTOGRAPHY cannot be obtained without the proper exposure. Numerous tables have been published for the calculation of exposures for the Auto- chrome plate but these do little more than indicate the approximate ex- _ posure and the factors on which their successful use depends are very difficult to estimate accurately, so the use of tables is not very satis- factory. The only satisfactory method lies in the use of an actinom- eter such as the Watkins or Wynne to which reference has already been made in a previous chapter. Special color plate meters are supplied by the makers and the use of these is preferable to the regu- lar form because those designed particularly for color work are pro- vided with scales which take into consideration the failure of the rec- iprocity law which occurs with plates of very low sensitiveness as the Autochrome plate. Owing to the low working speed of the Auto- chrome plate, the reciprocity law according to which exposure is the product of time and intensity, which are inversely proportional, does not hold. Therefore, in working in feeble light or with a small dia- phragm the increase in exposure is more than that which would be indicated by the law. According to M. Fauchet, reduction of inten- sity by one half increases the exposure by about 2.25. Development.—In 1907 when the Autochrome plate was introduced a pyro-ammonia developer was recommended. This, however, has subsequently been replaced by one of metoquinone and while many of the older workers prefer the former, metoquinone is the best for the novice. The formula is as follows: Metoquinone « «0055.0 4 ..0k ba au vas Clee YY oz. I5 gm. Sodium sulphite (dry }-.. ..05 so 3% oz. 100 gm. Ammonia 920 (22° Baume) 0.2. 225.9 eee 657° nO Potassium bromntide: .$ 2.0 0¢a000 vent go gr. 6 gm. Distilled water to... isn d.50er es oe eee $0 208) 1000 cc. For time development dilute one part of the above concentrated stock solution with four parts of water and develop exactly 2% min- ~ utes at 60° F. The Watkins Meter Company supply a special ther- mometer which shows by the height of the mercury the time for de- velopment at any temperature. Development for a fixed time is suitable only for plates which have been correctly exposed. For all others, preference should be given to a controlled method based upon the time of appearance of the image. ; To develop by this method one begins development in a diluted de- — ae ee eS se ae NATURAL COLOR PHOTOGRAPHY 581 veloper, taking the time of appearance of the image in this solution. On the appearance of the outlines of the image (the sky being dis- regarded) the developing solution is strengthened by the addition of a certain amount of concentrated metoquinone developer according to the time required for the first appearance of the image. For a plate up to 4x6 inches in size one may begin development in a solution composed as follows: SRT ie oa vce aks Me eee ve 80 cc. 24 oz. Concentrated metoquinone developer.............. 5 cc. 85 min. The following table then shows the amount of concentrated developer to be added upon the appearance of the image and the total duration of development. Appearance of Outlines of Im- Quantity of Developer A to Total Duration of Develop- age (Disregarding Sky) Add en Appearance of ment From Immer- Atter Immersion. First Outlines. sion of Plate. Seconds Minutes. Seconds. I2to 14 15 c.c.s. (4 02.) I 15 15 to 17 do. do. I 45 1to2r. do. do. 2 15 a2 10 27 do. do. 3 fe) 23 t0:33 do. do. = 30 S410 30°, do. do. ae. 30 _ Extreme 40 to 47 A5 c.c. (1% ozs.) z O under-exposure { Above 47 45 c.c. (13 ozs.) 4 O (If it is thought desirable for any reason to use a larger volume of developing solution all the quantities given should be increased ac- cordingly. ) M. F. Dillaye recommends that the exact time of appearance be taken by transmitted light and then watching for the moment at which the image, which first appears as a negative, seems completely extinguished, the whole plate presenting the appearance of an even, diffused. density. It is at the moment at which this occurs that development should be stopped. If development is continued the image appears as a positive and will be over developed. This method while possibly practical for the advanced worker is not one which the novice should attempt. One may of course use a desensitizer in which case development may be conducted in a comparatively bright light which makes it easier to determine the appearance of the image. The makers supply in tube form a desensitizer for this purpose, or one may use Aurantia (am- monia salt) at a concentration of 1 part to 1000 of water. Pina- 582 PHOTOGRAPHY kryptol Green may also be employed but some difficulty is experienced — at times in removing the stain of phenosafranine from the film so it is better to avoid this agent. Reversal of the Image.—I{ we were to fix the image at this stage, we would secure a negative image in complementary colors: ‘The image secured by development represents exactly the reverse of what we require; the silver deposit obstructing the light which should be transmitted while that which should be stopped is being transmitted. It is necessary, therefore, to reverse the image. This involves (1) the removal of the developed silver image and (2) the redevelopment of the remaining silver salt to form the positive image. Accordingly as soon as development is complete the plate is rinsed in a tray of clear, cold water and slipped into the following solution of potassium per- manganate which dissolves the silver image: Potassium. permanganate, ..< ..2¢)0) sole 30 gr. 2 gm. Sulphuric acid 66° 25555 5 08 Gens ee coe ee 3 athe 10 cc. Water to make wo. 0.0.0 sien as eee 35 OZ. 1000 cc. As soon as the plate is covered with this solution the darkroom may be left and all succeeding operations conducted by full daylight, pref- erably near a brightly illuminated window. In this solution the image rapidly disappears and in 30 or 40 seconds is gone completely. The plate is then taken from the solution and carefully washed for about half a minute in running water and then replaced in the first developer, which should be retained for this purpose. In this the image re- appears, this time as a positive, and development is complete within four to five minutes. There is no fear of over development, however, while complete conversion of the silver salt to the metallic state is es- sential to the brilliancy and the permanency of the image. Care should be taken, therefore, that development is not stopped too soon. After this second development, the plate is washed for three or four minutes in running water, taking care that the water does not strike the plate with any undue force as the film is very tender at this stage. It is then placed on the drying rack and dried as quickly as possible by ‘means of an electric fan if available. On no account must heat or alcohol be used. Varnishing.—Although this operation is not absolutely eontial it is to be advised since it increases the brilliancy of the colors and serves to protect the image from injury. Varnish for this purpose may be — secured from the makers of the plates or prepared according to the 4 following formula: } t NATURAL COLOR PHOTOGRAPHY . 583 Pere EAU GODENZENE aes ki bles See sca dateaaes 100 cc. 5 Oz. Re nV fie ne dw ds Axcesa¥e fh ala a 20 cc. oye This is flowed over the plate in the usual manner after which the plate is placed on the drying rack in a place away from dust where it must not be disturbed until completely dry. No varnish containing alcohol must be used. After-Treatment of Autochromes.—If after development the trans- _ parency lacks brilliancy and appears dull and brownish a clearing bath may improve matters. For this purpose a 2 per cent solution of sodium bisulphite may be employed. A general thinness and lack of body in the colors may be due to either over exposure or sometimes, but less often, over development. Intensification will make some improvement. After-treatment of any kind is risky, however, as the film is apt to soften and frill and if carried out directly after development a hardening bath of formaline or alum should be employed. Another point which requires attention when a color plate must be intensified is complete development, other- wise the reduction which takes place in the fixing bath will render the plate useless. Therefore, if intensification appears to be necessary one should make sure that the second development is carried to completion. For intensification the makers recommend : Be se hc bce ede tee pe sin 3 gm. 45. gf. Re gs ee Se cnn utp 3 gm. ae: 0 Br Peigcea water 100. i.e. } te deh ig Si ae 1000 cc. ase "oe eT TRUE i ied hae bk weeds ould eles 5 gm. We ogy, Wy eS os a a i a ae 1000 cc. 34 oz. For use take Solution A 10 parts, B 1 part. The chromium intensifier may also be used, in fact any method which does not produce a colored deposit. | With the formula given intensification is quite rapid, from 20-30 seconds being sufficient in most cases. The solution slowly turns yellow and becomes turbid and the plate will then be stained unless transferred immediately to a fresh solution. After intensification the plate is cleared by immersion for a few seconds in a 0.001 per cent solution of neutral potassium permanganate, then after a short wash- ing it is placed for two minutes in an acid hypo fixing bath prepared as follows: | Rl rR er ithe isch sd. cheis- [hs 6-4 dia'a ide aie wo 150 gm. 5% oz. Saturated solution sodium bisulphite............ 50 cc 134 Oz MNT Oe im, esis og an exces eden a's's 1000 cc. ae Oe, Fixing must not be omitted when the plate has been intensified. ) g 1 ‘as! aah oe ) ee ae 584 PHOTOGRAPHY. is : A final wash of four to five minutes completes the process. The Agfa Color Plate-——The Agfa color plate, like the Lumiere Autochrome, is a mosaic, multi-color, screen plate. The plate itself, as well as the operations of producing color transparencies with it, very closely resembles the Autochrome. The individual color elements are about the same average size as in the Autochrome plate but are more uniform, varying in size from 0,008-0.017 mm. The screen as a whole, however, transmits very nearly twice as much light as the Autochrome; the relative transmissions being 14 per cent for the Agfa — plate and 7.5 per cent for the Autochrome. The manipulation of the — Agfa color plate differs from that of the Autochrome only in some minor details. Duplicating Processes of Screen-Plate Color Photography.—De- spite the obvious advantages of a duplicating process employing a separate taking screen, such methods have not met with commercial success. One of the earliest of such plates was the Joly-MacDonough, issued about 1892, but discontinued on account of difficulties met with in the production of the taking screen-plate. The Thames plate, in- troduced several years later, enjoyed a brief spell of popularity and was finally replaced by the Paget Duplicating Process which was es- sentially an improved Thames plate. This was probably the most successful of the separate screen-plate methods but was discontinued early in 1925. Soon after the disappearance of the Paget method a similar process, but of higher speed, was announced by Chas. Baker of High Holborn, London. This is the only representative of sepa- rate-screen methods now on the market. The Duplex Method.—The exposure is made with the special taking » screen in contact with the panchromatic emulsion specially provided for the process. After exposure, the plate is developed in the usual manner, a desensitizer being employed if desired. Intensification or reduction of this negative may be carried out exactly as with other negatives. From this negative any required number of transparencies may be made on black-tone transparency plates and these positives when superimposed in exact register on the viewing screen reproduce | the colors of the original subject. 7 The process thus permits of unlimited duplication without loss of | quality since as many transparencies as desired may be made from the original negative by the simple operation of contact printing. Besides this important advantage there is another no less important: i.e. the greater speed of the separate plate method. As shown by Mr. F. E. — NATURAL COLOR POT OGRA PEL Y. 585 Ives the tri-color filters used for making the three color-sensation nega- tives should divide the spectrum into approximately three equal parts, while the three filters used for viewing purposes should transmit only very narrow bands of the three colors. With the combined plate Fic. 238. Duplex Screen naturally a compromise must be made for one screen must serve both purposes, but with separate screens the taking screen can be made lighter, thus reducing the exposure required. The Paget process was considerably faster than the Autochrome plate and the new Duplex method is from four to five times as fast as the latter. With a lens having an aperture of F'/4.5 full exposure in bright light will be secured at about 44 9 of a second, thus permitting hand camera ex- posures under favorable conditions. Not all is plain sailing, however, for there are some tran bact to the separate screen method. The most important is the parallax error arising when the image is not viewed at exactly right angles. When examined from any other than a right angle the patch of silver deposit in the transparency is not in line with its appropriate color screen but the one to the left or right of it. The colors vary, therefore, with the angle from which the image is observed and only by looking at it perpendicularly can the proper colors be seen. This defect of parallax is present to a greater or less degree in all separate plate processes and 586 ) ePHOTOGRAPE YT unfortunately the smaller the color elements the greater is the parallax — error, Other drawbacks are the difficulty of securing perfect contact be- — tween the taking screen and the sensitive plate and between the view- ing screen and the positive transparency—a condition which becomes increasingly difficult with an increase in the size of the transparency. Registration also presents some difficulties at times but these are but minor matters which do not radically affect the performance of the process. | GENERAL REFERENCE WorKS AsNnEy—Color Measurement and Mixture. BoLas, TALLIENT AND SENIOR—Photography in Colors. BrowNn—Color Photography. (Photo-Miniature No. 128.) CLERC AND CAMELS—La Reproduction Photographique des Couleurs. Ducos pu Havron—La Triplice Photographique des Couleurs. Husit—Three-Color Photography. (English Translation by H. O. Klein.) Hust—Die Dreifarbenphotographie. Jounson—Photography in Colors. , Konic—Natural Color Photography. (English Translation by E. J. Wall.) KronE—Die Darstellung der naturlichen Farben. Mees—Color Photography. (Photo-Miniature No. 183.) VALENTA—Die Photographie in naturlichen Farben. VipaAL—Photographie des Colours. VipaLt—Traite pratique de Photochromie. Watit—Practical Color Photography. Wati—History of Three-Color Photography. Color Photography—lInstructions. (Photo-Miniature No. 147.) Per NN DX List OF THE PRINCIPAL REFERENCE WORKS ON PHOTOGRAPHY In ENGiisH, FRENCH AND GERMAN REFERENCES TO TECHNICAL JOURNALS APPENDIX A LIST OF MORE IMPORTANT REFERENCE WORKS ON PHOTOGRAPHY Note.—The following list contains the titles of general reference works only. For works relating to any particular subject see the short bibliographies at the end of each chapter. Works are classified according to the language in which originally printed. Translations are also listed where published in book form. Works which are now out of print have been included where especially valuable. Although these can no longer be obtained from the publishers, they may be located from time to time by the large dealers in second-hand technical works. REFERENCE WoRKS IN ENGLISH ABNEY—Instruction in Photography, 10905. AsNEy—Treatise on Photography, 1903. AsnEY—Photography with Emulsions, 1806. BayLEy—The Complete Photographer, 1923. BrotHers—A Manual of Photography, 1890. Dererr—Photography for Students of Physics and Chemistry. Jones—Science and Practice of Photography. Jones—Photography of Today. Jones—Cassell’s Cyclopedia of Photography, 1912. MEES AND SHEPPARD—Investigations on the Theory of the Photographic Process, 1907. . MeELpoLAa—The Chemistry of Photography. MorTIMER AND Watt—The Dictionary of Photography. RorBucK—The Science and Practice of Photography. WatTxins—Photography—Its Principles and Applications, 1912. Woopspury—Dictionary of Photography, 1897. FLrint—The Chemistry of Photography, 1918. - The Physical Chemistry of the Photographic Process, 1923. Photography as a Scientific Implement, 1923. REFERENCE WorkKsS IN GERMAN Davip—Lehrbuch der Photographie. Davin—Photographisches Praktikum. Eper—Ausfiihrliches Handbuch der Photographie, 1885-1903. In four volumes. (The most complete and authoritative work on the subject in existence.) 