THE Franklin institute LIBRARY The Elihu Thomson CoUecHon Given by Mrs. Elihu Thomson CLASS BOOK /PS 7y^ ACCESSION y^i^/^^ TEXT-BOOKS OF SCIENCE ADAPTED FOR THE USE OF ARTISANS AND STUDENTS IN PUBLIC AND SCIENCE SCHOOLS PHOTOGRAPHY Text-Books of Science. ABNEY'S PHOTOGRAPHY, 3J. M. ANDERSON'S STRENGTH of MATERIALS, 35. (>d. ARMSTRONG'S ORGANIC CHEMISTRY, 3s. 6rf. BALL'S ELEMENTS of ASTRONOMY, 6^. BARRY'S RAILWAY APPLIANCES, 3-f- 6s. BAUERMAN'S SYSTEMATIC MINERALOGY, 6^. BLOXAM & HUNTINGTON'S METALS, s^- GLAZEBROOK'S PHYSICAL OPTICS, 6^. GLAZE BROOK & SHAW'S PRACTICAL PHYSICS. 6^. GORE'S ELECTRO-METALLURGY, 6^. GRIFFIN'S ALGEBRA & TRIGONOMETRY, 3.?. dd. Notes, 3J. dd. HOLMES'S THE STEAM ENGINE, 6^. JENKIN'S ELECTRICITY & MAGNETISM, ^s. Gd. MAXWELL'S THEORY OF HEAT, 3^. 6d. MERRIFIELD'S TECHNICAL ARITHMETIC-s^. 6d. Key 3^-. 6d. MILLER'S INORGANIC CHEMISTRY, 31. 6d. PREECE and SIVEWRIGHT'S TELEGRAPHY, 5^. RUTLEY'S PETROLOGY, or Study of Rocks, 4s. U. SHELLEY'S WORKSHOP APPLIANCES, 4^. 6d. THOME'S STRUCTURAL & PHYSIOLOGICAL BOTANY, 6$. THORPE'S QUANTITATIVE ANALYSIS, 4^. 6d. THORPE & MUIR'S QUALITATIVE ANALYSIS, 3J. 6./. TILDEN'S CHEMICAL PHILOSOPHY, 3^. 6d. ; or, with Answers, 4^. 6d. UNWIN'S MACHINE DESIGN, (>s. WATSON'S PLANE & SOLID GEOMETRY, 3s. e,d. London : LONGMANS, GREEN, & CO. A TREATISE ON PHOTOGRAPHY BY CAPT. W. DE WIVELESLIE ABNEY R.E., F.R.S. FIFTH EDITION REVISED AND ENLARGED LONDON LONGMANS, GREEN, AND CO. AND NEW YORK : 15 EAST 16* STREET 1888 All rights reserved PRINTED BY SPOTTISWOODE AND CO., NEW-STREET SQUARE LONDON LIBRARY •/ ' ' THE " ' FRANKLIN INSTITUTI PREFACE. The numerous advances in the art of Photography have made it incumbent on the author to make a thorough revision of the previous editions of this work. This has not only necessitated the addition of new matter, but also the excision of some which it was felt might be omitted without detriment to the completeness of the subject. The additions have, however, considerably overbalanced the excisions, with the result that the pages have been considerably increased in number. The aim of the author has been to give a rational explanation of most of the different phenomena to be met with in Photography, and at the same time to give sufficient practical directions to enable the student to produce a picture which should be technically good, and also to show how Photography may be made an aid to research. The theories which were put forward in the older editions have so far stood the test of time, and the author has not had to modify or change them in the vi Preface. present one. It is a matter of regret to him that amongst photographers, both amateur and professional, there are but few who take what may be called a scientific interest in the art they practise, and it would be a source of gratification to him if this work should be the means of adding even a few earnest workers in the wide field which is still open for investigation. Some of what may be called the commercial applications of Photography have not been dealt with in this work, as it was found impossible to introduce them into the available space. For these the reader is referred to ' Instruction in Photography,' by the same author, or to some of the various manuals which are to be met with. CONTENTS. CHAPTER I. HISTORICAL SKETCH OF THE DISCOVERY AND PROGRESS OF PHOTOGRAPHY. PAGE Scheele and Ritter's Experiments ...... i Wedgwood and Niepce ........ 2 Daguerre's Discovery ........ 3 Talbot's Photogenic Drawing ....... 4 Positive and Negative Pictures ....... 4 Tlie Calotype Process ........ 5 Tile Collodion Process ........ 5 Processes with Gelatine, &c. ....... 6 CHAPTER n. EXPERIMENTS WITH LIGHT. The Prismatic Spectrum ........ 6 Light, Heat, and Actinic Rays ....... 8 Limits of the Spectrum . . . . . . . .9 Work caused by the Absorption of Rays . . . . .10 CHAPTER in. THEORY OF SENSITIVE COMPOUNDS. Arrangement of Molecules . . . . . . . il Chemical Effects not an absolute measure of Work expended . 12 viii Contents. I'AGE Comparison of Photographic. Compounds with Explosives . -13 Oscillations of a Molecule 14 The co-existence of Longer Waves with Shorter Waves . 15 The Visible and Invisible Photographic Image . . . .18 Methods of Development . . . . . . -19 Fixing the Photographic Image . . . . . . .21 CHAPTER IV. THE ACTION OF LIGHT ON VARIOUS COMPOUNDS. Action of Light on Silver Chloride . . . . . .21 Action of Light on Silver Iodide . . . . . • 2,5 Absorbents of the Liberated Atoms . . . . . . 25 Action of Light on Silver Bromide . . . ... 26 Action of Light on Organic Compounds of Silver . . . 27 Action of Light on Ferric and Uranic Compounds . . 29 Action of Light on Chromium Compounds . . . -31 Action of Light on Asphaltum, Dyes, and a Mixture of Chlorine and Hydrogen ......... 33 Light causing a Combination between Sulphur and Antimoniuretted Hydrogen .......... 34 CHAPTER V. ON THE SUPPORT AND THE SUBSTRATUM. Essentials of a Support ........ 35 The Vehicle Holding the Sensitive Compounds on the Support . 36 CHAPTER VI. THE DAGUERREOTYPE. Preparation of the Sensitive Surface ...... 39 Development of the Image ....... 40 Strengthening the Image by Gold ...... 41 Reproductions of Daguerreotypes . . . . , -41 Contents. IX CHAPTER VII. COLLODION. PAGE Action of Sulphuric Acid on Organic Matter . . . .42 Action of Anhydrous Nitric Acid on Cotton . . . -43 Preparation of Pyroxyline ........ 45 The Solvents of Pyroxyline ....... 49 Formula for Plain Collodion ....... 50 Order of Sensitiveness of Sensitive Silver Compounds prepared from different Metallic Compounds . . . . -52 Formulas for Iodized Collodion ....... 53 Testing Plain Collodion ........ 54 CHAPTER VIII. PREPARATIONS NECESSARY FOR WORKING THE WET PROCESS. Cleaning the Plate ......... 55 The Sensitizing Bath 58 Distilling Water 5g Formulae for tha Sensitizing Bath . . . . . .61 Keeping the Bath in Order 62 CHAPTER IX. ON THE DEVELOPMENT OF THE PHOTOGRAPHIC IMAGE. Theoretical Considerations of Development . . . -63 On Different Strengths of Developers ...... 65 On Viscid developing Solutions . 66 Formulas for Developers ........ 67 CHAPTER X. GIVING INTENSITY TO AND FIXING THE IMAGE. Different Methods of giving Intensity .70 Formulae for Intensifiers 71 X Contents. PAGE Action of the Solvents in Fixing the Image . . . -74 Formulce for Fixing Solutions 75 Varnishing the Collodion Film 7^ CHAPTER XI. MANIPULATIONS IN WET-PLATE PHOTOGRAPHY. Cleaning the Plate . . -77 Coating the Plate w ith Collodion ...... 78 Sensitizing 79 Development ^° Intensifying the Image . . . • • • • .81 Fixing the Image 83 Varnishing the Negative . . . ■ • • • . 84 CHAPTER XII. DEFECTS IN NEGATIVES. Defects due to the Chemical Processes 85 Defects due to the Lens 86 Irradiation 87 CHAPTER XIII. POSITIVE PICTURES BY THE WET PROCESS. Formulae for Sensitizing Bath 89 Formulae for Developer and Collodion 9° CHAPTER XIV. DRY PLATE OR ALKALINE DEVELOPMENT. Development . . . . • • • • • • 9^ Experiments with Alkaline Developers 92 Comparison of the Methods of Development . . . .98 CHAPTER XV. DRY PLATE PROCESSES WITH THE BATH. Substrata . . . 100 Collodion loi Contents. xi PAGE Sensitizing .......... 102 Applying the Preservative . . . . . . .103 Drying the Plate ......... 104 Details of the Gum Gallic Process ...... 104 P'ormula; for Developers . . . . . . . .107 Albumen-beer Process no CHAPTER XVI. COLLODION-EMULSION PROCESSES. Experiments with Silver Bromide Emulsion . . . -113 Canon Beechey's Unwashed-Emulsion Process . . . .116 CHAPTER XVn. WASHED-COLLODION EMULSIONS. Washed-Emulsions 118 Preparation of Emulsion 120 CHAPTER XVni. THE GELATINO-BROMIDE PROCESS. Preparation of Plates 123 Emulsification of the Silver Haloids . . . . .125 Adding the Gelatine . . . . . . . .126 Washing the Emulsion . . . . . . . .127 Coating and Drying the Plates . . . . . .128 CHAPTER XIX. EXPOSURE AND DEVELOPMENT OF GELATINO-BROMIDE PLATES. Exposure of Dry Plates .131 Development .......... 132 Intensification . . . .134 Defects , . , . 136 xii Contents. CHAPTER XX. PAPER NEGATIVES. PAGE Preparation of Iodized Paper . . . . . . -137 Sensitizing 138 Developing .......... 139 Waxing the Paper ......... 141 Le Gray's Process ......... 142 Gelatine Bromide Paper ........ 142 CHAPTER XXI. SILVER PRINTING. Experiments with Sensitized Paper . . . . . .144 Theory of Toning .... ..... 147 Fixing the Print 151 CHAPTER XXn. MANIPULATIONS IN SILVER PRINTING. Preliminary Preparation of the Paper . . . . -154 The Sensitizing Bath . . . . . . . ■ '55 Printing the Picture . . . . . . . .156 Toning the Print . . . . . . . • -157 Fixing the Print . . . - • . . . -159 Washing the Print . 160 Testing for Sodium Hyposulphite . . . . . .161 Defects in Prints 162 CHAPTER XXin. COLLODIO- AND GELATINO-CITRO-CHLORIDE PROCESSES. Formula for Collodio-Chloride . . . . . . .162 Paper for Collodio-Chloride . . . . . . .163 Formula for Gelatino-Chloride ....... 164 CHAPTER XXIV. PRINTING WITH IRON AND URANIUM COMPOUNDS. Printing Processes with Salts of Iron . . . . .166 Chrysotype 167 Contents. xiii PAGE Salmon and Garnier's Process ....... i68~ Poitevin's Process with Ferric Chloride and Tartaric Acid . .169 Printing Process with Uranium Salts . . . . .170 CHAPTER XXV. THE PLATINOTYPE PROCESS. Theory of Platinum Process . . . . . • 171 Preparation of Paper . . . . . . . .172 Development .......... 173 CHAPTER XXVI. PRINTING WITH CHROMIUM SALTS. Swan's Process ......... 175 J. R. Johnson's and Sawyer's Improvements . . . .177 Preparation of Tissue .180 Sensitizing the Tissue . . . . . . . .182 Developing the Print 184 CHAPTER XXVII. MISCELLANEOUS PRINTING PROCESSES WITH CHROMIUM SALTS. Willis's Aniline Process . . . . . . . .186 The Powder Process ........ 187 Woodbury type . . . . . . . . . 1S8 CHAPTER XXVIII. PHOTO-LITHOGRAPHIC TRANSFERS. Requisites in the Paper and Ink 190 Formula for Preparing the Paper . . . . . .191 Inking the Printed Picture and Developing the Transfer . .192 Papyrotype 194 Photo-Lithography in Half-Tone 195 CHAPTER XXIX. PHOTO-ENGRAVING AND RELIEF PROCESSES. Niepce's Process 196 Ehrard's Process . . . . , . , . .198 Talbot's Photo-Engraving Process 159 xiv Contents. CHAPTER XXX. PHOTO-COLLOTYPE PROCESS. PAGE General Principles of the Photo-collotype Processes . . . 202 Albert's Process . . 203 Process by Capt. Waterhouse ....... 204 CHAPTER XXXI. ELEMENTARY PHOTOGRAPHIC OPTICS. Laws of Refraction ......... 205 Dispersion and its Correction ....... 207 Spherical Aberration ........ 209 Use of the Diaphragm or Stop ....... 210 Distortion of the Image . . . . . . . .211 The Optical Centre 213 Portrait Lenses ......... 216 Landscape Lenses . . . . . . .■. .217 Further Uses of the Stop . . . . . . . .221 Definition of the Focus of an Object . . . . . .221 CHAPTER XXXn. APPARATUS. Cameras 225 Hare's Changing Box . . . . . . . . 230 Warnerke's Roller slide . . . . . . . .231 The Camera Stand ......... 233 Drop Shutter . 235 Rouch's Dark Tent 238 The Dark Room 238 CHAPTER XXXHI. ON THE PICTURE. Principles to be followed in Choosing a Point of View . . 239 Examples illustrating Landscape Photography . . . .241 Examples illustrating Groups ....... 252 Epitome of Simple Rules to be followed in Landscape Com- position . . . . . . . . . . 254 Contents. xv PAGE Placing the Camera. ........ 257 Architectural Pictures 258 Exposure . . . . • • . . . . 259 CHAPTKR XXXIV. PHOTO-SPECTROSCOPY. Herschell's Experiments ........ 260 Arrangement for a Photo-Spectroscopic Apparatus . . . 261 Camera ........... 265 Heliostat . 268 Lockyer's Photo-Spectroscopic Apparatus ..... 270 Method of Examining Metallic Vapours 273 Miller's Researches in Absorption Spectra .... 274 CHAPTER XXXV. EFFECT OF THE SPECTRUM ON THE HALOID SALTS OF SILVER. Influence of the Spectrum on Silver Iodide .... 277 Influence of the Spectrum on Silver Bromide .... 284 Influence of the Spectrum on Silver Chloride .... 287 Thermal end of the Spectrum Mixtures of Silver Haloids . .291 CHAPTER XXXVI, ON THE APPARENT DESTRUCTION OF THE ACTION OF LIGHT ON THE PHOTOGRAPHIC IMAGE. Destruction of the Daguerrean Image ..... 303 Reversal of Images ......... 307 CHAPTER XXXVII. ORTHOCHROMATIC PHOTOGRAPHY. Eff'ect of Different Dyes .316 Ammoniacal Solutions of Dyes. ...... 317 Luminosity of the Spectrum . . . . . . .318 Luminosity of the Spectrum through Yellow Glass . . . 319 Theory of tlie Action of Dyes . . . . . . .321 xvi Contents. CHAPTER XXXVIII. LIGHT FOR THE DARK ROOM. PAGE Absorption Spectra of Dyes and Glasses 323 Artificial Lights 325 CHAPTER XXXIX. ACTINOMETRY. Draper's Experiments Bunsen and Roscoe's Silver Chloride Actinometer Roscoe's Actinometer ....... Reading the Tints of the Actinometer .... Discussion of the Tru'.h of Light and Shade in Photographs Spinge's Sensitometer ....... Density of Deposit in Negatives 327 328 330 331 333 335 337 CHAPTER XL. CELESTIAL PHOTOGRAPHY. Solar Photography vk^ith a Fixed Telescope .... 340 The Photo-Heliograph 343 Solar Eclipse Photographs ....... 346 Lunar Photography 347 Stellar Photography . . 3^1 CHAPTER XLI. PHOTOGRAPHY WITH THE MICROSCOPE. Apparatus required -53 Monochromatic Light 356 APPENDIX. Table of Elements 359 Comparison of the Metrical with the Common Measures . . 360 INDEX 363 A TREATISE ON PHOTOGRAPHY. CHAPTER I. HISTORICAL SKETCH OF THE DISCOVERY AND PROGRESS OF PHOTOGRAPHY. To the alchemists of the sixteenth century belongs the honour of having first noticed the change that took place m silver chloride (known to them as ' Luna cornua') by ex- posure to light, but they regarded the darkening as a species of transmutation of metals, and it remained for Scheele, the Swedish chemist, in 1777, to investigate the properties of this compound, though his researches led at the time to no practical end. Scheele found, when he exposed silver chloride to the action of light beneath water, that in the fluid was dissolved a substance which, on the appHcation of silver nitrate, gave once more silver chloride, and that, after applying ammonia to the blackened body, an insoluble residue of metallic silver remained behind. These were the only facts elicited at the time, and a delay of more than half a century occurred before they were put to really good pur- pose. In 1 80 1 Ritter, of Jena, repeated the experiments of Scheele, and discovered that the chloride darkened rapidly in those rays of the spectrum which lie beyond the extreme violet. To him also is due the announcement that the red 2 Historical Sketch. rays have the property of undoing the work effected by the violet, though he attributed the effect to the wrong cause. In 1802 Thomas Wedgwood read a paper before the Royal Institution, entitled ' An account of a Method of Copying Paintings on Glass ^ and of making Profiles by the agency of Light upon Nitrate of Silver.' With these experiments of Wedgwood's Sir Humphry Davy was associated, and in their record we find it stated that muriate of silver was more readily acted upon by light than the nitrate, and that white leather used as a basis gave better images than paper. Images obtained by the solar micro- scope were impressed without any serious difficulty, but no means was discovered of rendering them anything but tran- sitory when exposed to daylight. For Charles, a French- man, has been claimed the credit of employing at an earlier date the same method of obtaining black . profiles by the action of light, but there seems to be no authentic proof extant that this claim should be allowed. Dr. Wollaston, in 1803, discovered that gum guaiacum, when exposed to the action of the blue rays of light, became changed in colour, and that on exposing those altered portions to the red rays, the original tint was restored. In 1814 Photography was to receive a new votary in the person of Joseph Nic^phore de Niepce. Leaving the salts of silver, he devoted himself to the study of the action of light on resins. After several years of research, he at length completed the process known as heliography, which consisted in the production of a picture m bitumen on a polished metal plate. The discovery he made m regard to this resin was that, after insolation, it became insoluble in its ordinary solvents. An exposure of many hours in a camera obscura was necessary to produce the required effect; hence, as may be imagined, the views taken by this means 1 \ mistake often occurs in the reading of this sentence. Wedg- wood'did not make the copies on glass, but copied paintings which ■were dra^wn on glass. Daguerre's Discovery. 3 were wanting in vigour, owing to the shifting direction of the sunlight, and, as we shall see later on in this work, from other causes, were of necessity deficient in delicate lights and shades. In 1827 Nie'pce came over to England, with the intention of drawing the attention of the Royal Society to his discovery, but his process being secret, his communication was not received, and he returned to France. In 1824 Daguerre, a French painter, began a series of ex- periments in the same direction, and in 1829 he and Niepce entered into a partnership, and presumably it was the knowledge of the latter's method of working which gave the former the idea of the daguerreotype. Nie'pce had employed silver plates- covered with asphaltum, which, after expo- sure and application of the solvent, left the metal bare in parts. The image thus formed was brown ; the shadows being represented by the metallic surface. In order to produce a proper effect, it was necessary that the parts covered by the bitumen should be whitened and the bare parts darkened. After various experiments, he applied iodine to the picture, subsequently removing the bitumen. It is to be presumed that Daguerre noticed the action that the light produced on those portions of the plate which had been converted into iodide. At any rate to Daguerre belongs the glory of the discovery that an image could be produced on a silvered plate which had been exposed to the vapour of iodine, though it was by fortuitous circumstances that he hit on the method of developing an invisible image. In January 1839 the discovery of the daguerreotype pro- cess was first announced, and in August of the same year the details of production were given to the world, Daguerre and Nie'pce the younger (the successor of Nice'phore), obtain- ing a pension from the Government of France. Whilsf Daguerre was working in France, we find that one of our own countrymen, Fox Talbot, had been experimenting in another direction. Bearing in mind the work of Scheele and Wedgwood, he devoted himself to the production of B2 ^ Historical Sketch. drawings, &c, on silver chloride, and in January 1839 he read a paper before the Royal Society on 'Photogenic Drawincrs ' His method of procedure was somewhat as follows ° Writing paper was coated with a solution of common salt, and after drying was brushed over with silver nitrate • by this means silver chloride was obtained, with a slight excess of the nitrate, in which condition it proved excessively sensitive to light. Various bodies, such as lace and ferns, were laid on this paper, and a reversed facsimile of them in black and white was produced, and he fixed the Fig Fig. 2. impressions by solutions of bromides and chlorides. When such a reversed facsimile was placed over similarly-prepared paper, and the light allowed to act through it, the result was the formation of a facsimile, only this time not reversed in These two prints were respectively named the negative and the positive (fig. i and fig. 2). Comparing this process with the former we see what an immense advantage Talbot's process had over the daeuerreotype. With Talbot's any number of copies of a subj^^^^^^^^^ be cheaply produced, whilst with the latter Caiotype. 