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THE ADVANCE OF PHOTOGRAPHY

THE ADVANCE OF PHOTOGRAPHY

ITS HISTORY AND MODERN APPLICATIONS

BY

A. E. GARRETT, B.Sc.

FELLOW OF THE PHYSICAL SOCIETY OF LONDON

FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY

FELLOW OF THE ROYAL GEOGRAPHICAL SOCIETY

ETC.

WITH ILLUSTRATIONS

mfeB»MM»VWM

LONDON KEGAN PAUL, TRENCH, TRUBNER & CO. L™

1911

BRI BC

PREFACE

Nearly forty years have now elapsed since the first edition of VogeFs " Chemistry of Light and Photography " was issued, and no revision has been made of that work since 1883.

During the last thirty years the science of photography has developed with very rapid strides. The perfection of the dry plate, the introduction of orthochromatic photography, and the scientific investigation of the many problems confronting the practical photographer have been landmarks in that progress.

Then, again, much interest has centred round the application of photography for recording results obtained in various branches of scientific research ; while the advent of Rbntgen-ray photography, the transmission of photographs from place to place by electrical means (photo-telegraphy), and the improvements made in the photography and projection of photographs of moving objects, have opened up new fields of interest for workers in this science.

On the other hand, VogeFs " Chemistry of Light and Photography " is still considered a classic with respect to those branches of the subject with which it deals. Thus, although the present book is of necessity practically a new work, it is based upon the lines laid down in the original publication.

The historical account given in VogeFs edition is so

VI THE ADVANCE OF PHOTOGRAPHY

complete and is of such interest, that it has with slight alterations been adopted in toto.

I should like to express my thanks to the many firms which have so readily come to my assistance in the loan of blocks, etc., for the illustrations, and especially to thank the following gentlemen :

Professor Hale for the excellent photographs obtained at Mount Wilson, Professor R. W. Wood for the photo- graphs obtained with invisible light, Mr T. Thorne Baker for the illustrations of the Korn process and Telectograph process, Mr C. 0. Bannister for microphotographs of metallurgical specimens, Messrs Swain & Co. for illustra- tions of the three-colour process and the effect of line screens, Messrs H. Cox & Co. for the radiographs, Messrs Ilford for the illustrations concerning ortho- chromatic plates, and Messrs Martin Hood & Larkin for the fine collotype which appears as frontispiece and the illustration of lithographic work.

A. E. Garrett.

London, June 1911.

CONTENTS

CHAP. PAGE

I. Historical Survey 1

II. The Chemical Action of Light ... 36

Pseudo-Photographic Effects ... 72

III. The Photographic Importance of the Chromium

Compounds ...... 78

IV. Lenses ........ 104

V. Camera Appliances . . . . . .134

VI. Dry Plates, Films and Papers . . . 159

VII. Art in Photography 199

VIII. Some Early Applications of Photography . 214

IX. Photography in Natural Colours . . . 227

X. Book Illustrations ..... 264

XI. Astronomical Photography .... 281

XII. Micro-photography and Projection Apparatus 321

XIII. Kontgen-Ray Photography

XIV. Photo-Telegraphy . XV. Animated Photography .

342 360

368

Index 377

LIST OF ILLUSTRATIONS

Collotype

Leaf photographs Camera obscura Leaf photograph

Wet plate dark slide Printing-frame . Vignetting Wave illustration Diagram to illustrate

Refraction .

,, in raindrop prism

Spectrum . Solomon's lamp Flash bag Diagram to illustrate

Path of sun's rays .

Lines of latitude

Crystal of chrome alum

Electrotyping

Gelatine relief

Action of light on bichromated gelatine

Refraction of light

,, through plate glass

,, ,, glass prism

Connection between lens and prism

Action of lens on parallel rays

Conjugate foci

Formation of image by lens

Types of lenses

Spherical aberration

Focal lines .