39 589 590 PHOTOGRAPHY EpER AND VALENTA—Beitrage zur Photochemie, 1904. GotpBerc—Der Aufbaudes der Photographischen Bildes, 1920. LIESEGANG—Photographische Physik. LiesEGANG—Photographische Chemie. Lupro-CraMER—Kolloidchemie und Photographie, 1921. LuTHER—Die Chemischen Vorgange in der Photographie, 1899. LaNIER—Photochemische Chemie und Photochemie, 1899. Metruie—Lehrbuch der Praktischen Photographie. Pi1zzIGHELLI—Handbuch der Photographie. PLoTNIKOW—Photochemische Versuchstechnik. PLotNikow—Grundriss der Photochemie, 1923. ScHMiIptT—Kompendium der Photographie, 1920. ScH Mipt—Photographiren. ScHMipt—Vortrage uber Chemie und Chemilalienineae fur Photographierende. StoLz—Chemie fur Photographen. VALENTA—Photographische Chemie und Chemikalienkunde, 1920. WENTzEL—Die Photographisch-chemische Industrie, 1926. REFERENCE WorKS IN FRENCH BreLtin—Precis de Photographie Generale, 1905. Braun—Dictionnaire de Chimie Photographique, 1904. CLerc—La Photographie Practique. Coustet—Out en est la Photographie. DavannE—La Photographie, Traite Theoretique et Practique, 1888. Faspre—Traite Encyclopedique de Photographie, 1889-1906. Eight volumes (The standard reference work in French.) HeEnri—Etudes de Photochemie, 1919. MatTHET—Traite Chimie Photographique. PouLENc—Les Produits Chemiques en Photographie. SEYEWETZ—Le Negatif en Photographie. REFERENCES TO TECHNICAL JOURNALS Chapter I. The Development of Photography (For list of general reference works see page 34) CromMER—Deux Details Historiques. Bull. Soc. franc. Phot., 1923, p. 250. CroMER—Une Lettre de Nicephore Niepce. Bull. Soc. franc. Phot., 1922, p. 60. PoTONNIEE—Baynard and The Invention of Photography. Brit. J. Phot., 1914, 61, 43. , PotonniEE—The Cardinal Plate of Niepce. Brit. J. Phot., 1920, 67, 29. PoTtonNniEF—Date of the Invention of Photography. Bull. Soc. franc. Phot., F025, 0 312. PotonNiEE—The Origin of the Camera Obscura. Bull. Soc. franc. Phot., 1923, p. 52. TENNANT-Woops—Early Daguerreotypers in the United States. Brit. J. Phot., 1920, 67, 420. WATERHOUSE—History of the Camera Obscura. Phot. J., 1900, 40, 270. WATERHOUSE—The Development of Photography with Salts of Silver. Phot. J., 1903, 43, 159. W ATERHOUSE—Robert Hooke’s Portable Camera Obscura. Phot. J., 1909, 49, 348. WATERHOUSE—Robert Boyle’s Portable Camera Obscura. Phot. J., 1900, 49, 333- Chapter II. The Camera and Darkroom THe ARRANGEMENT OF THE DARKROOM Brown—Fitting up the Darkroom. B. J. Almanac, 1913, p. 523. Davis—The Arrangement of a Darkroom. Amer. Phot., 1913, 7, 108. GEAR—Fitting up the Darkroom. Phot. J., 1911, 51, 338. Kinc—That Model Darkroom. Amer. Phot., 1920, 14, 67. Krart—Shutter for Darkroom Window. Amer. Phot., 1915, 9, 664. LaFER—My Darkroom. Amer. Phot., 1913, 7, 579. Roserts—The Evolution of a Darkroom. Amer. Phot., 1916, 10, 16, 238. WEston—Darkroom Fittings. Phot. J., 1921, 61, 25. On Darkroom SAFELIGHTS Hartrice—Darkroom Safelights. Brit. J. Phot., 1915, 63, 503. HickMAN—Illumination of the Darkroom by Means of Lamps in Liquid Cells. Phot. J., 1920, 60, 147. Merres—Darkroom Illumination by Reflected Light. Brit. J. Phot., 1915, 62, 603. Mees AND BAKER—Measurement of the Efficiency of Darkroom Light Filters. Phot. J., 1907, 47, 276. NEUGEBAUER—Preparation of Darkroom Safelights. Brit. J. Phot., 1923, 70, 397- 591 592 PHOTOGRAPHY PLepGE—Darkroom Illumination. Brit. J. Phot., 1921, 68, 249. STENGER—Liquid Darkroom Safelights. Brit. J. Phot. 1905, 52, 732; Zeit. wiss. P., 1905, 2, 233. | TRIVELLI—Lights for the Darkroom. Brit. J. Phot., 1911, 58, 474, 404, 533, 628, — 777, 872, 957; 1912, 59, 22. | Chapter III. Photographic Optics (For list of general reference works see page 84) Foca, LenctH AND Its DETERMINATION Jostinc AND Satt—Measurement of Focal Length by Clay’s Method. Brit. J. Phot., 1922, 69, 137. JoHnson—Focal Length of a Lens or Lens Combination. Phot. J., 1906, 46, 300. JOHNSON AND GLEICHEN—Summary of Laws Relating to Focal Length. Phot. J., 1913, 53, 183. | LAMBERT—Measuring the Focal Length of a Lens. Phot. Journal of America, © 1923, 60, 87. Locxett—A New Method for Finding the Focal Length of Lenses. Brit. J. Phot., 1915, 62, 411; Brit. J. Phot., 1922, 69, 434. . —.—Measuring Focal Length. (Summary of Methods.) Brit. J. Phot. — 1916, 63, 79. DeptH oF Focus BrowN—Theory and Practice of Depth of Focus. Brit. J. Phot., 1922, 69, 492, 507, 521, 534. BrowNnE—A Simple Depth Chart. Brit. J. Phot., 1923, 70, 775. Cottins—Depth of Focus and Its Graphical Representation. Brit. J. Phot. — 1920, 67, 645, 659. Fraprig—Table of Hyperfocal Distances. Brit. J. Phot., 1915, 62, 795. Jounson—Calculating the Distance Beyond Which Everything is in Focus. — Phot. J., 1906, 46, 320. | Lee—Chart for Finding the Depth of Focus. Phot. J., 1922, 62, 229; Brit. J. — Phot., 1922, 69, 135. PipeR—Depth Simplified. Brit. J. Phot., 1905, 52, 1004. Prrer—Depth and the Sine Condition. Brit. J. Phot., 1906, 53, 125. Pirper—Causes of Variation in Depth of Focus. Brit. J. Phot., 1903, 50, 666, — 687. 1 RupotpH—A New Depth Test Object. Phot. Rund., 1921, p. 266. SCALE OF OptTIcAL REPRODUCTION BrowN—Scale of Optical Reproduction. Brit. J. Phot., 1921, 68, 667, 685, 702. Loss or Licut 1n Lens SysteMs By ABSORPTION AND REFLECTION CuHESHIRE—The Loss of Light in Lenses. Brit. J. Phot., 1912, 59, 507, 645. : Morritr—The Light Absorbed by Lenses. Phot. Journal of America, 1920, 59, q AII. - Nuttinc—The Brightness of Optical Images. Phot. J., 1914, 54, 187. : BEPERENCES TO -TECHNICAL JOURNALS | ‘593 OpENcRANTS—The Experimental Determination of the Luminosity of Photo- graphic Objectives. Nord. Tids. Fot., 1925, 9, 21; S. I. P., 1925, 5, 87. ZscHOKKE—Factors Other than Aperture in the Rapidity of a Lens. Brit. J. Phot., 1912, 59, 823. Chapter IV. The Aberrations of the Photographic Objective (For list of general reference works see page 103) ON THE ABERRATIONS OF PHOTOGRAPHIC OBJECTIVES AND THE TESTING OF OBJECTIVES Bennett—Aberrations of Long Focus Anastigmatic Objectives. Bur. Stand- ards Paper, No. 404. _ Bow—On Photographic Distortion. Brit. J. Phot., 1861, 8, 417. Bow—On the Curvature of the Image. Brit. J. Phot., 1863, 10, 228. Bow—On the Loss of Light from Obliquity of Incidence. Brit. J. Phot., 1866, . 13, 159. Carson—The Correction of the Aberrations of a Photographic Objective. Phot. J., 1903, 43, 188, 278. CHALMERS—The Aberrations of Photographic Objectives. Phot J., 1007, 47, 374- CLay—Determining the Focal Length and Aberrations of a Photographic Ob- jective. Phot. J., 1904, 44, 180. Grusps—On the Equalization of the Photographic Image in Fields of Large Angle Projected upon a Flat Surface. Brit. J. Phot., 1863, 10, 4or. Gruss—Depth of Focus and Spherical Aberration. Brit. J. Phot., 1867, 12, 61. Houpaitte—Sur D’essai Scientifique et Pratique des Objectifs Photographiques. Bull. Soc. franc Phot., 1893, 9 (2 Series), 257. Librarie Gauthier-Villars, 1893. KoHLrANscH—Testing Photographic Objectives. Phot. Korr., 1920, p. 45. KoLtLMorcEN—Achromatic and Apochromatic Correction. Phot. J., 1902, 42, 180. LenouveL—Methode de Determination et de Mesure des Aberrations des Sys- tems Optiques. S. T. I. P., 1924, 4, 33. Morssarp—Appareil pour L’Etude Experimentale Complete des Lentilles et des Objectifs Photographiques. Bull. Soc. franc Phot., 1889, 5 (2 Series), 124. RHEDEN—Reflections in Lenses. Phot. Rund., 1921, 101. “Rooms ”—Lens Corrections. Phot. J., 1907, 47: Part I, p. 24; Part I, py, eae; Partiiil, p,.279; Part IV, p..330; Part V, p, 351. “ Rooms ”—Achromatism. Phot. J., 1908, 48: Part I, p. 320; Part II, p. 333; Pare), 344. Part 1V, p. 375; Phot: J., 1000, 49: Part V, p. 54; Part VI, p. 126. “ RHoms ”’—Astigmatism. Phot. J., 1909, 49, 417. “Ryoms”—An Exact Formula for Spherical Aberration. Phot. J., 1900, 49, 381. | STEINHEIL—Das Prufen und Wahlen der Photographen-Objectiv. Phot. Korr., 1869, 6, 49. Taytor—Axial Aberrations of Lenses. Brit. J. Phot., 1918, 65, 101, 113, 124. TayLor—Lens Testing Instruments. Phot. J., 1902, 42, 40. 594 PHOTOGRAPHY TILLYER AND Kerr—Lens Testing Instrument. U. S. P., 1, 383, 578. TwyMANn—On the Use of the Interferometer for ‘Tevine Photographic Ob- J jectives. Phot. J., 1919, 59, 239. fl TuHompson—Zonal Aberration and its Consequences. Phot. J., 1900-01, 40, sau British Journal Almanac, 1902. Chapter V. The Photographic Objective (For list of general reference works see page 146) PAPERS ON THE DEVELOPMENT OF THE OBJECTIVE BuNnGer—Genesis of Modern Lenses. Brit. J. Phot., 1907, 54, 638, 660, 736. CLray—The Photographic Lens from a Historical Point of View. Phot. J., 1922, 62, 459. DaLLMEYER—The Evolution of Modern Lenses. Phot. J., 1900-01, 40, 64. LUMMER—Beitrage zur photographischen Optik. Zeitsch. Instrument, 17, 208, 225, 264. Von Rour—Uber die Bedingungen fur die Verzeichnungsfreiheit optischer oe teme mit besonderer Bezungnahme auf die bestehenden typen Photo- graphischer. Zeitsch. Instrument, 1898, 17, 271. Von Rour—Beitrage zur Kenntniss der geschichten Entwicklung der Ansichten uber die Verzeichnungsfreiheit photographischer Objectiv. Zeitsch. In- strument, 1898, 18, 4 i Von Rour—Uber die Lichtvertheilung in der Brennebene photographischer Ob- jectiv mit besonderer Berucksichtigung der bei einfachen Landschaftslinsen und symmetrischen Konstruction auftrenden Unterschiede. Zeitsch. In- strument, 1898, 18, 171, 197. Von Rour—Die Entwicklungeschichte der getrauchlichen Typen Photograph- — ischen Objectiv. Eder’s Jahrb., 1900, 14, 106. Von Rour—Development of Symmetrical Objectives with Central Diaphragm, Composed of Equal or Similar Halves up to the Time of the Aplanat. Central Zeitschrift Mech. Optik., 1921, p. 327. E Von Rour—Contributions to the History of the Photographic Objective in — England and America between 1800-1875. Phot. J., 1924, 64, 3590. — ; Papers RELATING TO INDIVIDUAL LENSES Ax.pis—Astigmatism and a New Stigmatic Lens. Phot. J., 1895, 35, 117; Brit. — J. Phot., 1896, 43, 262, 280. Beck—A New Principle in Photographic Lens Construction. Phot. J., 1 44, 172. Brck—The Isostigmar. Phot. J., 1907, 47, 191. a Hartinc—Recently Discovered Objectives of Petzval and Zinc-Sommer. Phot. — Ind., 1924, p. 1030. . KiuGHarpt—The Ernemann Ernostar F/2 Anastigmat. Phot. Ind., 1924, p 1008. a Lre—The Taylor, Taylor and Hobson F/2 Anastigmat. Trans. Opt. Soc., 1924, — 25, 240. i. MertE—The Tele-tessar. Cent. Zeit. Mech. Opt., 1921, p. 245. MieTtHE—Symmetrisches Objectiv ohne Astigmasie. Phot. Mitt., 1888, 25, 123. REFERENCES TO TECHNICAL JOURNALS 595 Puyo anp PutticNy—The Anachromats. Brit. J. Phot., 1906, 53, 184. RupoLpH—Anastigmatic Aplanatism and the Zeiss Lenses. Brit. J. Phot., 1803, 40, 481. RopENSTOCK—Bistigmatsatz. Phot. Rund., 1901, —, 37. Von Rour—Uber das Planar. Eder’s Jahrbuch, 1898, 12, 70. Von Rour—Uber altere Portratobjektive. Zeitsch. Instrument, 1901, 21, 40. WenuHAM—Achromatic Periscope. Brit. J. Phot., 1874, 21, 507, 621; Brit. J. Phot., 1875, 22, 22. PATENTS ON LENSES (B. P., British Patent; D. R. P., German Patent; U. S. P., United States Patent; B. F., French Patent) ABBE AND RupotpH—Photographic Triplet. D.-R. P. 55,313/1890, B. P. 6020/00. A.pis—The Stigmatic. B. P. 16,640/95, D. R. P. 92,582/95. A.tpis—The Aldis Triplet. B. P. 5170/02. ARBEIT—Symmetrical Anastigmatic Objective. D. R. P. 135,742/o1, D. R. P. ery a . . 16,431/11. Beck—Isostigmar. B. P. 27,180/1906, B. P. 14,673/1908, D. R. P. 104,267. Beck—Neostigmar. B. P. 2619/1911, B. P. 3399/1911, B. P. 4714/tTo11. BoorH—Pentac. B. P. 151,506/20. CLrarK—Objective. U.S. P. 399,400/1880. DALLMEYER—Rapid Rectilinear and Modified Petzval Lens. B. P. 2502/1866. DALLMEYER—Rectilinear Landscape Lens. B. P. 1853/1888. DALLMEYER—Single Landscape Lens. B. P. 2530/1864. DALLMEYER—The Achromatic Triplet. B. P. 3096/1866. Gorrz—Cf. Von Hoegh and Zschokke. 5 Grar—Graf Anastigmat. U. S. P. 1,463;132/1923, B. P. 22,400/1o910, U. S. P. 981,412/II. Gruss—Grubb’s Lens. B. P. 1968/1871. GunpLacH—Turner-Reich Anastigmat. Harrison—The Globe Lens. B. P. 2496/1860. KaAEMpFER—Kollinear. D. R. P. 00,482/1895, D. R. P. 91,883/1805. KoLttMorcen—Aristostigmat. D. R. P. 125,560. Lan—Davis-Serrac. B. P. 27,518. Lacour BertHiotr—Anastigmat. B. F. 374,045/1907. Lacour Bertuiot—Stellor. B. F. 456,484/1913. Lee—Unsymmetrical Anastigmatic Objective. B. P. 200,371. Lertrz—Unsymmetrical Anastigmatic Objective. D. R. P. 116,440. Martin—Omnar. O. P. 8364/1901. Morrtson—Wide Angle Lens. U. S. P. 126,970. Potak—Hyperchromatic Objectif. B. P. 201,920. Rupo_tPH—Doppel-Anastigmat. B. P. 4692/1803, B. P. 19,500/1804. Rupotra—tThe Planar. D. R. P. 92,313, B. P. 27,635/1806. © RupotpH—Unar. D, R. P. 134,408/1899, B. P. 24,080/1809. Rupo.pH—Tessar. D. R. P. 142,294/1902. RupotpH—Protar Series VIIa (New Construction). D. R. P. 228,667/19009, B. P. 23,604/1909. RupotpH—Plasmat. B. P. 161,091/1920, 596 PHOTOGRAPHY REICHERT—Solar. D. R. P. 180,255/1904. REICHERT—Combinare. D. R. P. 153,525. ReirzscHEL—Linear. D. R. P. 118,466/1808. RopENstockK—Imagonal. D. R. P. 177,266. ScHROEDER—Concentric Lens. B. P. 5194/88. ScHROEDER—Achromatic Periscope. U.S. P. 554,737/06. SmitH—Air Space Doublet. B. P. 133,459. STEINHEIL—Orthostigmat. D. R. P. 76,662/93. Orthostigmat Type II. D. R. P. 88,505/03. STEINHEIL—Antiplanet. D. R. P. 16,354/81, B. P. 1602/81. STEINHEIL—Unofocal. D. R. P. 133,957. STEINHEIL—Portrait Aplanat. B. P. 1124/74. STEINHEIL—Group Aplanat. D. R. P. 6189/79. STEINHEIL—Periskop. B. P. 2037/65. Simon—Octanare. D. R. P. 168,977. Taytor, H. D.—Cooke Triplet. B. P. 22,607/93, D. R. P. 81,825/o04, B. P. 15,107/95, D. R. P. 86,757/05, B. P. 24,391/1905, B. P. 3398/1905, B. P. 7661/1906, B. P. 3799/1912. Von HorcH—Dagor. D. R. P. 74,437/92, B. P. 23,378/o92. Von HoecH—Improved Form of the Dagor. B. P. 13,162/95. Von Horcuo—The Celor and Syntor. D. R. P. 1009,283/08, B. F. 278,768/08, B. F. 320,304/03, D. R. P. 202,083/07. VoIGTLANDER—Modification of the Petzval Objective. D. R. P. 5761/78, B. P. 4756/78. VoIGTLANDER—Euryscope. D. R. P. 5761/78, B. P. 1938/77. ZeIss—Triplet Anastigmat. D. R. P. 86,757/95, B. P. 6328/13, B. F. 455,546/13. ‘Ze1ss—Four Lens Symmetrical Anastigmat. U. S. P. 1,479,197/23. ZsCHOKKE—Dogmar. D. R. P. 258,495/12, B. P. 833/13, B. F. 453,230/13. THeE TELEOBJECTIVE Reference Works DALLMEYER—Le teleobjectif et la photographie. (French translation by L. P. Clerc. 1904. English edition out of print.) Lan Davis—Telephotography. ScuHmMipt—Das Teleobjectiv. Von Rour—Zur Geschichte und Theorie des Photographischen Teleobjektivs mit besonderer Berucksichtigung der durch die seiner Strahlen Begrenzung bedingen Perspectiv. 1897. W HEELER—T elephotography. Papers DALLMEYER—The Adon and Notes on Telephotographic Systems. Phot. J., 1902, 42, 97. DALLMEYER—A New Teleobjective for Photography. J. Cam. Club (London), 1892, p. 10, Eder’s “ Photographischen Objektiv,” p. 171. : DaLLMEYER—The Compound Telephoto Lens. J. Cam. Club (London), 1802, Eder’s “ Photographischen Objektiv,” p. 175. Lre—Principles and Construction of the Telephotographic Lens. Phot. J., 1925, 65, 392. REFERENCES TO TECHNICAL JOURNALS 597 MretHe—Fin neues telephotographisches System. Eder’s Jahrb., 1892, 6, 152. Von Rour—Petzval’s Orthoskop. Eder’s Jahrb., 1900, 14, 108. Von Ronr—Zur Entwicklungsgeschichte des Teleobjektivs und seiner Theorie. Eder’s Jahrb., 1897, 11, 181. WaterHOUSE—Lens Systems and the Genesis of Telephotography. Phot. J., 1902, 42, 4. PATENTS ON TELEOBJECTIVES ae AND STUART—The Telecentric. B..P.. 2349/12. Bootu—The Dallon. B. P. 151,506/19, B. P. 151,507/19, U. S. P. Spokane Booth—The Telic. B. P. 1,156,743/1915. Lan Davis—The Large Adon. B. P. 1185/14. LrE—Teleobjective. B. P. 132,067/1018. Martin—The Busch Bis-Telar. B. P. 15,732. Merte—The Tele-tessar. B. P. 145,548/19, B. P. 170,520/21, U. S. P. 1,467,- 804/23. STUART AND BreLtickKE—The Teleros. B. P. 188,621/22. W HEELER—Telephotographic Lens. B. P. 18,121. ZE1ss—Teleobjectives. B. F. 363,499/06, B. P. 4532/06, D. R. P. 227,112/08, B. F. 11,701/09, B. P. 19,580/09. Chapter VI. The Photographic Emulsion (For list of general reference works see page 171) GELATINE IN PHOTOGRAPHY FinpLAy—Some Properties of .Colloidal Matter and Their Applications in Photography. Phot. J., 1920, 60, 223. . SHEPPARD—Colloid Chemistry and Its Relation to Photography. Phot. J., 1900, 49, 320. SHEPPARD—The Modern Chemistry of Gelatine. Brit. J. Phot., 1922, 69, 677, 695, 706, 710; J. Ind. and Eng. Chem., 1922, 14, 1023. SHEPPARD, ELLIOTT AND SwEET—Photographic Properties of Gelatine. Physical Chemistry of the Photographic Process. (Published as Transactions of the Faraday Society.) SHEPPARD—Photographic Gelatine. Phot. J., 1925, 65, 380. . StapE—Colloid Chemistry in Photography. (Bibliography.) Brit. J. Phot., 1920, 67, 645. . SLATER-Price—Gelatine. Phot. J., 1922, 62, 356. SLATER-Price—Certain Fundamental Problems in Photography. J. Roy. Soc. Arts, 1924, 72, 725, 739, 753- ScHAUuM—Photographic Binding Media and Other Gels. Koll.-Zeits., 1925, 36, 199. (Zsigmondy-Festschr.) THEORY OF EMULSION PROCESSES Bancrortr—The Photographic Plate. J. Phys. Chem., 1910, 14, 12, 97, 201, 620. Ex_spen—On the Formation of a Chemical Compound of Ammonia with Silver Bromide. Phot. News, 1881, 25, 174. 