5 one positive was the sole result, unless expensive electro- chemical means were resorted to. The Rev. J. B. Reade was also an ardent experimentalist in this process, and to him is to be ascribed the discovery of the accelerating power of gallic acid, in the presence of silver nitrate, for the production of an image, and also for the development of the invisible image by the same agency. From this discovery, together with that of Daguerre's, Fox Talbot reasoned out the caiotype process, which he patented in 1841. By it an invisible image is formed on silver iodide on paper, and developed by gallic acid. In this process of Talbot's a negative image was formed, while by the first process the positive pictures ■ were produced ; and it should be remarked that the same method of pro- ducing silver prints obtains to the present day with scarcely any alteration. Sir John KerscheV had drawn attention to the possi- bility of producing photographic pictures on glass, and in 1843 had actually printed, in a camera obscura on silver chloride, deposited on such a plate, a picture of his 40-foot telescope. Niepce de St. Victor made a further great advance when he succeeded in holding the sensitive salts of silver on glass by using albumen as a ^■ehicle, but to Le Gray must be accorded the credit of suggesting collodion as suitable for retaining them m situ. Scott Archer, with whom was asso- ciated Dr. Hugh Diamond, in 185 1, however, introduced the collodion process in the practical form in which it exists to-day, and it may safely be said, that, with the exception of the daguerreotype process, no more important discovery in photography has been made. In 1839 Mungo Ponton published the fact that potas- sium dichromate, when applied to paper and dried, altered in composition when exposed to the influence of light. This announcement caused much investigation into the subject, and it was subsequently discovered that not only was the chromate altered in composition, but that the sizing 6 Experiments with Light. of the paper was oxidised. Gelatine, gum, starch, albumen, were all found to become insoluble when exposed in contact with it; and Poitevin utilised this fact in the pro- duction of pictures in powdered carbon by a process analogous to that subsequently to be described in these pages. Swan, Johnson, Woodbury, and others, have more recently extended its application by the production of images formed in gelatine, coloured with pigments; whilst a still wider field has been opened by Albert, Edwardes, and others, in the production by mechanical means of prints in printers' ink from a gelatine image, founded on the fact that oxidised gelatine is incapable of absorbing water. CHAPTER II. EXPERIMENTS WITH LIGHT. Before entering into the theory of photography, it will be convenient to enter briefly into some of the phenomena of light ; for it is with this form of energy that the photo- grapher has to deal. It will be as well first to tr>' one prac- tical experiment with light, in order to clear up certain difficulties which may present themselves. Darken a room of some 12 or 14 feet in length by means of shutters (light wooden frames covered with opaque paper will answer), and in one fix a small plate of glass, a, which has previously been covered with tinfoil, and on which, with a sharp knife, has been cut a straight line laying bare the glass ; on a table place a glass prism, B, the centre of which is at the same height as that of the slit, and have a screen, covered with white paper, c, and a lens, e, of about 1 2 inches focus, ready at hand; outside the window arrange a mirror, d — an ordinary looking-glass will answer — in such a position as to reflect the sunhght on to the slit: interpose the lens, e, 8 Experiments with Light. about 6 inches from a, in such a manner as to cause the beam to fall upon the prism, b. The floating dust in the room will immediately show that the original beam of white light has been split up into a series of coloured rays, and a position for the screen may then be found which will cause the top and bottom edge of the spectrum (as this glorious band of colour is termed) to be sharply defined; and if the cut in the tinfoil be fine enough, a series of dark lines will traverse it vertically. With these, however, we have no- thing to do at present. Now experiments, the results of which have formed the groundwork of mathematical reason- ing on the theory of light, have conclusively proved that light as light is merely a sensation. Permeating all known space is assumed to be an imponderable and elastic fluid known as ether, and in it a luminous or heat source is able to generate a series of ripples or waves, flowing unbrokenly and continuously from it. What the prime form of these undulations may be we cannot tell. They may be, and most probably are, compounded of an almost infinite number of different undulations, when ordinary' white light is the impression given to the eye, and each of these series of undulations vary in length from crest to crest. Those of certain lengths are able to affect the nerves which line the retina of the eye; whilst some of these are able to affect other nerves lying in our bodies, producing the sensation of heat ; others again, though incapable of producing the sen- sation of light or heat, exhibit themselves by their effect on certain compounds, causing chemical combination or decom- position. Of those waves whose impact on the eye produces the sensation of light the shortest is about 600 millionths of a millimetre, whilst the longest is about 350 millimetres. The former give the sensation of a violet colour, the latter of a brilliant red. Examining the spectrum thrown on the screen, the intermediate colours of blue, green, yellow, and orange are seen, and the wave-lengths producing their effects ' i.e. not polaiised. Waves. 9 bn the eye lie intermediate between the limits given above. There is uncertainty as to the lower limit to which the heat- producing rays are refracted, but probably to a length equal to that of the visible spectrum, whilst the range in length of the chemically active waves other than those situated in the visible portion of the spectrum, and which lie beyond the violet (being called the ultra-violet or fluorescent rays), is, if anything, still more uncertain. It will be evident on reflection that it must be accidental that, between certain limits, the waves should be capable of producing a sensa- tion of light or of heat. The exact upper limit of the thermal spectrum is unknown, but from theory it must be co-termi- nous with the chemically active rays, as will be seen further on, the inferior limit of the capacity of any waves to pro- duce decomposition is as yet unascertained. All those series of waves which effect decomposition in any compound are called actinic rays, and, as will be seen, the range of these vary for every ordinary photographic compound. It may help us in a right comprehension of our subject if reference is made here to one quality of these undulations. The interstellar ether in which these waves ripple is assumed to permeate every body, solid, liquid, and gaseous ; and it depends upon the disposition of the ultimate molecules of the body whether it is opaque or transparent to any of the visible or dark rays of light. It must be borne in mind that the molecules of every substance are presumably in a state of vibration, the extent and velocity of which depend partly upon the temperature, and partly upon the nature, of the substance, and that this must ever be so unless the purely theoretical condition of absolute cold be arrived at. Sup- posing, then, we have a glass, which with white light falling on it allows only the transmission of red light, and we look through it at the spectrum formed by white hght, we should find that it cuts off the whole of the colours excepting the i-ed, obliterating them more or less perfectly, that is, in tech- nical language, it absorbs them. Now, according to all 10 Experiments with Light. ideas of the conservation of energy, this absorption must indicate the performance of some kind of work. It may be that it causes the already vibrating molecules of the glass to take up and swing in some complicated manner with those rays particularly absorbed, and thus to cause a rise in tem- perature in the body, so small indeed, perhaps, as to be mdistinguishable, owing to the rapid cooling due to radiation; or it may be that work is performed in effecting chemical decomposition, for even glass is thus affected by light. The rays which simply pass through the glass produce no effect on it — their energy is unimpaired. It should also be noted that where light is not entirely absorbed, but is only reduced in intensity, even then also work must be performed by it ; for the intensity of any coloured or white light is dependent on the extent, or ampHtude, as it is termed, of the wave or waves ; and any diminution of the amplitude indicates that a portion of its available energy has been exhausted, and that therefore a transference of the portion so expended must have been made to the body through which it passed. This exchange or transference of energy is an important subject m all photographic matters ; it explains many of the phenomena in photography which often present a great difficulty to the beginner or to the rule-of-thumb photographer, whilst it is all-important in the right understanding of the revelations which are made by the spectroscope. It may then be laid down as an unalterable law, that where there is absorption of light {whether of dark or visible rays) by any body, work of some description must have been per- formed in that body. An account of the valuable experi- mental research of Joule on the mechanical values of light and heat is given in the ' Philosophical Magazine ' for 1843, and is deserving of special study. Tiieory of Sensitive Compounds, II CHAPTER III. THEORY OF SENSITIVE COMPOUNDS. Every particle of matter may be considered to be made up of molecules, each molecule consisting of constituent atoms. Thus a particle (and when we say particle we mean to con- vey the idea of the smallest visible quantity of matter) of silver iodide is composed of molecules of a like definite composition, the components being— two atoms of iodine and two of silver, or multiples of these numbers. The physical aspect of matter often conveys to the mind an idea of a certain kind of arrangement in the molecules ; as does the analysis of a compound, if not of the absolute arrange- ment of the atoms, at all events of the arrangements which they cannot take. Oxalic acid, for instance, we know is composed of carbon, hydrogen, and oxygen, having the formula C2 H2 O^, or the exact equivalents of water, H2 O, carbon monoxide C O, and carbon dioxide C O2, yet the compound is totally different in its physical cha- racters and chemical reactions from any of these. From this we can argue that the atoms of its molecules must be separated in such a manner that the oxygen molecules cannot seize Fig. 4. upon the hydrogen to form water, or on carbon to form carbon mo- noxide or dioxide. When the atoms are so arranged as to be mcapable of forming a molecule of a simpler type, they occupy a position of excessively stable equilibrium, and it would be necessary to expend a large amount of work to separate them. On the other hand, where the atoms of the mole- cule are so grouped that by rearrangement they may form perhaps more than one molecule, each of which may be 1 2 Theory of Sensitive Compounds. of less complex character, it often happens that all the atoms are in state of stable, though verging on indifferent, equilibrium. We may take as an illustration of this state of equilibrium the frustum of a pyramid standing base uppermost, on a narrow section parallel to the base. It is apparent that the work expended in order to cause the frustum to find a new position of more stable equilibrium (or, in other words, to fall on to one of its sides), may be made as small as we please by diminishing the area of the section on which it stands. Whilst falling, the body can do a certain amount of work, which will be quite independent of the amount of work expended to cause its fall. So with the atoms of a molecule which are in this state of almost indifferent equilibrium ; a very small amount of work need be expended in order to cause them to take up more stable positions ; but the kinetic energy they may possess whilst passing to this new state, need be no measure of the work performed upon them. A measurement of the work per- formed by their re-arrangement would principally tell what amount of work had been expended in some chemical pro- cess, in order to place them in that state bordering on indifferent equilibrium. It is possible, however, under cer- tain circumstances, to compare two or more energies with one another, by comparing the effects they produce on such molecules. Extending our previous illustration, supposing we had a row of such frusta of pyramids, and that it was found that one pellet of a number (all being of equal weight) when striking one frustum with a certain velocity, was able to cause it to fall, and also that in every case the accuracy of aim was undoubted, and that in falling one frustum did not strike its neighbour ; then at any interval after the commencement of a bombardment the amount of work expended in projecting the pellets could be compared by simply counting the number of frusta which had fallen. It is in a manner akin to this that the comparative values of the intensity of those rays which produce chemical Ejfect of Vibrations. 13 decomposition in sensitive compounds are found. The molecules of the compound answer to the frusta, and the pellets to the number or amplitude of the waves impinging on them. The method of estimating the number of mole- cules altered in composition, is by noting the colour or the attractive power on other matter which they possess. In our illustration we assumed that one frustum never interfered with another during its fall, and, so far as the compounds, which are photographically sensitive, are concerned, this is a correct assumption, for the alteration in one molecule does not cause an alteration in the neighbouring one. In other sensitive compounds this may not be the case. It is frequently the case that the rearrangement of the atoms of a molecule calls into play such a large amount of kinetic energy (it may be in the form of heat), that the neighbouring atoms are caused to rearrange themselves, and so on. In this case the destruction of the original form of the molecules may be so rapid, and the potential energy converted into kinetic may be so large, that we may have a compound which is an explo- sive. With these latter compounds the energy existing in a vibration is often sufficient to cause explosion. The vibration may be that of the longer waves produced in the medium we have already discussed, or may be those pro- duced in the atmospheric or other gases. Thus, radiant heat may cause -it, as also sound. It has been experi- mentally proved that many explosives are particularly sensi- tive to vibrations of a definite wave length ; thus, the vibration to which nitro-glycerine is most sensitive is not the best with which to cause the explosion of gun-cotton. It has also been asserted that the atoms of the molecules of iodide of nitrogen can be caused to be dissociated by the at- mospheric waves which are due to sound of a particular pitch. In order to understand more readily how it is that the molecules of such bodies may be disturbed by waves of a certain length, it must be recollected that they are in a state of agitation. In solids the paths they describe are limited, 14 Theory of Sensitive Compounds. though the excursions they take will be the greater, the higher the temperature of the body ; and from analogy it may be assumed that the agitation is really a definite oscilla- tion, though the paths described may be very complex. Now, ordinary white light, as has already been pointed out in the last chapter, most probably consists of an almost infinite series of undulations of varying length, traversing a medium, and it is quite conceivable that the molecules of a body, whose oscillations synchronise with one of these series of ethereal waves, may have their paths altered in form, and their amplitude increased to such a degree, Fig. s. that a rearrangement of the atoms must en- sue. In order to illustrate the effect of one oscillation upon another, the late Professor Rankine employed the following contrivance. ^ A is a lath to which is suspended a leaden bob, B, some six or seven pounds in weight ; c is a string attached to b, by which is sus- pended a wooden bob, d. The whole is caused to oscillate on an axis placed at x. When the length of the string is such as to cause the heavy and the light pendulum to c synchronise accurately, a slight horizontal displacement of B will cause the length of I amplitude of the oscillations of d to increase Qi' to such an extent that the latter will pass the semicircle and tumble. When the syn- chronism is only nearly perfect, the amplitude of d will at first increase, gradually stopping the oscillation of b, when it will diminish, and finally come to rest and bring b into oscillation once more, and so on. If we take the swing D as the type of the oscillation molecule, and that of b as the oscillation of the ethereal medium, it will be seen how perfect and nearly perfect synchronism will increase the oscillation of the molecule. The same illustration applies to a part of the theory of explosives, whether caused to Waves of Varying Length. 15 explode by the energy of radiant heat, or by that of atmospheric or gaseous waves. This is in accordance with what we have aheady advanced : it is only those waves which are entirely or partially absorbed, and whose amplitude is consequently annihilated or reduced, which can do work on a body : therefore, in choosing any particular ray of hght with which to cause this class of decomposition in a compound, it is a sine qua non that it must be absorbed ; in addition to which, some atoms must be less loosely bound to the molecules than are others. It is found prac- tically that the bodies employed for photographic purposes are affected principally by the waves of short length, and that as a rule those of greater length are inoperative ; and here we come to a great distinction existing between the re- arrangement of the atoms of the molecules, in explosives and in photographic compounds. The short wave lengths do not affect the former, though the longer ones, which we call radiant heat, can do so. Now the energy transmitted from a hot and luminous body by the medium lies principally in those waves which are capable of pro- ducing what we call heat (in fact the energy can only properly be estimated by ascertaining the heating effect due to the radiations) and as the heat produced in a body by the waves of lengths such as 450 niillionths of a milli- metre is insignificant, and when they are of a length of 200 millionths of a millimetre is at present immeasurable, it it is evident that the energy expended on the production of these last wave-lengths is small ; at the same time it happens that their production, as a rule, necessitates the existence of those of greater length. Thus, a platinum wire inserted in an electrical circuit may be heated, and yet only radiate dark rays ; by increasing the current it may become cherry-coloured, and a spectroscopic examination will demonstrate that only red rays are emitted, whilst at the same time it may be shown that the intensity of the dark rays is increased. By further increasing the current, the 1 6 Theory of Sensitive Compounds. yellow, green, blue, violet, and ultra-violet rays may in suc- cession be caused to radiate from the wire ; all the first emitted rays increasing in intensity. Fig. 6 shows the relatively greater increase in an indigo ray, compared with a yellow ray, emitted from a carbon filament heated in vacuo by an electric current. Fig. 6. In order, therefore, to displace the molecules of small stability of the photographic compound which are in equili- brium, it is as a rule necessary to produce waves of great length as well as waves of short length, and this may mean the existence of a great heat energy at the primary source of radiation, though not necessarily at a reflecting surface. Now, the usual result of the displacement of an atom from what we may call the sensitive molecule is to form a fresh Molecular Vibration. 