Circle of least confusion

Distortion of image by lenses

Frontispiece

Figs.

Page

1,2

5

3,4

7

5

18

6

19

7

27

8

30

9

32

10

37

11

41

12

41

13

42

14

43

15

49

16

50

17

52

18

54

19

79

20

83

21

86

22

87

23

104

24

104

25

105

2G

105

27

106

28

106

29

107

30

107

31

107

32

109

33

110

34

112

34a

112

35

113

THE ADVANCE OF PHOTOGRAPHY

Diagram to illustrate

Compound lens

Teleplioto lens Bapid rectilinear lens . Eoss homocentric lens . Portrait lens

Dallmeyer-Bergheim portrait lens Diffusion of focus apparatus Wide angle lens Rapid wide angle combination .

Front combination .

Back Lens with teleplioto attachment A don telephoto lens Busch Bis-Telar Photographs to show effect produced

extension lenses

by usin

g Cooke

An early camera Diagram to illustrate

Formation of image in pinhole camera

Effect of size of pinhole Sanderson stand camera

patent camera front Panros camera on stand The Kodak camera

" " " *

Houghton folding reflex camera

Naturalist camera

Bebe camera

The Ticka camera

Panoramic camera

Stereoscopic camera

The stereoscope

The stereoscope lenses .

Diagram to illustrate

Path of light through lenses American stereoscope

Enlarging easel lantern

Studio camera on stand

Thornton-Pickard shutter

Photograph showing halation effect

of same, using backed plate

Figs.

Page

36

114

37

118

38

123

39

124

40

124

41

125

42

126

43

127

44

128

45

128

46

128

47

129

47a

131

48

132

To face page

49, 50, 51

133

Page

52

134

53

135

54

136

55

140

56

141

57

143

58

144

59

145

60

146

61

146

62

147

63

147

64

148

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149

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151

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151

69

152

70

153

71

155

72

155

73

156

74

157

Between

pages

75 \

160

76 J & 161

LIST OF ILLUSTRATIONS

XI

Sensitivity of eye and dry plate Photograph (landscape) obtained with

Ordinary plate

Orthochromatic plate Photograph (coloured ^object) obtained with-

Ordinary plate

Orthochromatic Photograph with ordinary light

,, ultra violet light

infra red light

Burroughs & Wellcome^exposure meter

Tabloid outfit .

Diagram to illustrate Solids

Effect of shading . Formation of image on the screen Position of eye when viewing picture Effect of distance ....

Bust to show effect of distance

Effect of distance on appearance of hollow bodies ,, height of eye

Diagram to illustrate

Method of photographing temperature records Stein's apparatus for photographing inner ear

heliopictor Photographs of flying birds Marey's pistol Ives' kromskop Sanger-Shepherd's repeating back

,, camera with repeating back

Arc lamps for process studio . Process studio camera (front) .

(back) .

,, ,, (stand) .

Effect produced by half-tone screen Printing-frame for process-work Holt etching-machine

Black impression yellow plate First working yellow Second working red Progressive red over yellow Third working blue Finished print .

Fitrs.

77

Page 162

Between pages

78

79

80 81

166 & 167

81a

170

81b

- &

81cJ 171

82

Page

173

83

189

84

199

85

200

86

201

87

202

88

203

89

204

90

206

91

207

92,93

208

94, 95, 96

209

97

214

98

216

99

217

100, 101

225

102

241

103

242

104

243

To face page 105 264

106

265

107

265

108

266

109

268

110

273

111

273

Betwee 112 ,

i pages

113 114

278 - & 279

112a 113a

114aJ

Xll

THE ADVANCE OF PHOTOGRAPHY

Arrangement of telescope for photography Refracting telescope

Eclipse of sun .

Eclipse of sun .

Corona during eclipse .

Diagram to illustrate

Arrangement of telescope (Rutherford) Spectroheliograph ....

