The science and practice of photography; an elementary textbook on the scientific theory and a laboratory manual

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Preface

Many students are attracted by a course in photography who have had little acquaintance with science and less understanding of its spirit. There is no reason why the methods of modem science, as well as its attitude, cannot be taught by a course in photography as well as by a course in quantitative chemistry or in the theory of electricity. It has the enormous advantage over many other subjects of starting with a vigorous curiosity on the part of the student, and of maintaining his interest by the strong desire to master the subject for his own use. In all the author's experience there has been no opening for criticism of the amount of work his students were willing to do ; the difficulty has been to keep the laboratory closed at unreasonable hours.

The methods of science are necessarily quantitative where such methods can be applied. The quantitative study of the dry plate offers a very attractive example to induce the student to chew with some relish the tough grist of close thinking. For the student who has already developed some powers of mastication, the case is not particularly difficult, and its understanding is absolutely necessary for an intelligent use of any of the photographic processes. No collection of apparently empirical rules can take its place. It was this attitude, determining the treatment, which persuaded the author to undertake this text. Also the available books offered nothing at all suitable for a laboratory manual where even a small number of students were to be cared for. In consequence, the broad treatment as well as the

^ PREFACE

details are the product of experimental selection in this laboratory. The course has now been given for seven years, during which time both text and laboratory manual have been rewritten twice in mimeograph form, besides receiving inntunerable additions and corrections. The author hopes that this summary of his experience will be found of value to other instructors and students, as well as to many not in university work yet who are interested in the real understanding of their subject.

While the primary purpose of the book is a class-room text, the author has aimed to make it useful to many earnest amateurs by collecting and systematizing for them material now widely scattered through the literature of photography. For the aid of such workers and for the instructor many references are given to the literature which will draw attention to and aid in the study of the original work.

In the British Journal Almatuic for 1916 (at page 503) will be found a list of the journals of the world which print photographic information. For an English-speaking worker the most generally useful are: the British Journal of Photography (U. S. agent, George Murphy, 59 E. 9th Street, New York), the Journal of the Royal Photographic Society (Harrison and Sons, 45 Pall Mall, London, S. W.), the Photo Miniature (Tennant and Ward, New York), American Photography (221 Columbus Avenue, Boston, Mass.), and possibly still more popular ones, as the Photo Era and the Camera,

Any teaching in science should keep before the student that he is just beginning and that the most experienced worker carries in his head but a small proportion of the known facts and even of the accepted generalizations relating to his subject. The literature is the record of all this information, and the sooner a student can be brought to turn to it for chance reading as well as for information on specific problems, the more satisfactory will his progress

PREFACE vii

be. It is not easy to get him interested to the extent that he will read voluntarily, and it will aid materially if a few current numbers of the scientific as well as the more popular journals are kept in the laboratory, where they will be seen thus announcing their existence and be picked up during waiting moments. For the library it hardly needs saying that the more current journals in all langus^es that are kept, and the more complete the files of back numbers, the better for the real worker.

To some not directly in touch with research it may be advisable to point out some justification for burdening the practical photographer with so much work in mathematics, in physics, and in chemistry. There is no doubt that many photographers doing excellent technical work know very little of these subjects whose application is the basis of the methods they use. This last statement contains the justification, and if an experimental proof is desired, it is only necessary to point out the painfully slow progress of the methods of time and factorial development in ousting the old-time guess work with the tray. Further, if there were an intelligent demand from the user, manufacturers would soon mark their plates with the actual development speeds and the actual sensitiveness. Hardly anyone doubts that the empirical worker could have acquired his skill with less experience, and could probably improve it now, by understanding the relationships among the variables with which he works.

The course as outlined in this book requires an elementary knowledge of mathematics, of physics, and of chemistry. Good high-school algebra as well as good high-school courses in physics and in chemistry will be found su£ficient to enable the student to take the further steps required.

No extended description of the common photographic apparatus is included in the text since it can be given much

▼Ill PREFACE

better by the instructor with the apparatus beside him. The student finally becomes familiar with it by use. Any discussion of the variations, with uses and advantages, would extend the text unprofitably.

The discussion of lenses has to stop short of any real mathematical treatment for which the average undergraduate has no preparation. There are many excellent special treatises now to be had. The careful testing of lenses requires an expensive optical outfit, some advanced mathematicsy and considerable practice. In any case the main problems of the young photographer are not with his lens performance, and very few expert photographers are competent to pass upon the optical questions involved ; hence it has seemed profitable to limtit the discussion of lenses to practically that contained in a good book on college physics.