17 solid body, and consequently the potential energy of the molecule is small, also the number of these molecules acted upon in a given time is small in comparison with the total ; hence the kinetic energy (which may take the form of heat) that may be generated by the chemical decomposition and recombination falls far short of that required to produce even red light, much less waves of still shorter length. We thus see that although one molecule of an explosive per se, after its potential energy has become kinetic, can cause vibrations of such a character as to effect a disruption of the neighbouring molecules, yet a similar disturbance produced in a molecule of a photographic compound is not capable of caus- ing an extension of the action beyond the molecule itself, and that it requires a renewed action of the disturbing force to do it. At first sight this seems unfortunate, but when we con- sider what would happen were such an event possible, it is apparent that the production of a photographic image in such a case would be impossible. In a succeeding chapter it will be found that a molecule of chloride of silver responds principally to the swing of the ultra-violet waves in the spectrum, and that it undergoes a change, owing to the throwing off of one of its consti- tuent atoms ; yet the same body may, by the aid of an artifice, be fused by the dark rays of heat, which are comparatively of great wave-length, and though it in itself becomes luminous, emitting the very same rays that, when falling on it, can cause one of its atoms to be shaken off, yet it remains unaltered. In this last case the vibrations of the molecules are not of the definite character needed to cause the change. A small force, applied at definite in- tervals, may cause a body to attain a great amplitude of vibration. A boy may cause a violent oscillation of a church bell if he time his pulls at the rope properly, and the accumulated energy may be such that it may drag the ringer up, though the work he may have executed at each pull of the rope may be very small. On the other hand, the ringer 1 8 Sensitive Coinpoiinds. may expend the same amount of energy at the wrong time, and the effect on the bell will be insignificant. The ex- periment given at p. 14, fig. 5, illustrates this effect. As before stated, the number of molecules affected in a short interval of time by light may be so small that their change in atomic composition may be invisible to the eye, or in physical appearance may be of a similar nature to the com- pound from which they are derived, in which case even a pro- longed exposure to the actinic rays would produce no visible effect. When the sensitive compound is formed in a thin layer held in situ on some substratum, such as paper, glass, &c., the light reflected and radiating from an object after passing through a lens may be caused to fall upon its surface and form an image. When the rays are of such a nature as to cause the equilibrium of the constituent molecules to be disturbed, the change will take place only m such parts of the thin lamina as are illuminated ; and thus an invisible ^mage formed by the shaken compound may be impressed if the time of exposure be short, or the change produced be such as not to be within the scope of oui" vision. Otherwise upon long exposure a visible image may be produced, the resulting compound being different in appearance from the original. As the point is of great importance, we must again direct attention to the fact, that the two images are exactly alike in chemical composition, one differing from the other solely in the number of molecules altered. Fortunately, methods exist of rendering visible to the eye what is ordi- narily and primarily invisible, and this operation is termed the development of the image, The invisible image is frequently termed latent, an appellation which, though convenient, is yet open to some criticism. We will now discuss the various ways in which development may be effected. ist Method. — The new compound may possess an attrac- tive force. If a rod or wire of zinc be placed in a solu- The Effects of Light. 19 tion of lead acetate, chemical operations immediately com- mence. The outside particles of the zinc enter into com- bination with the acetic acid of the lead acetate, and par- ticles of lead are deposited upon the rod in their stead. As the action continues the lead further reduced is, by a certain well-ascertained law, attracted to the lead already deposited. Spangles of the metal in a crystalline form at- tach themselves to the rod and then to one another, until what is known as a lead tree results, just as a magnet will a string of nails suspended from one of its poles. In a similar way a silver tree may be formed from a solution of its salts, provided the reduction be slow. So the action of light on cer- tain sensitive compounds, especially amongst which may be mentioned those of silver, is to cause the formation of a body which is capable of attracting the metal (of which it is itself a salt), when slowly deposited from a solution. This first de- posit is capable of attracting still more of the metal, and thus an image is completely built. This action is more fully treated at p. 64. 2nd Method.— TYit altered compound may be able to effect a reduction to a metallic state of a metal from a solution of its salt, which the original compound may be incapable of doing. In this case the metal would be natu- rally precipitated on the altered compound, and the attrac- tive force of the freshly-deposited metal would determine the attraction of any other that might be caused by extra- neous causes to deposit itself. In this method, as in the last, it is evident that the minutest portion of the altered com- pound is able to effect a building up of the image. 2,rd Method. — The image may be formed by the partial reduction, to a more elementary state, of the altered com- pounds, when treated with certain solutions, which reduction in the original compound was impracticable ; also in this reduced state it may exercise the same attractive force as above. We shall have an example of this in alkaline de- velopment. 20 Sensitive Compounds. ^th Method.— The altered compound may be capable of forming a coloured body when treated with metallic or other solutions. In this case it is manifest that the image must be due solely to the amount of the sensitive salt originally altered in composition, and its vigour must consequently - depend upon the time the light has acted. Of this method of development we shall have examples in the more sensitive ferric salts. t^th Method.— The attractive force of an altered mole- cule may be utilised by causing metallic or other vapour to condense upon it in preference to the neighbouring mole- cules which may not have been changed by light. This first condensation may determine the following condensa- tion. Of this we have an example in the development of the daguerreotype plate. dth Method.— The alteration in the compound may be shown by its incapacity to absorb moisture. Method.— The new compound may be incapable of entering into solution, though the original compound may be readily soluble. The chemical agents which are utilised in order to allow the development of the latent image to take place will be discussed as each method is brought under particular con- sideration. It is to be .remarked that these agents are technically called developers, a term which, critically speak- ing, is a misnomer, as in the majority of cases the part they play is a secondary one, and one which they fill whether applied to development or not. The term is convenient, however, and will be adopted in this work, though the student must in his own mind make the reservation indicated when coming across the term. Intensifying an image already developed or visible is a term applied to a process whereby the image is (i) rendered more visible to the eye, or (2) rendered more absorbent of, and therefore less transparent to, some particular kind of light, be it white, blue, red, yellow, &c. Both of these results The Action of Light. 21 can be obtained by following the methods indicated for developing the image. Fuller information regarding the necessary procedure will be given as various processes are described. Fixing an image is rather a vague term. It is intended to express that the image due to the first exposure and sub- sequent development shall be so treated as to undergo no change, leading to obliteration. This is usually effected by clearing the image of all that portion of the sensitive com- pound which has not been acted upon by light, and thus of rendering it incapable of being obliterated by fresh exposure or appearing indistinct. If the sensitive compound were absolutely colourless, and the action of light were to leave the new compound colourless, the developed image would need no clearing or fixing ; but, since all the sensitive com- pounds are either coloured themselves, or are converted by light into others possessing colour, there is evidently no safety, except in their entire removal CHAPTER IV. THE ACTION OF LIGHT ON VARIOUS COMPOUNDS. The action of light on various substances must have been a matter of remark from the earliest times. The tanning of the skin, the fading of colours, must all have been noted long before an attempt was made to ascertain the cause of such alteration. However, as we pointed out in the historical sketch, silver chloride was the first substance whose behaviour was philosophically examined ; and we propose to study the principal silver compounds before proceeding to other sensitive bodies. Scheele, as we have seen, found that chlorine was given off during expo- sure from the chloride, and that after treatment of the 22 Tlie Action of Light. blackened body with ammonia, metallic silver was left behind. There is not much need to carry the investigation further than Scheele, only the conclusion that he accepted, viz. that metallic silver was separated at the time of ex- posure, should be viewed with much doubt, particularly when it is found that the darkening action of the chloride takes place even when immersed in the strongest nitric acid. The accepted theory seems to be that exposure to light reduces any chloride to the state of subchloride, thus : Silver Chloride = Silver Subchloride + Chlorine Ag^Cl^ = Ag.Cl + CI When the same compound is moistened the reaction appears to be different, as chlorine decomposes the water with which it is in contact, forming hydrochloric acid (HCl) whilst the other atom of hydrogen and the oxygen atom in the molecule of water combine with another atom of chlorine to form hypochlorous acid (HCIO). If, instead of exposing the silver chloride in a dry state or in the presence of moisture, it is exposed in presence of free silver nitrate, fresh silver chloride is formed, and this same compound of chlorine and oxygen liberated ; and it is found generally that the darkening takes place much more rapidly when any body which will take up the chlorine is in contact with it. Thus, stannous chloride will cause more rapid darkening, from the readiness with which it absorbs chlorine. The student would do well to repeat the experiments of Scheele and those subsequently indicated, in order to convince him- self that these reactions really occur. The easiest method of procuring pure silver chloride is to precipitate it from a solution of silver nitrate by an excess of pure hydro- chloric acid, and to wash it thoroughly by decantation, re- peating the washing to such a point that the supernatant water shall no longer show acidity when tested with blue litmus paper. This method of procedure prevents the possibility of contamination by the organic matter of filter paper. The Silver Chloride and Iodide. 23 silver chloride, if required in a dry state, should be dried in the dark over a water-bath, in a watch-glass or porcelain capsule. A test-tube is a convenient vessel in which to give the exposure to the light, and the subsequent washings are conveniently carried out by simple agitation and pouring off the liquid. It may be noted here that perfectly dry silver chloride when exposed to light in vacuo remains white. This is probably due to the absence of a second substance in the tube, and a consequent inability to be decomposed. It must ever be remembered that there is no experiment properly carried out, with a set object in view, which is not worthy of record. The most trivial deviation from the expected results of an experiment often causes some new line of thought to be taken up, and may suggest important investigations. The next silver salt that requires a careful study is the iodide ; and it is owing to certain peculiarities in its beha- viour when exposed to light that so much difficulty has arisen in defining the true changes that take place in it. Silver iodide may be produced in two or more ways. The most common is by treating a silver nitrate solution with a soluble iodide, such as ammonium. If the former be in excess, even in minute proportions, after most careful washing, it will be found that the compound darkens slightly on exposure to light, whilst if the latter be in excess, there is no apparent change in colour. To explain this last phenomenon is somewhat difficult, but it must be remem- bered that even with the most thorough washing any salt which may have been in excess cannot be really eliminated. The iodide of the alkali is itself sensitive to light, liberating iodine, and it seems probable that, both being acted upon by light, there is merely an interchange of iodine atoms. Also it must be borne in mind that, whilst chlorine and bromine are gaseous at ordinary temperatures, iodine is solid, and cannot, therefore, so readily escape. There is also reason to believe that the molecules of iodine are more complex than 24 The Action of Light. those of the other halogens, and we can thus understand that a difficulty exists in causing it to change. The shocks which will break up the chlorides or bromides are insufficient to produce any alteration when nothing but pure iodide is present. When, however, there is an excess, however slight, of silver nitrate, the conditions are quite altered ; for then there is a compound at hand which is ready to seize any iodine which may be brought near it. Thus, when the silver nitrate is present, the molecule of iodide is at once changed in chemical composition, and a subiodide is formed in a similar way to the formation of subchloride from the chloride. Silver Iodide = Silver Subiodide + Iodine Agjiz = AgJ + I. It may here be reniarked that in one respect iodine is unlike chlorine in behaviour ; it is incapable of forming hypoiodous acid (HIO), though chlorine, as already pointed out, forms HCIO ; hence there is some difficulty in ascertain- ing theoretically the exact-reaction which takes place between the liberated iodine and the silver nitrate which is neces- sarily present to produce the change. A simple experiment, however, which it is well to repeat, throws light upon it. Take washed silver iodide, and place it in a test- tube containing in solution silver nitrate which has previously been thoroughly boiled in order to expel any air which it may contain. If an air-pump or an exhausting syringe be at hand, the boiling may be dispensed with, and the same end attained by creating a vacuum in the tube. Now expose to light ; in a short time bubbles of gas will be found collecting in the solid iodide, and with care these may be collected, and on testing by the ordinary means will be found to contain oxygen. From this we may suppose that the liberated iodine decomposes the water m contact with it (as does chlorine), and produces hydroiodic acid (HI) and oxygen. Absorbents. 25 The former combines with the surrounding silver nitrate, and we have a total reaction, as follows : — Silver Silver ^ Silver , Owtr^n a. Silver Iodide ^ Iodide + Nitrate + ^^^^"^ " Subiodide+ ^'^^?>^^ + (newly formed) + Ag,I, +AgN03+ = Ag,I + O + Agl + Nitric Acid HNOg ' If any iodine absorbent be placed in contact with washed silver iodide, prepared with an excess of soluble iodide, the reaction that takes place is apparently more simple, the iodine atom combining directly with such a body. It may thus be stated as a law that in order to produce a change by the action of light in silver iodide^ so?ne body must be present which can absorb iodine} There are one or two suggestive experiments which may impress this on the mind. The first is to silver a glass plate as if for a mirror, and then to expose it to the action of iodine vapour (as in the daguerreotype process) to such a degree, that the whole of the extremely thin film of metal is converted into iodide. On exposing such a plate to sunlight no change is visible, nor can one be brought to the cognisance of the senses by bring- ing developing agents in contact with it. If the film be not wholly converted into iodide, this result will not occur, as the metallic silver is an iodine absorbent. Another ex- periment, which is very conclusive, is as follows : Prepare a film of silver iodide, as in the wet process, and im- merse it in potassium iodide solution till any excess of silver nitrate is converted into silver iodide, and wash thoroughly for an hour, and dry. Next take a small square piece of silver leaf and apply it to one portion of the iodide surface, brushing it well down, in order that real contact may be obtained. To another small portion apply a solution of tannin in alcohol, and after drying expose the plate to the ' This law seems to have been first emphatically enunciated by Vogel, though a claim has been made by Poitevin. 26 The Action of Light. light. On developing, as indicated at page 71, a darken- ing action will be apparent after a short interval of time on those portions of the plate treated with the silver leaf and the tannin. The action will be most intense in the latter, as might naturally be expected, the whole thickness of the iodide being in the one case brought in contact with the absorbent, whilst only those particles which form the surface are brought in contact with it in the latter. The experiment is more telling if the plate be exposed behind a negative, with the uncoated side next the image. If instead of the silver leaf a thin silvered plate be pressed into firm contact with a sensitive collodion film, prepared as above, it will be found that even a fair exposure is sufficient to cause the for- mation of an image on both, which, though unrecognisable to the senses, is yet capable of being developed by the proper methods. This experiment thus serves to show conclusively that iodine is liberated by the impact of light ; for, were the change one merely of molecular arrange- ment (as many enquirers have held to be the case), no image could be formed on the metal plate, the possibility of de- veloping an image on it being dependent on the presence of silver iodide (see the daguerreotype process). The behaviour of silver bromide is similar to that of silver chloride ; hypobromous acid being formed under similar circumstances to those in which hypochlorous acid is produced. The chloride and bromide are both soluble in ammonia (which is an important point when dry plate pro- cesses are considered), whilst the iodide is not. It may be here recorded that with the chloride and bromide, as with the iodide, the presence of silver nitrate increases sensitive- ness to a very high degree. Besides the salts already mentioned there are other in- A lii Organic Salts of Silver. 