Direct photograph of sunspot (Mount Wilson) Photograph, calcium flocculi ,,

hydrogen

,, sunspot, using Ha line ,, ,,

Tower telescope, Mount Wilson

Spectroscope .....

with photographic attachment Diagram to illustrate

Measurement of distance of star .

Transit of Venus ....

Effect of exposure of plate to weak light . Solar microscope ....

Modern Microscope ....

Diagram to illustrate formation of image in microscope Camera attached to microscope (horizontal) ,, » » (vertical)

Micro-photograph transverse section, leaf tea-plant

Arrangement for photographing larger objects . Embryo of armadillo ....

Microsummar lens ....

Camera for micro- photography in metallurgical laboratory

Carbon steel (micro-photo)

Silver cupellation bead (micro-photo)

Small model micro- photography metallurgical laboratory The stage of the microscope Diagram to illustrate path of rays of light Early projection lantern Modern projection apparatus Diagram to illustrate Path of rays of light

Fies. 116

Page 286

117

288

To face page 118 290

119

290

120

291

121

Page 293

122

295

Between 123 ^

pages 296

124 \

&i

125 J

297

To face page 126 298

127

299

128

Page

307

129

308

130

312

131

313

132

318

133

321

To face page 134 322

135

Page 324

136

325

137

326

To face pag-e 138 328

Between 138a ^

139 /

pages 328

fe329

140

Page 329

Between

141 \

142 I

143 J

pages 330

&

331

144

Page 331

145

333

146

334

147

336

148

337

149

339

LIST OF ILLUSTRATIONS

Diagram to illustrate Path of rays of light Projection of opaque objects Parts of Ruhmkorff s coil .

Coil as used for radiographic work

Rontgen-ray tube

Diagram to illustrate

Arrangement with electrolytic break

Water cooled tube

Apparatus for obtaining instantaneous radiographs

To^illustrate photograph obtained with soft^tube To illustrate photograph obtained with correct tube » hardAtube

Diagram to illustrate effect due to selenium cell' Belin's method

Photograph obtained by Korn's process

telectrograph process

Camera for obtaining cinematograph films Combination apparatus (ordinary projection and cinemato

graph) Cinematograph reversing apparatus

Figs.

Xlll

Page

150 340

151 341

152 344

153 346

154 349

155 351

156 352

157 352

158 355

Between pages

159 \ 356

160 \ & 160a J 357

Page

161 361

162 362 To face page

163 362

164 366 Page

165 369

166 167

372 373

OTHER ILLUSTRATIONS

Map to illustrate lithographic process Plate I.

itrations of effects produc Plates II., III., IV., and V.

To facepape

100

Between pages

Illustrations of effects produced by diffusion of focus ^ 126

./ & 127

THE

ADVANCE OF PHOTOGRAPHY

ITS HISTORY AND MODERN APPLICATIONS

HISTORICAL SURVEY

Action of Sunlight. The light which radiates from the great central body of our planetary system produces manifold effects upon the animate and inanimate world, some of which are at once evident to the senses, and have been known for thousands of years, while others, again, are not so apparent to the eye, and have been discovered, examined, and utilized only through the observations of modern times.

The first effect which every person, however unculti- vated, notices when, after the darkness of night, the sun rises, is that bodies become visible. The rays from the source of light are thrown back (reflected) from various bodies, they reach our eyes, and produce an impression upon the retina, the result of which is the perception of material objects by the eye. But soon another effect is observed, not by the eye, but by the sense of feeling. The sun's rays not only illumine bodies upon which they fall, but heat them, as is felt when the hand is held in the rays. Both effects, the shining, or illuminating, and the warming effects of the sunbeams, differ very essentially from each other. The illuminating effect we perceive instantane- ously ; the heating effect is only felt after a certain time,

2 THE ADVANCE OF PHOTOGRAPHY

which may be shorter or longer, as the heating power of the sun is stronger or weaker.