The number of photographic reproductions in the text has been limited by consideration of the fact that they are reproductions varying necessarily in character from the originals, and by the fact that negatives and positives themselves should be displayed in the laboratory and the instructor should call attention to the points in the student's own work.

My thanks are due to Mr. R. C. Williamson, who so ably assisted me for three years in giving this course, and to whose criticism and suggestion many improvements in both laboratory course and text are due.

John R. Roebuck.

Physics Laboratory

UniversUy of Wisconsin 1917

Part I General Theory Chapter Page

L HISTORICAL DEVELOPMENT

Art.

1. The Beauty and Utility of an Image . . 3

2. Niepce 4

3. Daguerre 5

4. Talbot 6

5. Wet Collodion 7

6. Collodion Emulsions 9

7. The Development of the Gelatine Dry Plate 10

8. Gelatine 11

9. Silver Haloids 12

10. Dry Plate Making 13

11. Ammonia Modification . . . .16

12. Centrifuge . 16

13. Processes in General 17

14. Summary 18

IL PROPERTIES OF THE GELATINE DRY

PLATE— EXPOSURE AND DEVEL- OPMENT

15. The Problem 19

16. Hurter and DriflFeld 20

17. Law of Absorption of Light . . .21

18. Photometer 23

20. Ferrous Oxalate 27

21. Growth of Density 28

22. Constant Density Ratios .... 29

23. Application in Practice . . . •31

24. Intensification 32

25. Characteristic Curve 33

26. Period of Correct Exposure • • • 37

ix

CHAPTER

IV. Contents

2y, Perfect Negative

28. Period of Underexposure . • . •

29. Period of Overexposure . . . .

30. Period of Reversal

31. Dependence of Slope on Development .

32. Development Factor

33. Practical Control of Amount of Develop-

ment (Contrast)

33a. Constant Time at Const. Temperature

33b. Time Adjusted to Temperature .

33c. The Watkins Factorial System .

34. Contrast

35. Thickness of Film

30. Like Negatives from Different Exposures

(Latitude)

37. Organic Dry Plate Developers .

38. The Choice of a Developer

39. Estimation of Exposure ....

40. Shutters

41. The Stop

42. Dry Plate Speeds . . . .

43. Preliminary Exposure ....

44. Character of Subject

45. Lighting

PROPERTIES OF THE GELATINE DRY PLATE— COLOR SENSITIVENESS

46. Colored Object with Ordinary Plate

47. Color Sensitiveness Curves

49. Absorption Curves

50. Absorption and Sensitizing

51. Practical Application .

52. Screening Action

53. Color Screens

54. Rendering Color Contrast .

55. Basic Facts ....

PAGE

39 40

43 43 45

47 48

49 50

58 61

59- 60.

Theories Physical Modification Silver Grain Silver Sub-bromide Solid Solution .

Conclusion .