27 organic compounds of silver, such as the fluoride, phos- phate, sihcate, which are altered by the action of light, but these are comparatively unimportant. There are, however, certain organic compounds formed, the action of light upon which can only be briefly noted here, though a fuller de- scription of the phenomena will be given in a subsequent chapter. When organic matter is brought into contact with a soluble salt of silver, a definite compound is often formed, and the effect of impact of light upon this is somewhat com- plex to trace. Thus, if we form an albuminate of silver by bringing a solution of silver nitrate in contact with one of albumen, and expose it to light whether there is an excess or defect of the silver salt present, a darkening of the com- pound results. The blackened compound is not a true silver oxide, though chemical considerations lead us to infer that the colouration is dependent on the formation of silver oxide, in combination with organic matter. The same results are obtained if gelatine or other kindred body is substituted for albumen. It will be as well if the student experimentally compare the effect of light on an organic silver salt with that on silver chloride, as both are employed in the silver printing process. The following experiments will naturally suggest them- selves. Take sodium chloride and dissolve in water and add an excess of silver nitrate to it, by which we have precipitated silver chloride formed; also take the same solution and allow the sodium chloride to be in excess. Carefully spread the moist chloride on pieces of glass, and expose to light. Both will readily darken, more especially the former, which will gradually assume an inky black tint, whilst the latter remains a pale violet. From what has already been said, the cause of this phenomenon will be apparent, the chlorine liberated in the first case is rapidly absorbed, whilst in the second it is merely held in 28 The Action of Light. solution, clinging as it were to the silver subchloride, and ready to reduce it back to the same state as before. If to the silver chloride, in which the sodium chloride is in excess, we now add a little stannous chloride which is ready to absorb chlorine, the blackening will proceed as rapidly in the one case as in the other. Now treat all these residues with nitric acid, and they will all be found to remain unattacked by it, but instantly yield to a strong solution of sodium hyposulphite, leaving metallic silver in small quantities behind. Next precipitate albumen in ex- cess, or otherwise of silver, and expose to light ; the darken- ing will proceed more rapidly and to a greater depth in the one case than in the other. Treated with ammonia but little alteration is visible, but on applying nitric acid, the oxide at once disappears. If, however, it be treated with sodium hyposulphite, it will remain nearly unaltered in appearance. Next treat the undarkened albuminate of silver with hypo- sulphite, and it will dissolve, leaving a milkiness in the solu- tion ; on further adding ammonia to the solution, however, this will disappear entirely. If both the darkened bodies are treated, after the sodium hyposulphite has been applied, with a solution of hydrogen sulphide (HjS), the former will blacken from the formation of silver sulphide, the latter will bleach from the formation of a new organic compound; the bearing of this experiment will be seen when we consider the fading of silver prints. Again, to a similarly treated precipitate of chloride and al- buminate add potassium cyanide; the one will be but slightly acted on, whilst the other will be speedily attacked. In deter- mining the fixing agent to employ in silver printing, this point has to be taken into consideration. If experiments with other organic bodies be carried on in a similar manner, it will be found that the same phenomena will be observed; the dis- tinction between the nature of the reduced organic com- pound will be seen in the different colours they assume From these simple experiments, then, we learn, that the Uranium and Iron Salts. 29 darkening action of silver chloride is aided by the presence of a chlorine absorbent; that the subchloride thus formed is unaltered by nitric acid (in fact the darkening action takes place as rapidly in the presence of nitric acid with silver nitrate as if the latter be alone in excess); that the subchloride is split up by sodium hyposulphite into metallic silver and silver chloride, the latter being destroyed by it as shown at p. 74. That in organic matter which forms a compound with silver nitrate, when acted upon by light, the silver is reduced to a state of organic oxide, and that the presence of an excess of silver nitrate is not absolutely necessary ; that the dark- ened compound is unaffected by sodium hyposulphite ; that potassium cyanide is a solvent of this oxide, and not of the metallic image formed from the sub-chloride. The next metallic salts to which we shall refer, in regard to their behaviour when exposed to light, are those of iron and uranium. Their reactions are almost precisely similar. To Sir John Herschel we owe most of our knowledge of the iron compounds ; whilst to Niepce de St. Victor is probably due the discovery of the particular properties of uranium. If we brush over a piece of paper a neutral solution of ferric chloride, and, after allowing it to dry, expose it to light, the yellow colour imparted to it will be found gradually to disappear, leaving the surface appa- rently bleached. If, now, we allow a solution of potassium ferri-cyanide to flow on to the exposed paper, it will be found that a deep blue colouration is immediately produced, whilst if applied to the unexposed paper no such phenomenon would be observed. From chemical experiment we know that, in order to produce the blue precipitate, it is necessary to have in contact with the potassium ferricyanide some ferrous compound. Since it was a ferric compound, viz. ferric chloride, which was applied to the paper, we are led to con- clude that the action of light has been to reduce this salt to the state of ferrous chloride. By similar experiment we become convinced that the 30 The A ction of L igh t. action of liglit on all ferric salts, under certain conditions, is to reduce them to the ferrous state. It may be remarked that in order to produce the requisite reduction, the presence of organic matter, such as the size of the paper, with some of these iron salts seems a necessity; if this be absent, the action is very slow. And, again, the organic compound should be of such a nature that it is ready to combine with the atoms thrown off, in the same way as that already indicated for silver iodide. There are a variety of bodies which will combine with these atoms ; but unfortunately, as a rule, they have a greater affinity for the atoms than has the iron compound with which they are only loosely combined. The organic matters with which they will combine without being torn away from the iron are rather slow absorbents, and therefore generally the sensitiveness is not great. For, as with the silver iodide, the sensitiveness depends chiefly on the readiness of the neighbouring matter to absorb what is thrown off. In order, then, for iron salts to become as sensitive to light as silver salts, some body must be found which, per se, will not reduce them to the ferrous state or decompose them, yet which, when the atom is liberated, will seize it with greater facility than any body with which we are as yet ac- quainted. As a rule, the development of these pictures is carried out by either method 2 or 4 (p. 19), the details of which will be given in the section on printing with these salts. Since these compounds are comparatively but little sensitive to light, they are chiefly used for obtaining positive prints; an exposure in the camera to produce a developable image would have to be very prolonged. The same experiments carried out with regard to the uranium' compounds give identical results. The uranic compounds are reduced to uranous, and the methods of development are similar. To the same class of metals belongs vanadium, the interesting compounds of which were investigated by Pro- fessor Roscoe. The reactions are similar to the above. On Chromium Salts. 31 The last metallic compounds to which we shall refer at length are those of chromium combined with the alkalis. The salts found most sensitive to light are the dichro- mates, though the chromates are also, to a certain extent, capable of being acted upon. Mungo Ponton first indicated the principle which governs their employment. If a solu- tion of a dichromate, such as that of potassium, be brushed over paper, and be allowed to dry, and be then exposed to light beneath an engraving, it will be found that in those portions corresponding to the white paper the orange colour will gradually assume a delicate brown tint, whilst on the parts shaded by the lines the salt remains unchanged. The eye then at once tells that some chemical change has taken place in the chromium compound. Chemists are accustomed to employ the dichromate to convert a ferrous salt into a ferric, and by having it in a solution of known strength, and ascertaining when the reaction is complete, the amount of iron in the ferrous solution can be estimated quantitatively. Thus we have, say, the amount of ferrous chloride to test quantitatively : the amount is calculated by applying the following equation : Ferrous Potassium _^ Hydrochloric _ Ferric ^ Potassium ^ Chloride Dichromate Acid "Chloride Chloride 6FeCl2+ KjCrA + 14 HCl^ =3Fe2Cl«+ 2 KCl + Chromic Chloride + Water Cr^Clg + 7 H2O It will be seen that the potassium dichromate readily parts with its oxygen and potassium, and becomes converted into a pure chromium compound. The change induced by the light is analogous to this, there being every reason to believe that the following equation is a type of the reaction, though carbon dioxide may be a product to form potassium carbonate — Organic Matter + Potassium Dichromate = Potassium Hydroxide + C^UyO^ + K^Crp, = 2KHO + Chromic Oxide + Organic Matter Cr^Oj + Cx^y~2^z-\-s 32 The Action of Light. An analogous reaction of a chromium salt in the presence of an organic compound, without the impact of light, is found in chromium trioxide. If alcohol be dropped on these dry crystals, oxygen is evolved so rapidly that the spirit is ignited by the energy of the act of combination. Now the dichromate contains less oxygen than the acid (H2Cr04) formed by the trioxide (CrOg), hence the evo- lution of oxygen from it is likely to be less easily effected by organic matter than from the latter. The swing caused by the waves of light is sufficient to effect the change, that is indicated by the equation above. It will be noticeable that not only is the chromium compound altered in composition, but that also the organic matter is deprived of hydrogen ; and it is the fact of this deprivation, or change in organic matter, that renders the dichromates valuable for photogra- phic purposes. It will be found, after experiment, that the dichromatised paper prepared as above is nearly insensitive when moist, and that the image can be formed most readily when it is dry. The reason of this is probably that when dry the organic matter and dichromate form a real compound, which is, however, readily split up on remoistening. If, however, the contact be long continued, an alteration in the position of the atoms of the molecules probably commences. This might account for the insolubility of old carbon tissue, and it may be presumed that the change which is rapidly effected by light is much more slowly accomplished by the long contact even in the dark. The development of pictures taken on ordinary sized paper is usually effected by method 4 (p. 20), and will be noticed when treating of the aniline process. When, however, the paper is coated with a layer of organic matter, such as gelatine or albumen, the development of the picture may be effected by methods 6 or 7. Colloidal bodies available for photographic purposes, when oxidised, are changed in physical as well as in chemical properties, ist, they cannot after oxidation be dissolved by water, either On Organic Bodies. 33 hot or cold, though before oxidation they may be easily soluble. 2nd, they will not absorb water, and consequently will not increase in bulk, if the impact of light be pro- longed. These modes of development will be entered into fully when treating of the carbon and collotype printing processes. It is scarcely necessary to refer to the salts of other metals ; they are mostly too insensitive to the action of light even for contact printing. Robert Hunt, in his excel- lent ' Researches on Light,' has entered fully into the phenomena observable with most of these compounds, and the student should study that work for further information. Of organic bodies there are a variety which respond to the chemical vibrations. First and foremost, as being of prac- tical utility, is the substance known as asphaltum, or bitumen of Judaea. It is the substance which was first employed by Nie'pce for practical photography, and it still retains its place amongst useful photographic compounds. It is readily soluble in a variety of menstrua, such as benzole, chloro- form, and turpentine. After exposure to light, it loses its excessive solubility, and it is not only possible, but practi- cable, to dissolve away from a thin layer of it all those por- tions which have not been acted upon by light. For certain photo-engraving and relief-printing processes it is still em- ployed, on account of its resistance to the action of acids (see p. 197). It seems that during exposure it becomes oxidised to a certain extent. Amongst other sensitive organic compounds may be named the extracts of flowers and leaves and certain dyes. The sensitiveness of the last has recently been found to be of use in what is known as ortho-chromatic photography. Among gaseous bodies which are sensitive to light we may name chlorine, when exposed in the presence of hydro- gen. If in a dimly lighted room the proper combining volumes of these two gases be mixed in a glass bulb, or other convenient holder, and then exposed to the direct rays of D 34 The Action of LigJit, the sun, or other strong source of light which emits the shorter wave-lengths, it will be found that tliey combine with explo- sive violence to form an equal volume of hydrochloric acid. In diffused daylight the combination takes place much more quietly, and attempts have been made to utilise this action to measure the ' actinism ' of any light to which it may be exposed (see ' actinometry '). The affinity of chlorine for hydrogen is so great that it causes a decomposition of the water in an aqueous solution, when exposed to the light, though it has no power to do so if kept in the dark. The latest discovery of light causing a combination between a gas and a solid is due to Mr. Francis Jones, of Manchester. He found that in sunlight, if sulphur was brought in contact with antimoniuretted hydrogen or stibine, the orange sulphide of antimony was formed. The equation representing the re- action is as follows : — Antimoniuretted Hydrogen + Sulphur = Antimony Sulphide + Hydrogen Sulphide 2 SbHg + 6 S = Sb^Sj + 3 H2S. With aiseniuretted hydrogen (As H3) a like reaction takes place.' After this brief resume of the sensitive compounds the student will at once distinguish the advantage to be gained by the employment of the simpler salts of silver for obtaining images in the camera. It is these alone which are susceptible of rapid development by exercising an attractive force when the altered molecules are few in number; whilst all the other compounds require a large number of particles to be changed in order that the image may be made visible at all. With the iron salts per se the development by attraction may be resorted to ; but it will be found on experiment that the attractive force is so small that it does not nearly equal that of the silver compounds. Hence we may assert that, for pro- ducing developable images in the camera, the chief portion of the sensitive salt must consist of one of these silver salts, ' As being substituted for Sb in the above equation. On the Support and Substratum. 3 5 and that other metallic salts can best be utilised for obtaining impressions by long exposure, and are therefore chiefly adapted for obtaining positive proofs from negatives. ' CHAPTER V. ON THE SUPPORT AND SUBSTRATUM, In judging of the kind ot support on which to receive an image, whether it be developed or formed by the con- tinued action of light, it must be considered for what pur- pose the image is to be employed. If it is to be em- ployed as a screen, or a negative from which to form a picture complementary to it in photographic density and position, then evidently the more transparent the support is, the better it will be for this particular purpose. As a rule, it is only images impressed in a camera which are employed as ' negatives,' and as these may be said to be invariably taken upon the sensitive salts of silver, which are easily acted upon chemically by extraneous matter, it is evident that a substance should be employed which is unaf- fected by them and by the agents which cause the develop- ment of the image. In addition to this quality, a certain amount of rigidity in the support is convenient, though not essential, as the operations involved are of such a character as to cause this to be a desideratum. Evidently glass answers the object most thoroughly. Less fully does paper when waxed answer these requirements ; for with it there is trans- lucency and not transparency, and none of the other qualities. A support of collodion and india-rubber com- bined, such as was at one time advocated by Warnerke, answers to the first two requirements, and, since the opera- tions involved in his method of working do not necessitate rigidity, it is a suitable one. 36 On the Substratum. To give a correct idea of an image by reflected light, that is, to look at it as a picture is looked at, it should appear as highly coloured or as black as possible when contrasted with the ground on which it rests. Formed by develop- ment (after exposure in the camera), it possesses always more or less density, the density approximately varying inversely as the intensity of the reflected actinic light which has acted on it. If the developed image be of a dark colour, the proper effect of light and shade will be reversed. To give a correct representation, the deposit must be transparejttly white, and the support dark- coloured or black. In the daguerreotype process the support is a metal plate, which is by contrast dark when compared with the mercurially-developed image. In the collodion pro- cess, if the image can be made to appear white by reflected light, the support may be of glass, if it be backed with some dark-coloured substance, such as black velvet or var- nish, or it may be of metal darkened with some substance that is unaffected by the chemical agents employed (an example of this we have in the ferrotype plates). On the other hand, if the picture be produced by a subse- quent operation from a negative, the image should be as transparently dark as possible, and the ground white. In this case the support (unless other considerations forbid it) may be white paper, opal glass, or any other white medium. It is also worthy of notice that in order to pro- duce a proper representation of light and shade the ground should always be in contact with the image. An example of a certain false gradation given in an image, in which this important rule is neglected, is to be found in the old collodion positives on glass. Heretofore nothing has been said regarding the vehicle employed for holding the sensitive compounds in situ on the support, and this requires a detailed consideration. It need scarcely be said that some sort of vehicle is generally necessary. The fact that most of the compounds employed On the Support. 