In addition to these two effects of sunlight, there is a third, which generally requires a still longer period to make itself noticed, and cannot be directly perceived either by the eye or by the sense of feeling, but only by the peculiar changes which light produces in the material world. This is the chemical effect of light.

Physical v. Chemical Change. If we take a piece of wood, and bend or saw it, we change its form ; if we rub it, it becomes warm we change its temperature, but it still remains wood. These changes, which do not affect the substance or matter of a body, we term physical.

But if we set fire to a piece of wood, strong-smelling gases ascend, ashes are deposited, and a black residue remains, which is totally different from the wood. By this process a new substance charcoal has been pro- duced. Material changes of this kind we term chemical changes ; and such chemical changes are, in an especial manner, produced by heat. If, for instance, we heat a bright iron wire red hot, it undergoes apparently only a physical (not a material) change. But if Ave allow it to cool, we find the bright rod has become dull and black ; that it has acquired a brittle, black surface, which easily breaks away on bending the rod, and differs entirely from the bright, tough, flexible iron. Here again a chemical change that is to sa}^, a change of substance has taken place ; the iron has been converted into another body, into iron scale, because it has combined with a component part of the surrounding air with oxygen.

Chemical changes of this kind are not only produced by heat, but also by light.

Fading of Coloured Fabrics. It has long been known that when the colours with which fabrics are dyed are not the so-called fast colours, they fade in the light that is, become paler. In this case the colouring matter changes

HISTORICAL SURVEY 3

into a colourless or differently coloured body ; and that this is the effect of light is evident from the fact that those parts of the material in question which are covered up from the light beneath the folds, for example remain unchanged. This effect of light upon colour has been long turned to practical use in the bleaching of linen. The unbleached fabric is spread out in the sunlight, and repeatedly moistened with water ; and thus, through the combined effect of light and moisture, the dark colouring substance becomes gradually soluble, and can then be removed from the linen by boiling it in alkaline lye.

It was formerly believed that the changes we have just described were caused by the heat which is produced in bodies by the sun's rays. That this is an erroneous view is evident from the fact that fabrics dyed in colours which are not fast can be exposed for months together in the temperature of a hot oven without any bleaching effect ; and further, that wax, which the sunlight likewise bleaches, becomes darker, rather than paler, through heat.

As we remarked before, the bleaching effect of sunlight is a slow process, and this circumstance renders the phenomenon less striking. A sudden and rapid occur- rence surprises us, and stirs us up to inquire and to reflect.

Silver Chloride and Silver Nitrate. In the mines of Freiberg is occasionally found a vitreous dull-shining silver ore, which, on account of its appearance, is called horn silver (chloride of silver).

This horn silver consists of silver and chlorine in chemical combination, and this compound can be artifici- ally produced by passing chlorine gas over metallic silver, or by adding a solution of nitrate of silver to a solution of common salt (sodium chloride). This horn silver in its original position in the earth's crust is completely colour- less, but when it is exposed to the daylight it assumes, in a few minutes, a violet tint.

In another substance containing silver this phenomenon

4 THE ADVANCE OF PHOTOGRAPHY

is still more apparent. Silver, placed in nitric acid, is dissolved with effervescence, and if the solution is evapo- rated, a solid mass of crystals is obtained. This is not silver, but a combination of this body with nitric acid. This nitrate of silver is totally different from ordinary silver ; it is easily soluble in water ; it has a bitter, dis- agreeable taste ; it fuses readily and destroys organic matter ; and it is therefore used as a corrosive agent, under the name of lunar caustic.

Fingers which have grasped lunar caustic, skin which has been cauterized by it, or in fact any light coloured objects sprinkled with a solution of it, quickly assume a dark colour. This can be at once observed by moistening a small piece of paper with a silver nitrate solution, allowing it to dry, and then placing it in the light.