78 80

83 84 87

91 92

93 94 97

CONTENTS xi

CHAPTER PAGE

V. NEGATIVE DEFECTS

62. Classification 99

63. Thin Negatives 99

64. Intensification 10 1

65. Overexposure 103

66. Overdevelopment 104

(fj. Fog 104

68. Reduction 107

69. Local Reduction 109

70. Conclusions 109

71. Frilling no

72. Dust .Ill

73. Air Bells 112

74. Black Spots 112

75. Finger Marks 113

76. Pressure Marks 113

77. Halation 114

78. "Oyster Shell Marking" . . . .118

80. Bubbles 118

81. Drying Troubles 119

VI. POSITIVE PROCESSES

82. Transparencies 121

83. Lantern Slides .121

84. Blue Prints 124

85. Printing Out Paper (P. O. P.) . . . 125

86. DevelopitK^ Paper 127

87. Sulphide Toning 132

88. Platinotype 133

89. Carbon Printing 135

90. Photo-engraving 139

VIL LENSES

91. Pinhole Images 142

93. Images 146

94. Size of Image 149

95. Defects in Images Formed by Lenses —

Aberrations 150

96. Spherical Aberration . . . • • 151

98. Chromatic Aberration 153

99. Achromatism 154

100. Curvature of Field 159 xn

CHAPTER

CONTENTS loi. Astigmatism

102. Distortion .

103. Flare .

104. Unequal Illumination

105. Depth of Focus .

106. Speed of Lenses .

107. Lens Types .

108. Telephoto Lens .

VIIL COLOR PHOTOGRAPHY

109. Classification of Methods no. Lipmann Process

111. Pigment Processes

112. Three Negatives .

113. The Additive Method .

114. The Subtractive Method

115. Carbon Process .

116. Pinatype Process

117. Traube

118. Photo-engraving .

119. Mosaic Screen Process

120. Autochrome

121. Mosaic Screen Making

122. Conclusion .

123. Bleach-out Process .

IX. GOOD PICTURES

124. Definition

125. Pictures

126. Picture Composition • • • .

127. Unity

128. Simplicity

129. Foreground

130. Sky-line

132. Aerial Perspective . . . .

133. Color in Subject

135. Notan

136. Balance

137. Focal Length of Lens ....

138. Distortion from Short Focal Length .

139. Angle of View

140. Focusing

141. Distortion from Inclined Plate .

142. Lens Axis

PAGE

159 160

162 163

165 167

174 178

185 186

187 190

193 193

195 196

20&

213 213

CONTENTS xiii

CHAPTER PAGE

143. Exposure and Development . . . 214

144. Printing 214

145. Mounting 216

146. Conclusion 217

WATKINS SPEED LIST 220

TABLE OF PLATE SPEED NUMBERS . . .225

Part II Preface 229

GENERAL DIRECTIONS 231

EXPERIMENTS

1. Negatives 234

2. Negatives Repeated 235

3. Solution Making . . . ' . . . 236

4. Prints on Developing Paper . . . 237

5. To Test Systematically for Correct Ex-

posure 239

6. Factorial Development . . .241

7. Contact Lantern Slides .... 242

8. Lantern Slides by Reduction . . . 245

9. Halation 246

10. Intensification 247

11. Reduction 251

13. Spectrum Photography for Plate Color-

, Sensitiveness 253

14. Dying and Testing of Plates . . . 256

15. Microphotography 257

16. Blue Print Paper 258

17. Gelatino-chloride Paper Also Called

Printing Out Paper (P. O. P.) . .260

18. Enlarging 262

19. Sulphide Toning (sepia) .... 264

20. Wet Collodion Plates 265

21. Pictures Showing Clear Distance and

Clouds 269 xiv CONTENTS

EXPEBIMENTS PAGE

22. Autochromes 269

23. Carbon Printing 271

24. Characteristic Curve 273

Finishing Up Work • 274

APPENDICES

Laboratory Apparatus 276

Special Apparatus .••••• 277

Supplies per Student 278

Chemicals per Student 279

Photometers 280

Order Slip 288

Photography Laboratory Record . . . 288

INDEX 289

PARTI

GENERAL THEORY

Chapter I

raSTORICAL DEVELOPMENT

I. The beauty and utility of an image formed by a lens or curved mirror inspires every observer with the desire to make it permanent. The direct method for accomplishing this is to have the light forming the image act upon some substance in such a way as to leave a record. The history ^ of photography begins probably with the observation of the action of light on substances, and a great variety of such observations are recorded. In particular the Swedish chemist, Scheele, studied the action of light on silver chloride, proving the decomposition of the

substance, thus

AgCl - Ag+Cl

Along with such observations one must place also studies on lenses and on the camera obscura. The idea of the latter is attributed to an Italian in the sixteenth century. A certain amount of chemical

' For more detail in the history, see W. J. Harrison, "History of Photography."

knowledge was essential to progress in photography, particularly the chemistry of the salts of silver which have always occupied the leading place among the substances affected by light. Toward the end of the eighteenth century chlorine, bromine, and iodine had all been discovered and studied so that Sir Humphry Davy and Thomas Wedgwood were able to experiment more intelligently than their predecessors. They coated plates with the salts of silver, silver nitrate, silver chloride, silver bromide, and silver iodide, and, exposing them to light, made pictures. They discovered that the action of light differed somewhat with the different substances used and with the color of the light allowed to fall upon them. But they could not make their pictures permanent, as they knew of no way to remove the unacted on silver salt, so that exposure to light in the course of time turned it all black.