37 for photographic purposes are sohds, and whether formed by precipitation from or evaporation of a solution are in a pul- verulent state, at once clearly demonstrates the necessity. We must distinguish between two cases. 1. In the case in which the image is formed by develop- ment, it is essential that the developing agent should cause no chemical change or discolouration in it any more than in the support. Now the sensitive compounds may be formed in the support itself Thus a solution of potas- sium bromide brushed on to paper, and then followed by a similar application of silver nitrate, will cause the forma- tion of silver bromide in the paper. The paper will in this case act as a support and vehicle too. An invisible impressed image can be developed on it, but it would be found that it is Hable to stains due to the organic matter of the sizing. It is also only translucent and not trans- parent. And it is therefore, as a rule, unadvisable to use it for holding the sensitive compounds in situ when 'nega- tives ' are required. Again, if the support be not at the same time a medium, it is essential that the latter should adhere to the former during the operations of development. 2. When the image is to be formed entirely by the direct action of light, the above conditions are not a necessity. Then paper will be suitable for a medium, as there is nothing extraneous to the sensitive compound to act upon it. At the same time there is nothing to forbid the employment of all other vehicles which are not acted upon by the sensitive compound. In the production of camera pictures collodion was till recently alone' employed ; and it is particularly suit- able for holding precipitable silver compounds in position, as it is a ready solvent of most of the bodies which it is neces- sary to use, and the precipitation can be affected in the viscous collodion itself The silver compounds are thus formed in a finer state of division than they would be if precipitated from an aqueous solution. This point is most important, for ' For exception see the geiatino-bromide process. 38 The Daguerreotype. light impacts upon surfaces : and as the surfaces of similar particles increase as the square of the diameter, whilst their masses increase as the cube, it is evident the smaller the particles are the larger will be the available area for the same quantity. Collodion is a transparent, semi-viscous fluid, made by dissolving pyroxyline (gun-cotton) in a solution of ether and alcohol, and is for ordinary purposes totally unacted upon by the sensitising solution of silver, though, when specially prepared, it is beUeved that an organic compound of silver is formed. Other advantages of collodion are to be found in the property it has of setting in a gelatinous form, previous to its final desiccation, and also that the sol- vents used evaporise rapidly at ordinary temperatures, leav- ing the salts, which are to form the precipitate, if not in solution, yet with their individual particles in such an ex- tremely minute state of division as to be undistinguishable to the eye. Aqueous solutions of gelatine and albumen are equally good solvents of the salts alluded to. When, however, gela- tine is employed, as in the gelatino-bromide process, the direct combination between it and the silver is avoided. CHAPTER VI. THE DAGUERREOTYPE. Under the head of silver processes, the first that would naturally occupy the attention is that due to Daguerre. It is even at the present date adopted for some kinds of work; for instance it was employed by the French expeditions, sent to observe the transit of Venus in December 1874. The daguerreotype process consists, as already stated, of the formation of a sensitive surface of silver iodide, or silver iodide and bromide, on a silvered plate, by means of the direct Manipulations. 39 action of iodine, or iodine and bromine. One of the most difficult (and difficult only because the greatest cleanliness in every detail is required) parts of the whole process is the pre- paration of the silvered surface before its coming in contact with the halogen. The plates are usually copper, on which a film of metallic silver is deposited by the electro-plating pro- cess, and when they leave the silvering solution have, as a rule, a frosted appearance. After being cut to the proper size, and the corners clipped of about ^th of an inch for convenience' sake, they are ready for polishing. A plate in this state may be placed on a flat table, and four thin strips of wood nailed round it to prevent it slipping. To the surface is then applied tripoli powder in alcohol with Canton flannel, and worked about to such an extent that it is per- fectly free from scratches, and is fairly smooth. The next operation consists in polishing it. I'his is effected by means of a buff. That which the writer has found effective is made by enclosing a wooden ball, of the size of a small apple, in a skin of felt and then of cotton wool. Over this is stretched a piece of the finest Chamois leather. On to the surface of the silver is then scattered a small quantity of jeweller's rouge, and the buff is caused to travel over the plate from end to end and side to side alternately till it becomes of the highest polish. This polishing should take place almost im- mediately before the sensitising operation is commenced, otherwise there is a liability of the „ surface attracting impurities from the atmosphere. To sensitise the required. An illustration of that ^ i.., . employed when daguerreotype was commonly practised for portrait work will give an idea of the sort of contrivance required. On the bottom of the box c is placed iodine in powder ; a is a piece of card- board, which fits into grooves as shown, b b are the sup- plate two sensitising boxes are 40 TJie Daguerreotype. ports on which the silvered plate is to rest.' The iodine will volatilise at ordinary temperatures, and condense on the surface of the cardboard next to it. When a plate is to be sensitised the cardboard is reversed, and the iodine volatilises from the top surface on to the silver plate. The plate gradually receives a thin coating of iodide, passing through various stages of colour. When a ruddy colour is reached, it is placed m a similar box to that already described (omitting the cardboard), at the bottom of which is a mix- ture of bromine and calcium hydrate. The bromine attacks the surface, and with the iodide forms silver bromo-iodide. When the surface assumes a steel-gray or violet colour the plate is removed, and once more placed in the iodine box for a third of the time originally necessary. In this state the plate is exceedingly sensitive, and is ready for exposure in the camera. The exposure may be made at once, or it need not take place for several hours; Claudet, in fact, found that, by keeping, the sensitiveness increased. The time necessary to impress an invisible but developable image is very short, a few seconds being all that is necessary. Practice alone can tell the exact time required, but it is soon learned approximately after a few trials. The development is ac- complished by exposing the impressed surface to the vapour of mercury. A cast-iron tray, with wooden sides and lid, is convenient : it may form a box similar to that shown for the iodising operation. At the bottom is placed a thin layer of mercury, the temperature of which is raised to about 150° Fahr. The plate is placed in the box, face downwards, on the supports, and the development is allowed to proceed, the process being watched as it progresses by inspecting it from time to time in a non-actinic light. If the exposure be riglit the image will be brilliant, if under-exposed it will be weak ; whilst if over-exposed it will be covered with a veil of mercury. ' When smaller sizes are to be used they may be held in frames similar to the inner frames of a camera sHcle. Intensification of the Image. 41 The development, it will be remarked, is due to the attraction of the subiodide for the metallic mercury vapour, and to no other cause. In order to fix the image the plate is immersed in a 10 per cent, solution of sodium hypo- sulphite. After a few seconds the unaltered iodide Ag2l2 and the Agl of the subiodide (Agal) are dissolved away, and the image is left as a white amalgam of mercury and silver on a darker coloured background. After a thorough washing in distilled water the picture is permanent, but its appearance may be improved by toning it ; i.e. intensifying it with gold to darken the silver, and render the amalgam still purer in colour. This is accomplished by pouring over it, in such a quantity as just not to run over the edges, 1. Gold trichloride .... "i gramme. Distilled water 50 cc. 2. Sodium hyposulphite ... '4 gramme. Distilled water . . • . 50 cc. The two solutions are well mixed together, and, after flowing them on the plate, a spirit-lamp is moved about beneath its bottom surface, until the toning action com- mences. The more rapid the deposition of the gold, the more satisfactory the image. When complete, the plate must be well washed in a dish of cold water, and finally rinsed with distilled water. Drying is best accompUshed by gentle heat, applied first at one end, and gradually moved down. Any large drops of water should be absorbed by blotting-paper. Daguerreotypes may be reproduced by electrotypy, if the plate be immersed almost immediately after toning in the copper solution. The ordinary electrotyping process answers every purpose : for the details, reference must be made to books treating specially of the subject. The fact is mentioned here, as it shows that the image, after all these operations, is in relief, though naturally to a very limited extent, yet still sufficiently to cause the reflected light to give all the necessary gradations of light 42 TJie Collodion Process. and shade. Sir W. Grove also introduced a method of etching daguerreotype plates by means of the battery, immersed the plate in a solution of hydrochloric acid two parts, and water one part, and opposed by a platinum plate placed at "2 inches from it. When the current was generated by a couple of Grove's cells, an oxy-chloride of silver was formed, and after thirty seconds the plate Avas found to be sufficiently bitten. The cxy-chloride was removed, and for fine work was found of sufficient depth to allow it to be printed from with printer's ink in the printing-press. This process has not come much into vogue, as it is one which is too delicate for ordinary opera- tions, and the silvered copper is expensive in comparison with the other metals employed for the purpose. The most recent development of photo-engraving and the production of reliefs are described in a subsequent chapter. CHAPTER VIL COLLODION. Pyroxyline is prepared by acting upon cotton, paper, or other kindred substances with a mixture of nitric and sulphuric acids. For an example of the process we may take cotton, which has a definite formula of CgHioOa. Sulphuric acid has the property of absorbing water from any organic substance with which it is in contact; for instance a drop of oil of vitriol on cloth or paper rapidly chars it, owing to the destruction of the constituent atoms, through its affinity for water. Thus, if we take the cotton itself, it will be seen that each molecule contains 6 equivalents of carbon, and just sufficient hydrogen and oxygen to form 5 molecules of water ; the oil of vitriol is thus capable of splitting up the molecule of cotton, appropriating the 5 molecules of The Chemistry of Gun Cotton, 43 water, and leaving the carbon behind.^ Another good ex- ample of the abstraction of an equivalent of water from a molecule is in that of ethyl alcohol, or spirits of wine. If this be distilled over in the presence of concentrated sulphuric acid, we have ether ^ as the product. When the acid is diluted with water, its destructive power is limited, though as the water evaporates from it the power returns. Now the strongest nitric acid which is usually obtainable contains a large proportion of water. Thus nitric acid, if 1-457 at 60° F. contains only 84 per cent, of HNO3, hence it is that when this is mixed with sulphuric acid, the water is abstracted from it, and the true nitric acid (HNO3) is left to act on any body with which it is brought in contact. This is undiluted, and is capable of acting on cotton in a some- what peculiar way. It abstracts either 2 or 3 atoms of hy- drogen (according to the strength of the acids employed, and the temperature), and replaces them by 2 or 3 molecules of nitrogen tetroxide (NO2) with the formation of water. The formula stands thus : — ^ . ., n 1 i' • -J Water combined with the Cotton + Nitric acid + Sulphuric acid + ^^j^^.;^ sulphuric acids C6H,o05 + 2 HNO3 + HjSOj + Aq. Water formed by the decomposi- = PyroxyHne -t- Sulphuric acid + tion of the cotton, and that combined with the acids = CeH3(N02)A+ H.SO, + 2H,0 + Aq. Or, Cotton + Nitric acid + Sulphuric acid + Water = Gun Cotton + C6H,A+3HN03+ H.,S04 + Aq = C^H^lNO^laOs + Sulphuric acid -l- Water + Water. H2SO4 +3H2O+ Aq. > It is for this reason that if the most dilute sulphuric acid be spilt on the clothes, or passed through a filter-paper, and be allowed to dry, a charring takes place. In the first case neutralisation with an alkali, or in the second very thorough washing, will prevent the disaster. ^ Hence the name sulphuric ether. The following equation exhibits the reaction. Ethyl alcohol = Ether -I- Water abstracted by the sulphuric acid. C,H,A =C,H,oO+ H^O 44 Collodion Processes. The first being the gun-cotton, as used for collodion, and the second being the well-known explosive com- pound. It will be noticed that the sulphuric acid remains unaltered in composition, its sole function being to ab- sorb the water formed by the operation. The fact of the existence of the tetroxide of nitrogen in the altered cotton can be demonstrated in its combustion in an exhausted glass vessel by the red fumes which tinge the gaseous pro- ducts. The same reaction as the above can be obtained by em- ploying potassium nitrate (KNO3) instead of the nitric acid, though in this case a portion of the sulphuric acid becomes converted into potassium sulphate. Thus : — Potassium nitrate -I- Sulphuric acid = Potassium sulphate + Nitric acid. KNO3 + H2SO4 = KH(S04) + HNO3. The above equations represent the reaction that theo- retically takes place when cotton is treated with nitric and sulphuric acid in the above proportions, but there are other points to be attended to in practice. The proportion of the acids to each other materially affects the properties of the pyroxyline. Sulphuric acid parchmentises paper when it is immersed in it or floated in it, that is, renders it tough and of close texture. The chemi- cal effect produced by the sulphuric acid is hardly known, but if prolonged, it is known that the paper is dissolved. Parch- mentised paper treated with nitro-sulphuric acid has different qualities to that in which the parchmentising is omitted. With the former a tough collodion results, though it is more pow- dery. An excess of sulphuric acid beyond that necessary to produce the reaction shown in the equations acts in a similar way to treating the cotton first with the acid, for it partially parchmentises the cotton previous to its conversion into pyroxyline, and as this is beneficial to a collodion, vi'hen not carried beyond proper limits, an excess of this acid is always employed. The amount of dilution of the acids with water also Eifect of the Dilution and the Temperature of Acids. 45 largely modifies the resulting compound. When little or no water is added, the pyroxyline gives an unevenly flowing collodion which is strongly contractile when drying. When a large proportion of water is added, the collodion is limpid, flows readily, and is apt to give a matt appearance on drying. By increasing still further the amount of water, the cotton when immersed will entirely dissolve in the acids. Evidently then a mean between no water and the amount necessary to produce dissolution should be employed. The effect of the temperature of the acids on the cotton is also marked by the behaviour of the resulting pyroxy- line, as the effect of heat is to aid chemical change. Pyroxy- line made at low temperatures forms a collodion that is al vays glutinous and difficult to flow over a plate, whilst the higher the temperature the more easily will it flow. It should be remarked that the same effect is produced by the addition of more or less water to the acids. It is, there- fore, possible by diminishing the amount of water and increasing the temperature to obtain the same amount of fluidity in a collodion as would be gained by the full amount of water at a lower temperature. With the above facts before us we can evidently manu- facture various qualities of pyroxyline which may be suitable for different purposes. With the wet process, where a so- lution of silver nitrate comes in contact with the soluble iodides, &c., dissolved in the collodion, its conditions should be : — Firstly, that it should be fairly porous ; and, secondly, that it should be fairly tough. This is efl"ected by adding a moderate proportion of water to the mixed acids, and by immersing the cotton in it at a medium temperature. The following are the proportions which Hardwich (who was the first to thoroughly investigate the manufacture of pyroxyline fit for collodion) states should be observed : — Sulphuric acid, sp. gr. 1-842 at 15° C. . 500 cc. Nitric acid, sp. gr. i -456 .... 166 6 cc. Water . . . 