Indelible Ink. These properties were soon made use of to produce a so-called indelible ink, which is nothing more than a solution of one part of nitrate of silver in four parts of water, mixed with a thick solution of gum. Written characters traced with this ink upon linen cloth are pale ; but, when dried in the sunlight, quickly become dark brown, and are not injured by washing. A quill, not a steel pen, must be used, as steel decomposes the nitrate of silver.

From the discovery of the blackening of paper saturated with lunar caustic to the invention of photography there was but a step ; yet it was long before anyone thought of producing pictures by the help of light alone, and still longer before these attempts were crowned with success.

Experiments of Wedgwood and Davy. Wedgwood, the son of the celebrated manufacturer of porcelain who produced the popular Wedgwood ware, and Davy, the celebrated chemist, made the first attempts in the 3-ear 1 802. The}7 placed flat bodies, such as the leaves of plants, upon paper prepared with nitrate of silver. Light was thus kept from the parts of the paper covered by the

HISTORICAL SURVEY

objects, these parts remained white, whilst the uncovered portions of the paper were blackened by the light ; and thus was produced a white outline, or " white silhouette/' of the superimposed objects upon a black ground (see figs. 1 and 2).

Wedgwood produced in this manner copies of drawings upon glass, in white lines upon a black ground ; and this process became the basis of a mode of treatment which attained great importance, under the name of the lichtpaus process.

Unfortunately these pictures were not durable. They

Fig. i.

had to be kept in the dark, and could only be exhibited in a subdued light. If they remained long exposed to the light, the white parts also became black ; and thus the picture disappeared. No means were then known to make the pictures durable— that is to say, to make them unalterable by light, or, as we now say, to fix them. But the first step towards the discovery of photography was made ; and the idea of producing pictures of objects with- out the help of the draughtsman became, after these first attempts, so extremely attractive that, from that time, both in England and in France, a large number of private persons occupied themselves with the subject with the greatest enthusiasm.

6 THE ADVANCE OF PHOTOGRAPHY

It is clear that by the process of Wedgwood and Davy only flat bodies could be copied, and, notwithstanding all the improvements of which the process was still susceptible, it admitted of only a limited application.

The Camera Obscura. But Wedgwood had already conceived the idea of the possibility of producing pictures of any bodies whatsoever by the action of light on sen- sitized paper. He tried to effect this by the aid of an interesting optical instrument which has the property of forming flat images of solid objects. This instrument is the camera obscura. The origin of this camera is by no means certain ; it has been attributed by some to Leonardo da Vinci, and by others to Porta, and although there is no mention in their writings of any apparatus of this nature, it is probable that a form of this camera was introduced by da Vinci at the beginning of the sixteenth century, and an improved form by Porta about the middle of that century. That it was possible to project outside objects through a small hole into a darkened room was known by Euclid, and Levi ben Gerson appears to have made some apparatus at the beginning of the fourteenth century for this purpose. The first record of apparatus of this kind was Alberti's show-box somewhere about the year 1437, and the first instance in which a definite mention occurs of the use of a convex lens in connection with the camera appears to be in D. Barbaro's " Treatise on Perspective/' 1568. The first portable camera obscura was in all probability made by Kepler.

The formation of the image by means of this instru- ment will be easily understood from the following brief description.

If a small hole be made in the window shutter of a completely darkened room on a sunny day, a clear image of the landscape will be seen on the opposite wall of the room.

Let a be a poplar, o the hole, and w the back wall of the

HISTORICAL SURVEY

room, then from each point of the poplar rays of light will travel towards the hole, and beyond that in straight lines to the wall. It is now clear that to the point a' light can only arrive from the point a of the poplar, which is

Fig. 3.

situated on the extension of the line a' o. Therefore this point of the wall can only reflect light, which in its colour and position corresponds to the point a. The same remark applies to the points / and g, and the result accord- ingly is that on the wall an inverted image of the tree is visible.