2. Niepce. — Following this a Frenchman, Niepce, succeeded in making pictures by using the fact that light renders bitumen insoluble in certain oils (probably on account of the oxidation of the bitimien — compare the present day carbon process). He coated a metallic surface with a layer of the bitumen and exposed it to light in a camera sometimes for a .whole day, and then with the oil washed away the parts still soluble. Since all the insoluble part was on the surface and the remaining soluble part next the metal, this washing removed all the fine lines (detail) where the insolubility did not extend right through to the metal. Nevertheless he obtained permanent pictures and probably the first ones.

3. Daguerre. — Later Niepce and Daguerre worked together in partnership but it was not till after the former's death that Daguerre succeeded in making permanent pictures. Neglecting variations during the process of discovery, the final process consisted in making a polished silver plate, exposing it to iodine vapors, thus forming silver iodide on the surface. This plate was then exposed in the camera, and later developed by exposing to the vapor of mercury, this process of development being discovered by Daguerre quite by accident but followed up with great skill. The picture was then fixed, early in the work by boiling in strong sodium chloride solution, but later, when the solvent action of sodium hyposulphite solution on the salts of silver was pointed out to him, he changed over to it. The compound itself as well as its action on the salts of silver had only been discovered a few years before. The pictures were very fine, full of detail, and more permanent than most of our prints now. It quickly leaped into popularity and won a pension for Daguerre from the French government. Good examples may still be seen. The exposure was of the order of five minutes, but this was lessened later by using a mixture of the haloids, chlorine, bromine, and iodine, instead of iodine alone, to act on the silver plate. This process held the field for about 15 years, when it was displaced by the collodion process.

4. Talbot.— At this same time, 1840- 1850, Talbot in England worked out a process which was in some respects superior to Daguerre's though not on the whole. Paper was brushed over with silver nitrate solution and dried. It was then wet in potassium iodide solution and made still more sensitive to the action of light by soaking in a gallic acid solution — this really acting also as a developer. He fixed them at first by soaking in potassium bromide solution but later used hyposulphite solution. This gave a negative, and by waxing or oiling the paper, it was possible to lay it on another piece of paper similarly treated and by exposing to light make a print which was a positive. The process allowed of duplication and was cheaper than Daguerre's, but the latter were much finer in detail and more permanent, so that the Daguerreotypes held the popular fancy, and Talbot's process was never generally used.

During these years Herschel suggested the use of glass for support of the sensitive substance, and a second Niepce actually covered a plate with albumen to hold the silver salt, but the exposure was long, — 5-10 minutes, and the process did not attract much attention.

5. Wet Collodion.— The foundation for the collodion process * vjas laid when the Swiss chemist, Schonbein, discovered the action of nitric and sulphuric acids on ordinary cotton. Acting for a long time with very concentrated acid makes guncotton an explosive, but if for a shorter time or in more dilute solution it forms a substance called soluble pyroxyline. A solution of this in a mixture of alcohol and ether is called collodion, and is still used in surgery for covering up cuts and abrasions, as it dries to a strong film. The soluble pyroxyline mixed with some camphor makes celluloid. Scot Archer applied these discoveries of soluble pyroxyline in the collodion process, which, as it came from his hands, quickly displaced all previous processes. It held the field till about 1880 and is still used in many of the photo-engraving establishments. Either the soluble pyroxyline or its solution called collodion can now be bought from photographic supply houses, but for many years the photographer had to make his own pyroxyline by soaking cotton in a

* For an excellent detailed description of the modem process, see Foxlee, "Wet Collodion Process," British Journal, 54, 483 mixture of equal parts of nitric and sulphuric acids and washing thoroughly. The product was dissolved in a mixture of alcohol and ether, a little soluble iodide and bromide added, and a film formed on clean glass by pouring the solution onto the glass plate so as to cover it all, and letting the excess drain off while the rapidly .drying material left a coating on the glass surface. As soon as the film hardened it was dipped in a solution of silver nitrate, thus precipitating silver bromide and silver iodide in the film. It was exposed while wet and developed while yet wet by a ferrous sulphate solution, which reduced to metallic silver the silver haloid which has been acted on by light. The plate was fixed by flooding it with a solution of sodium hyposulphite, in strong solution, which dissolves out the unacted on silver haloid. Many workers preferred a solution of potassium cyanide for this purpose but it is exceedingly poisonous. The plate was finally washed in water. These reactions may be represented in chemical symbols thus,