598 PHOTOGRAPHY GarpicKE—Ammoniakraucherung bei Trockenplatten. Eder’s Jahrbuch, 1913, 27, 62. JarMAN—Photographic Emulsions. Photominiature No. 179. JoHNson—Gelatino-Bromide of Silver Emulsions Treated with Ammonia. B. J. Almanac, 1877, p. 95. ie KnocHEe—Researches on Photographic Ripening. Phot. Ind., 1924, p. 1169. — LizeseEGANG—Uber die Reifung von Silberhaloidemulsion. Zeit. Phys. Chests IQI0, Pp. 75, 374- LirsEGANG—Ripening of Silver Halide Emulsions. Zeit. wiss. Phot., 1923, 22, 81. LizeseEGANG—Intermediate Stages in Emulsion Making. Phot. Ind., 1925, p. 111. LoreNz—Kolloidchemie und Photographie. Koll.-Zeits., 1918, 22, 103. Luppo-CRAMER—The Ripening Process. Zeit. wiss. Phot., 1924, 23, 84, III; 1925, 23, 137. ; Luppo-CrRAMER—Latent Fog in Emulsions. Z. Angew. Chem., 1924, 37, 40. MitryaTtaA—Manufacture of the Photographic Dry Plate. J. Chem. Ind. Japan, 1921, 24, 884, 906. Papyrus—The Ripening of Washed Emulsions. Influence of Foreign Sub- stances. Phot. Ind., 1925, p. 372. QuincKkE—Die Bedeutung der Oberflachenspannung fur die Photographie mit Bromsilbergelatine und eine Theorie des Reifungsprozesses der Bromsil- bergelatin. Eder’s Jahrbuch, 1905, 19, 3 REINDERS—Studien uber die Photohaloide. Zeits. Physik. Chem., 1911, 77, 213, 357; 677. ReNwick—The Manufacture of Sensitive Emulsions as a sti and an Art. J. Soc. Chem. Ind., 1923, 42, 43. RENwicK—Note on the Factors Affecting Grain Size in Bhoweeiitle Emul- sions. Phot. J., 1924, 64, 324. ScHARLOW—Preparation of a Silver Bromide Emulsion for Diapositive Plates and Bromide Papers. Phot. Ind., 1924, p. 233. STEIGMANN—Remarks on Ripening. Phot. Ind., 1925, p. 88. TRIVELLI—Beitrage zu einer Theorie des Reifungsprozesses der Silberhaloide. Zeit. wiss. Phot., 1910, 8, 17. TriveLLI—Influence of Silver Iodide on the Sensitivity of Silver Bromide. 1 Ph Soc. Chem. Ind., 1923, 42, 908. GRAIN SENSITIVITY BrooksBANK—The Darkening of Silver Bromide Grains on Exposure to Light as Further Evidence of their Heterogeneity in Photographic Emulsions. Phot. J., 1921, 61, 421. CrarK—Grain Structure vs. Light Quanta in the Theory of Development. Brit. J. Phot., 1922, 69, 462. CLrarK—The medgeune Centers of a Silver Bromide Emulsion. Phot. J., 1923, 63, 230. Brit.-J. Phot., 1923, 70, 227. CLaRK—Sodium Arsenite aod the Plate. Brit. J. Phot., 1923, 70, 717. CLark—On the Sensitivity of the Silver Haloid Grains of a Photographic Emul- sion. Phot. J., 1924, 64, 91. 3 7 ; ; a q 4 eS ae : : : REFERENCES TO TECHNICAL JOURNALS 599 CirarK—On the Sensitivity of a Silver Bromide Emulsion. The Physical Chem- istry of the Photographic Process. (Published by the F araday Society.) CLrarK—Reversal by Hydrogen Peroxide, Sodium Arsenite and Light. Phot. J., 1924, 64, 363. CLarK—The Action of Arsenites on the Photographic Plate. Brit. J. Phot., 1925, 72, 155. 7 | Renwick—The Sensitive Centers of Silver Bromide Grains. Brit. J. Phot., 1923, 70, 359. SILBERSTEIN—Quantum Theory of Photographic Exposure. I. Phil. Mag., 1922, 44, 257; Il. Ibid., 1922, 44, 956; III. Ibid., 1923, 45, 1062. SHEPPARD, WIGHTMAN AND TRIVELLI—Exposure Theories. The Physical Chem- istry of the Photographic Process. (Published by the Faraday Society.) SHEPPARD AND TrRivetLI—Influence of Crystal Habit on the Photochemical De- composition in Silver Bromide Crystals. Phot. J., 1923, 63, 334. SHEPPARD AND WiGHTMAN—Note on the Theory of Photographic Sensitivity. Science, 1923, 58, 80. SHEPPARD AND TRIVELLI—Structure of the Photographic Emulsion. Trans. Faraday Soc., 1923, Ig, 270. SHEPPARD, TRIVELLI AND WiGHTMAN—Relationship of Photographic Emulsion Fog to Grain Size. Phot. J., 1925, 65, 134. SHEPPARD, WIGHTMAN AND TrivELLI—The Action of Hydrogen Peroxide on Photogranhic Gelatino-Silver Halide Emulsions. J. Frank. Inst., 1923, 195, 337. SHEPPARD, WIGHTMAN AND TRiveLLI—Action of Hydrogen Peroxide on Plates with a Single Layer of Grains. S. I. P., 1925, 5, 50. SLADE AND Hicson—Photochemical Investigation of the Photographic Plate. Proc. Roy. Soc., 1920, 98, 154. SLADE AND Hicson—Action of Light on the Photographic Plate. Phot: J., 1921, 61, 35, 144, 252. SLATER-Price—The Modern Conception of the Sensitivity of Photographic Emulsions. Phot. J., 1925, 65, 208. SVEDBERG—Size and Sensitiveness of the Grains in a Photographic Emulsion. Zeit. wiss. Phot., 1920, 20, 36. SvEDBERG—The Se ticibility of the Individual Halide Grains in a Biciasehie Emulsion. Phot. J., 1922, 62, 183. SvEDBERG—On the Relation between Sexsitiveness and Size of Grain in Photo- graphic Emulsions. Phot. J., 1922, 62, 186. SvepBERG—The Interpretation of Light Sensitivity in Piseenoly: Phot. J., 1922, 62, 310. SVEDBERG, SCHUNK AND ANDERSSON—Relation between Exposure and the Num- ber of Developable Centers. Phot. J., 1924, 64, 272. Toyv—The Theory of the Characteristic Curve of a Photographic Emulsion. Phil. Mag., 1922, 44, 352; II, 1923, 45, 715. _ Toy—The Quantum Theory of Photographic Exposure. Brit. J. Phot., 1922, 69, 443. Toy—The Mechanism of Latent Image Formation. The Physical Chemistry of the Photographic Process. (Published as Transactions of the Faraday Society. ) 600 | PHOTOGRAPHY Toy AND Epcerton—The Relation between the Light Frequency and the Num- q ber of Developable Centers. Phil. Mag., 1924, 48, 947. SIZE-FREQUENCY DISTRIBUTION GERMANN AND HyLan—Dispersity of Silver Halides in Relation to their Photo- graphic Behavior. Science, 1923, p. 332. Second Paper, J. Phys. Chem., 1924, 28, 449. . Hicson—The Emulsion for a Process Plate. Phot. J., 1919, 59, 260. SHEPPARD, WIGHTMAN AND TRIVELLI—Size-Frequency Distribution of Particles of Silver Halide in Photographic Emulsions and its Relation to Sensi- tometric Characteristics. I..J. Phys. Chem., 1921, 25, 181; II. Ibid., 1921, 25, 501; III. Ibid., 1923, 27, 1; IV. Ibid., 1923, 27, 141; V. Ibid., 1923, 27, 440. SHEPPARD—Dispersity of the Silver Halides in Relation to their Photographic Properties. First Colloid Symposium Monograph, 1923. SHEPPARD—Grain-Size and Distribution in Emulsions. Phot. J., 1925, 65, 31. SVEDBERG—Size and Sensitiveness of the Grains of a Photographic Emulsion. Zeit. wiss. Phot., 1920, 20, 36. WIGHTMAN, TRIVELLI AND SHEPPARD—Photographic Densities Derived from Size-Frequency Data. J. Phys. Chem., 1924, 28, 520. Chapter VII. Orthochromatics (For list of general reference works see page 199) Cotor SENSITIZING OF GELATINE EMULSIONS ADAMS AND HALLER—The Kryptocyanines—A New Series of Photosensitizing Dyes. J. Amer. Chem. Soc., 1920, 42, 2661. ApripAt—Sur la Preparation des Colorants Sensibilisateurs dans les Emulsions Photographiques. Bull. Soc. Franc. Phot., 1923, p. 283. BrAUNHOLTZ—A Comparison of Three Isometric Cyanines J. Chem. se (Lon- — don), 1922, 121, 160. : CAPSTAFF AND BoutoentPradticten of Panchromatic Sensitiveness welts Dyes. Brit. J. Phot., 1920, 67, 7109. . MEES AND GUTEKUNST—Some Sensitizers for Deep Red. Brit. J. Phot., 1922, 69, 474. Mitts AND Pope—The Isocyanine Dyes as Sensitizers. Phot. J., 1920, 60, 183. Mitts AnD Pope—The Carbocyanines as Photographic Sensitizers. Phot. J., 1920, 60, 253. | Mitts AND BraunHoLZ—The Thioisocyanines. J. Chem. Soc. (London), 1922, I2I, 2004. Mitts AND PorpE—2-p-Dimethylaminostyrylpyridine Methiodide—A ‘New Photo- graphic Sensitizer. J. Chem. Soc., 1922, 121, 946. | Mitts AND HamMEeR—The Cyanine Dyes. III. J. Chen Soc. (London), 1920, II7, 1550. a Mirts—The Cyanine Dyes. IV. J. Chem. Soc. (London), 1922, 121, 455. REPERENCES TO TECHNICAL JOURNALS 601 Mitts AnD BRAUNHOLTZ—The Cyanine Dyes. V. J. Chem. Soc., 1922, 121, 1480. Mrs AND BrAUNHOoLTZ—The Cyanine Dyes. VI. J. Chem. Soc., 1922, 121, 2004. | Mitts AND BrAUNHOLTZ—The Cyanine Dyes. VII. J. Chem. Soc., 1922, rar, 2804. MonpiLLarpD—Mixed Sensitizers. La Procede, 1906, February; Brit. J. Phot., 1906, 53, 245. Newton—Ortho Plates and Sensitizers. Phot. J., 1903, 43, 262; 1005, 45, 15; 1906, 46, II0, 300. Eper—Uber farbenempfindliche Platten zur Spektrumphotographie im Infrarot, Rot, Gelb, und Grun. (Pinacyanol blue, Pinachrome blue, Pinachrome violet, and Dicyanine A.) Phot. Korr., 1915, 51, 23; Brit. J. Phot., 1917, 64 (color supplement), 8. HAmMER—The Optical and Sensitizing Properties of Some Isometric Isocyanines. Phot. J., 1922, 62, 8. Hamer—The 6.6’-diacetylamino-1.1’-diethylcarbocyanine. J. Chem. Soc. (Lon- don), 1923, 123, 2333. : HamMeER—Derivatives of the Methylenediaminaldines. J. Chem. Soc., 1923, 123, 246. Husr—Absorption and Sensitizing Spectrum of the Cyanines. Phot. J., 1906 46, 133; Das Atelier, 1906, 6, 14. Husnix—Color Sensitizing in Theory and Practice. Phot. J., 1900-01, 40, 364. Konic—Sensitizing Plates by Bathing. Phot. J., 1905, 45, 370 (Abstract). Konic—Pinaflavol—A New Green Sensitizer. P. Rund., 1921, 6, 80; Brit. J. Phot., 1921 (color supplement), p. 16; P. Rund., 1921, 6, 193. Konic—The Quinocyanines—The Constitution of Pinacyanol. Ber. Deuts. chem. Ges., 1922, 55, 3203. Louse—Die Wirkung der Farben auf Bromsilbergelatineplatten. Eder’s Jahrb., 1894, 8, 271. MEES AND SHEPPARD—Estimating the Color Sensitiveness of Plates. Phot. J., 1906, 46, I10. MEEs AND WRATTEN—Dicyanine and Photography of the Infra Red. Phot. J., 1908, 48, 25. Newton—The Properties 6f Homocol as a Sensitizer. Phot. J., 1905, 45, 226, 264. ‘ RENWICK AND BLrocH—Auramine as a Sensitizer. Phot. J., 1920, 60, 145. ReNwick—The Action of Soluble Iodides on Photographic Plates. Phot. J., 1921, 61, 34. SHEPPARD—The Optical and Sensitizing Properties of the chat int Phot. J., 1908, 48, 300. SHEPPARD—The Action of Soluble Iodides and Cyanides on the Photographic Emulsion. Phot. J., 1922, 62, 88. VALENTA—Dyes for Color Sensitizing. Phot. J., 1905, 45, 370, 341. Watt—A Review of Recent Work in Color Sensitizing. Brit. J. Phot., 1907, 54, 365, 386, 406, 464. 602 PHOTOGRAPHY WALTERS AND Davis—Color Sensitive Photographic Plates and Methods of 4 Sensitizing by Bathing. Bulletin of the Bureau of Standards No. 422; J. Frank. Inst., 1922, 193, 103; Brit. J. Phot., 1922, 69, 416, 430. WATERHOUSE—Experiences with Red Sensitizers. Phot. J., 1904, 44, 165. WRATTEN AND Mees—Wedge Spectrographs. Brit. J. Phot., 1907, 54, 304. THE PuHotocRAPHIC LIGHT FILTER Davis—A New Method of Measuring the Factors of Light Filters. Phot. J., 1921, 61, 160. Hnatek—Light Filter Formule. Zeit. wiss. Phot., 1915, 13, 133, 271; Brit. J. Phot., 1921, 68, 95. HopcmMan—Light Filter Formule. Brit. J. Phot., 1922, 69, 6. MerEes—Luminosity Filters. Brit. J. Phot., 1906, 53, 430. PoTapENKo—Theory and Technique of Light Filters. (Appended to this paper is a very complete bibliography of 115 papers on light filters in various photographic and. other technical journals.) Brit. J. Phot., 1921, 68, 507, 522, 534- Renwick—Color Values in Monochrome and a New Viewing Filter to Assist in Obtaining Them. Phot. J., 1910, 59, 158. SmitH—Light Filter Making. Brit. J. Phot., 1921, 68, 459.. Chapter VIII. The Latent Photographic Image (For list of general reference works see page 225) REACTIONS OF THE SILVER HALIDES ON PROLONGED ExposuRE Eccert AND Noppak—Silver Halide Emulsions and the Law of Photo-chemical Equivalence. Z. Physik., 1925, 31, 922. Eccert AND NoppAak—The Photo-chemistry of Silver Compounds. Z. Physik., 1925, 31, 942. Garrison—Influence of Light on the Photo-magnetic Properties of the Silver Halides. J. Amer. Chem. Soc., 1925, 47, 622. Guntz—The Action of Light on Silver Chloride. Phot. J., 1905, 45, 131. Hartunc—The Action of Light on Silver Bromide. J. Chem. Soc. (London), 1922, p. 682. Hartunc—The Photo-chemical Decomposition of Silver Bromide. J. Chem, Soc. (London), 1924, p. 2198. KocH AND SCHRADER—The Action of Light on Silver Chloride, Silver Bromide and Silver Iodide. Z. Physik., 1921, 28, 127. Kocu AND Kretss—Change of Mass of Silver Halides on Intense Illumination. Z. Physik., 1925, 32, 384. ScHWwarz AND Stock—Photo-chemical Decomposition of Silver Bromide. Z. q anorg. Chem., 1923, 132, 380. ScHWARZ AND Gross—Photo-chemical Decomposition of Silver Chloride. Z. 4 . anorg. Chem., 1924, 133, 380. Stock—The Photo-chemical Decomposition of Silver Bromide. Zeit. wiss. Phot., 1925, 24, 132. ee a ee ee REFERENCES TO TECHNICAL JOURNALS 603 WEIGERT—Photo-chemistry of Silver Compounds. Sitzungsber. preuss. Akad. Wiss., IQ2I, p. 641. Quantity oF LicgHT REQUIRED TO Propuce DEVELOPABILITY Hetmick—On the Quantity of Light Required to Render Developable a Grain of Silver Bromide. J. Opt. Soc. Amer., 1922, 5, 908. : HrLtmMick—The Average Quantity of Ultra Violet Energy Required to Render Developable a Grain of Silver Bromide. J. Opt. Soc. Amer., 1924, 9, 521. SHEPPARD AND WIGHTMAN—Energy Exchanges in the Formation of the Latent Image. J. Opt. Soc. Amer., 1922, 5, 913. ACTION OF OxipIzING AGENTS ON THE LATENT IMAGE Citark—Sodium Arsenite and the Plate. Brit. J. Phot., 1923, 70, 717. CLiarK—Action of Arsenites on the Photographic Plate. Brit. J. Phot., 1925, 72, 155. Lupro-CRAMER—Fog by Arsenite. Phot. Ind., 1923, p. 456. Lupro-CrAMER—Latent Image Reactions. Phot. Ind., 1924, p. 1007. RusseL—Action of Resin and Allied Substances on the Photographic Plate. Phot. J., 1890, 39, 345. SHEPPARD AND Merges—Action of Substances on the Latent Image. Phot. J., 1907, 47, 65; Brit. J. Phot., 1907, 54, 33. SHEPPARD, WIGHTMAN AND TRIVELLI—Fffect of Oxidizers on the Sensitivity and on the Latent Image. J. Frank. Inst., 1924, 198, 507. SHEPPARD, WIGHTMAN AND TRIVELLI—Action of Arsenite and Oxidizing Agents. J. Frank. Inst., 1924, 198, 629. - Sterry—The Action of Oxidizers on the Latent Image. Phot. J., 1907, 47, 170; Brit. J. Phot., 1907, 54, 166, 171, 206; Eder’s Jahrbuch, 1907, 21, 364. RETROGRESSION BAEKLAND—Photo-retrogression. Zeit. wiss. Phot., 1905, 3, 58. CHANNoN—The Influence of Time on the Latent Image. Phot. J., 1917, 57, 72. Strauss—The Retrogression of the Latent Image. Kinotechnik, 1924, 6, 315. DEVELOPMENT AFTER FIXATION LEFFMANN—Development after Fixation. Brit. J. Phot., 1924, 71, 40. LUMIERE AND SEYEWETZ—Bull. Soc. franc. Phot., 1911, 2, 264, 373. LUMIERE AND SEYEWETZ—The Latent Image after Fixation. Compt. rend., 1924, 179, 14. é LUMIERE AND SEYEWETZ—Development of the Latent Image after Fixing. Compt. rend., 1924, 178, 1765. LUMIERE AND SEYEWETZ—Causes of the Destruction of the Latent Image after - Fixation. Bull. Assn. Belg. Phot., 1924, 46, 74; S. I. P., 1924, 4, 130. Luppro-CraMER—Physical Development after Fixation. Phot. Ind., 1924, p. 780. 604 PHOTOGRAPHY REVERSAL AND SOLARIZATION ARENS—Significance of Photographic Reversal. Zeit. Phys. Chem., 1925, 114, 337- CiarK—Reversal by Sodium Arsenite, Hydrogen Peroxide and Light. Phot. J., 1924, 64, 363. . Eper—The Solarization of Photographic Plates. S. I. P., 1925, 5, 131; Brit. J. Phot., 1925, 72, 459. ENncLiscH—Studien uber die Solarisation bei Bromsilbergelatine. Arch. wiss. Phot., 1900, 2, 242. Lupro-CRAMER—Solarization. Phot. Ind., 1924, p. 1174; Zeits. Physik., 1924, 29, 387. SCHEFFERS—Studies on Solarization. Zeits. Physik., 1923, 20, 100. SLATER-PriceE—On Solarization. Brit. J. Phot., 1925, 72, 506. TRIVELLI—Beitrag zur Kenntnis der Solarisationsphanomens und weiterer Eigen- schaften des latenten Bildes. Zeit. wiss. Phot., 1909, 6, 197, 237, 272. THEORIES OF THE LATENT IMAGE Asecc—Die Silberkeim Theorie oder subhaloid Theorie. Brit. J. Phot., 1899, 46, 773; Phot. J., 1800, 39, April. ALLEN—The Formation of the Image on the Photographic Plate. Phot. J., 1914, 54, 175. | - Bancrortr—The Latent Image. J. Phys. Chem., 1912; Brit. J. Phot., 1912, 59, 881. Banxs—The Theory of the Latent Image. Brit. J. Phot., 1898, 45, 117. BoTtHAMLEY—The Latent Image. Phot. J., 1890, 39, Jan. Braun—Uber die Natur des latenten Bildes. Zeit. wiss. Phot., 1904, 2, 2090. Butt-—The Latent Image. Brit. J. Phot., 1906, 53, 160. Eper—Silber sub-bromide in latente Lichtbilde auf Bromosilber und die Silber- keimtheorie. Phot. Korr., 1899, 36, 276. . Eprr—Die Silberkeim Theorie und Verwachtes. Brit. J. Phot., 1899, 46, 788; Phot. Korr., 1899, 36, 1650. Emicuo—Zur Geschichte des latenten Photographischen Bildes. Zeit. wiss. Phot., 1907, 5, 183. HomotKka—The Latent Image and Developers. Brit. J. Phot., 1917, 64, 81. IpzErpA—Zur Theorie des latenten Bildes. Zeit. wiss. Phot., 1910, 8, 234. KincGpon—Considerations on the Nature of the Latent Image. Phot. J., 1906, 46, 57; Brit. J. Phot., 1906, 53, 36. Luppo-CRAMER—Studien uber die Natur des latenten Lichtbildes. Phot. Korr., IQ0I, 39, 145, 218, 550, 643; Brit. J. Phot., 1901, 48, 520, 552, 569, 820; B. J. Almanac, 1903. Luppo-CRAMER—Neue Untersuchungen zur Theorie des photographischen Vor- gange. Phot. Korr., 1904, 42, 12, 118, 159, 254, 310, 374, 432, 478, 573. Luppo-CRAMER—Remarks on Some New Work on the Latent Image by Mr. F. F. Renwick. Phot. Korr., 1920, 57, 250, 285. Luppo-CrRAMER—History and Theory of the Latent Image. I. Zeit. wiss. Phot., 1924, 23, 91; II. Ibid., 1925, 23, 122; III. Ibid., 1925, 23, 216. - —— ; E i REFERENCES TO TECHNICAL JOURNALS 605 LuTHEeR—Edersche Versuch und das latenten Bild. Brit. J. Phot., 1899, 46, 664; Phot. Korr., 1899, 36, 584. LutTHER—On the Present State of our Knowledge of the Nature of the Latent Image. Brit. J. Phot., 1910, 57, 651. Mercator—The Nascent Silver and Sub-Haloid Theories. Brit. J. Phot., 1800, 46, 628. ODENCRANTS—Was muss man von einer Theorie des latenten Bildes fordern. Zeit. wiss. Phot., 1919, 16, 261. Rawiinc—The Mystery of the Latent Image. Phot. J., 1923, 63, 482. RENWIcK—Photographic Images—Visible and Invisible. Brit. J. Phot., 1920, 67, 447, 463. ReENwicK—The Gelatine Emulsion and the Latent Image. Brit. J. Phot., 1923, 70, 382. ScHAuM—Zur Theorie des Photographischen Processe. Arch. wiss. P., 1900, 2, 9. ScHUMANN—Theory of the Latent Image. Phot. J., 1890, 39, 313. SEYEWETZ—The Latent Image. Chemie et Industrie, 1925, 13, 355. STEIGMANN—Theory of Photographic Light Sensitivity. Chem.