1457 cc. 46 Collodion Processes. The nitric acid and water are first poured into a strong glazed porcelain dish, and well mixed, the sulphuric acid is added last, the Hquid being kept well stirred as it is poured in. The temperature will generally rise to 75° or 85° (if to the latter, it may be suspected that the acids are too dilute), and it must then be allowed to cool gradually to 65°. A dozen balls of cotton wooV weighing about i-| grammes each, having been prepared, should be immersed separately in the fluid, and after thorough soaking (assisted by a glass or porcelain spatula, fig. 9), be allowed to remain at the bot- tom of the vessel. The immersion should take place rapidly, otherwise decomposition takes place, and this, when once commenced, will cause the temperature to rise rapidly, and the whole of the cotton will be dissolved with the evolution of nitrous fumes. The balls must be left in the acid from ten minutes to a quarter of an hour, and they are then presum- ably in a state ready for washing. The longer the immer- sion, the more likely are they to become insoluble in ether and alcohol, approaching more nearly the state of explosive gun-cotton. They are next raised by the spatula, the excess ^ of acid as far as possible squeezed out of them against the side of the vessel, and then they are dashed into a vessel holding a large quantity of water. All traces of the acids are eliminated by washing in frequent changes of water, or, better still, in running water. To test when this is complete, a piece of blue litmus-paper should be pressed against the wet cotton, and if after two minutes it remains unaltered it may be assumed that the washing is complete. The pyroxy- line should now tear easily, and not be readily separable into the original balls, and should weigh about 30 grammes. If the original fibre be easily distinguishable, the temperature probably fell during the operation, or sufficient water was ' The cotton should have previously been well steeped in soda and water, and be then thoroughly washed and completely dried. - If the precaution be not taken of squeezing out the acids, there is a great probability of a solution of a portion of the cotton taking place. Manufacture of Pyroxyline. not added. If the weight fall much below that indicated, the water was probably a little in excess, or the temperature was too great. It cannot be too strongly impressed upon the student that the strength of acids is all-important, and if the amount of water present with them be above that indicated, that so much water must be deducted from that given in the formula. A specific gravity bottle is a very convenient means of ascertaining the strength of the solu- tion, for, the specific gravity once known, the amount of true acid present can be found from the tables given in the ap- pendix Other methods for ascertaining the specific gravity will be found in most works on Chemistry. The next formula for preparing pyroxyhne of the same character is given without comment, as the above remarks apply to it. Sulphuric acid, 1-842 ... . . 170 cc. Dried potassium nitrate ' . . . 1 10 grammes Water ...... 28-3 cc. Best dried cotton wool ... 4 grammes. Hardwich states that the chances of failure with this process are very slight if the potassium nitrate be not too much contaminated with potassium chloride. In the above operations a thermometer is absolutely necessary. It should not be mounted in wood, but should be graduated on the stem itself It may be supported in a clamp, as shown in the figure. For dry processes the foregoing formulae give pyroxyline, which some consider as too tough and homy, and some hold, though the writer does not, that this is especially the case for processes where the sensitive salt of silver is formed in the collodion itself (see chap. xvi.). A modification in the proportions of acid and water can be made to suit those who prefer a more limpid collodion. It has also been found by some workers that in the latter process the presence of a little nitro-glucose is a desideratum. ' The potassium nitrate should be dried at a temperature of about 120°. Placing it in an air-bath is the most convenient method of obtaining the temperature. ■48 Mamifactiwe of Pyroxyline. The following method secures its formation, though, if the resulting pyroxyline be well washed, it is in a great measure eliminated. It would seem better to add the nitro-glucose to the collodion, but as this has not been established from long experience, it has been thought better to give the pro- cess as published by M. Leon Warnerke, in a communi- cation to the Photographic Society of Great Britain. Six grammes of the finest cotton-wool are put into a porcelain jar, and 2 grammes of gelatine dissolved in the smallest quantity of water are added. The cotton is impregnated with the gelatine by pressing it with a wooden spatula, and when this is effected the cotton is carefully dried by the aid of heat. It is then ready for immersion in acids which are of the following strength : — Nitric acid, I -45 . > . . . . , 175 cc. Water . . . . . . . 68-3 cc. Sulphuric acid, I '840 ..... 262 -5 cc. Fig. g. Or, Nitric acid, i -42 . Water Sulphuric acid, 1 '840 . . 194' I cc. . 49-2 cc. . 262"S cc. Solvents of Pyroxyline. 49 The acids and water are mixed in the order named, and when a steady temperature of 70° is obtained the gelatinised cotton is immersed in it for twenty minutes. With some cotton the amount of water given above is inadmissible, as it immediately dissolves. The proportions of acids should be kept, and the water diminished to such a degree that the solvent action is reduced. After washing and drying, the resulting pyroxyline will be found to have lost considerably in weight, and it should be almost powdery in appearance, and readily disintegrable. It will be found highly soluble in a mixture of ether and alcohol, and as much as 2 per cent, may be required to give a sufficient body to the collodion. Hitherto cotton has alone been mentioned as capable of forming pyroxyhne ; but it may be stated that every analo- gous substance may be similarly treated. Thus linen and paper are amenable to the above treatment, and for some purposes they give superior results ; for instance, Whatman's drawing paper has been found by Warnerke to give better results than the gelatinised cotton in the last process. Being already sized with gelatine, there is no need for the preli- minary treatment pointed out. The action of the solvents employed in the collodion on . the pyroxyline deserves a passing remark, as many modifi- cations in the resulting film can be caused by judiciously varying their proportions. The specific gravity of the alcohol employed should invariably be ascertained, as the condition of the sensitiveness of the plate depends much upon its strength. With a collodion made at a low tempera- ture, the presence of a certain percentage of water is advis- able, as its horny nature is thereby modified, and a certain degree of porosity obtained. A specific gravity of '820 is in this case admissible. With the pyroxyline such as that obtained by the last formula, the water should be a minimum, as it is already porous, and the presence of water is apt to make it reticulated and rotten. The specific gravity in this case should rarely be over -812. An excess of alcohol £ 50 Collodion Processes. also tends to give porosity, and therefore sensitiveness; but if the addition be carried to an extreme, the very porosity dimi- nishes sensitiveness, as the sensitive salts formed in the film coagulate into too large particles. Alcohol also diminishes the rapidity of setting. Ether, on the other hand, tends to close the pores of the film, as is demonstrated by coating a plate made with an excess of it, when it will be found that a con- traction takes place, causing the film to leave the edges of the plate, or to split on drying. The ether employed should be as pure as possible (this is not insisted on by manufac- turers of collodion) as otherwise it is apt to liberate the halogen from the dissolved salts, giving rise to an alkaline reaction which is one cause of rottenness in the film, and an apparent want of body in collodion.^ The following are collodions for different processes. For the wet process : — No. I. Pyroxyline, Hardwick's formula Alcohol, '820 Ether, 725 No. 2. Pyroxyline, Hardwick's formula Alcohol, -820 Ether, 725 No. I is most suitable for cold, and No. 2 for warm weather. 12 to 14 grammes 450 cc. 550 cc. 12 to 14 grammes 500 cc. 500 cc. For dry processes with the bath : — No. 3. Pyroxyline, first formula Pyroxyline, last formula Alcohol, -813 or -814. Ether 725 Water 10 to 12 grammes 4 g-rammes 500 cc. 500 cc. Quant, sufif. The water is shown in No. 3 to remind the student it puts a power into his hand of naodifying the collodion in structure by its addition. It frequently happens that No. i or 2 ' The student would do well to try the experiment of adding a small quantity of caustic potash to a phial of collodion, and noting the action that takes place. On the Salts dissolved in Collodion. 51 formula may also be improved for dry processes by attend- ing to the amount of water present. The next point to be determined is the amount of bromide and iodide to be dissolved in the collodion, and to determine their proportions it will be well to enter into detail as to their behaviour when converted into the silver compounds and exposed to the light. Iodide of silver in a film is capable of forming a dense image with a short ex- posure, but the gradations in density are often wanting when the light is extremely bright; added to which, if organic matter be present with it even such as is to be found in many collodions, the picture is apt to be veiled and wanting in vigour. Bromide of silver, on the other hand, is especially adapted for those collodions which have an organic re- action. It has usually been accepted that the iodide is the more sensitive of the two salts, but recent investigations tend to show that the bromide has the advantage, both as regards sensitiveness and delicacy, when developed by method 3 (p. 19). The failure of the bromide when developed by method 2 (p. 19), which is wet-plate de- velopment, consists in its comparative insensitiveness to very faint light as found in deep shadows. A bromo-iodide of silver, however, combines the advantages of the bromide with that of the iodide ; for the wet process and certain of the dry processes it possesses every essential quality for the production of a good picture. The proportions of bromine and iodine in combination vary considerably, from i part of the former to 10 parts of the latter (which is just suffi- cient to secure cleanliness and freedom from veil with all ordinary preparations of collodion and bath) to 25 parts to I. The latter proportion is never employed except in dry-plate processes. The iodide is usually fixed at about from 6 to 10 grammes per litre. The sensitiveness of the surface in all cases depends on the mode of development employed. Thus for a wet plate, Vogel has found that 52 Collodion. the proportion of iodine to bromine should be about 4 to i to secure the greatest sensitiveness, whilst with the alka- line method it is diminished to the smaller proportion, or it may be omitted altogether when a dry process is in question. At present we are considering the wet process, and not the more modern dry processes where the conditions are different. The metal with which the iodine and bromine are combined when introduced into the collodion serves to exercise a great influence on the sensitiveness of the sur- face. Not long ago Warnerke has stated that the metals combine with the pyroxyline and form compounds whose composition is as yet undetermined, and thus the difference in structural effect and viscosity exhibited between two identically similar collodions when iodised with a cadmium and an alkaline salt may be accounted for in a great measure. For experiment, it will be advantageous if the student iodise two portions of collodion ; one with 4 grains of cadmium iodide, and the other with 4 grains of potassium iodide, and note the difference in their behaviour when poured on a plate. With the latter he will find a freely flowing fluid; with the former one which is more glutinous, and difficult to manipulate. It also appears that the different metallic salts in solu- tion cause different degrees of sensitiveness in a film. This has been investigated by Warnerke, who places them in the following order for imparting sensitiveness and intensity: — Order of sensitiveness . . . Zn Cd Na Fe NH4 K U Order of intensity of image . Zn U NH4 Cd Na K Fe The alkaline iodides are those which are. most prone to decompose under the action of ether, particularly if it be methylated, hence, for a collodion to keep long, it is neces- sary that the purest form be employed. As before shown, when the iodide is decomposed, the alkali decomposes the pyroxyline, rendering it very fluid and defective in setting Formula for Sailed Collodion. 53 qualities, whilst the iodine itself increases the density of the image, probably by the formation of a silver iodate. In bromised collodion it is very rare for bromine to be set free, and in bromo-iodised collodion the dark colour obtained by long keeping is invariably due to the iodine liberated, for uncombined bromine will always displace iodine. To increase the density of a developed image it is always advisable to add a little tincture of iodine to a collodion. In choosing the iodide or bromide of any particular metal for iodising or bromising a collodion, it must be remembered that it is the amount of iodine and bromine that are the essentials, and not the metal. Hence, 4 grains of ammonium iodide and 4 grains of cadmium iodide mean a totally different quantity of iodine. The amounts may be calculated from the combining weights given. The following formulae will be found to give collodion suitable for the ordinary wet process: — No. I. Ammonium iodide ..... 7 grammes Cadmium bromide . . . . .4 grammes Plain collodion ' . . . . .1 litre. No. 2. Ammonium iodide 8 grammes Cadmium bromide 2-5 grammes Plain collodion i litre. No. 3. Cadmium iodide 9 grammes Cadmium bromide 4 grammes Plain collodion i litre. Nos. I and 2 are speedily ripe enough for use; with a little alcoholic tincture of iodine added they may be em- ployed immediately. No. 3 requires keeping, as at first it will not flow freely. A sample of collodion such as No. 3 has been kept two years without deterioration, the precaution being taken to keep it in the dark and in a cool place. It must be borne in mind that the collodion may be made by i, 2, or 3 formula, and the pyroxyline may be of the varying types shown at p. 50. Formula No. 2 is that usually to be recommended. 54 Collodion Processes. No. 2 is suitable for dry-plate work and for interiors, but as a staple article No. i is recommended. For a simple iodised collodion the following formula may be adopted: — No. 4. Ammonium iodide .... 8 grammes Plain collodion , . .1 litre Or, No. 5. Cadmium iodide .... 10 grammes Plain collodion .... i litre No. 4 should be used immediately after making, whilst No. 5 will keep almost indefinitely. The next formula is for a simple bromised collodion : — No. 6. Zinc bromide 16 grammes Plain collodion . . . _ . i litre For all the above iodides and bromide substitution may be made with others, and it by no means follows that those chosen as examples will prove the most sensitive, though experience has shown they give good results. It is cus- tomary in preparing plain collodion to omit half of the alcohol, and to employ that half as a solvent for the haloid salts. This is convenient but not absolutely necessary. It is a good plan to make a note of the date of the manufacture of the collodion, as also of its iodising; useful information is often given by such memoranda. Testing Plain Collodions. Plain collodion should be tested before iodising, and the following tests may be applied, recollecting that a film that may not be suitable for the bath process may still be suitable for an emulsion process, and vice versa. Coat a plate (in the manner described at p. 79), and ascertain if when dry the film dry dead white, opalescent, or transparent. If the first, it is unsuitable for any process; if the second, it may be employed for emulsion work; whilst if the third, it may be suitable for any process. Coat a plate, and, after the collodion has set, mark if it Cleaning the Glass Plate. 55 is powdery to the touch, or if on applying the finger it comes away in strips. If the former, it may be good for dry- plate work; if the latter, for both dry plates or the wet process. Coat another plate, and, after setting, wash the film under the tap till all the solvents are washed out, and note if it take an even film of water or if it repels it at parts. If the latter it is too horny to use in the bath processes ; a litde potassium carbonate may improve it. Note if the collodion flows freely, viscously, or lumpily. Too hmpid a collodion will fail to give density; too viscous a collodion is unsuitable for any but small plates, whilst a lumpy collodion will give irregular images. The flowing qualities of a collodion arising from the pyroxyline may often be corrected by altering the proportions of ether and alcohol. If the film be reticulated, having marks like a crape pattern on it, the solvents may not be sufficiently anhydrous, or the pyroxyline may be in fault, as before stated. The collodion should also be tested after iodising ; the defects will be noticed when treating of the defects in negatives produced by the various processes. CHAPTER VIII. CLEANING THE GLASS PLATE. The plate, before being taken into use, should be most carefully cleansed from dirt of any description. The success of a photographer may be said to depend in a great measure on the eff'ectual manner in which he completes this opera- tion. The dirt that is to be looked for on a glass plate is that due to the manufacture, that due to subsequent ex- posure to the atmosphere and to the hands of the packers, 56 Collodion Processes. and sometimes that due to the chemical compounds with which it may have been in contact. Ordinary plates are sometimes found to be gritty on what should be the polished surface, and the application of acid may dissolve the grits away. Hence it is a good plan to treat all new plates with a solution of dilute nitric acid (lo parts of water to i of acid). This will not rid them of mechanical dirt, such as dust or grease. The presence of dust is readily ac- counted for, but the origin of the greasy matter is far more difficult to understand. If a plate that is thoroughly cleaned be put away in a plate box for a few days, and be then exa- mined by breathing on it, it will be found that it shows signs of repelling the aqueous vapour from the breath in certain parts, and that a subsequent cleaning of the plate is neces- sary to render it fit for use. This phenomenon can be accounted for on the supposition that organic matter of a fatty nature is to be found in the atmosphere, and when we remember that the lungs expire not only carbon dioxide, but also various organic matters, we should expect that in an inhabited house this latter might condense on some dry cool surface. The danger of using plates on which this deposit exists will be apparent by a simple experiment. Rub a warm finger or hand over the plate, coat with collo- dion, sensitise, but do not expose to light ; then apply the developing solution and watch the result. It will be found that where the contact has been made, a reduction of metallic silver will take place, and as development proceeds a dark stain will be produced. Imagine a similarly treated plate, prepared as before, exposed in the camera and developed : a dark deposit will take place both where the hand has touched and also where the invisible image has been im- pressed. It may be said that all animal organic matter has the property of causing a tendency for metallic silver to be reduced from the solutions of its salts. A similar ren:ark applies to the mercury compounds which sometimes get in- visibly reduced in the surface of the glass. The composi- Detergents. 