AgNO,-f-KBr = KNO,-h AgBr and AgNO,+KI=KNO,+AgI

and in development

m

FeS04+AgBr = Ag+Fe(SO«)Br

On attempting to fix in weak hypo solution AgBr+Na«S20s=NaBr+AgNaS20i (insoluble)

and in strong hypo solution

2AgBr+3Na4S20, = aNaBr+AgjNaiCSaOs)! (soluble)

and in potassium cyanide solution

AgBr+2KCN=KBr+KAg(CN), (soluble)

This process is a difficult one on account of the great care required, but it gives beautiful negatives, very clear with abundant detail, and is still unexcelled for lantern slides. For the enthusiast it has the advantage that the whole process is entirely in his own hands and also for this reason gives the clearest laboratory insight to the photographic process. We will spend a short time on it in the laboratory. (See Exp. 20.) 6. Collodion Emulsions. — On account of the

m

difiiculty of handling the plates wet, persistent efforts were made to perfect some way to use them dry, but the sensitiveness to light was always much reduced, though a goodly number of ways were worked out as possible. The best of these modifications, which might have held the field but for the gelatine plate, was Bolton's "Washed Collodion Process." * The collodion with soluble bromide in it, not iodide, was mixed with silver nitrate dis-

'See also article on Orthochromatic Collodion Emulsions in British Journal, 55, 418 (1908).

lo PHOTOGRAPHY

solved in alcohol. The resulting liquid is called an emulsion on account of its resemblance to a wellshaken mixture of oil and water. It is filled with fine particles of silver bromide, which is not soluble in the alcohol-ether mixture, and besides, holds in solution the other products of the reaction as well as possibly some silver nitrate if it was present in excess. The emulsion was then dried in a thin layer and thoroughly washed to remove the soluble salts, dried and again dissolved in the alcohol-ether, when it was ready for coating the plates, which were used after they were dry, but were moistened again with silver nitrate solution before development. Plates made on this plan were on the market for some years. The great difiiculty with all these dried collodion plates is that the collodion dries to such a hard and compact film that it does not absorb the solutions used for development.

7. The development of the gelatine dry plate soon put all these processes out of use except for very special purposes. A great many early workers experimented with gelatine, but with many failures till about 1880, when the plates were perfected sufficiently to outclass, the collodion so completely as to displace it in three or four years, their great advantages being their keeping qualities, portability, and speed. It will be sufficient for our purpose to describe one method for making gelatine plates and

HISTORICAL DEVELOPMENT ii

not to attempt to trace the modifications during discovery.

8. Gelatine itself is a very interesting substance.* It is obtained from animal tissue, horns, skin, hoofs, etc., by digesting them in hot water, for the highgrade gelatine without boiling, but for the lower grades and for glue even heating in a closed boiler under pressure. It has to be freed as completely as possible from all fat and also from all inorganic salts. Isinglass is made from the air bladder of a fish, the sturgeon, in much the same way. Gelatine belongs to the class of bodies called colloids; it is made up principally of two substances, both organic and nitrogenous, called glutin and chondrin, the former being in excess in photographic gelatines, making them set more readily and melt at a higher temperature — ^that is "hard*' gelatine. Dried in the air it still retains 15-20% of water, and if put in water it swells up, absorbing several times its own volume of water, becoming very soft and mechanically weak but not fluid. If while still swelled the temperature be raised, depending on the gelatine used, to 30-40° C, it melts to a thick liquid, and on cooling it hardens, "sets,'' "gelatinizes," or "gels," again but slowly, not immediately on cool-

* Sec also Abney, "Photography with Emulsions/' p. 102 ; Brothers, "Manual of Photography," p. 257; Burton and Pringle, "Processes of Pure Photography," p. 51.

ing. In the dry form it keeps indefinitely but when wet or dissolved in water it quickly putrifies, as it makes an excellent medium for the growth of molds and bacteria, and it then generally loses its setting power, becoming liquid or remaining liquid. Boiling in water steadily lowers the melting point till finally the setting power is destroyed. It absorbs chlorine, bromine and iodine readily, which is important in its photographic use, as it thus allows the more complete decomposition of the silver salt by the light, the removal of the halogen from the field of action preventing it from recombining with the silver. When boiled with silver nitrate the gelatine solution turns red and the silver salt is decomposed. Treatment of the swelled gelatine with quite a large number of substances, such as alum, chromalum, formalin, etc., raises the melting point or even renders it quite insoluble in water. It is not soluble in strong alcohol, and is precipitated from water solution by the addition of alcohol ; even the swelled gelatine when immersed in alcohol loses water and shrinks down to a hard film.