-Ztg., 1924, 48, 234. THORNE-BAKER—Cause of Sensitivity of Silver Bromide Emulsions. Phot. J., | 1924, 64, 369. TRIVELLI—Beitrag zur Kenntnis der Silberhaloide. Zeit. wiss. Phot., 1909, 6, 358, 438. ' TRIVELLI—Beitrag zur Photochemie der Silber (sub) haloide. Zeit. wiss. Phot., IQII, 8, 113. TRIVELLI, SHEPPARD AND LovELAND—The Formation of the Latent Image. J. Frank. Inst., 1925. Toy—The Mechanism of the Latent Image. The Physical Chemistry of the Photographic Process. (Published as the Transactions of the Faraday Society.) We1sz—Researches on the Latent Image by Means of Plates Free from Colloid. Brit. J. Phot., 1907, 54, 960. Chapter IX. Sensitometry (For list of general reference works see page 253) GENERAL PAPERS ON SENSITOMETRY Bioco—Sensitometry. The Physical Chemistry of the Photographic Process. (Published as the Transactions of the Faraday Society.) BrowNn—The H. and D. Doctrine. Brit. J. Phot., 1921, 68, 335, 354, 372, 386, “401, 415. Eprer—System der Sensitometrie Photographischen Platten. Phot. Korr., 1900, 37, 241, 304, 364, 441, 495, 567, 625; 1902, 39, 386, 449, 504. MerrEs AND SHEPPARD—Instruments for Sensitometric Investigation. Phot. J., 1904, 44, 200. (With excellent bibliography.) MEES AND SHEPPARD—The Sensitometry of Photographic Plates. Phot. J., 1904, 44, 282. (With excellent bibliography.) 40 606 PHOTOGRAPHY ODENCRANTS—Sensitometrische Apparate und deren Fahlerquellen. Zeit. wiss. Phot., 1919, 16, 69 (Bibliography). STANDARD LiGHT SOURCES BoTHAMLEY—The Amyl-Acetate Lamp. Phot. J., 1894, 34, 231. Eder’s Jahr- buch, 1890, 54. Dispin—Light Standards. Phot. J., 1894, 6, 712. EpEr—Employment of Magnesium as a Secondary Source. Brit. J. Phot., 1925, ype BW SNS To. LR Se EncLtisH—Eine Amyl-Acetate lampe fur Sensitometrische Zwecke. Phot. Mitt., IQOI, 28, 157. Fasry—Luminous Standards for Sensitometry. S. I. P., 1925, 5, 121. Frery—An Acetylene Standard. Phot. J., 1905, 45, 132. Jonges—Light Standards for Sensitometry. S. I. P., 1925, 5, 123. Jouaust AND BatLaup—Color Temperature of the Acetylene Flame. S. I. P., 1925, 5, 124. Jouaust—The Incandescent Lamp as a Sensitometric Standard. S. I. P., 1925, 5, Taz LuTHER—Constant Light Source. Phot. J., 1925, 65, 60. Merrs—Screened Acetylene Light. Brit. J. Phot., 1906, 53, 890. MEES AND SHEPPARD—Investigations on Standard Light Sources. Phot. J., 1910, 50, 287; Brit. J: Phot., 1910, 57, 627. NauMANN—Artificial White Light for Photographic Purposes. Phot. J., 1925, 65, 348. NAuMANN—Colored Filters for Sensitometric Light Standards. Zeit. wiss. Phot., 1925, 23, 303. WatsH—Standards of Light for Photographic Sensitometry. Phot. J., 1925, 65, 52. Exposing APPARATUS BRIEFER—Improvements in the Disk Densitometer of H. and D. Trans. Motion Picture Engineers, 1925, No. 21, p. 85. CALLIER—The Construction of Photometric Instruments. Phot. J., 1913, 53, 242; Brit. J. Phot., 1913, 60, 951, 972. Davipson AND BaLMAIN—A New Form of Exposing Apparatan Phot... J; 1925, 65, 60. Eprr—The Eder-Hecht Wedge in Sensitometry and Photometry. Phot. Korr., 1920, 56, I, 41. Fasry—The Measurement of Density by Photographic Methods. S. I. P., 1925, 5, 128. . Harpy—A Non-intermittent Sensitometer. Jl. Opt. Soc. Amer., 1925, 10, 149. Hicson—A Simple Non-intermittent Sensitometer. Phot. J., 1920, 60, 235. Hitcuins—A Non-intermittent Sensitometer. Bull. Soc. franc. Phot., 1921, p. 74. Jones—A Simple and Inclusive Method of Testing Pilates. (Chapman Jones plate speed tester.) Phot. J., 1901, 41, 246; Eder’s Jahrbuch, 1901, 15, 491. REFERENCES TO TECHNICAL JOURNALS 607 Jones—A Non-intermittent Sensitometer. Phot. J., 1920, 60, 60. RAWLING—Exposure Mechanism. Phot. J., 1925, 65, 64. RENwicK—Sources of Error and Differences in Dry Plate Sensitometers. Brit. J. Phot., 1910, 57, 626. RENwick—Note on Exposure Mechanisms. Phot. J., 1925, 65, 74. EFFECT OF INTERMITTENT EXPOSURE AND THE RELATION OF TIME TO INTENSITY AsnrEY—The Effect of Intermittent Exposure and the Relation between Time and Intensity. Treatise on Photography, 1903, p. 391. Bitocuo—Plate Speeds, Failure of the Reciprocity Law. Phot. J., 1917, 57, 51. Husit—Determination of Schwarzschild’s Index and its Significance. Phot. Korr., 1919, p. 363. Jones AND Huse—On the Relation between Time and Intensity in Photographic Exposure. J. Opt. Soc. Amer., 1923, 7, III5. MA.LLET—Photographic Plates and the Law of Schwarzschild. S. I. P., 1923, 3, 1. RENwicK—Some Deductions from Schwarzschild’s Rule. Phot. J., 1916, 56, II. Ropertson—Determination of the Schwarzschild Constant. Jl. Opt. Soc. Amer., 1923, 7, 900. ScHWARZzSCHILD—Uber die Wirkung intermittentender Belichtung auf Bromsil- bergelatine. Phot. Korr., 1899, 36, 109, 171. StrAuss—The Schwarzschild Exponent. Kinotechnik, 1924, 6, 125. WALLACE AND LeEmon—The Reciprocity Law. Brit. J. Phot., 1909, 56, 378. WERNER—Das photographische Reziprozitatsgesetz fur Bromsilbergelatine bei Erregung mit Licht verschiedener Wellenlauge. Zeit. wiss. Phot., 1907, 5, 382; 1908, 6, 25. DENSITOMETERS AND Density MEASUREMENT BaKker—A Photo-Electric Photometer and Densitometer. J. Scient. Inst., 1924, I, 345. BuLt AND CarTWRIGHT—The Measurement of Photographic Density. Phot. J., 1924, 64, 180. Butt AND CartwricHt—An Evaluation of the Light Scattered by Photographic Densities. Phot. J., 1925, 65, 177. CALLIER—Absorption and Scatter of Light by Photographic Negatives Meas- ured by Means of Marten’s Polarization Photometer. Phot. J., 1900, 49, 200; Zeit. wiss. Phot., 1909, 7, 257. CAPSTAFF AND GREEN—A Motion Picture Densitometer. Phot. J., 1924, 64, 97. CarNEGIE—Modification of the H. and D. Photometer. Brit. J. Phot., 19009, 56, 197. CLavieR—Influence of Non-uniformity on Photographic Plates on Photometric Measurements. S. I. P., 1924, 4, 9. Cousin—A New Photographic Photometer. Brit. J. Phot., 1907, 54, 24. CLERc—The Measurement of Density and the Expression of the Results. S. I. Eo 1oes, 6 128... df ee Uo ae , 608 | PHOTOGRAPHY Dozsson—A Flicker Type of Photo-electric Photometer. Proc. Roy. Soc., 1923, A104, 248. EccErRT AND ARCHENHOLD—The Optical Scattering Power of Photographically Developed Silver Layers. Z. physik. Chem., 1924, 110, 407. Frercuson—A New Density Meter. Phot. J., 1911, 51, 405; Brit. J. Phot., 1912, 59, 24; Eder’s Jahrbuch, 1912, 26, 469. Frercuson—A Bar Photometer for Measuring Densities by Non-parallel Light. Phot. J., 1912, 52, 283; Brit. J. Phot., 1912, 59, 772. Frercuson—The F. R. B. Photometer. Phot. J., 1918, 58, 155; Brit. J. Phot., 1918, 65, 128, 213, 224. Fercuson—The Ferguson Density Meter No. V. “Phot. J., 1924, 64, 30; Brit. J. Phot., 1924, 71, 10. GoLpBERG—The Densograph. Brit. J. Phot., 1910, 57, 649; Eder’s Jahrbuch, IQIO, 24, 226. HartMAn—A New Photographic Photometer. Brit. J. Phot., 1900, 47, 67. Harrison—Precision Photometers for Photographic Photometry. Jl. Opt. Soc. Amer., 1925, 10, 157. HorFMANN—Ein neues Photometer zur Sensitometrie. Phot. Korr., 1901, 38, QI, 651. Hyort, Lowy anp Biackwoop—Optical Densitometer. JI. Opt. Soc. Amer., 1924, 9, 43. Jones—A Densitometer for the Measurement of High Photographic Densities. J. Opt. Soc. Amer., 1923, 8, 231. Jones—An Opacity Balance. Phot. J., 1898-0, 38, 90. Lux—A Densitometer. Phot. Korr., 1920, p. 13. Martens—Modified Konig Photometer. (Exact title not available.) Phot. Korr., 1901, 39, 528. PrrrRINE—Photo-electric Cell Photometer for the Measurement of Photographic Densities. Jl. Opt. Soc. Amer., 1924, 8, 381. Prunp—The Pfund Photometer. Brit. J. Phot., 1907, 54, 660. PatuEe—Influence of the Diffusion of Light in Photometric Miaharanients Brit. J. Phot. i025, 723°S.. 1. P., 1085, soa ReNwick—A New Form of Density Measuring Apparatus. Phot. J., 1910, 50, 177, ReNwick—The Measurement of Densities. Phot. J., 1912, 52, 250, 260; Brit. J. Phot., 1912, 59, 304; Eder’s Jahrbuch, 1914, 28, 384. RenwickK—The Effect of Inter-reflection on Density Values. Phot. J., 1013, 53, 204; Brit. J., 1913, 60, 611. RENwickK—An Improved Form of the Ferguson Bench Photometer. Phot. J., 1914, 54, 167. Renwick—An Instrument for the Measurement of Gamma. Phot. J., 1914, 54, 163. RENwick—How Should the Densities of a Photographic Deposit be Measured. Brit. J. Phot., 1924, 71, 65. SANGER-SHEPPARD—Density Meter. Brit. J. Phot., 1911, 58, 926; British Patent 23,429 of IgII. STENGER AND Kuyawa—The Measurement of Photographic Density. Zeit. wiss. Phot., 1924, 23, 80; Phot. Ind., 1924, p. 953. : : a f r 7 . ; : ee LE ee eens pee ee ee re ee ere, REFERENCES TO TECHNICAL JOURNALS 609 Toy AnD Rawiinc—Photo-electric Densitometer. Phot. J., 1924, 64, 189; Jl. Scient. Instr., 1924, 1, 362. Toy—The Standardization of Photographic Density Measurements. Phot. J., 1925, 65, 164. Weser—Causes of Error in Density Measurements. Zeit. wiss. Phot., 1924, 23, 175. Wiisry—Correction for Fog in Photographic Densities. Phot. J., 1925, 65, 454. DEVELOPERS AND DEVELOPMENT OF SENSITOMETRIC TESTS Braver—New Methods of Control for Thermostats. J. Ind. Eng. Chem., 1923, 15, 359. CLarK—Standard Development. Phot. J., 1925, 65, 76. HARRISON AND Dozgson—Note on the Uniform Development of Photographic Pilates, Phot. J., 1925, 65, 80. MerEES AND SHEPPARD—Instruments for Sensitometric Investigation. Phot. J., 1904, 44, 210. SHEPPARD AND Exrriotr—lInfluence of Stirring on the Rate and Course of De- velopment. J. Frank. Inst., 1924, 198, 333. WEDGE SCREENS AND THEIR USE IN SENSITOMETRY Biocx—Selective Absorption of Neutral Wedges. Phot. J., 1917, 57, 51. Eprr—Application of the Neutral Wedge Sensitometer. Phot. Korr., 1922, p. 17 (festnummer). GoitpBerc—Gelatine Wedges. Brit. J. Phot., 1910, 57, 642, 648. 5 GoLtpperc—Uber die Automatische Herstellung der Characterischen Kurve. Zeit. wiss. Phot., 1911, 9, 323; Brit. J. Phot., 1910, 57, 642, 664. Hicson—Wedge Method of Photometry. Phot. J., 1921, 61, 93. LANnGcEeR—Neutral Wedge Sensitometry according to the Normal Constant Sys- tem. Phot. Rund., 1924, 61, 59. LEHMANN—Neutral Wedges in Sensitometry. Zeit. wiss. Phot., 1922, p. 214 (festnummer). Lopet—Automatic Registration of the Characteristic Curve. Bull. Soc. franc. Phot., 1924, 11, 209. THE INTERPRETATION OF RESULTS BaAKeER—Interpretation of the Characteristic Curve. Phot. J., 1925, 65, 181. BiocH—Interpretation of Results. Phot. J., 1925, 65, 186. Eprr—For all papers see Beitrage zur Photochemie. HerypDEcKER—Rapid Solution of Some Common Problems in Sensitometry. S. eet 1OR6, 5, 21. LutTHER—Interpretation of the Characteristic Curve. Phot. J., 1925, 65, 185. Meres—The Interpretation of Sensitometric Tests. Brit. J. Phot., 1906, 53, 104, 126, 143, 179, 617, 636, 797, 857. Merrs—Report on the Present Condition of Sensitometry. Brit. J. Phot., 1909, 56, 685. 610 : PHOTOGRAPHY Mrres—The Photographic Reproduction of Tone. Phot. J., 1924, 64, 310. RayLeicgH—The General Problem of Photographic Reproduction. Phil. Mag., IQII, p. 734; Brit. J. Phot., 1911, 58, 904. RenwickK—The Under Exposure Period in Theory and Practice. Phot. J., 1913, 52. 127% . Renwick—Tone Reproduction and its Limitations. Phot. J., 1916, 56, 222; Brit. J. Phot., 1916, 63, 675. ReENwicK—The Under Exposure Period. Brit. J. Phot., 1912, 59, 280, 312; Eder’s Jahrbuch, 1912, 26, 106. THORNE-BAKER—Interpretation of the Characteristic Curve. Phot. J., 1925, 65, 181. Wartxkins—New Methods of Speed and Gamma Testing. Phot. J., 1912, 52, 207; Brit. J. Phot., 1912, 59, 316. Watit—Elementary Sensitometry. Amer. Phot., 1923, pp. 298, 356, 416. j Chapter XI. The Theory of Development (For list of general reference works see page 288) On THE THEORY OF DEVELOPMENT Asecc—Eine Theorie der photographischen Entwicklung. Arch. Wiss. P., 1899, I, 109. AsecGc—Theorie des Eisenentwicklers nach Luther. Arch. Wiss. P., 1900, 2, 76. Asecc—Zur Frage nach der Wirkung der Bromide auf die Entwickler. Eder’s Jahrb., 1904, 18, 65. ANDRESEN—Zur Theorie der Entwicklung des latenten Lichtbildes. Phot. Korr., 1898, 35, 445. ARMSTRONG—The Chemical Changes Attending Photographic Operations. I. The Theory of Development in Relation to the Essentially Electrolytic Character of the Phenomena and: the Nature of the Photographic Image. Brit. J. Phot., 1892, 39, 276. Bancrort—The Effect of Bromide. Brit. J. Phot., 1912, 59, 878. Banxs—The Theory of Development. Brit. J. Phot., 1896, 43, 677. BoTHAMLEY—Remarks on Some Recent Papers on the Latent Image and its — Development. Phot. J., 1890, 39, 123. Brepic—Die electromotorische Scala der photographischen Entwicklers. Eder’s Jahrb., 1895, 9, 19. DrsALME—The Chemical Theory of Development. Brit. J. Phot., 1910, 57, 653. FRIEDLAENDER—Zur Theorie der Entwicklung. Phot. Korr., 1902, 39, 252. Hurrer AND DrirFieLD—The Action of Potassium Bromide. Phot. J., 1898, 38, 360. KELLER—The Theory of Photographic Development. Koll. Zeits., 1923, 32, 304. KroHN—The Mechanism of Development of the Image in a Dry Plate Nega- tive. Phot. J., 1918, 58, 179; Brit. J. Phot., 1918, 65, 412. LUMIERE AND SEYEWETZ—Contribution a,L’Etude du Role des alcalis dans les Revelateurs organiques. Bull. Soc. franc. Phot., 1906, 16, 32. a ae ee ee eS = se CULT le eS ee ee ) ee a Fe eee | ; REPERENCES TO TECHNICAL JOURNALS 611 Luppo-CRAMER—Die verzogernde Wirkung der Bromide in den photograph- ischen Entwicklern als kolloidchemischer Vorgang. Koll. Zeits., 1900, 4, 92. Luppo-CRAMER—Uber den Einfluss der Bromide im Entwickler auf die topo- graphische Verteilung des Silbers im Negativ. Phot. Korr., 1912, 49, 383. Luppo-CrRAMER—Ueber die beranderung der konform der bromsilbers bei der Reduktion und die Nahronertheorie der Entwicklung. Phot. Korr., 1911, 48, 547. Lupro-CrRAMER—Zur theorie der chemischen Entwicklung. Phot. Korr., 1908, 35, 206. | Luppo-Cramer—Acceleration of Development by Soluble Iodides. Koll. Zeits., 1922, 30, 186. MATTHEWS AND BARMEIER—The Electro-potentials of Certain Photographic De- velopers and a Possible Explanation of Photographic Development. Brit. J. Phot., 1912, 59, 879. Merrs—Time Development. Phot. J., 1910, 50, 403; Brit. J. Phot., 1910, 57, 919; Eder’s Jahrb., 1910, 25, 161. Pinnow—Behaviour and Function of Sulphite in Developing Solutions. P. Rund., 1923, 60, 27. PrecHT—Beitrage zur Theorie der Photographischen Entwicklung. Arch. Wiss. P., 1900, 2, 155; Brit. J. Phot., 1900, 47, 650. Rees—Theory of Development. Phot. J., 1906, 46, 302; Bull. Soc. franc. Phot., 1904, 20, 324. ReNnwick—The Physical Process of Development. Brit. J. Phot., 1911, 58, 75. ScHAaumM—Zur theorie des photographischen prozesses. I. Das latenten Bild; II. Der Entwicklungsvorgang. Arch. Wiss. P., 1900, 2, 9. ScHILOW AND TIMTSCHENKO—Physikalisch-chemische Studien an photograph- ischen Entwicklung. JII. Hydrochinon als Induktor. Zeits. Elektro- chem., 1913, 19, 8106. SHEPPARD—Theory of Alkaline Development, with Notes on the Affinities of Certain Reducing Agents. J. Chem. Soc., 1906, 89, 530. SHEPPARD—Reversibility of Photographic Development and the Retarding Ac- tion of Soluble Bromides. J. Chem. Soc. (London), 1905, 87, 1311; Zeit. wiss. Phot., 1905, 3, 443. SHEPPARD—The Electro-chemistry of Development. Trans. Electrochem. Soc., I92I, , Pp. 429. . SHEPPARD—On the Silver Germ Theory of Development. Phot. Korr., 1922, 59, 76. | SHEPPARD AND Ettiotr—On the Theory of Development. Trans. Faraday Soc., 1923, 19, 355. SHEPPARD AND Mers—On the Chemical Dynamics of Development. Phot. ' J., 1905, 45, 281; Zeit. wiss. Phot., 1904, 3, 97. SHEPPARD AND Meres—Some Points in Modern Chemical Theory and their Bearing on Development. Phot. J., 1915, 45, 241. SHEPPARD AND Mryer—On Chemical Induction in Photographic Development. J. Amer. Chem. Soc., 1920, 42, 680. VotMER—The Theory of Development of the Latent Photographic Image. Zeit. wiss. Phot., 1921, 20, 189; Phot. Korr., 1921, p. 226. 