57 tion of dust is of a most varied nature, and not unfrequently consists of ferric oxide, sodium chloride, and other earthy constituents. The reduction of silver nitrate in the presence of some of these would be certain. Alkalis have the property of converting greasy into saponaceous matter, and spirits of wine will dissolve both soap and grease ; hence both are employed as detergents. Mechanical dirt requires friction to remove it, and this should be just sufficient for the purpose, yet not enough to injure the surface of the glass. Such bodies we have in tripoli powder and rouge. The former is recommended on account of its being less gritty than the latter. The most common cleaning solution is made as follows Spirits of wine . , • . . 50 cc. Tripoli powder : — Quantity sufficient to make a thin cream Ammonium hydrate. . , , . i cc. Mr. Warren De la Rue for his astronomical photography employed a solution of potassium dichromate and sulphuric acid. This is doubdess a most effective detergent, but the use of sulphuric acid is open to objection on account of the damage it may do to the dress or hands. The writer has heard of a process of cleaning recom- mended, in which it was proposed to employ potassium cyanide, followed by nitric acid. The student is earnestly recommended not to attempt this plan, as it is poisonous and highly dangerous (see p. 74). Boiling the glass plate in caustic soda or potash has also been proposed. This is apt to injure the surface of the plate, owing to the slight solubility of vitreous matter in solutions of the caustic alkalis. Perhaps no more effective method for securing a clean plate can be adopted than by first treating the plate wit'h a cold solution of caustic potash, rubbing it well in with a rag, and then immersing it in dilute nitric acid and washing under the tap. A final thorough rinse in distilled water, and a rapid drying in a 58 Collodion Processes. water-oven, will leave the plate in as clean a state as can be desired. SENSITISING BATH. The sensitising solution, that is, the solution in which the collodion containing the soluble iodides or bromides, or both, are immersed in order to form the iodide bromide, or bromo-iodide of silver, may be said to be invariably made of silver nitrate dissolved in water. The purity of both con- stituents is of the highest importance, as any extraneous matter may be fatal to obtaining good results in develop- ment. Distilled water is naturally the purest form of water that can be obtained, but even this is sometimes contami- nated with organic matter in solution, which is apt to react upon the sensitive salt. The manner in which am- FlG. lO. monia is carried over with the aqueous vapour is well known to any chemist, and in a similar way hydrogen sulphide can be carried over. The latter contamination is most hurtful to sensitiveness, and the former might cause fog. It may be use- ful to point out the best mode of distilling water in a small way, in order to obtain absolute purity, A glass retort is always clean, and dirt can be more Distilled Water. 5Q readily seen than if it be of metal. The form known as Liebig's condenser is therefore recommended instead of the ordinary still. The water should be placed to the level of the flask A shown in the diagram, and a little (say a gramme to a litre) caustic potash should be dissolved in it. This will free the water of any ammoniacal compounds when warmed. The distillation takes place through the glass tube, round which is placed a glass jacket, c, containing water. Cold water is allowed to enter the jacket by the tube, d, and the heated water is carried ofif by ^ ; an universal clamp, B, is useful for holding the condensing apparatus in position. The first 50 cc. of each litre distilled should be rejected, and the distillation should not be continued beyond that point where 100 cc. are left in the retort. The distillate may then be considered to be pure enough for photographic purposes. If an ordinary worm still be employed, care should be taken that the worm is clean, free from dust, and not of lead. The water should be distilled over as before, the first and last portions being rejected. If distilled water cannot be obtained for making up the solution, spring water, if not impregnated with sulphates, will generally answer. Failing these, river water, and lastly rain water, after twice filtering through charcoal, must be resorted to. At first it may seem strange to place rain water last on the list, but it should be remembered that it is almost invariably col- lected from the roofs of houses, and is consequently sure to be contaminated with organic matter, and also inorganic matter. Rain water, if it could be collected directly as it falls, would save the necessity for using distilled water. A method of purifying ordinary water for bath purposes is as follows. Boil and filter it, add a little barium nitrate to it, and see if it turns milky. If such be the case, add a small further quantity, together with a few crystals of silver nitrate to each litre of water, and place in the sunlight. After a few hours' exposure, the organic 6o Collodion Processes. matter and sulphates will be at the bottom of the con- taining vessel, and the supernatant water may be decanted, syphoned, or hltered off. An excess of barium nitrate is not hurtful to the solution, for, as will be seen at p. 62, its addition is recommended. Nitrate of silver should be pure. The uncrystallised will be found sufficiently free from nitric acid to be available for forming a bath solution needing no doctoring. It is sometimes adulterated ; if any suspicion of this arise, a cer- tain known quantity of the crystals should be dissolved up in water, and the amount of silver nitrate really pre- sent calculated by any of the methods usually adopted. Silver nitrate is readily soluble in its own weight of water, but this strength would be quite unsuitable for a sensitising solution for two reasons : first, silver iodide is soluble to a certain extent in silver nitrate solution. The stronger the latter, the greater the amount of iodide dissolved. A varia- tion in temperature also affects the quantity capable of being held in solution. Now, even supposing that at the temperature at which the bath was formed immersion of a,n iodised plate took place, the heat evolved in the act of combination between the soluble iodide and the silver nitrate to form the sensitive compound would be suffi- cient to cause the iodide in the film to be partially dis- solved out. Secondly, the formation of the iodide would be so rapid that there would be a coarseness in the particles unsuitable for rapidity. Sutton has demonstrated that where any iodide is in the solution, 10 per cent, is as great a strength as can well be managed, whilst a 5 per cent, solution is the limit in the other direction. When bromides alone are employed, the strength may be 15 per, cent, as the silver bromide is almost insoluble in silver nitrate solution. In preparing a bath it is generally saturated with silver iodide, to prevent the silver nitrate dissolving away portions of the sensitive surface. Some skilled photograpliers, how- ever, prefer the saturation to take place from the film The Silver Nitrate Bath. 6i itself, a method which is recommened to the student, if he exercise ordinary care in working his plate. The degree of acidity of the bath depends much on the iodising or bromising of the collodion. To secure the greatest degree of sensitiveness, if iodide alone be present the solution should only be faintly acid, with bromo-iodidc it should be distinctly acid, whilst with bromides alone it should be very acid. The rationale of the different degrees of acidity is as yet not known accurately, more investigation into the subject being required ; but it may be presumed that it is in a measure dependent on the behaviour of the silver bromide and iodide when exposed in the presence of silver nitrate. The following formula for the silver-bath solution is a standard one where iodide or bromo-iodide of silver is the sensitive salt to be produced : — The silver salt should be dissolved in a quarter of the water, and the potassium iodide added to it after solution in the least possible quantity of water. After shaking (which will cause a partial solution of the silver iodide first formed), the remaining water should be added, when a further emulsion of iodide will appear. When filtered out, the bath solution will be ready for use, supposing proper acidity to be attained. An excess of acidity may be corrected by the addition of a few drops of a sodium carbonate solution. When a per- manent precipitate is obtained, the requisite acidity should be given after filtering by adding a few drops of a 5 per cent, solution of nitric acid. Some photographers have recommended the employment of acetic acid instead of nitric acid, but the writer has never found any benefit re- sulting from it — in fact the reverse ; for although acetic acid added to silver nitrate will not at first form silver acetate, yet as the solution becomes contaminated by working Recrystallised silver nitrate Water .... Potassium iodide . . 80 grammes . I litre . "25 gramme 62 Collodion Processes. there is danger of compounds forming, which will combine with it, and finally cause decomposition between the new compound and the silver salt. As the bath solution gets worked, that is, has many plates immersed in it, the original purity becomes im- paired by the accession of ether, alcohol, and various nitrates from the collodion, besides any extraneous matter that may accidentally be carried in. After a time the vigour and cleanliness of the developed image will be found to diminish, and the strength, &c., of the bath has to be attended to. Gently warming it will get rid of the ether, and evaporating it to half its bulk will get rid of most of the alcohol. If organic matter be present, exposure of the bath (after neutralisation of the free acid with sodium carbonate) will cause metalhc silver to be precipitated, and itself to be oxidised by the liberated molecule of nitric acid, thus ren- dering it innocuous. With certain collodions acetic acid will find its way into the bath, and the best method of eliminating the silver acetates which will probably have been formed is to evapo- rate the bath to dryness and add some strong nitric acid. This will liberate the acetic acid, which may be driven off by a further application of heat. None of these modes of treat- ment will eliminate all the impurities, for all the foreign nitrates (except ammonium) remain almost unchanged, even by prolonged fusion ; nothing remains but to precipitate the silver as chloride, or in the metallic state. If a film, after withdrawal from the bath, presents an appearance as if fine particles of the sensitive salt had been sprinkled over it, the solution is ' over-iodized ' ; that is, it is super- saturated with silver iodide. The disturbance made by the immersion of the plate probably causes the deposit. Diluting to double its bulk, next filtering, and then making up the solution to proper strength, will be a cure, or, as some photographers aver, the addition of 2 per cent, of barium nitrate will answer the same end. Development. 6'3 CHAPTER IX. DEVELOPMENT OF THE PHOTOGRAPHIC IMAGE. The importance of a thorough understanding of the ra- tionale of developing an image in the silver compounds is not to be over-rated, as a close study of it furnishes clues to apparently mysterious results, which are so often met with by every student in the art. The method of developing the Daguerrean image has been already given, and we pro- pose in this chapter to confine ourselves to that employed in what is known as wet-plate photography and dry-plate photography, and also that followed in the calotype and other kindred processes. It will be recollected that by method i the invisible image was to be made visible by the attraction exercised by the new compound formed after the impact of light on the original one. As already announced in chap. iv. p. 24, the change effected on a molecule of silver haloid is its re- duction to a lower type, i.e. one containing a lesser number of atoms. Thus Agglg was reduced under certain circum- stances to Agal, the other atom of iodine being absorbed by some body in contact with it. A similar change was shown to be effected on the silver bromide and chloride. We may, therefore, take as a type any one of these. We will choose the iodide, and follow the development from the earliest stage, when used in the wet process. It has already been shown at p. 19 that the building up of the image is due to the well-recognised law that every minute freshly-formed crystal attracts every other of a similar nature, and that the formation of the tree is entirely due to this molecular attraction, and the slow reduction of the metal from its solution. If the metal were deposited rapidly the 64 Collodion Processes. same law would still hold good, but the attraction of one reduced molecule on its immediate neighbour would be greater than that exercised by the metal adhering to the rod, as the probable distances in the one case would be far less than in the other. A particle of such a size and weight would therefore be built up before the metal on the rod could draw it sufficiently near to overcome the force of gravity exercised on it ; hence it would sink to the bottoin of the containing vessel. If we take a solution of silver nitrate and add to it a solution of ferrous sulphate, we have an almost instantaneous reduction of metallic silver. Thus- Silver nitrate + Ferrous sulphate = Silver + Ferric sulphate 3 AgNOs + 3 FeSO^ = 3 Ag + Fe^ (80^)3 + Ferric nitrate + Fe(N03)3 Any other oxygen-absorbing medium which is incapable of causing double decomposition with the silver nitrate might be substituted for the ferrous sulphate. By adding an acid to the latter the same action takes place, but much more slowly, the time necessary to effect the total reduction being dependent on the amount of acid present. Supposing by some means or another we are able to cause the first crystals of the silver to deposit themselves in certain posi- tions, we may be certain from analogy that the remaining crystals will adhere to these and build up a miniature silver tree. In the wet process, and also in the dry, we have means of causing these first particles of silver to deposit on the invisible image. This invisible image is formed of subiodide of silver (Ag2l). Only one of these atoms of silver is saturated; the other is still ready to combine with any other atom with which it has an affinity. Such an atom it finds in freshly- deposited silver. The solution of silver nitrate is already present in the wet process, and in the dry processes it is Crystalline A ttraction. 65 added to the oxygen-absorbing agent, which is employed in both. The first deposited crystals attract others, and thus an image is built up. It may, however, be asked how it is that different density of deposit is caused. The answer to this is that the invisible image is formed of variable quantities of the subiodide, approximately proportional in fact to the intensity of light acting on it. At any spot on the sensitive surface it is the integral of the attractions of the different atoms lying close to one another that determines the amount of the first deposit, and the varying mass of this determines the dis- tribution of the subsequent deposit. It is an axiom that the stronger the solution of the reducing agent the more rapid must be the deposit, and it may be convenient here to discuss the bearing of this. Suppose adjacent particles of the sensitive surface possess separate attractions of, say, i, 2, 3, and 4 units, caused by the different intensities of light acting on those parts. The probabilities are that the first metallic silver atom deposited will be drawn to the spot possessing 4 units of attraction. If the interval in time for the reduction of the next atoms exceed that necessary for placing the first atoms in situ, the attraction originally equal to 4 units will become ap- proximately 5, and the probabilities are that the larger pro- portion of the next reduced atoms will be attracted by the 5 units than by the 3 ; and by the same action the 4 units may attract several atoms, whilst the 3, 2, and i units may have attracted proportionally less. If the reduction of a sufficient number of atoms to saturate the whole of the atoms of AgjI take place almost simultaneously, the probability is that the difference in the increase of attractive power will be less marked. Thus 4 may become 5 ; 3, 4 ; 2, 3; and 1 become 2. It may therefore be asserted that the position of the first deposition will determine that subse- quently taking place, provided the same rate of the reduction be maintained. From the foregoing reasoning it will be 66 Collodion Processes. apparent that the stronger the developing solution the less marked will be the variation in density due to the different intensity of light acting on the various portions of the sensi- tive salt. The more viscid a liquid and the smaller the mass of the particle, the slower will a particle travel through the liquid. An application of this law has been applied to development. A certain amount of colloidal substances, such as gelatine, albumen, or these bodies acted upon by acids, is added to the liquid in which the oxygen-absorbing agent is dissolved. Though the reduction of the silver nitrate to the metallic state, may take place as rapidly as in a solution in which the colloidal body is omitted, yet the time the metallic atoms take to travel through the viscous solution is lengthened to such an extent that an appreciable time is taken to form a visible particle of silver. The time, therefore, taken to build up an image is longer than with a solution in which the colloidal substance is absent ; it is found that a small quantity of the colloid will give sufficient viscidity to cause slow deposition. The examination under the microscope of an image de- veloped in the manner indicated above will perhaps throw more light on the subject than any verbal description that can be given. It will be found that the whole of the image is formed of these minute crystals, varying in size according to the length of time which they took to deposit. The ap- pearance of the film when the half-tones of the negative are thus examined, will be as though it had been sprinkled with the metallic granules by means of a pepper-box ; whilst the parts representing deep shadows will be represented by large patches of bare collodion, with here and there a crystal lying embedded in the film. The student should take every oppor- tunity of studying the effect of different kinds of develop- ment as regards the actual physical composition of the image ; and he may rest assured that the highest excellence in any negative can never be attained when the deposit is coarse and highly crystalline. With a 2-inch objective it should appear as a stain on the film of more or less intensity. ForimdcE for Developers. The following are the formulae usually employed in de- velopment : — No. I. Pyrogallic acid . . . . i gramme Glacial acetic acid . , . . 20 cc • . . . : Quant, suf. Water . . . . . . 500 cc. This developing solution is usually employed for simply iodised collodion, and is useful when great density in the lights is required. The iron developers of a weak and strong type are as follows : — No. 2. Ferrous sulphate .... 10 grammes Glacial acetic acid Alcohol Water . No. 3. Ferrous sulphate , Glacial acetic acid Alcohol Water . 