9, Silver Haloids.— Of the compounds of the haloids, chlorine, bromine, and iodine, with silver, the bromide is the most important one in photographic work. Stas,*^ a famous German chemist, studied the different forms in which it occurs and

how It may be changed from one to the other. Precipitated directly from water solutions, it is finely divided and a faint yellow, and quickly gathers into larger particles and settles to the bottom. If boiled for a long time in water out of the light the particles subdivide till it will remain suspended for a long time in water, and this finely divided form is exceedingly sensitive to light. It may be made also by precipitating dilute boiling silver nitrate solution with dilute boiling ammonium bromide solution. When precipitated from a solution containing a small quantity of one of many other substances, including gelatine, the particles of silver bromide carry down with them some of the foreign material — and this property has an important bearing on the manufacture of gelatine emulsions.

10. Dry PUtc Making.— As an example of the preparation of a gelatino-bromide emulsion the following from Abney ^ will be described. Materials and their quantities are as follows :

1. Pot. Iodide o.3g in 3.5 cc water

2. Pot. Biknmde S,yg in 40 cc water

3. Nelson's No. i Photo Gelatine 2g.

4. Silvef Nitrate ".4g in 15 cc water f Heimich's Gelatine log.

^' \ Nelson's No. i Gelatine 6g.

The presence of both bromide and iodide increases the sensitiveness over that with either alone.

•Abney, "Treatise on Photography."

The use of the two kinds of gelatine is for the purpose of making the film hard enough whefi wet to stick sufficiently strongly to the glass, and yet of low enough melting point to absorb the solutions readily. Both lots of gelatine are washed with water to free them from dust and then allowed to swell in water. To the silver nitrate a drop of hydrochloric acid is added to make the emulsion acid. No. 3 Gelatine is melted by standing the drained swelled gelatine in a water bath at 50° C, and the silver nitrate added and well mixed. Then No. 2 Pot. Bromide is added slowly drop by drop, with constant agitation, of course in the dark room, till ^ is added, when No. i Pot. Iodide is mixed with the remaining Pot. Bromide and the mixture added to the gelatine as before, drop by drop. This makes the precipitated haloid very finely divided, and the liquid with suspended precipitate is called an emulsion. It will be noted that the bromide and iodide are present in more than sufficient quantity to combine with all the silver nitrate, so that there is no free silver nitrate left to react with the gelatine. If a little of this emulsion is now examined by looking through it, it will be found to be ruby red. Place the flask in boiling water in the dark for 40 minutes. If plates be made from it at intervals, it will be found that the sensitiveness will be increasing steadily and the silver bromide particles increasing in size. At some time the sensitiveness will begin to decrease and the boiling should be stopped short of this point. It will be noted that part of the gelatine was reserved so that it would not be affected by the boiling. It is now warmed to melting and mixed with the emulsion, and the whole allowed to set.

In the emulsion besides the water, gelatine and silver haloids, there will be the following substances, KBr, KI, KNOs, HCI (see reactions in Art. 5), and if these latter are removed the plate will be much more sensitive. To this end the jelly is now well broken up by squeezing through strong cloth, and washed and soaked and washed for hours. It is then well drained and is ready to coat the plates as soon as it is melted by warming. If a small quantity of alcohol is added before coating the plates it will increase the sensitiveness somewhat.

To make the layer of gelatine adhere to the glass surface through all the later operations it is necessary that the glass surface be absolutely clean. No one who has not actually attempted to thoroughly clean a surface realizes how difficult it is. All grease can be removed by a hot alkali solution, particularly if assisted by friction. Hot chromic acid or nitric acid will destroy all other organic substances and dissolve a good many inorganic. Wash thoroughly, finishing with distilled Water. Many workers finish by polishing with French chalk (finely divided calcium carbonate) on clean chamois leather or cloth, but the latter must be clean and must not be touched by the fingers on the part used for polishing. A layer of hardened gelatine or of albumen adheres to the glass very strongly, and the emulsion adheres to this layer strongly also so that most plates on the market are made this way when the cleaning does not need to be quite so thorough.