612 : PHOTOGRAPHY On THE PHYSICAL CHEMISTRY OF PHOTOGRAPHIC DEVELOPMENT Birocu—Plate Speeds. Phot. J., 1917, 57, 51. DRIFFIELD—Control of the Development Factor. Phot. J., 1903, 43, 17. HurTerR AND DrirrieLD—Photochemical Investigations. J. Soc. Chem. Ind., 1890, 9, 455. HuRTER AND DrIFFIELD—Exposure and Development. Phot. J., 1805, 35, 372. HurtTER AND DrirFieLD—The Latent Image and its Development. Phot. J., 1898. FERGUSON AND Howarp—Control of the Developing Factor at Various Tem- peratures. Phot. J., 1905, 45, 118; Brit. J. Phot., 1905, 52, 249; Eder’s Jahrb., 1905, 21, 408. Frercuson—A New Method of Calculating the Time of Development at Various Temperatures. Phot. J., 1906, 46, 182; Brit. J. Phot., 1906, 53, 206; Eder’s Jahrb., 1905, 21, 474. : FEercuson—Investigations on the T.C. of a Pyro Soda Developer. Phot. J., IQ10, 50, 412; Eder’s Jahrb., 1910, 25, 506; Bull. Soc. franc. Phot., rozo, 27, 175. KroHn—The Mechanism of Development of the Image in a Dry Plate Nega- tive. Phot. J., 1918, 58, 179; Brit. J. Phot., 1918, 65, 412. LuTHER—The Physical Chemistry of Negative Processes. Phot. J., 1912, 52, 201. Mers—Interpretation of Sensitometric Tests. Brit. J. Phot., 1906, 53, 104, 126, 143, 179, 617, 636, 797, 857. Merres—Time Development. Phot. J., 1910, 50, 403; Brit. J. Phot., 1910, 57, 919; Eder’s Jahrb., 1910, 25, 161. Merrs—Physical Chemistry of Photographic Development. Brit. J. Phot., 1913, 60, 935. MeEES AND SHEPPARD—On the Development Factor. Phot. J., 1903, 43, 48. MEES AND SHEPPARD—On the Highest Development Factor obtainable on any Plate. Phot. J., 1903, 43, 199. MEES AND SHEPPARD—On the Sensitometry of Photographic Plates. Phot. J., 1903, 43, 199. MEES AND SHEPPARD—Some Points in Modern Chemical Theory and their Bearing on Development. Phot. J., 1905, 45, 241. Nietz—Theory of Development. Phot. J., 1920, 60, 280. Nietz—Theory of Development. Monograph No. 2 Eastman Research Lab- oratory, 1922. Pinnow—The Function of Sulphite in Alkaline Developers. P. Rund., 1923, 60, 27. Prper—Application of Physico-chemical Theories in Plate Testing and Experi- mental Work with Developers. Brit. J. Phot., 1913, 60, 110. REeNwick—The Physical Process of Development. Brit. J. Phot., ror1, 58, 75. Renwick—The Calculation of Gamma Infinity. Phot. J., 1911, 51, 213; Bull. Soc. franc. Phot., 1911, 352. Renwick—An Improved Method of Computing the Velocity Constant and Gamma Infinity. Phot. J., 1923, 63, 331. SHEPPARD—The Chemical Dynamics of Photographic Development. Proc Royal Society, 1904, 74, 457. REFERENCES TO TECHNICAL JOURNALS 613 Toy anp Hicson—Factors Determining Gamma Infinity. Phot. J., 1923, 63, 68; S. T. I. P., 1923, 3, 131. Watxins—On the Variation of the Temperature Coefficient with Different f iatege 2) NOt. )4 1010, 50, 411; Brit. J. Phot., 1911, 58, 3. Watxins—New Methods of Speed and Gamma Testing. Phot. J., 1912, 52, 200. Chapter XII. The Organic Developing Agents ON THE CONSTITUTION OF ORGANIC DEVELOPING AGENTS ANpDRESEN—Constitution der Entwickler. P. Mitt. 1891, 27, 124, 286, 206. ANDRESEN—Constitution organischer Entwickler. Eder’s Jahrb., 1893, 7, 418. ANDRESEN—Organische Entwicklersubstanzen. Eder’s Jahrb., 1903, 7, 486. ANDRESEN—Zur Charakterisirung der Entwicklersubstanzen. Phot. Korr., 1890, 36, 635. ANDRESEN—Zur Chemie der organischen Entwickler. Phot. Korr., 1900, 37, 185. Homo.tka—Beitrage zur Theorie der organischen Entwickler. Phot. Korr., 1914, 51, 256, 471. Homo,txa—The Latent Image and Development. Brit. J. Phot., 1917, 64, 81. LrerppeER—Photographic Developers. Brit. J. Phot., 1900, 47, 826. LirsEGANG—Die Constitution der organischen Entwickler. Photochemischen Studien, vol. II, 1894. Lopet—Der Ersatz der Alkalien durch Ketone und Aldehyde in den photo- graphischen Entwicklern. Eder’s Jahrb., 1904, 18, 103. LUMIERE AND SEYEWETZ—Die Bildung von Salzen mit Entwicklerfahigkeit aus Aminen und Phenolen. Eder’s Jahrb., 1899, 13, 306. LUMIERE AND SEYEWETZ—On the Developing Power of Hydrochinon Substitu- tion Compounds. Brit. J. Phot., 1914, 61, 341. | LUMIERE AND SEYEWETzZ—Sur les Substitutions Alkylees dans les Groups de la Function Developpatrice. Bull. Soc. franc. Phot., 1898, 14, 158. LUMIERE AND SEYEWwETz—Ueber die Additionsprodukte, welche die Gruppen mit entwickelnden Eigenschaften mit den Aminen und Phenolen bilden. Arch. wiss. Phot., 1899, I, 64. LuMizRE AND SEYEWETZ—Influence du Groupe Cetonique sur le Pouvoir De- veloppateur des Polyphenols. Bull. Soc. franc. Phot., 1897, p. 415. LUMIERE AND SEYEWETz—Sur la Constitution des Substances Reductrices, sus- ceptibles de developperd L’Image Latente sans entre additionees D’ Alkali. Bull. Soc. franc. Phot., 1903, ; Eder’s Jahrb., 1904, 18, 99. LUMIERE AND SEYEWETZ—Untersuchungen uber die chemische Konstitution der Entwickler-Substanzen. Eder’s Jahrb., 1898, 12, 100. LUMIERE AND SEYEWETZ—Sur la Fonction Developpatrice. Bull. Soc. franc. Phot., 1806, p. 268. SrYEWETz—The State of our Knowledge of Organic Developing Agents. Bull. Soc. franc. Phot., 1920, p. 129; Brit. J. Phot., 1920, 66, 186. 614 . PHOTOGRAPHY On OrcGANIC DEVELOPING AGENTS “ Acra ’”’—Photographic Developers. Paraphenylene Diamine with Amido or — Hydroxyl Groups. British Patent 11,872/1803. * q “ Acra ”—Photographic Developers. Para-oxy-phenyl-glycinamide. British _ Patent 9537/1905. q “ Acra ”’—Photographic Developers. Oxy-phenyl-alkyl-glycin. British Patent — 18,095/1913. 7 ANDRESEN—Photographic Developers of the Naphthalene Series. British Pat- — ent 5207/1889. a ANpbRESEN—Photographic Developers. Amido Derivatives of Naphthol. Brit- — ish Patent 25002/1893. a ANDRESEN—Bromhydrochinone als Entwickler. Phot. Korr., 1890, 36, 306. ANDRESEN—Die Isomeren des Amidols. Phot. Korr., 1894, 31, 505. | ANDRESEN—Verwendung von Derivaten des p-Phenylendiamins, sowie des p- — Toluylendiamins als Entwickler in der Photographie. Eder’s Jahrb., 1895, — 9, 50. ANDRESEN—Weitere Beitrage zur Kenntniss des Diamidooxydiphenyls als — Entwickler. Phot. Korr., 1899, 36, 208. BUCHERER—Photographic Developers of Paratoluolsulfonylaminophenol. D. R. P. 369,391/1921. BuneLt—Preservation of Diamidophenol Developers in Solution. Il. Prog. Fot., 192I, p. 204. CROWTHER—Preservatives of Amidol. Brit. J. Phot., 1920, 67, 642. DrEsALME—On a New Developer, Sulphinol. Brit. J. Phot., 1912, 59, 425. 4 DieTerLE—Photographic Developer of Sulpho-acid Aminophenol. United — States Patent 1,432,542/1922. 7 Druce—Stabilizing Solutions of Amidol. Brit. J. Phot., 1922, 69, 81. ErMEN—Rodinal Type Developers. Brit. J. Phot., 1920, 67, 611. 3 ErMEN—Preparation of Monomethyl Paramidophenol Sulphate (Metol). Phot. — J., 1923, 63, 223. * FaucHEy—Conservation of Amidol Developers. Bull. Soc. franc. Phot., 1923, — 10, 90. FiscHer—Developers Yielding Colored Images. British Patent 2562/1913. Grear—A New Developer (Ds50). Brit. J. Phot., 1921, 68, 307. Havurr—Amidophenol Developers. British Patent 15,434/1891. ‘_ Haurr—Photographic Developer of Ortho-para-diamidophenol Ortho-p-diam- — ido-ortho-cresol-p-diamidometa-cresol. British Patent 14,542/1892. 4 Haurr—Photographic Developers of Sulphonic or Carboxylic Acids of Ortho, or Para, or Ortho-para, Amidophenol (Neol). British Patent 154,198 of — 1920. q Homo_tkA—Hydrocoerulignon as a Developer. Phot. Korr. (Festnummer), 1922; Brit. J. Phot., 1922, 69, 397. Kinc—Photographic Developers of 2: 4 Diamidophenol and Stannous Chloride. British Patent 196,672/1922. Loset—Preservatives of Amidol in Solution. Brit. J. Phot., 1921, 68, 7o1. LUMIERE AND SEYEWETZ—Sur les Properties Developpatrices des Hydroxyl- amines Aromatiques. Bull. Soc. franc. Phot., 1894, , 487. REFERENCES TO TECHNICAL JOURNALS 615 LUMIERE AND SEYEWETZ—Sur les Proprietes Revelatrices D’une Nouvelle Com- binaison D’hydrochinone et de Paraphenylenediamine. Bull. Soc. franc. Phot., 1890, , 135- LUMIERE AND SEYEWETz—Sur la Preparation et les Proprietes Revelatrices de la Metoquinone-combinaison de metol et D’hydrochinone. Bull. Soc. francPhot,, 1903, , 231. LUMIERE AND SEYEWETZ—Sur la Preparation et les Proprietes Revelatrices die Chloranol (Chlorhydroquinonemethylparamidophenol). Bull. Soc. franc. Phot., 1913, p. 223. Luppro-CrAMER—Development with Amidol and Related Substances. Phot. Korr., 1921, 68, 121 (Festnummer). MeELpoLa—Eikonogen. J. Soc. Chem. Ind., 1889, , 958. Perk1N—Pyrocatechin. J. Chem. Soc., 1890, 57, 587. SCHERING—Benzyl Para-amidophenol Compounds as Developers. British Pat- ent 20,050/1907. STEWART—Photographic Developers of Aminophenol Derivatives. Canadian Patent 237,842/1920. “THeRmiIT ”"—Glycollic Acid as a Preservative of Amidol. Brit. J. Phot., 1921, 68, 125. VALENTA—Das Sulfinol als Entwickler fur Bromsilbergelatine Trockenplatten. Phot. Korr., 1915, 52, 206. VALENTA—4: Oxyphenylmethylglycin als Entwicklersubstanz. Phot. Korr., 1915, 52, QO. VALENTA—Ueber die Verwendbarkeit von Diamidophenolnatrium zur Entwick- lung von Bromsilbergelatine Trockenplatten. Eder’s Jahrb., 1905, 169, 122. MISCELLANEOUS CLARK—Chemical Tests for Developing Substances. Brit. J. Phot., 1918, 65, 499. CRABTREE—Photographic Methods of Testing Developers. American Annual of Photography, 1922, p. 184. DuUNDON AND CRABTREE—Fogging Properties of Developers. Brit. J. Phot., 1924, 71, 701, 719. ErMEN—Tests for Developing Agents. Brit. J. Phot., 1917, 64, 390. Kamu—The Solubility of Developing Agents. Phot. Korr., 1921 (Festnummer). LUMIERE AND SEYEWETZ—Influence de la Nature des Revelateurs sur la Gros- seur de Grain de L’Argent Reduit. Bull. Soc. franc. Phot., 1904, , 294. LUMIERE AND SEYEWETz—Sur Procede de Developpement Photographique con- duisant a L’Obtention D’Images a Grain Fin. Bull. Soc. franc. Phot., 1904, » 422. MEEs AND Piper—The Fogging Power of Developers. Phot. J., 1911, 51, 226; 1912, 52, 221. Brit. J. Phot., 1911, 58, 312, 491, 515; I912, 59, 337, 342, 428, 441, 465. Bull. Soc. franc. Phot., 1912, p. 44. 616 PHOTOGRAPHY a Chapter XIII. The Technique of Development ae : (For list of general reference works see page 337) ON THE TECHNIQUE OF DEVELOPMENT ALvEs—Time-Development: Its Excellences and Abuses. Brit. J. Phot., 1910, © 57, 378. | - Amor—Desensitizers and Chemical Fog. Brit. J. Phot., 1925, 72, 183. 4 BayLEy—Time Development. Brit. J. Phot., 1905, 52, 140, 168. | 4 ! BoTHAMLEY—Fundamental Points on Development. Brit. J. Phot., 1809, 46, 453; Arch. wiss. Phot., 1900, 2, 24; Bull. Soc. franc. Phot., 1900, 15, 520. CLEVELAND—Desensitizing with Phenosafranine. Amer. Phot., 1922, 16, 756. | z CrowTHER—Pinakryptol and Developers. Brit. J. Phot., 1922, 69, 351. Dawson—Time Development with the B. J. Pyro-Soda Formula. Brit. j.3 Phot., 1915, 62, 445. ErmMen—The Effect of Safranine on Development. Brit. J. Phot., 1921, 68, 7 445. . EIS HES os of Calculating the Time of Development at Various Tem- 4 peratures. Phot. J., 1906, 46, 182; 1910, 50, 412. 4 GLoverR—A Comparison of Develonment Methods. Brit. J. Phot., 1921, 63, i 183, 195. a HomoLtxa—New Desensitizers. Phot. Ind., 1925, p. 347. a Husit—Contribution to Our Knowledge of Desensitizing. Phot. Hund 1925, 62, 71. 4 Hupsit—Methylene Blue as a Desensitizer. Phot. Ind., 1925, p. 14. Krncpon—Causes of Variation in the Watkins Factor for Different Developers. Phot. J., 1918, 58, 270. a von Kienck—Thermo-Entwicklung. Phot. Mitt., 1902, 39, 232. ; Krart—Time Development. Amer, Phot., 1922, 16, 1; Brit. J. Phot., 1922, 69, 123. a Locxett—The Personal Element in Factorial Development. Brit. J. Phot., 1906, 53, 464, 502. — LUMIERE AND SEYEWETZ—Les Succedanes des ‘Atraila dans les Develonps ea ; Alcalins. Bull. Soc. franc. Phot., 1895, , 32. 4 LUMIERE AND SEYEwETz—Sur L’Emploi des Aldehydes et des acetones a presence du Sulfite de Soudre dans le Developpement de L’Image Latente_ . Photographique. Bull. Soc. franc. Phot., 18906, , 558. om LUMIERE AND SEYEWETz—Sur L’Utilisation Pratique de L’Acetone comme Suc- cedane des Alcalis dans les Developpateurs Alcalins. Bull. Soc. franc, Phot., 1897, , 550. LUMIERE AND SEYEWETZ—Sur L’Alteration a L’air du Sulfite de Sadr An- hydre. Bull. Soc. franc. Phot., 1904, , 226. LUMIERE AND SEYEwETz—Action of Alkalis in Organic Developers. Bull. Soci franc. Phot., 1906, 22, 32; Phot. J., 1906, 46, 160. - LUMIERE AND SEYEWETZ—Desensitizers for Plates and Papers. Brit. J. Pho 1922, 69, 351, 370. Luppo-CRAMER—Desensitization by Isocyanines and Carbocyanines in Present e of Soluble Bromides. S. I. P., 1923, 3, 58. ae ——— Oe a ee eS REFERENCES TO TECHNICAL JOURNALS 617 Lupro-CraMER—Destruction of the Latent Image and Desensitization. Phot. Ind., 1923, p. 236. Luppo-CraMER—Prevention of Chemical Fog with Desensitizer. Phot. Ind., 1924, Pp. 433. Luppo-CraMer—Desensitizing Colorants and their Leucobases. Phot. Ind., 1925, p. 56. Luppo-CRAMER—For all papers on Desensitization see Negativ Entwicklung Bei Hellem Lichte, 1922. Luppo-CraMER—On the Watkins System of Factorial Development. Phot. Rund., 1921, p. 81. Lupro-CrAMER—Development Paradoxes. Phot. Ind., 1924, p. 6. MerEs AND WRATTEN—Variations in the Watkins Factor. Brit. J. Phot., 1907, 54, 560. MeEES AND WratreN—Development by Time. Brit. J. Phot., 1910, 57, 376; Phot. J., 1910, 50, 403. ' Newton—A Pyro Developer for Great Contrast. Brit. J. Phot., 1916, 62, 62. PaTHE CINEMA (Research Laboratory)—New Desensitizers. Rev. franc. Phot., 1924, 5, 286.. PaTHE CINEMA (Research Laboratory)—Oxidation Fog and Desensitizers. Rey. franc. Phot., 1925, 6, 33. Rem—A Comparison of Desensitizing Agents. Brit. J. Phot., 1925, 72, 10. Rosst—Decoloration of Emulsions Desensitized in Safranine. Rev. Fot. Ital., 1923, 8, 100. Scottr—Preservation of Solutions of Sodium Sulphite. J. London Camera Club, 1923, I, 3. SHEPPARD AND ANDERSON—Equivalence of Sodium and Potassium Carbonates in Developers. Brit. J. Phot., 1925, 72, 232. SowErsy—Allowance for Subject in Time Development. Amat. Phot., 1915, » 439. STEIGMANN—Experiments on Desensitizers. Phot. Ind., 1923, p. 458. Watxins—Method and Instrument for Timing Development. Brit. J. Phot., 1894, 41, 120, 125; Phot. News, 1804, 38, IIS. Wartxkins—Control over Results in Development. Phot. J., 1895, 19, 161. WaTKINs—Some Developers Compared. Phot. J., 1900, 24, 221. WatTKiIns—Some Aspects of Photographic Development. Brit. J. Phot., 1902, 49, 1025. Watxkins—Developing Speed of Plates. Brit. J. Phot., 1908, 55, 382, 4or. WaTKINS—Time Development Calculator. Brit. J. Phot., 1908, 55, 646. Wartxins—Some Recent Aids to Time Development. Phot. J., 1909, 49, 367; Brit. J. Phot., 1900, 56, 913. WaTKINS—Time Development. Amat. Phot., 1910, 51, 481, 500; Brit. J. Phot., 1910, 57, 387. Watxkins—tTesting the Developing Speed of Plates. Brit. J. Phot. 1921, 68, 383. Watt—The Alkalis in Development. Amer. Phot., 1922, 481; Brit. J. Phot., 1922, 69, 634. Wati—Development in a Bright Light. Amer. Phot., 1921, 15, 651. 618 PHOTOGRAPHY a Wa.tt—Sulphites, Metabisulphites and Acid Sulphite. Amer. Phot., 1922, 16, 137, i Chapter XIV. The Laws of Fixation and Washing “= (For list of general reference works see page 358) a ELtiott, SHEPPARD AND SwEET—The Chemistry of the Acid Fixing Bath. ye Frank. Inst., 1923, 196, 45. ExLspEN—The coe and Practice of Washing. Phot. J., 1917, 57, 90; Brit, 14 Phot., 1917, 64, 120. | GAEDICKE—Rapid Washing of Plates. Phot. Woch., 1906, p. 41. HicKMAN AND SPENCER—Washing of Photographic Products. Phot. J., 62, 225; 1923, 63, 208. Brit. J. Phot., 1922, 69, 387, 400. * HIcKMAN AND SPENCER—The Washing of Photographic Products, Parts Iv, V,°ViL= Phot; Js ro24) GAnss0. ; LUMIERE AND SEYEWETZ—Action des Alums et des Sels d’Alumine sur la Gelawd tine. Bull. Soc. franc. Phot., 1906, —, 267. q LUMIERE AND SEYEWETZ—Sur L Tnsalublieaeee de la Couche Gelatinee des Plaques ou des Papiers Photographiques dans le Bain de Fixage. Bull. ~ Soc. franc. Phot., 1906, —, 306. a LUMIERE AND SEYEWETZ—Sur L’Insolubilisation de la Gelatine par Formalde- § hyde. Bull. Soc. franc. Phot., 1906, —, 364. LUMIERE AND SEYEWETZ—Sur la inate D’Emploi des Bains de Pica. Bull, Soc. franc. Phot., 1907, —, 10. 4 LUMIERE AND SEYEWETZ—Sur L’Emploi de L’Hyposulfite, D’Ammoniaque a D’un Melange D’Hyposulfite de Soude et D’un Sel Ammoniacal pour le Fixage des Plaques et des Papiers. Bull. Soc. franc. Phot., 1908, —, 217. + LUMIERE AND SEYEweTz—Sur L’Elimination par Lavage a L’eau de L’Hypoa sulfite de Soude Retenu par les Papiers et les dbigci Photographiques. 4 Bull. Soc. franc. Phot., 1902, —, 251. 7 LUMIERE AND Sivewerthe Time ak Fixing of Developing Papers. Brit. J. Phot., 1924, 7x, 108. Bull. Soc. francs Phot; 16247 pan a LUMIERE AND SEYEWETZ—The Fixing of Photographic Negatives. Rev. franc. Phot., 1924, 5, 61. a LUMIERE AND SEYEWETZ—The Rapid Washing of Photographic Negatives. Rev. franc. Phot., 1922, 3, 109. 4 LUMIERE AND SEYEWETZ—Fixing in Sodium Thiosulphate with the eee of Ammonium Chloride. Rev. franc. Phot., 1924, 5, 204. ae Piper—The Rate of Fixing. Brit. J. Phot., 1913, 60, 50. ; ae PiperR—Rapid Fixing Baths. Brit. J. Phot., 1914, 61, 193, 437, 458, 511. 