An improved instrument was soon obtained by using instead of the room a small box (fig. 4) which had a

r~- I

Fie. 4.

movable semi-transparent screen in place of the solid wall. On this screen the image of an object in front of the box is clearly visible, if a minute hole is made in the front side, which is best if composed of a thin tin plate. The light

8 THE ADVANCE OF PHOTOGRAPHY

should be excluded by covering the head with a black cloth, as indicated in the diagram.

These images appear still more beautiful if, instead of a hole, a glass lens is substituted. This lens, at a certain distance, which is equal to that of its focus, casts a distinct image of distant objects, which is much better defined and clearer than that which is produced by a hole.

In this improved form the instrument was emplo}Ted by Wedgwood and Davy. Their idea was to fix on sensitized paper the image produced upon the screen. They fastened a piece of paper saturated with a silver salt upon the place of the image, and left it there for several hours unfortunately without result. The image was not bright enough to make a visible impression upon the sen- sitized paper, or the paper was not sufficiently sensitive.

Action of Light on Asphalt Nicophore Niepce. It now became necessary to find a more sensitive preparation to catch the indistinct image ; and this was achieved by a Frenchman Nicophore Niepce. He had recourse to a very peculiar substance, the sensitiveness of which to light was before unknown to anyone asphalt, or the bitumen of Judaea. This black mineral pitch, which is found near the Dead Sea, the Caspian, and many other places, is soluble in ethereal oils such as oil of turpentine, oil of lavender as well as in petroleum, ether, etc. If a solution of this substance is poured upon a metal plate, and allowed to cover the surface, a thin fluid coating adheres to it, which soon dries and leaves behind a thin brown film of asphalt. This film of asphalt does not become darker in the light, but it loses by light its property of solubility in ethereal oils.

If such a plate, therefore, is put in the place of the image on the camera obscura, the asphalt coating will re- main soluble on all the dark places (shadows) of the image, whilst on the light spots it will become insoluble. The eye, it is true, does not perceive these changes ; the plate,

HISTORICAL SURVEY V

after being exposed to the influence of light, appears the same as before the exposure. But if oil of lavender is poured over the coating of asphalt, it dissolves all the spots that have remained unchanged, and leaves behind all those that have been changed by light that is, have been rendered insoluble. Thus, after several hours' exposure in the camera obscura, and subsequent treatment with ethereal oils, Niepce succeeded, in fact, in obtaining a picture. This picture was very imperfect, it is true, but interesting as a first attempt to fix the images of the camera, and still more interesting as evidence that there are bodies which lose their solubility in the sunlight. This fact was again made use of long after the death of Niepce, and it led to one of the finest applications of photography, that of heliography, or the combination of photography with copper-plate printing, which Niepce himself, to all appearance, had already known.

From experiments which have been recently carried out by Dr V. Vojtech it seems highly probable that this property of the insolubility of asphalt in turpentines after it has been exposed to light is entirely due to chemical changes, since no such action takes place when the asphalt is placed in an atmosphere of either nitrogen, carbon dioxide, or hydrogen during the exposure to light.

A Copper-plate Print. A copper-plate print is pro- duced thus : A smooth plate of copper is engraved with the burin (or graving tool) that is to say, the lines which should appear black in the picture are cut deeply in the plate. In producing impressions, ink is first rubbed into these cuts, and then a sheet of paper is placed upon the plate and subjected to the action of a roller press, whereby the ink is transferred to the paper and produces the copper- plate impression. Niepce endeavoured to utilize light in producing these engraved copper-plates in place of the laborious process of cutting. To effect this he covered the copper-plate with asphalt, as before stated, and

10 THE ADVANCE OF PHOTOGRAPHY

exposed it to the light beneath a drawing on paper. In this case the black lines of the drawing kept back the light ; and, accordingly, in these places the asphalt coating remained soluble ; under the white paper, on the contrary, it became insoluble. Therefore, when lavender oil was afterwards poured over the plate, the parts of the asphalt which had become insoluble adhered to the plate, whilst the soluble parts were dissolved and removed, and the plate in those places was laid bare. Thus a film of asphalt, in which the original drawing appeared as if engraved, was obtained on the plate.

If a corrosive acid is now poured on such a plate, it can only act on the metal in those places where it is not pro- tected by the asphalt ; and in such places this metal plate was in fact eaten into. Thus an incised drawing upon a metal plate was produced by the corrosive action of the acid, and a plate was obtained which, after cleaning, could be used for printing like an engraved copper-plate. Copper- plate prints of this kind have been found amongst the papers left by Niepce. These prints he called " helio- graphs," and he showed them to his friends as far back as 1826. This method, in an improved form, has been much used, especially in the printing of paper money, when it is requisite to produce a number of engraved plates which are all to be absolutely alike, so that one piece of paper money ma}' perfectly correspond to another, and may therefore be distinguished from counterfeits.

Daguerre' s Experiments. Niepce *s impressions were undoubtedly very imperfect, and therefore remained unnoticed. He himself gave them up, and again entered upon a series of experiments to fix the charming images of the camera obscura. In 1829 Daguerre joined him ; and both carried on experiments in common until 1833, when Niepce died without having obtained the reward of his long-continued efforts. Daguerre went on with the experiments ; and he would not, perhaps, have got further

HISTORICAL SURVEY 11

than Niepce if a fortunate accident had not worked in his favour.

He made experiments with iodide of silver plates, which he produced by exposing silver plates to the vapour of iodine, a peculiar and very volatile chemical element. Under this treatment, the silver plate assumes a pale yellow colour, which is peculiar to the compound of iodine and silver. These plates of iodide of silver are sensitive to light, they take a brown colour when exposed to it, and thus an image is produced upon them when they are exposed to the action of light in the camera. A very long exposure to light, however, is necessary to accomplish this end"; and the thought could scarcely have arisen of taking the likeness of any person in this manner, since he would have been obliged to remain motionless for hours.

One day Daguerre placed aside as useless, in a closet in which were some chemical substances, several plates that had been exposed too short a time to the light, and therefore as yet showed no image. After some time he happened to look at the plates, and was not a little astonished to see an image upon them. He immediately divined that this must have arisen through the operation on the plates of some chemical substance which was lying in the closet. He therefore proceeded to take out of the closet one chemical after the other, and placed there plates which had been exposed to the light, when, after remaining there some hours, images were again produced upon them. At length he thought that he had removed in succession all the chemical substances from the closet ; and still images were produced upon the plates. He was now on the point of believing the closet to be bewitched, when he discovered on the floor a dish containing mercury, which he had hitherto overlooked. He conceived the notion that the vapour from this substance for mercury gives off vapour even at an ordinary temperature must have been the magic power which produced the image.

12 THE ADVANCE OF PHOTOGRAPHY

To test the accuracy of this supposition, he again took a plate that had been exposed to light for a short time in the camera obscura, and on which no image was yet visible. He exposed this plate to the vapour of mercury, and, to his intense delight, an image appeared, and the world was enriched by a most valuable discovery.

Compared with the beautiful pictures which it is possible to produce now by means of the prepared sensitized papers, daguerreotypes were but very poor productions. The appearance of these pictures was injured by the ugly mirror-like surface, which prevented a clear view of them. No such objections were felt in the year 1839, when Daguerre's discover}7 was first spread abroad by report. Pictures were said to be produced without a draughtsman by the operation of the sun's raj-s alone. That was of itself wonderful ; but it was still more wonderful that, by the mysterious operation of light, every substance impressed its own image on the plate. How many ex- travagant hopes, how many evil prognostications, were associated with the report of this mysterious invention ?

It was prophesied that painting would come to an end, and that artists would die of starvation.