4 PipeErR—Further Experiments on Fixing. Brit. J. Phot., 1915, 62, 364. SHEPPARD AND Mres—Theory of Fixation. Phot. J., 1906, 46, 235. Warwick—Scientific Washing of Negatives and Prints. Amer. Phot., 17, 4 II, 317. PAPERS ON THE LAWS OF FIXING AND WASHING i Warwick—The Laws of Fixation. Amer. Phot., 1917, 11, 585. a Warwick—The Fixation of Prints. Amer. Phot., 1917, 11, 639. a " oy vie - ~, oe —, ee ee ae REFERENCES TO TECHNICAL JOURNALS 619 Chapter XVI. Intensification and Reduction (For list of general reference works see page 389) REDUCTION ANDRESEN—Hydrogen Peroxide as a Reducer. Phot. Korr., 1890, 36, 256. BacHrRAcH—The Mercury-Cyanide Reducer. Brit. J. Phot., 1916, 63, 163. BayLtEy—Persulphate and Sulphocyanide Reducer. Phot. News, 1900, 44, 174. BENNETT—Ammonium Persulphate Reduction. Phot. J., 1907, 47, 328. BoTHAMLEY—Some Minor Processes in Photography. Phot. J., 1918, 58, 48. DrEBENHAM—The Hypochlorite Reducer. Brit. J. Phot., 1916, 63, 487, 538. Drecx—The Permanganate-Persulphate Reducer. Brit. J. Phot., 1916, 63, 301. Dopcson—Notes on the Action of Ammonium Persulphate as a Reducer. Phot. daetoit, St. 205, 302: Brit. J. Phot., 1911, 58, 503, 742. Hetain—The Theory of Persulphate Reduction. Bull. Soc. franc. Phot., 1899, 15, 304. Hicson—Reaction between the Persulphates and Silver. J. Chem. Soc. (Lon- don), 1921, 119, 2048. Hicson—History of Persulphate Reduction. Phot. J., 1921, 61, 237. (Full bibliography. ) Hicson—Potassium Persulphate as a Reducer. Phot. J., 1922, 62, 08. Huse Aanp Nerrz—Proportional Reducers. Brit. J. Phot., 1916, 63, 580. Huse anp Neirz—The Hypochlorite Reducer. Brit. J. Phot., 1917, 64, 143. LUMIERE AND SEYEWETZ—The Action of Persulphate of Ammonia on Metallic Silver. Brit. J. Phot., 1808, 45, 473. LUMIERE AND SEYEWETZ—The Theory of Persulphate Reduction. Bull. Soc. franc. Phot., 1800, 15, 226. J.UMIERE AND SEYEWETZ—Reducers. Brit. J. Phot., 1900, 47, 805. LUMIERE AND SEYEWrTz—On the Irregularities in the Action of the Persulphate Reducer. Brit. J. Phot., 1921, 68, 124. Luppo-CraMER—The Chemistry of Persulphate Reduction. Brit. J. Phot., 1901, 48, 89; Phot. Korr., 1901, 38, 17. Lupro-CRAMER—The Action of Reducers and its Dependence on the Constitution of the Image. Eder’s Jahrb., 1906, 20, 237. Lupro-CRAMER—The Composition of Negative Substances and its Influence on Reduction. Phot. Korr., 1907, 54, 940. Lupro-CRAMER—The Action of Reducers. Phot. Korr., 1907, 54, 230. Luppo-CRAMER—Absorption Complexes in the Silver Grain as the Cause of the ; Persulphate Effect. Phot. Korr., 1908, 45, 159. . Lupro-CRAMER—Reduction with Oxidizers containing Halides and with Per- sulphate. Phot. Korr., 1910, 47, 489; I911, 48, 466. Lurro-CRAMER—The Dispersoid Theory of Persulphate Reduction. Phot. Korr., IQI2, 49, 118. Lupro-CRAMER—The Theory of Persulphate Reduction. Phot. Korr., 1914, 51, 240. Namias—Ammonium Persulphate Reduction. Phot. Korr., 1890, 36, 86, 144, 216, 620 PHOTOGRAPHY Namras—The Use of Ammonium Persulphate. Eder’s Jahrb., 1901, 15, 165. Namias—A Comparative Study of Photographic Reducers. Il. Prog. Fot., 1922, 29, 161. Patmer—A Copper Bromide Reducer for Decreasing Contrast. Phot., 1915, Pp. 420. Pinnow—Reduction with Persulphate. Zeit. Wiss. Phot., 1908, 6, 130. Puppy—The Sulphocyanide-Persulphate Reducer. Phot., 1900, p. 99. ScuErrer—Researches on the Action of Reducers. Brit. J. Phot., 1908, 55, 472. ScHULLER—The Theory and Practice of Reduction. Phot. Rund., 1910, 24, 113, 161. ScHULLER—Persulphate Reduction. Eder’s Jahrb., 1913, 27, 419; Phot. Rund., — 1912, 26, 270. SHEpparp—The Effect of the Iron Content of Ammonium Persulphate on its Photographic Reducing Power. Brit. J. Phot., 1918, 65, 314. Syepparp—The Action of Soluble Chlorides and Bromides on Reduction with Ammonium Persulphate. Phot. J., 1922, 62, 321. SuHEepparpD—Persulphate Reduction. Phot. J., 1921, 61, 450. SmitH—The Cobaltine Reducer. Brit. J. Phot., 1914, 61, 59. STENGER AND HELLER—Reduction with Persulphate. Zeit. Wiss. Phot., 1911, 9, 73- STENGER AND He_tER—The Persulphate Reducer. Zeit. f. Reproductionstechnik., — IQIO, 12, 162, 178; I9II, 13, 5, 20, 34, 50, 70, 84, 100. STENGER AND HeLter—Reduction with Persulphate. Part Il. Zeit. Wiss. Phot., : IQII, 9, 389. STENGER AND Hetter—Reduction with Persulphate. Part III. Zeit. Wiss. Phot. — 1913, 12, 300. STENGER AND HELLER—Reduction with Persulphate. Part IV. Zeit. Wiss. Phot. 1915, 14, 177. SrieGMANN—Persulphate Effect with a Bleacher of Mercury and Copper. Phot. a Rund., 1921, 4, 52. ST1EGMANN—Mercuric Nitrate and Sulphate as Proportional Reducers. Phot. F Ind., 1921, p. 697. Wusey—Intensification and Reduction with Pyro Developers. Brit. J. Phot, @ 1910, 66, 721. _____ Softening Contrast by Re-Development. Brit. J. Phot. 1914, 61, 788. — INTENSIFICATION Baxer—The Theory and Practice of Intensification. Brit. J. Phot., 1906, 53, 264, 284, 309. CatLier—Powerful Intensification of Gelatine Plates. Brit. J. Phot., 1911, 58, 7 452. Cuartes—A Bichromate-Mercury Intensifier. Brit. J. Phot. 1919, 66, 172. CLErRc—Desalme-Intensification with Copper and Tin. Brit. J. Phot., 1912, 59, 3 215, 266; Bull. Soc. franc. Phot., pp. 96, 99. CrowTHER—Chromium Intensification with Chlorochromate. Brit. J. Phot. — 1919, 66, 709. CuNNINGHAM—Intensification. Brit. J. Phot., 1915, 62, 818. ee ee To eS a ee Te ee Cg Ee ge OR ee REFERENCES TO TECHNICAL JOURNALS 621 Eper—Modern Intensifiers for Gelatino-Bromide Plates and their Effects. Brit. J. Phot., 1900, 47, 68; Phot. Korr., 1900, 37, 23. Eper—Fffect of Intensification. Brit. J. Phot., 1900, 47, 460. Ives—Intensification by Dye Toning. Brit. J. Phot., 1921, 68, 187. JaNKo—A Comparative Table of the Effects of Various Intensifiers. Brit. J. Phot., 1900, 47, 518. Jones—Intensification with Mercuric Chloride and Ferrous Oxalate. Phot. J., IQIO, 50, 238. Jones—On the Proposed Substitutes for the Ferrous Oxalate Developer. Phot. J., 1910, 50, 242. LUMIERE AND SEYEWETZ—Sur L’emploi de L’iodure Merique comme Renforca- teur. Bull. Soc. franc. Phot., 1890, p. 472. LUMIERE AND SEYEWETZ—Sur L’emploi des Quinones et de leurs derives Sul- foniques pour Renforcer les Images Argentiques et pour les Virer en Dif- ferentes Couleurs. Bull. Soc. franc. Phot., 1910, p. 360. Brit. J. Phot., 1910, 57, 949; I9II, 58, 460. LUMIERE AND SEYEWETZ—Intensification with Salts of Chlorochromic Acid. Phot. Korr., 1920, 57, 282. LUMIERE AND SEYEWETZ—Toning and Intensification with Toluquinone. Rev. fr. Phot., 1922, 3, 203. Namias—The Mercuric Iodide Intensifier. Il. Prog. Fot., 1921, p. 103. NamiAs—Extreme Intensification. Brit. J. Phot., 1922, 69, 149. Neitz anD Huse—The Sensitometry of Photographic Intensification. Phot. J., 1918, 58, 81. Pirper—Chromium Intensifiers. Brit. J. Phot., 1907, 54, 3 Piper—Intensification by Increase of the Bulk of the Image Compound. Brit. J. Phot., 1908, 55, 195. Pirper—Physical Intensification with Mercury. Brit. J. Phot., 1916, 63, 1, 67. SmirH—Silver Intensification. Brit. J. Phot., 1909, 56, 82. Witsey—Intensifying by Redevelopment with Pyro. Brit. J. Phot., 1919, 66, 721. WELLINGTon—Intensification with Silver. Brit. J. Phot., 1911, 58, 551. Intensification. Brit. J. Phot., 1915, 62, 570. Intensification by Re-Development. Brit. J. Phot., 1915, 62, 426. Chapter XVII. Printing Processes with Silver Salts (For list of general reference works see page 414) THE SENSITOMETRY OF SILVER DEVELOPMENT PAPERS BLAcKsTRoM—Sensitometry of Photographic Papers. Nord. Tids. Fot., 1922, 6, 121. ForRMSTECHER—Absolute Gradation as a Characteristic Ganstant of Photo- graphic Papers. Zeit. wiss. Phot., 1922, 21, 21. GLover—Experiments with Bromide and Gaslight Papers. Brit. J. Phot., 1920 67, 139, 151, 169. GLover—Contrast Rating of Gaslight and Fromide Papers... Phot, ‘J.,.1022, 62, 132; Brit. J. Phot., 1922, 69, 156. 41 622 PHOTOGRAPHY Goopwin—Capacity of Printing Processes for Rendering Gradation. Brit. = Phot., 1909, 52, 187, 207, 227. HENDERSON—Speed and Gradation of Papers. Brit. J. Phot., 1916, 63, 311. HurTER AND DriFFIELD—Relation between Photographic Negatives and their Positives. J. Soc. Chem. Ind., 1891, 10, 100; Eder’s Jahrb., 1893, 7, 18; H. and D. Memorial Volume. Jones, NutTINc anpD MrEes—Sensitometry of Photographic Papers. Phot. J., 1914, 54, 301; Brit. J. Phot., 1915, 62, 9, 22, 38. JonEs AND Firt1us—The Gloss Characteristics of Photographic Papers. Brit. J. Phot., 1922, 69, 216, 220. OpENcRANTS—The Investigation of Development Papers. Nord.. Tids. Fot., 1922, 6, 70. RENwick—The Sensitometry of Photographic Papers. Phot. J., 1915, 55, 20. THE HANDLING OF DEVELOPMENT PAPERS BarnEes—Glazing Prints. Brit. J. Phot., 1923, 70, 798. BrowNn—Practical Notes on Printing Processes. British Journal Almanac, IQ16, p. 342. Davis—Controlling Tone Values by Compensating Positives. Phot. Era, 1921, p. 231. GLoverR—The Case for the Factorial Development of Bromide Paper. Brit. J. Phot., 1921, 68, 503, 519. GLOvER—The Development of Gaslight Papers. Amer. Phot., 1923, 17, 20. GLoveErR—The Development of Bromide Paper Prints. New Photographer, 1923, p. 64. Kruc—Just Plain Prints. Amer. Phot., 1922, 16, 60. Jones AND FawxKes—Sensitometric Study of the Reduction of D-O-P Paper Prints. Brit. J. Phot., 1921, 68, 275. JoNES AND CRABTREE—A New Densitometer for Determining the Time of es . posure in Positive Printing. J. Soc. Mot. Pict. Eng., 1923, p. 89. Jorpan—Still Another M-Q Developer for Gaslight Papers. Amer. Phot., 1923, 17, 139. LAMBERT—A Consideration of the Technical and Artistic Qualities of Printing Processes. Phot. J., 1924, 64, 266. Mayer—The “ Drem” Exposure Meter (for positive printing). Phot. Rund., 1924, 61, 12. Chapter XVIII. Projection Printing (For list of general reference works see page 444) Bani—Positives Direct on Bromide Paper. Phot. Journ. of Amer., 1922, 60, 440. BRAWTREE—Positives by Reversal on Dry Plates. Brit. J. Phot., 1914, 61, 320. Canpy—The Best Lighting System for the Amateur’s Enlarger. Amer. Phot., 1919, 13, 200. aS at oe ee eet OO ee. ee ee ~~ & ea ee . | | REFERENCES TO TECHNICAL JOURNALS 623 Canpy—Selection, Application and Manipulation of Condensing Lenses for Pro- jection Printing. Amer. Phot., 1923, 17, 588. CuHaArLEsS—Enlarging without Condensers. Brit. J. Phot., 1921, 68, 600. Cottins—Exposure, Scale, Aperture and Distance in Lantern Reproduction. _ Brit. J. Phot., 1923, 70, 31. Cousms—Illuminating Factors in Pareie: Brit. J. Phot., 1917, 64, 16. DrirrigLp—The Principles Involved in Enlarging. Brit. J. Phot., 1804, ay 714, 721; H. and D. Memorial Volume. oF ee reneiain-— Making a Parabolic Illuminator for Enlarging. Phot. Era, 1915, p. 66. Frary, MitcHELL AND BAKER—Positives Direct by Reversal. J. Soc. Chem. Ind., 1912, p. 901; Brit. J. Phot., 1912, 59, 788. GaILLArD—A Vertical Enlarger. Brit. J. Phot., 1915, 62, 812. GumBEert—Positives Direct with Thiocarbamide. Amer. Phot., 1915, 8, 124; Brit. J. Phot., 1915, 62, 167. HENDERSON—Finding Exposures in Bromide Enlarging. Brit. J. Phot., 1915, 62, 448. Jacoss—Exposure in Artificial Light Enlarging. Amer. Phot., 1921, 15, 490. Kinc—Calculation of Exposures in Enlarging. Brit. J. Phot., 1906, 53, 188. Kruc—Speeding up the Enlarger. Amer. Phot., 1923, 17, 453. Locxett—The Calculation of Exposures in Danio Halereine. Hrit..J.-F not. 1905, 52, 845. Lockett—Enlarging to Scale. Brit. J. Phot., 1917, 63, 350. Locxett—A Suggested Type of Enlarging Lantern. Brit. J. Phot., 1918, 66, 393. Locxett—Enlarging to Scale with Supplementary Lenses. Brit. J. Phot., 1920, a. . 07, $71. ; Locxett—A Self-Focussing Vertical Enlarger. Brit. J. Phot., 1923, 70, 760. ‘Locxetr—A Focussing Scale for Enlarging. Brit. J. Phot., 1924, 71, 171. MarsHALL—A Vertical Enlarger for Artificial Light. Brit. J. Phot., 1917, 64, 160. : Moyne—Enlarging Easel. British Patent 124,639/1918; Brit. J. Phot., 1919, 66, 325. 2 ‘Pica ” —Correction.,of Distortion when Enlarging. MHarrington’s Photographic Journal, 1916, p. 323. Piper—Correction of Distortion Produced by Tilting Camera. Brit. J. Phot., 1908, 55, 604. | SELLors—Method of Calculating Exposures in Enlarging. Brit. J. Phot., 1923, 79, 349. “ Tuermit ”—An Easel for Rapid Enlarging. Brit. J. Phot., 1923, 70, 36. THompson—A Portable Enlarger for Gaslight Papers. ee Phot., 1913, 7, 142; 1914, 8, 150. THomson—A Vertical Enlarger. Brit. J. Phot., 1921, 68, 746. Youne—Portable Enlarging Apparatus. Camera Craft, 1921, p. 161. 624 PHOTOGRAPHY Chapter XIX. The Lantern Slide (For list of general reference works see page si Acra—Quinone Bleach for Dye Toning. British Patent 180,202; Brit. ae Phot 1922, 69, 426. v 77: BenNnetr—Lantern Slides Direct by Reversal. Amat. Phot., rort, p. 55. BrowNn—Lantern Slide Making. British Journal Almanac, 1912, p. 405. CuHarLEs—Slide Making Attachment for the Enlarger. Brit. J. Phot., — 69, qi 232. . GLover—Factorial Development for Lantern Slides. Brit. J. Phot, —_ 67 Grovea‘Thictarhamide and Blue Toned Lantern Slides. Brit. J. Phot, 192 7°, 135. a << GreENALL—Control in Lantern Slide Making. Phot., 1916, p. 118. Ives—Dye Toning. Brit. J. Phot. (color supplement), roro, 66, 1. 186; 1921, 68 foolar odcoienene mi JoHNson—Personal Practice in Lantern Slide Making. Phot. J., r918, oe JouNson—The Technics of Lantern Slide Making. Brit. J. Phot., 19RD Bs 237; Phot. J» 1923, 63, 58. Phot. J., 1911, 51, 159—first paper; Phot. J., 1917, 57, 138—correction « ol first paper. eh Kettey—Copper Bichromate Bleach for Dye Toning. British. Patea, 137/1921; Brit. J. Phot., 1922, 69, 330. ee Power—Lantern Slides by Reversal. Brit. J. Phot., rort, 58, 104, Powrer—Dye Toning of Lantern Slides. Brit. J. Phot., 1912, 59, 503. . Powrer—Dye Toning. Brit. J. Phot. 1912, 59, 41. Rosach—New Method of Dye Toning. Brit. J. Phot., 1923, 70, 363. i Trause—Copper Morgass Process. British is 163387 /1o8; ae " P British Patent 163,336, 163,337; Brit. J. Phot. ste 68. “(olor supp ment), 32. 2 Wirson—Dye Toning. Brit. J. Phot., 1912, 59, 503. Be hike Lantern Slides (Decennia Praction): Brit. J. Phot., 1916, 63, su, ee eS 23) Seal ea. REFERENCES TO TECHNICAL JOURNALS 625 Chapter XX. The Toning of Developed Silver Images (For list of general reference works see page 478) Sepia Toning by the Hypo-Alum Process DrINKWATER—Sulphur Toning. Brit. J. Phot., 1923, 70, 204. E. K. Company—Hypo-alum-gold Toning Bath. Phot. Era, 1911, p. 258. SEDERQUIST—Gold in the Hypo-Alum Toning Bath. Brit. J. Phot. 1920, 67, 437. THEermMit—aAccelerated Hypo-alum Toning. Brit, J. Phot., 1922, 69, 126. Ripening with Ammonia. Brit. J. Phot., 1922, 69, 126. Hypo-alum-gold Toning Bath. Brit, J. Phot., 1921, 68, 650. Toning with “ Liver of Sulphur” and the Polysulphides Butirocx—Polysulphide Toning. Brit. J. Phot., 1921, 68, 393. Fenske—Liver of Sulphur Toning, B. P. 18545 of 1012. LUMIERE AND SEYEWETZ—Sulphuration Directe des Image Argentique sur Papier au Moyen du Foie de Soufre. Bull. Soc. france. Phot., 1923, p. 320. UnperserG—Toning with the Polysulphides. Brit. J. Phot., 1924, 71, 50. Vero—Liver of Sulphur Toning. Brit. J. Phot. 1912, 59, 774. WoopMan—Liver of Sulphur Toning. Brit. J. Phot., 1912, 59, 565. Raw.iines—Liver of Sulphur Toning. Brit. J. Phot., 1914, 61, 218. Liver of Sulphur Toning. Brit. J. Phot., 1916, 63, 505, 606. Toning by the Indirect Sulphiding Process Atitport—An Iodine Bleacher for Sulphide Toning. Amat. Phot. (London), 1923, 55, 407. Baker—Factors in Sulphide Toning. Brit. J. Phot., 1912, 59, 609, Bu._tock—Experiments in Sulphide Toning. Brit. J. Phot., 1921, 68, 442, 447. Baxer—Non-Bromide Bleach for Sulphide Toning. Brit. J. Phot., 1916, 63, 626. - Carngecir—The Chemistry of the Sulphide Toning Process. British Journal Almanac, 1907, p. 676. Goutpinc—The Quinone Bleacher. Brit. J. Phot., 1915, 62, 725. GREENALL—A Phosphate Ferricyanide Bleacher for Sulphide Toning. Phot., IQI2, p. OI. GreenaALL—A Non-Acid Permanganate Bleacher for Sulphide Toning. Brit. J. Phot., 1916, 63, 621; Brit. J. Phot. 1917, 64, 371, 382. Greenatt—Acid Bleachers for Sulphide Toning. Brit. J. Phot., 1917, 64, 30. HrrMANSoN—Range of Tones in Indirect Sulphide Toning. Brit. J. Phot., 1916, 63, 626. Lumitre aND Sryewetz—Sulphiding with Sulphoxyphosphate. Rev. france. Phot., 1921, Supp. 4. LuMI&RE AND SEYEWETZ—Toning Red with Silver Sulphide. Rev. franc. Phot., 1923, P. 133. 626 PHOTOGRAPHY LUMIERE AND SEYEWETZ—Toning with Quinone. Brit. J. Phot., 1921, 68, 6. Namias—Barium Sulphide for Sulphiding. P. Mitt, roz1, 7, 100; Brit. J. Phot., 1911, 58, 324. Punnett—Sulphocyanide-sulphide Toning. Amer. Phot., 1907, p. 25. SmitH—Bleaching of Sulphide Toned Prints. Brit. J. Phot., 1914, 61, 402. SmitH—Reducing Sepia Toned Prints. Phot. J., 1907, 47, 281; Brit. J. Phot., 1907, 54, 595. Strauss—Contribution to Sulphide Toning. P. Ind., 1924, p. 78. THoMSoN—Sepia Tones by the Sulphide Process. Amer. Phot., 1921, 15, 610. Miscellaneous Processes of Sulphur Toning BLAKE-SMITH—Single Solution Sulphide Toner. Brit. J. Phot., 1911, 58, 140. Kropr—Single Solution Sulphide Toner. Brit. J. Phot., 1910, 57, 836; Phot. Rund., 1910, 21,:245. PuNNEtTT—Single Solution Sulphide Toner with Ammonium Sulphocyanide. Brit. J. Phot., 1910, 57, 860. SHAw—A New Method of Cold Sulphide Toning. Brit. J. Phot., 1923, 70, 267. — SHAw—The Theory of Nitro-Sulphide Toning. (In reply to Sheppard.) Brit. J. Phot., 1923, 70, 591. SHAw—An Improved Method of Single Sotutian Cold Sulphide Toning. Brit. J. Phot., 1923, 70, 750. SHEPPARD—The Theory of Toning with Nitro-Suiphide Bodies. Brit. J. Phot., 1923, 70, 547. TriePeEL—Cold Single Solution Sulphide Toner. B. P. 24,378 of 1910; Brit. J. Phot., 1911, 58, 657. VALENTA—Single Solution Sulphide Toning. Brit. J. Phot., 1912, 59, 313. Journ. Almanac, 1916. Toning with Copper, Uranium and Iron CosENzL—Iron, Copper and Uranium Toning Processes. Phot. Korr., 1922, 59 (Festnummer), p. II. . GREENALL—Intensified Copper Toning. Amat. Phot., 1919, p. 27. Lupro-CrRAMER—Clearing the Whites of Images Toned with Uranium and Iron. Camera (Luzern), 1923, 2, 177. _ . MurpHy—Copper-Tin Toning. Amat. Phot., 1922, p. 547. Namias—Copper Toning. P. Korr., 1907, p. 229; Brit. J. Phot., 1907, 54, 303. ; Namias—lIron and Vanadium Toning. Il Prog. Fot., 1922, 29, 85. SEDLACZEK—Ferricyanide Toning. (Uranium.) P. Ind., 1924, pp. 205, 234; Amer. Phot., 1924, p. 4. _ Srrauss—Toning with Copper. Phot. Rund., 1922, 59, 147. Strauss—Copper-Chromium Toning. Brit. Journ. Almanac, 1923, p. 367. THomson—Uranium as a Toner and Intensifier. Amer. Phot., 1920, 14, 648. Warp—Copper-Sulphide Toning. B. P. 8002 of 1912; Brit. Journ. Almanac, — 1914, p. 659; B. P. 6026 of 1913; Brit. Journ. Almanac, 1914, p. 423. Sulphur Toning in an Acid Solution. Phot. Era, 1915, p. 127; Brit. — oT) hme) ee ee . REFERENCES TO TECHNICAL JOURNALS 627 Toning with Cobalt, Tin and Vanadium Druce—Toning with Tin. Phot. J. of America, 1922, 60, 355; Brit. J. Phot., 1922, 69, 433. FoRMSTECHER—Toning with Stannous Compounds. P. Rund., 1921, p. 277; Brit. J. Phot., 1921, 68, 750. LAMBERT—Toning with Vanadium. Brit. Journ. Almanac, 1923, p. 666. MurpHy—tTin and Copper Toning. Amat. Phot., 1922, p. 547. NamiAs—Toning with Vanadium. Rev. franc. Phot., 1924, 5, 76. Namias—Toning with Iron and Vanadium. II Prog. Fot., 1922, 29, 85. RicHARDSON—Toning with Stannous Compounds. Amat. Phot., 1923, 55, 469. SOMERVILLE—Toning with Vanadium. Photogram, 1906, p. 265. StRAUSS—Toning with Cobalt. P. Rund., 1923, 60, 69; Brit. J. Phot., 1923, 70, 352. StTrAuss—Toning with Cobalt. P. Ind., 1924, p. 232. Watit—Toning with Vanadium. Phot. Journ. America, 1921, 59, 96. Miscellaneous Toning Processes FoRMSTECHER—Toning with Selenium. B. P. 169,378/1920; Brit. J. Phot., 1921 68, 650. FoRMSTECHER—Toning with Palladium. P. Ind., 1922, p. 774. ForstMANN—Two Color Tones with Selenium. Brit. J. Phot., 1921, 68, 410. Gaupet—Toning with Colloidal Silver. French Patent 514,016. Mimosa Axkt.—Toning with Cadmium and Mercury. B. P. 130,517; Brit. J. Phot., 1920, 67, 290. NaAmiAs—Sulpho-Selenium Toning. Brit. J. Phot., 1920, 67, 648. NamiAs—Sulpho-Selenium Toning. Il Prog. Fot., 1922, 29, 203. RAWLING—Toning with Colloidal Sulphur. Phot. J., 1922, 62, 3. SEDLACZEK—Toning with Mercury. Brit. J. Phot., 1906, 53, 624, 645. STEIGMANN—Mercury Toning by the Orywall Process. P. Ind., 1921, p. 797. STEIGMANN—Toning with Sodium Hydrosulphite. P. Ind., 1924, p. 649; Sci. et Ind. P., 1924, 4, 75. Watit—Selenium Toning. Amer. Phot., 1922, p..55. Chapter XXI. Platinotype and Iron Printing Process (For list of general reference works see page 488) ANpDERSON—The Choice of a Printing Paper with Special Reference to Platinum. Amer. Phot., 1913, 7, 336, 384. 2 Brown—Practical Notes on Printing Processes. British Journal Almanac, 1916, p. 343. Burtan—An Iron-Cobalt Printing Paper. Atelier, 1921, 28, 42; Phot. J. of Amer., 1922, 60, 318. Hawxs—The Kallitype Process. Brit. J. Phot., 1916, 63, 415. Jacospy—On the Use of Japine Platinotype Paper. Brit. J. Phot., 1906, 53, 807. 628 PHOTOGRAPHY Jacopy—A Sepia Platinum Paper. Phot. Korr., 1922, 59, 31. Lre1icHtoN—A Silver, Iron, Mercury Printing Paper. British Patent No. 11,- 610/1910; Brit. J. Phot., 1911, 58, 502. ScHwARz—Silver-Iron Sensitizer. Brit. J. Phot., 1922, 69, 219; British Patent 175,317/1920. SmitH—Modifications Produced by Variations in Strength and Temperature — of Developer. Phot. J., 1911, 51, 3. SmitH—Palladiotype. Brit. J. Phot., 1917, 64, 60, 334. SmMitH—Satista Paper. Brit. J. Phot., 1914, 61, 808. THomson—Kallitype. Amer. Phot., 1923, 17, 422. THomson—A Silver-Platinum Printing Paper. Amer. Phot., 1915, 9, 630. TuHomson—Possible Substitutes for the Platinum Print. Amer. Phot., 1917, I1, 642. | VALENTA—Ferro-Prussiate Sensitizer. Brit. J. Phot., 1917, 64, 70. : VALENTA—Kallitype. (An excellent summary of previously published papers on the subject.) Bibliography. Das Atelier, 1920, 27, 10. Watt—The Iron Salts. A summary of iron printing methods. Amer. Phot., 1922, 16, 677, 766; 19023, 17, 4. : Recovering Platinum from Waste Baths. Brit. J. Phot.; 1920, 67, 393. Chapter XXII. Printing Processes Employing Bichromated — Colloids, I. (Carbon and Carbro) (For list of general reference works see page 510) BrenneTt—Some Improvements in Sensitizing Carbon Tissue. Phot. J., 1904, 44, 7: f ‘ ' ; BraHAM—The Carbro Process. Phot. J., 1922, 62, 16; Brit. J. Phot., 1922, 69, 4. CarRANzA—A Quick Drying Sensitizer for Carbon Tissue. Brit. J. Phot. (col. supp.), 1914, 61, 3. pies Cuerrit—Multiple Carbon Printing. Phot., 1906, p. 327. ; FARMER—The Carbro Process. Amat. Phot., 1919, p. 285; Brit. J. Phot., 1910, 66, 583; Amer. Phot., 1920, 14, 92. FEeLLEos—Decorative Application of Carbon Printing. (Method of Preparing Carbon Tissue.) Brit. J. Phot., 1920, 67, 481. Garon—Revised Formule for Carbro. Brit. J. Phot., 1921, 68, 327. Hat~t—Control in Carbro Printing. Brit. J. Phot., 1922, 69, 783. Harris—Tank Development of Carbon Prints. Brit. J. Phot., 1914, 61, .214. LUMIERE AND SEYEWETZ—Sur la Composition de la Gelatine Insolubilisee par les Sels de Sesquioxyde de Chrome et Theorie de L’action de la Lumiére sur la Gelatine Additionee de Chromates. Bull. Soc. franc. Phot., 1904, » 73 LUMIERE AND SEYEWETZ—Sur la Composition de la Gelatine Impregnee de Bichromate de Potassium Insolubilisee par Lumiére et sur la Theorie de cette Insolubilisation. Bull. Soc. franc. Phot., 1905, , 440. a LUMIERE AND SEYEWETZ—Sur la Composition de la Gelatine Insolubilisee par la Lumiére en Presence de L’acid Chromique et des Principaux Bi- chromates Metalliques. Bull. Soc. franc. Phot., 1905, , 461. REFERENCES TO TECHNICAL JOURNALS 629 LUMIERE AND SEYEWETZ—Sur la Composition de la Gelatine Bichromatie In- solubilisee Spontanement dans L’Obscurite. Bull. Soc. franc. Phot., 1905, » 541. ‘MippLETON—Some Experiments and Notes on Pictures in Pigments. Brit. J. Phot., 1923, 70, 735. Namras—Reaction of Various Compounds of Chromium with Gelatine. Phot. J., 1902, 42, 195. PretrascH—The Development of Over Exposed Carbon Prints. Phot. Rund., 1912, p. 57; Brit. J. Phot., 1912, 59, 217. Watit—The Chromium Salts. Amer. Phot., 1922, 16, 613. Wati_—Substratum for Carbon Transparencies. Brit. J. Phot., 1914, 61, 459. Watit—The Carbon Process. Amer. Phot., 1924, 18, 1, 86. WarsurcG—Dyes as Sensitizers of Carbon Tissue. Phot. J., 1917, 57, 160. Chapter XXIII. Printing Processes Employing Bichromated Colloids, II. (Gum-Bichromate and Allied Processes) (For list of general'reference works see page 523) ANDERSON—The Gum Pigment Process. Amer. Phot., 1913, 7, 504, 584, 648, 700, 707; 1914, 8, 8, 12, 76. ANDERSON—Multiple Gum Printing. Amer. Phot., 1912, 6, 676. Battry—A Simplified Method of Printing in the Gum-Bichromate Process. Phot. J., 1923, 63, 308. | Davis—Gum-Bromide Printing. Amer. Phot., 1921, 15, 53. GRANDMAITRE—Multi-Layer Gum Process. Bull. Soc. franc. Phot., 1923, 10, 16. Kruc—Gum Printing. LetcHton—A Method of Working the Gum-Bichromate Process. American Annual of Photography, 1924, p. 40. Lrspy—Multiple Gum. American Annual of Piawseriniy, 1922, p. 124. MacnaMARA—Multiple Gum. Brit. J. Phot., 1919, 66, 320. Mente—Glue Printing. Camera (Luzern), 1922, I, 144. MorrpyKE—Multiple Gum Process. Camera Craft, 1921, p. 308. Owen—A Gum Printing Frame. Amer. Phot., 1923, 17, 416. RicHER—The Glue Print. Amer. Phot., 1923, 17, 38. STARNES—The Gum-Bichromate Process and a New Colloid. Phot. J., 1918, 58, 287; Brit. J. Phot., 1919, 66, 5o. ZeERBE—Method of Registration for Multiple Printing. Camera Craft, 1923, p 214; American Annual of Photography, 1923. ZERBE—The Gum-Platinum Process. Amer. Phot., 1910, April. Chapter XXV. Copying BraAMWELL—A Copying and Enlarging Cabinet. British Patent 155,906/I1919; Brit. J. Phot., 1921, 68, 142. BraMwett—Focussing Enlarged Copies. Brit. J. Phot., 1916, 63, 267. Brown—Vertical Copying and Enlarging Apparatus. British Patent 133,- _143/1918; Brit. J. Phot., 1920, 67, 39, 259. 630 PHOTOGRAPHY Cuartes—A Method for Exact and Rapid Copying to Scale. Brit. J. Phot., — 1919, 66, 736. CuarLtEs—Determination of Exposures in Copying re Artificial Light. Brit. J. Phot., 1922, 69, 700. Cierc—Contact Reproductions by Reflected Light (Ullman’s Process). Brit. J. Phot., 1921, 68, 65, 645. FERRARS—Vertical Stand for Reproduction Work. Camera Craft, 1923, p. 384. Gear—Copying Line Subjects. Phot. J., 1916, 56, 177; Brit. J. Phot., 1916, 63, ; 381. HANsEN—Vertical Copying and Enlarging Apparatus. British Patent 135,- 484/1918; Brit. J. Phot., 1921, 68, 52. HeypecKer—Reproduction of Documents by Contact using Reflected Light. — (Revival of Playertype.) Brit. J. Phot., 1923, 70, 445. MarrraGE—Copying Half-Tone Illustrations. Brit. J. Phot., 1916, 63, 162, MULLER AND GANZ—Vertical Copying and Enlarging Apparatus. British Patent 123,531/1920; Brit. J. Phot., 1920, 67, 304. Pascautt—A._ Vertical, Self-Focussing Copying Apparatus using Artificial =a Light. British Patent 150,912/1919; Brit. J. Phot., 1920, 67, 633. PowEr—Copying to Same Size. Brit. J. Phot., 1916, 63, 439. Pratr—Backgrounds for Small Objects Photographed in the Studio. Camera Craft, 1923, p. 3. RosE—Vertical Arrangement for Copying Small Objects. Brit. J. Phot. 1919, 66, 338. St1LEs—Copying. Amer. Phot., 1922, 16, 634. Watit—The Playertype Process. Amer. Phot., 1923, 17, 686. Westcotr—Test Object for Sharp Focussing. Amat. Phot., 1921, p. 106. WINKLER—Reproductions by Reflected Light. French Patent 556, — Dy LoP Seo st ia a Miner Mine Focussing by the Parallax Method. Brit. J. Phot., 1917 64, 322. Copying. (Decennia Practica.) Brit. J. Phot., 1916, 63, 7. ie eee ee ees ee SUBJECT INDEX Aberration, chromatic, 85 comatic, 92, 93 spherical, 89, 90, QI zonal, QI Absorption densitometers, 231 Absorption, loss of light in lenses by, 79 Accumulator lighting for projection printing, 423 Acetone, 321 Acetylene, for projection printing, 422 Achromatic, 89 lenses, single, 105, 106 Acid hypo fixing baths, 348 : extra hardening baths, 350 troubles with, 350 Actinometers, 262 correction for special when using, 264 for carbon printing, 497 Adapting paper to negative, 396 Additive methods of trichromatic photography, 570 Adon, 146 large 142 Adurol, 2098 Advertising, lantern slides, 453 Agfa color plate, 584 Aldis lenses, 135 Alkali, acetone as a substitute for, in development, 321 carbonates in development, 319 caustic, in development, 321 function in development, 271 proportion of, to developing agent 320 Amidol, 299 for bromide paper, 399 preservatives of, 300 Ammonia, ripening of emulsions with, 30, 158 -Ammonium chloride, influence on ra- pidity of fixation, 341 subjects Angle of view, 68 Angstrom unit, 172 Aperture, effective, 78 inconstancy of, 79 relative, variation with subject, 82 Aplanat, 109 Apochromat, 89 Aristostigmat, 120 Artificial latent images, 201, 202 light, for copying, 553 for projection printing, 419 Astigmatism, 99 curves, IOI Atmosphere, effect on time of expo- sure, 254 Autochrome plate, 577 after treatment of, 582, 583 development of, 580 exposure of, 570 filters for, 578 reversal and _ redevelopment of, 582 Aviar, 134 Back, swing, 43 - Baynard’s work in photography, 18 Bichromated colloid printing processes, history of, 32 chemistry of, 492 Binding lantern slides, 453 Bis-telar, I41 Black and white subjects, copying, 561 Bleaching, of bromide prints for bromoil, 536 and tanning baths for bromoil, separate, 537 chemical theory of brcmoil, 538 in sulphide toning, 467 Blue printing, 487 Bromide, advantage of excess in emul- sions, 155 density depression with, 285 631 632 effect of soluble, on characteristic curve, 285 effect on the development of oe 287 effect on velocity constant and gamma infinity, 286 reaction in development, 271 Bromide papers, amidol developer for, 399 fixing and washing of, 403 M-Q developer for, 390 safelight for, 400 sensitometric characteristics of, 390 Bromoil, the bromide print for, 534 bleaching, 536, 538 pigmenting, 539, 542 producing relief for, 530 transfer, 543 CoM." S., 234 Calotype process, 20 Camera, box, 35 copying, 554 enlarging, 415 hand, 37 ' miniature, 35 professional, 39 reflex, 40 trichromatic, 571 Camera obscura, history of, 1 with lens, 4 Carbon process, actinometers for, 497 continuing action of light in, 499 development of prints, 499 exposure in, 407 sensitizing tissues, 495 tissues for, 404 transfer in, double and single, 495, 500 transferring to rough papers. 502 Carbro process, 503 bromide print for, 504 development in, 507 multiple printing by the, 509 PHOTOGRAPHY sensitizing of tissues in the, 505, 506 transfer in the, 507 Catalytic theory of persulphate reduc- tion, 377 Celluloid as a glazing material, 410 Celor, 118 ; Central speed method, Watkins’, 240 Characteristic curve, method of. ob- taining, 236 significance of, 237 Chloranol, 315 Chromatic aberration, 85 correction of, 87 over-correction of, 86 under-correction of, 86. Chemical transfer, Zaepernick’s method of, 548 Chromium intensifier, 383 for prints, 408 Clouds in enlargements, 440 Collinear, 116 Colloid, bichromated, processes, 480, 402 Colloid .silver theory of latent image, 222 Collodio-chloride paper, 31 Collodion process, Archer’s, 23 inconveniences of, 23 modifications of, 25 Collodion emulsion, 26 Color-contrasts, photographing, 105 Color photography, dye bleach proc- esses, 568 light interference processes 569 screen plate processes, 575 trichromatic, 570 plates, Agfa, 584 Autochrome, 577 Duplex, 584 history of, — Condensing lenses, in projection, 425 ic i with diffusing media, 428 — Conjugate focal distances, 60 | in projection printing, — 427, 436 : SUBJECT INDEX Constant density ratios, 245 Contrast, alteration of, with develop- ment papers, 406 alteration of, in the platinotype process, 482 _ gamma as a measure of,. 248 Cooke triplet anastigmat, 132, 134 Copper intensification, 385 Copying, artificial light for, 553 black and white subjects, 561 cameras for, 554 exposure in; 83, 558, 560 focussing in, 555 illumination in, 552 monochromatic subjects, 563 objectives for, 554 to scale, 556 Curvature of field, 94 Cyanine, 182 Dagor, 114 alternate construction of, I15 Dalmac, 131 Daguerre, life and work, 12 Daguerreotype process, 15 Dallon, 143 Darkroom, arrangement of, 46 illumination of, 50 sinks, 48 size of, 45 ventilation, 46 water supply, 47 Defatting oil prints, 542 Defects in negatives, the why of, 359 Density, Beer’s law, 232 constant, ratios, 245 definition of, 232 dependence on method of meas- urement, 231 growth with exposure, 235 ratios and opacity ratios, the dif- - ference, 246 Densitometers, H. and D., 230 absorption and polarization, 231 Depth of focus, factors controlling, 73 © Plasmat and enhanced, 75 theory of, 7I 42 633 Desensitizing, agents, 322 of autochromes, 581 practice of 323 value of, 322 Developing agents, classification of, 280 reduction potential of, 287, 289 relative reducing energy of, 287 slow and rapid, 206 source and manner of deriva- tion, 290 Development, and the reproduttion of contrast, 245 and the perfect negative, 247 and the structure of gelatin, 267 as a reversible reaction, 270 effect of temperature on, 278 factorial, 272, 325 function of the alkali in, 271 induction period of, 271 invasion phase of, 266 physical, 208 precipitation phase of, 269 reduction phase of, 268 the latent image in, 260 thermo, 320 time of, for given gamma, 275 time of, for various temperatures, 284 velocity of, 272, 273 Diaphragm, Le Clerc focusing, 556 systems of notation, 77 variation in value of, with distance of subject, 82 Dichloric fog, 364 Dispersion of light, 61, 88 Dispersoid theory of persulphate re- duction, 377 Distances, conjugate focal, 69 extra focal, 71 Distortion, 95 Dogmar, II9 Drying, cabinet for, 57 D-O-P prints, 405 gum bichromate prints, 515 634 lantern slides, 452 oil, bromoil and transfers, 532 Duplex method of color photography, 584 Duplicating screen plate processes, 584 Duratol, 315 Dye sensitizing, development of, 174 known facts of, 175 theory of, 177 Dyes, local intensification with, 389 color sensitizing, 178, 179, 180, 181 | Dynar, ‘137 Edinol, 301 Effective aperture, 78 Electron theory of the latent image, 220 Emulsion, appearance under micro- scope, 163 advantages of excess halide in, 155 centrifugal separation of, 162 classes of, 149 digestion of, 157, 159 emulsification, 155 fog in, 159 gelatin in, 151, 152 grain sensitivity of, 164 grain-size distribution and prop- erties of, 168 history of gelatine, 26, 28, 29, 30 iodides in, 157 molecular states of silver halide in, 153 Eosin, 178 Ernostar, 138 Erythrosin, 178 Exhausting the fixing bath, 344 Exposing, autochrome plates, 579 carbon tissues, 497 developing papers, 396 for enlarged negatives, 442 for speed determination, 233 gum-bichromate prints, 516 lantern slides, 447 oil papers, 528 platinotype papers, 481 PHOTOGRAPHY Exposure, quantum theory of, 165 Exposure, growth of density with, 235 Exposure, in negative making, 254 actinometer, use of, in determin- ing, 262 atmosphere, effect of, on, 254 methods of determining, 