Then came sceptics who declared the whole thing impossible. These persons were reduced to silence by the testimony of Humboldt, Biot, and Arago, the three cele- brated scientific men to whom Daguerre disclosed his secret in 1838. The excitement grew. Through the influence of Arago an application was made to secure to Daguerre a yearly pension of 6000 francs, provided he made public his discovery. The French Chamber of Deputies agreed ; and, after a long and tiresome delay, the discovery was at length disclosed to the expectant world.

It was at a memorable public stance of the French Academy of Sciences in the Palais Mazarin, on the 19th of August 1839, that Daguerre, in the presence of all the great authorities in art, science, and diplomacy,

HISTORICAL SURVEY 13

who were then in Paris, illustrated his process by experiment.

Arago declared that " France had adopted this dis- covery, and was proud to hand it as a present to the whole world " ; and henceforth, unhindered by the quackery of mystery, and unfettered by the right of patent,1 the discovery of Daguerre made the round of the civilized world.

Daguerre quickly gathered round him a number of pupils from all quarters of the globe ; and they trans- planted the process to their homes, and became in their turn centres of activity, which daily added to the number of disciples of the art.

Sachse, a dealer in art living in Berlin, was initiated into Daguerre's discovery on the 22nd of April 1839, and was appointed Daguerre's agent in Germany. On the 22nd of September, four weeks after the publication of the discovery, Sachse had already produced the first picture at Berlin. These pictures were gazed at as wonders, and each copy was paid for at the rate of from £1 to £2 ; while original impressions of Daguerre fetched as much as £5. On the 30th of September Sachse made experiments in the Park of Charlottenburg, in the presence of King Friedrich Wilhelm the Fourth. In October the earliest types of Daguerre apparatus were being sold in Berlin. The first set of apparatus was purchased by Beuth for the Royal Academy of Industry at Berlin, and is still to be seen there. After the introduction of the apparatus, it was in the power of every one to carry out the system ; and a great number of daguerreotypists started into existence.

The first objects photographed by Sachse were archi- tectural views, statuary, and paintings, which for two years found a ready sale as curiosities. It was in 1840 that he first represented groups of living persons, and in

1 It was only in England that the process was patented, before its publica- tion, on the 15th of July 1839.

14 THE ADVANCE OF PHOTOGRAPHY

this way photography became especially an art of por- traiture. It made the taking of portraits its principal means of support, and in two years there were daguerreo- typists in all the capitals of Europe.

In America a painter, Professor Morse, afterwards the inventor of the Morse telegraph, was the first to prepare daguerreotypes ; and his coadjutor was Professor Draper.

The Daguerreotype. Let us now consider more closely the process employed in producing daguerreotype plates. A silver plate, or in the place of it a silver-plated copper- plate, serves to receive the image. It is rubbed smooth by means of tripoli and olive oil ; and then receives its highest polish with rouge and water and cotton- wool. It is only a perfectly polished plate that can be used for the process. This burnished plate is placed with its polished side downward upon an open square box, the bottom of which is strewn with a thin layer of iodine. This iodine evaporates, its vapour comes into contact with the silver, and instantly combine with it. By this means the plate first assumes a yellow straw-colour, next red, then violet, and lastly blue. The plate, protected from the light, is then placed in the camera obscura, where the image on the ground-glass slide is visible, and " exposed " for a certain time. It is afterwards brought back into the dark, and put into a second box, upon the metal floor of which there is mercury. This mercury is slightly warmed by means of a spirit lamp. At first no trace of the image is visible on the plate. This does not appear until the mercury vapour is condensed on those parts of the plate which have been affected by the light ; and this condensation is in proportion to the change which the light has caused. During this process the mercury is condensed into very minute white globules, which can be very well discerned under the microscope. This operation is called the development of the picture.

After the development the remaining iodide of silver,

HISTORICAL SURVEY 15

which is still sensitive to light, must be removed to render the image durable that is, "to fix it." This is effected by using a solution of hypo-sulphite of soda, which dissolves the iodide of silver. Nothing more is required after this than to wash with water and dry, and the