261 speed of plate, effect of variations in, 261 subject, influence of, 256, 2x7, 258, 259 variation month and hour, 255 __ visual exposure meters, 264 Extra focal distances, 71 hardening baths, 350 F system, 77 Factorial development, accuracy of, 327 basis of, 272, 325 developing factors for, 326 of bromide papers, 401 of lantern slides, 451 Farmer’s reducer, 372 Filters, autochrome, 578 contrast, 189 effect on lens definition, 564 graduated, I9I orthochromatic, 190 theory of compensating and selec- tion, 188 Fixing, action of sodium thiosulphate in, 338 ammonium chloride, rapidity of, 341. influence of concentration of hypo on rapidity of, 340 influence of temperature on rate of, 340 mechanism of, 339 necessity for complete, in sulphur : toning, 460 of P-O-P, 414 of lantern slides, 452 of prints, 346, 403 physical development after, 208 velocity constant of, 339 in light intensity by effect on i , \s 3 " - ~ . Se ee ee a ee a en eS eee a ee ee SUBJECT INDEX 635 Fixing bath, acid, 348 acid fixing and hardening, 349 troubles with, 350 exhaustion of, 344 extra hardening, 350 plain, 347 Flare and flare spot, 102 Focal length, 65 and size of image, 66 Focal distances, conjugate, 69 in projection printing, 427 Focus, 65 Focusing, diaphragm for, 556 in projection printing, 437 parallax method, 556 use of swing back in, 43 Fog, chemical, 362 dichloric, 364 emulsion, 159 general light, 361 local, 360 Gamma, calculation of, 250 definition of, 248 mathematical expression for, 249 relation to the characteristic curve, 249 time of development for given, calculating, 275, 277 Gamma infinity, 251 bromide, effect of soluble on, 286 determination of, 252 emulsion, effect of, 251 Gauss theory, image formation ac- cording to, 63 Gaussian objectives, 119, 120 Gelatine, physical properties of, 151 photographic properties, 152 gelatine-X, 152 Gelatino-citro-chloride paper, 31 Glass, introduction of, as negative ma- terial, 21 Glycin, 303 time development formula, 334 Green tones, with iron, 476 with vanadium, 476 Group relations, effect on developing property, 294 Gum-bichromate process, coating papers, 514 development, 516 drying sensitized paper, 515 exposure, 516 formulas for coating, 512 materials for working the, 512 registration in multiple print- ing, 517 the negative for, 512 Gum-bromide, 519 Gum-platinum, 519 Halogen, evidences for liberation of, on exposure, 215 Hard printing papers, 304 Heliar, 136 Homocol, 181 Hydrochinon, 304 contrast formula, 305 reactions in development, 269 Hypo, action on silver halides, 338 eliminators, 357 tests for presence of, 356 toning with acid, 464 Hypo-alum, toning with, 461 accelerated methods of ton- ing, 462 controlled methods of toning, 462 Illuminants for projection printing, 419 Illumination of the darkroom, 50 Illumination, unequal, 96 Image formation according to the Gauss theory, 63 Image, intensity of optical, 75 transference of, 209 Inconstancy of aperture, 79 Indirect sulphide toning, 467 and redevelopment, 470 Indoxyl development, 210 Induction period in development, 271 Inertia, as a measure of speed, 238 variation of, 239 Inspection, development by, 324 Instantaneous toning of P-O-P, 412 Intensification, how secured, 379 636 local with dyes, 388, 389 of lantern slides, 456 of prints, 407 sensitometry of, 386 Intensifying processes, 387 ) chromium, 383 copper, 385 lead, 385 mercuric-iodide, 381 mercury, 379, 380 Monckhoven’s method, 381 silver, 382 sulphide, 386 uranium, 384 Intermittency in exposure, effect of, 229 Invasion phase of development, 266 Iron printing processes, 486 toning processes, 475 Isostigmar, 125 classification, Kallitype process, 486 Landscape ‘photography, orthochro- matic methods in, 191 Lantern slide, making advertising, 453 binding, 453 by reduction, 447 developers and development, 449, 451 exposing, 447 fixing, washing and drying, 452 masking, 452 negative for, 445 physical development of, 455 plates for, 445 printing frame for contact printing of, 446 reduction of, 456 spotting, 452 thiocarbamide, development with, for warm tones, 455 toning of, 456 warm tones by development on, 454 PHOTOGRAPHY Latent image, artificial, 201 formation of, at low tem- peratures, 220 function of, in development, 269 halogenizing agents, action on, 211 indoxyl development of, 210 oxidizing agents, action on, 2iI photoregression, 206 photosalts, 209 . reversal of the, 204, 205 silver solvents, action on, 207 transference of, 200 Latent image theories, 211 colloid-silver theory, 222 electron theory, 220 metallic silver theory, 217 molecular strain theory, 218 orientation hypothesis, 223 ‘oxy-halide theory, 212 sub-halide theory, 214 Latitude of sensitive materials, 243 Lead, intensification with, 385 Lenses for copying, 554 Lenses for projection printing, 429 Lenses, image formation by, 62 Lenses, loss of light in, by seesicten: and reflection, 79 Lenses, speed of, 77 Light, and color, 172 continuing action of, 499 dispersion of, 61 refraction of, 50 Light filter, autochrome, 578 compensating, 190 contrast, 189 graduated, 191 theory of, 188 Light-interference processes of color — photography, 560 ey Local reduction and intensification, 388 hast bench for, 389 Luminosity, visual and photochemical, 173 ioe Magnar, 142 SUBJECT INDEX Maximum black, of printing papers, 392 Mercuric-iodide intensifier, 381 Mercury intensification, 379 Mercury-sulphide toning, 471 Metallic silver theory of the latent image, 217 Metol, 306 Metoquinone, 308 Molecular strain theory of the latent image, 218 Monckhoven’s intensifier, 381 Monochrome subjects, copying, 563 Monomet, 308 Negative, H. & D. definition of per- fect, 241 for gum-bichromate, 512 for lantern slides, 445 for projection printing, 433 ' Neol, 308 Neostigmar, 126 Objective, achromatic, single, 105, 106 Objective, aplanatic, 109 Objective, cemented symmetrical an- astigmatic, 114 Collinear, 116 Dagor, 114 Holostigmat, 115— Orthostigmat, 116 Protar, 116 Turner-Reich, 117 Objective, Gaussian anastigmatic, 118 Aristostigmat, 120 Homocentric, 121 Omnar, 121 Opic, 121 Planar, 119 Objective, meniscus, 104 Objective, Petzval portrait, 110 : modifications of, 112 Objective, semi-achromatic, 107 Objective, symmetrical air space. an- astigmatic, 118 Celor, 118 Dogmar, I19 Syntor, 118 637 Objective, telephoto, 138 adon, 145 anastigmatic, 142, 143, 144, 145 compound, 139 early fixed magnification, 141, 142 Objective, triple, 108, 109 Objective, triple anastigmatic, 132 Aldis, 135 Aviar, 134 Cooke, 132 Dynar, 137 Ernostar, 138 Heliar, 136 Pentac, 137 Objective, unsymmetrical anastig- matic, 127 Dalmac, 131 Protar, 127 Radiar, 131 Serrac, 130 Tessar, 120 Unar, 128 X-press, 131 Oil process, 524 brushes for, 525 drying, 532 exposing, 528, 531 papers for, 525 pigmenting in, 520 pigments for, 526 sensitizing, 527 Organic developing agents, classifica- tion of, 289 influence of. group rela- tions on, 204 slow and rapid, 206 source of, 290 Orthochrom T, 179 Orthoscopic lens, 141 Ortol, 309 Oxy-halide theory of the latent image, 212 Ozobrome, 491 Ozotype, 490 Panchromatic sensitizing, 178 638 Paper, early negative processes, 20 Paper, printing, bromide, 32, 390 bromoil, 33, 533 : developing, 32, 390 iron, 486, 487 oil, 525 platinum, 480 P-O-P, 411 Satista, 484 silver-platinum, 484 Papers, printing, adapting to negative, 304 sensitometric 390 Parallax focusing method, 556 Paramidophenol, 310 . Pentac, 137 Permanganate, as hypo eliminator, 357 Permanganate, reduction, 374, 408 Permanganate, test for hypo, 356 Persulphate reduction, catalytic theory of, 377 dispersoid theory of, 377 practice of, 377 sensitometric action of, 371 Peroxide, artificial latent images with, 202 Perspective, 67 Petzval portrait lens, 110 : Phenosafranine, 322 Photochemical action, constants of, early records of, 5 Photo-electric effect, 220 Photophysical and _ photochemical change, 200 Photoregression, 206 Physical development, after fixation, 208 of lantern slides, 455 Pigmenting, of bromoil prints, 539, 542 of oil prints, 529 relation to character of the image, 540 Pinachrome, 180 blue, 180 violet, 181 Pinakryptol, 322 "PHOTO GRAPHY Plasmat, 122 rs. Plate, agfa, sf orthockrons aa ‘¥ panchromatic, 178 process, 561 _— wet, 23 482 +P ace Polysulphides, nee wit Powder processes, 520 Precipitation phase of 269 . ee Principal foe 65 2 Printing, bromide, 390 bromoil, 533 carbon, 493. -carbré; 503 gum-bichromate, II . iron, 4865 <-..9unee lantern slides, 4 Aq Oil, S842 oe 5 platinotype, 479 we P- O- P; 411 419 eee in, : _ daylight for, Oe fof oe ‘tana for a : SUBJECT INDEX Quantum theory of photographic ex- posure, 165 Radiar, 131 Rebleaching of sulphide toned prints, 470 Redevelopment of bromide print after carbro, 507 Redevelopment, sulphide toning with intermediate, 470 Reducers, Belitzski’s, 373 Farmer’s, 372 iodine-cyanide, 374 mercury-cyanide, 373 permanganate, 374, 408 persulphate, 377, 408 proportional, 371, 375 subtractive, 371 superproportional, 371, 376 Reduction, chemical, in development with hydrochinon, 269 Reduction, lantern slides by, 447 Reduction, local, 388 of prints, 407 Reduction potential agents, 287 Refraction of light, 59 Reflection, loss of light in lenses from, 79 Relative aperture, variation with sub- ject, 82 Relative exposures in copying and en- larging, 82, 439 Rendering power of printing papers, 392 | Resinopigmentype, 521 Restrained development, warm tones on slides by, 454 Reversal, of autochroms, 582 of latent image by chemical re- agents, 205 of developing of latent image by continued ex- posure, 204 Rough papers, transferring carbon prints to, 502 Safelight, efficiency of, 53 for developing papers, 400 639 Satista paper, 484 Schellenwert method of speed deter- mination, 226 Screen plates, 575 Secondary spectrum, 89 Sensitivity, centers, 224 nature of the substance, 167 of the silver halide grain, 164 Sensitizing blue print paper, 487 carbon tissue, 405 carbro tissue, 506 oil papers, 527 Sensitizing, dye, history of, 174 known facts of, 175 theories of, 177 Sensitizing substance in gelatine, 152 Sensitometry, central speed method, 240 characteristic curve, 236 constant density ratios, 245 correct reproduction, 242 definition of, 226 densitometers, 229 density-exposure relation, 242 developers and development in, 233 development and contrast, 247 development and reproduction of contrast, 245 gamma, 248 H. & D. system, 226 inertia, 238, 239 latitude, 243 perfect negative, 241 Schellenwert method of, 226 sensitometers, 228, 229 standard light sources, 227 Sensitometry of intensification, 386 of printing papers, 390 Serrac, 130 Silver, action of solvents of, on latent image, 207 Silver halide, action of hypo on, 338 sensitivity of grain of, 164 Silver intensifiers, 382 Silver-platinum papers, 484 Silver printing processes, history of, 30 640 Silver stains on negatives, 367 Silver, sub-halides of, 215 Silver salts, light sensitiveness of, 153 Sinks, 48 Spectrum, 172 Speed of lenses, 77 Spherical aberration, 89 Spots on negatives, opaque, 369 transparent, 368 yellow, 370 Stains on negatives, developer, 365 miscellaneous, 367 silver, 367 Starch-iodide test for hypo, 357 Stigmatic, 126 Sub-halide, theory of the latent image, 214, 216 Sub-halides, evidence for the exist- ence of, 215 Subtractive printing processes, 575 reducers, 371 Sulphite, control of developer stain with, 319 forms of, 317 stock solutions of, 318 theory of action in development, 270 Sulphur toning, direct, 461, 462, 463, 464, 465, 466 indirect, 467 indirect with redevelopment, 170 OO influence of development, 459 influence of emulsion, 460 mercury-sulphide, 471 necessity for complete fixing, 460 rebleaching of toned prints, 470 Superproportional reducers, 371 Swing back, 43 Table, correspondence of systems of plate marking, 260 light sensitiveness of silver salts, 153 of factors required in determine tion of k, 279 PHOTOGRAPHY variation in light intensity by — month and hour, 255 Telecentric, 142 Tele-Dynar, 145 Telegor, 144 Teleobjective, anastigmatic, 142, 143, 144, 145 compound, 139 early fixed-magnification, 141, 142 principle of, 138 Teleros, 145 Teletessar, 144 Telic, 143 Temperature, calculation of time of development for given, 284 sensitiveness of plate at low, 220 Temperature coefficient, calculation of, 281 ‘ factors controlling, 281 mathematical expression for, 280 of developing agents, 281 Tessar, 129 constructions based on, 130 spherical aberration of, 92 Thermo development, developers and — tables for, 332 efficiency of, 335 principles of, 329 with glycin, 334 Thiocarbamide, development with ad- dition of, 455 Time of development at various tem- peratures, calculating, 284 for given gamma, finding, 275 Tissues, carbon, 494 Toning, accelerated hypo-alum, 462 — hypo-alum, 461, 462 indirect sulphide, 467 indirect sulphide with redevelop- ment, 470 iron, 475 liver of sulphur, 464 mercury-sulphide, 471 nitro-sulphide, 466 of lantern slides, 456 of P-O-P, 411 * polysulphide, 464 SUBJECT INDEX 641 uranium, 473 vanadium, 476 Total scale of printing papers, 392 Transfer, carbon, 495, 500, 502 carbro, 507 multiple bromoil, 543, 549 papers, 495, 545 presses, 546 Zaepernick’s chemical transfer method, 548 Transparency, sensitometric definition of, 231 Trichromatic color photography, 570 negative making, 571 printing processes, 573 Triplet, anastigmatic objectives, 132, 134 ‘Dallmeyer’s, 109 Sutton’s, 109 Trox film washer, 354 U. S. system, 77 Unar, 128 Unequal illumination of image, 96 Unofocal, 123 Uranium intensification, 384 Uranium toning, 473 Vanadium, toning with, 476 Varnishing of autochroms, 583 Velocity constant, 274, 275 Velocity of development, 272 formula for, 274 method of determining, 273 Velocity of fixation, 339 View, angle of, 68 Washers, centrifugal, for prints, 405 Neblette’s, for cut film, 354 rotary for prints, 405 Trox for roll film, 354 Windoe’s, for plates, 353 Washing, efficiency of devices, 352 mechanism of, 351 of prints, 355, 405 Watkins factor, for common develop- ing agents, 326 controlling influences, 326 theory of, 272 INDEX OF AUTHORS Abegg, 208, 217 Abney, 31, 155, 157, 173, 205, 261, 338 Aldis, 135 Allen, 222 Anderson, 294, 500, 514 Aristotle, 1 Bacon, 2 Baekeland, 32 Baynard, 18 Beck, 125 Beechey, 26 Belitski, 373 Bennett, 20, 155, 386, 471 Blake-Smith, 466 Bolton & Sayce, 26 Booth, 143 Bow, 98 Brown, 97, 98 Bullock, 465, 468 Butler 572 Callier, 410, 433 Charles, 556, 550 Cheshire, 80 Clark, 167 Collins, 4390 Cros, 560, 572 Daguerre, 12 Dallmeyer, 106, 107, 109, I12, 139, 145 Da Vinci, 2 Debenham, 83 Dewar, 220 Dillaye, 581 Draper, 17, 568 Drinkwater, 462 Du Hauron, 573, 574, 575, 576 Duvivier, 533 Eder, 104, 155, 158, 160, 175, 176, 200, 208, 373 Eder & Toth, 472 Farmer, 372, 491, 492 Ferguson, 278, 472 Fox-Talbot, 18, 19, 20, 489 Gard, 428 Gleichen, 104 Glover, 535 Goddard, 106 Graf, 124 Greenall, 467, 471 Grubb, 106 Guttmann, 539 H & D, 277, 433 Harting, 104, 136 Herschel, 1, 21, 32, 487, 568 Hickman & Speveee 355 Hodgson, 165 Hoegh, 115, 118 Homolka, 210, 214 Houdaille, 278 Hunt, 32, 568 Inston, 484 Ives, 560, 571, 572, 574, 584 Jobling & Salt, 79 Johnson, 29, 30 Jones, Chapman, 221, 433 Kaempfer, 116 Kellner, 91, 92, 100 Kennett, 29 Kepler, 4 King, 438 Kollmorgen, 120 Konig, 175 Kropf, 466 Lambert, 438 Lea, Carey, 200, 214 Le Clerc, 386, 556 642 I ee ee 2 ee INDEX OF AUTHORS Lee, 121, 143 ee ibtay 3 Liesegang, 560 Lippmann, 596 Lockett, 327, 438 Lumiére & Seyewetz, 208, 204, 342, 344, 345, 365, 376, 460, 465, 538, 576 Luppo-Cramer, 167, 176, 209, 222, 377 Luther, 160, 240 Maddox, 26 _ Manly, 33 Mansfield, 29 Marion, 490 Martin, 121 Maxwell, Clerk, 571 Mayer, 534, 536, 544, 548 McDonough, 576 Mees, 169, 192 Mees and Gutekunst, 175 Mees and Sheppard, 227, 252, 275, 278, 351 Meldola, 174, 338 Mercator, 208 Monckhoven, 158 Mortimer, 483 Namias, 201, 374, 472, 476, 521, 534, 542 Neitz & Huse, 371,. 375, 386 Neuhaus, 506 Nicol, 486 Niepce, 8, 11 Norris, 25 Oswald, 217 Owen, 517 Partington, 540 Petzval, 110 Piper, 33, 79, 340, 342, 492 Poitevin, 33, 480, 401, 568 Ponton, 32, 480 Pope, 175, 182 “Yorta, 3 Pouncy, 32, 489 Prett, 547 Punnett, 466 643 Rawlings, 33, 492 Reinders, 214 Renwick, 167, 209, 210, 222 Rohr, 75, 104 Ross, I10 Rudolph, 116, 119, 122, 127, 128, 120 Russell, 202 Scheele, 7, 214 Scheffer, 377 Schuler, 377 Schulze, 5 Scott-Archer, 21 Sedlaczek, 384, 474 Shaw, 466 Sheppard, 168, 223 Sheppard & Wightman, 167, 160 Simpson, 31, 568 Smith, 569 Snodgrass, 399 Stas, 153 Steinheil, 109, 116, 123 Stenger, 377 Stokes, 285 Svedberg, 165 Swan, 33, 490 Symes, 543 Taylor, 132 Thomson, 485 Triepel, 466 Trivelli, 214 Underberg, 465 Vallot, 569 Venn, 537, 538 Vogel, 174 Voigtlander, 110, 112 Wall, 33, 477, 492, 520 Warmisham, 134 Waterhouse, 175 Watkins, 240, 278, 282, 325, 320 Wedgwood, 8 Wellington, 382 644 Willis, 32 Windoes, 353 Worel, 569 Young, 570 D.VAN NOSTRAND COMPANY are prepared to supply, either from their complete stock or at short notice, Any Technical or Scientific Book In addition to publishing a very large and varied number of SCIENTIFIC AND ENGINEERING Books, D. 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