30 to 40 cc. Quant, suf. I litre 100 grammes 40 cc. Quant, suf. I litre These formulae give the limiting proportions of ferrous sulphate to water admissible, but any quantity between the two may be taken. For ordinary work, about 40 grammes is usually taken, as giving the best results. The double sulphate of ammonium and iron may also be substituted for the ferrous sulphate, and it has the advantage of re- maining in solution unchanged for a long period. The addition of copper sulphate to an extent equal to half the quantity of ferrous sulphate employed is also recommended by some operators, and it has doubtless in some cases a beneficial effect. The addition of various colloidal substances to the de- velopers, as already stated, may sometimes be desirable, particularly where great density and fine deposit are requisite. Perhaps the best of any is that proposed by Mr. Carey Lea: it is made as follows :— 30 grammes of French glue, or ge- latine, is softened in 50 cc. of water, to which 3 A- cc. of sulphuric acid is added. The water is next boiled, and the 68 Collodion Processes. gelatine dissolves, and, after adding another lo cc of water, tlie boiling is continued for a couple of hours. Five grammes of metallic zinc are next added, and the boiling continued one hour and a half longer. The solution is allowed to settle, and the clear liquid decanted off. To every 3 grammes of ferrous sulphate, i to 2 drops of this solution suffices to give sufficient restraint, without the addition of any acetic or other acid. Ferrous sulphate is a very unstable body, and will absorb oxygen from the air, and speedily attain the ferric state ; and as the latter salt is incapable of absorbing more oxygen, it is evident that the developing qualities are thus annihilated. It has been in effect found that ferric sulphate is a retarder, that is, a body which prevents the rapid deposition of the metalHc silver from the nitrate solution. The lesson to be learnt from this is, that when the developer attains a red colour it must of necessity be slower in action than when of the ordinary apple-green tint. A simple experiment with a developer containing ferric sulphate is worthy of trial by the student. Take, say 3 grammes of ferrous sulphate, and having dissolved it in 50 cc. of water, boil with strong nitric acid to such an extent that the addition of a drop of the solution to one of potassium ferricyanide produces no blue precipitate. Next precipitate the iron as ferric oxide by ammonia, filter, wash well, and dissolve up in the least pos- sible quantity of sulphuric acid, taking care to leave a slight residue undissolved. Make up the quantity of liquid to 10 cc, and add 2 cc. to a solution of ferrous sulphate made according to formula No. 3, omitting the glacial acetic acid. Develop a picture with it, and note the result. Attention should be paid in all cases to the crystals of ferrous sulphate employed. They are frequently mixed with a yellowish powder, due to the decomposition of the salt. In common specimens this often bears a considerable proportion to the ferrous salt itself, and must be allowed for in making up the solutions. The strength of the acetic acid is also Alcohol in the Developer. 69 important. What is commonly sold as glacial is often below strength. Its value should be estimated as given in various works on chemistry. In warm weather, owing to the in- creased rapidity of chemical action, more acetic acid is re- quired to control the reduction of the silver nitrate. Hence these quantities shown may require modification accord- ing to the temperature. The amount of alcohol required is invariably shown as 'quant, suf No definite quantity could be given, as it varies according to the amount of alcohol present in the bath solution. With a new bath none at all is required, whilst with one in which a large number of plates have been sensitised as much as 40 cc. to the litre may be neces- sary. A deficiency or excess of the alcohol is shown by the solution refusing to flow evenly over the surface of the sen- sitised collodion, and running into rivulets and tears. This is caused by the difference in surface tension of the fluid on the plate and the developer. Any body which reduces the de- ficiency may take the place of the alcohol. Thus a more viscid solution, such as that given by the gelatine retarder, is effective, no alcohol being required with it, even when the bath is very old. Methylated alcohol ^ should be avoided as far as possible, stains and disfigurements in the developed image being often attributable to it. CHAPTER X. GIVING INTENSITY TO THE IMAGE. Any method of increasing the apparent depth or black- ness of the image when received by reflected light, or of increasing the opacity of the image to non- actinic, or to ' Spirits of wine, sold as methylated, sometimes contains a certain quantity of resinous substance, in order to satisfy the excise requirements. 70 Collodion Processes. visual rays, is termed intensifying the image, and in both cases the result can be brought about by the same procedure. The following are modes of giving intensity to the image. I St. We may continue the development of the image by method i, if we supply more free silver nitrate solution to it when exhausted , and this will give us the necessary inten- sity. The theoretical considerations before noted need not be again brought before the student, neither is any special experiment necessary to impress them on his mind. 2nd. We may produce opacity to actinic rays by in- creasing the deposit by other means. As an example of what is meant, we may apply to the silver image a solution of mercuric chloride. Mercuric chloride + Silver = Double chloride of mercury and silver HgCU + Ag = AgHgCl,. This at first is grey (probably due to the formation at first of silver subchloride), but it finally becomes a pure white. It will be noticed that each atom of silver attracts one atom of HgClg. As regards opacity without regard to colour, the image must evidently be more opaque. It is, however, as regards actinic rays much less opaque than when the image was of the grey due to the silver. An application of ammonium hydrate to it, however, converts it into a jet black or deep brown. Here we have a still further deposition on the silver atom, which is therefore denser, and, being black, is very opaque to actinic rays. As another example of this mode of intensification we may instance the effect of copper bromide on metallic silver, and the subsequent treatment of the deposit thus formed with silver nitrate.^ Silver forming Copper _ Silver ^ Copper the image bromide ~ bromide sub-bromide Ag + CuBr2 - AgBr + CuBr For a detailed account see Photographic yournal, April 1877. Fonniilce for Intensifiers. 71 When treated with silver nitrate we have — Copper Silver Copper Silver _ Silver Silver nitrate in bromide"*" sub-bromide "'^nitrate "bromide sub-bromide solution AgBr -f CuBr^ -t-2AgNo3= AgBr -t- Ag^Br + CuCNO^), It will be seen how immensely the deposit on the image is increased by this method. Lastly, intensity in an image may be secured by sub- stituting some other metal for the silver by chemical means. For example, we may apply a solution of platinum tetra- chloride; the silver will be converted into chloride, and the platinum will be deposited in its place. The silver chloride may be subsequently dissolved away by sodium hyposulphite or ammonia, or by many of its well-known solvents. From a study of these methods it will be apparent that methods 2 and 3 must each be carried out on an image from which everything else is removed but the metallic silver; method i may be employed without such removal. The formulge for the first method are as follows : — No. I. Pyrogallic acid Citric acid . Water No. 2. Ferrous sulphate Citric acid . Water 4 grammes 4 to 8 grammes I litre 10 grammes 20 grammes I litre With the latter intensifying solution detail in the shadows is often brought out, though absent in the development, but the former is the most efficacious for rapidly giving opacity to the image. With each of the above a few drops of a solution of Silver nitrate .... 20 grammes Water ..... 500 cc. must be added immediately before application to the film. These intensifying solutions may be applied to the image either before or after fixing ; those which follow, however, 72 Collodion Processes. require the unaltered iodides and bromides to be previously dissolved away. Iodine 'I gramme Potassium iodide . . . . -2 gramme Water 50 cc. The iodine (which is held in solution by the help of the potassium iodide) converts a portion of the reduced me- tallic silver into iodide, and when continued but for a short time the image has a bluish-green tint, which is more non-actinic than if it were left in the metallic state. If this be not sufficient a solution of Potassium permanganate . . • l'5 gramme Water 50 cc. may be flooded over it. The permanganate is decomposed in coming in contact with the silver iodide, and insoluble manganic oxide is precipitated on the image. Another form of intensifier is made by — Mercuric chloride .... '2 gramme Water 750 cc, and, Potassium iodide . . . . i gramme Water 50 cc. The latter is added to the former till the red precipitate of mercuric iodide is on the point of becoming permanent. This solution applied to the image converts the silver into double iodide of mercury and silver, which is very non-ac- tinic in character ; other similar methods may be adopted, all depending on the formation of double metallic com- pounds. By converting the silver image into iodide by the application of the iodine solution, and then flooding with sodium sulph-antimoniate (NajS, SbSg) commonly known as Schlippe's salts, a scarlet deposit is produced of silver sulph-antimoniate in which 2 atoms of silver replace the 2 atoms of sodium, the iodine combining with the sodium. This method of intensification is due to Carey ScJdippe's Salts. 73 Lea, who described it in a paper which appeared in Feb. 1865, in the American 'Journal of Photography.' ScWippe's salts are prepared by taking Antimony bisulphide (finely powdered) 18 parts Dried sodium carbonate . . .12 parts Caustic soda 13 parts Sulphur Z\ parts These are ground up into a fine paste with a little water, and transferred to a well-closed stopped bottle, completely filled with water. After digestion and agitation for twenty- four hours, the clear liquid is filtered off, and allowed to evaporate spontaneously in a closed vessel over sulphuric acid, till lemon-coloured crystals of a regular tetrahedral shape are obtained. These are dissolved in water imme- diately before use, as the solution deposits an antimony compound when kept. The mother liquor may be employed for intensifying, but does not answer so well as the salt itself The quality of the colour is dependent on the amount of silver converted into iodide or chloride. When great density is required without gradation of shade, the following formula is efficient when preceded by a saturated solution of mercuric chloride. The effect of this compound, as already pointed out, is to form a double chloride of silver and mercury, grey at first, but which subsequently becomes converted into a pure white deposit. When in this state if is applied, a double sulphide is formed of an intense black. Dilute ammonium hydrate may also be employed, as already stated, in place of the sulphide. As regards the treatment of an image with copper bromide, this salt may be formed by dissolving i gramme of copper sulphate in 10 cc. of water and adding an equivalent of potas- sium bromide to it. This solution is flowed over the plate, and after a whitening action on the film and thorough washing, a 20 per cent, solution of silver nitrate is applied. Ammonium sulphide Water 50 cc. I litre 74 Collodion Processes. Fixing the Image. As regards fixing the image, nothing need be said ex- cepting that the solvent used must be incapable of readily attacking the metallic image, and such are the solutions of sodium hyposulphite and potassium cyanide. It will be useful here to point out the mode by which this solution is effected. Supposing, for instance, the image be developed on the iodide of silver; we have on addition of sodium hyposulphite either Silver Sodium _ Double hyposulphite Sodium iodide. ^ hyposulphite ~ of silver and sodium iodide. Agl + Na^SPa = AgNaSaOg + Nal or, 2 Agl + 3 Na.,S,03 - AgjNa^ slS.OJ + 2 Nal The first silver hyposulphite is very soluble in water whilst the last is very insoluble ; we have, therefore, in using sodium hyposulphite, a danger of the formation of the insoluble compound — a danger not to be under-estimated in the matter of silver prints, when the elimination of the less soluble compound is a matter of great difficulty. With potassium cyanide the danger does not exist, for though silver cyanide is formed, yet it is readily soluble in a small excess of the potassium compound. Silver Potassium Double cyanide of silver and Potassium iodide. cyanide. potassium iodide. Agl + 2 KCN = AgCN-KCN + KI Instead of Agl in the above equations, we may substi- tute nearly every silver compound— thus AgNOg, AgCl, AgBr, AgOSi02. Potassium cyanide, however, has the drawback that it is excessively poisonous, and that the presence of acid causes it to evolve hydrocyanic acid, a gas the deadly effects of which it is unnecessary to comment on. Another drawback to its use is the danger that exists of its dissolving up the finely deposited metallic silver, of which the half-tones of the image is composed. If used Vaiiiishing. 75 in a sufficiently weak solution, however, the solvent action need not be feared. All traces of the hyposulphite and cyanides should be removed by thorough v»'ashing, otherwise the transparent parts of the image might discolour, or a dis- integration of the film might take place through crystallisa- tion. The following solutions are those generally employed: — Sodium hyposulphite . . . loo grammes Water ...... 500 cc. And, Potassium cyanide . . . .30 grammes Water 500 cc. Varnishing the Film. The collodion film being excessively delicate and easily torn or scratched, photographers have adopted the plan of covering it with a transparent film of hard resin. This is effected by dissolving the resin in spirits, such as alcohol, and flowing it over the surface. In practice it will be found that, in order with safety to cover the film without dissolv- ing or disintegrating it, the specific gravity of the methy- lated alcohol, with which for economy it is made, should be greater than that employed in the manufacture of the collo- dion. It may at first sight seem strange that alcohol should be capable of attacking the pyroxyhne, but it must be re- membered that imdiluted methyl compounds are solvents of it, and, unless sufficient water be present in the varnish to check the tendency, a disintegration at least will take place. It must also be remembered that the rate at which the solvent evaporates will cause a difference in the transparency of the coating. If it be allowed to evaporate spontaneously, the alcohol evaporates first, and leaves the water behind, and, as anyone will find if he drop a little varnish into water, the resin at once separates in minute particles, which, when so united together, give a translucent deposit, caused by the re- flections of the various surfaces. On the other hand, if heat 76 Collodion Processes. be applied, and the water be caused to disappear as rapidly or nearly as rapidly as the alcohol, the resin will dry transparent, the heat being sufficient to cause the particles to be bound one to another, thus eliminating all chance of particular reflection. The resin should be as colourless as possible, as even the thin coating given to a negative picture is often sufficient to cut off much of the actinic light if it be of a red or yellow tint. As an experiment, it is only necessary to dissolve red Australian gum in spirit or water, and apply it to a portion of a glass plate, when it will be found that sensitive chloride paper darkens much less rapidly where covered with it than where it is bare. The constituents of most varnishes usually comprise amongst them lac and sandarac, but it is a matter of the greatest nicety to proportion them in such a manner that the film shall not split after exposure to any great variation in temperature. The cause of the contraction that takes place is not accounted for; it seems that some resins have a property of attracting moisture, and almost becoming hy- drates. This might cause an expansion of the film, whilst a rise of temperature might cause contraction. The whole blame, however, must not be laid upon the varnish, as the collodion film, when not free to expand and contract as it likes, may often produce the same effect. The following varnishes have been found satisfactory: — Unbleached lac .... 65 grammes Sandarac 6$ grammes Canada balsam .... 4 grammes Oil of thyme or lavender . . 32 cc. Alcohol, -830 500 cc. Or, Seed lac 120 grammes Methylated spirit . . . . i litre The lac is allowed to remain in contact with the solution for two or three days, with occasional shaking ; after which the supernatant liquid is decanted off, and thinned down to proper fluidity. Cleaning the Plate. 77 CHAPTER XL MANIPULATIONS IN WET-PLATE PHOTOGRAPHY. Cleaning the Plate. The glass plate must first be cleaned with one of the detergents indicated in the last chapter. If the tripoli powder cream be employed, it should be applied with a small pledget of cotton wool or soft rag, taking care that both sides of the plate are covered with it. It may either be al- lowed to dry on the plate, or, whilst still wet with the alcohol, may be wiped off with a soft diaper duster. In the latter case a little practice is required to prevent markings on the plate as shown by breathing on it. When the tripoli powder is all rubbed off, a final polish should be given to both surfaces with a chamois leather or old silk handkerchief^ The polishing should be such as is used in french-polishing a table ; not too heavy a pressure should be exercised, and there should be a continuous circular motion. It should be remembered that the effect of rubbing silk on glass is to generate electricity, which is sometimes a cause of non- adherence of the collodion to plates. The electricity in the glass should be allowed, or caused, to be dissipated before collodion is applied. There are various appliances for hold- ing plates during cleaning, some of which are excellent in their way, whilst others are toys made on principles unme- chanical. They are not necessary for the size of plate with which an amateur is likely to work. If a plate be clean the moisture from the breath will leave it evenly. It must be freed from dust before coUodionising, by passing a badger- hair brush over its surface. ' Before taking these into use, they should be thoroughly cleaned from all greasy matter by washing with soda. y8: Collodion Processes — Wet Plate. Coating the Plate with Collodion. The plate may be held in its centre by a pneumatic holder, such as that in figure ii, or at the corner by the fingers, if care be taken that no portion except the Fig. II. edges are touched. P'rom a half-filled 6-ounce bottle, or from what is known as a collodion pourer (fig. ii), the collodion should be care- fully poured upon the plate, so as to form a cir- cular pool at the end farthest away from the manipulator, and gradually be allowed to cover the entire surface, the wave flowing from the right-hand to the left-hand top corner, from thence to the left-hand and right-hand bottom corners, and finally into a stock bottle, whence, after decantation, and (if necessary) dilution with 2 parts ether to i of alcohol, it can be again employed. When the collodion is thus poured off, the plate will be in nearly a vertical position, and a gentle rocking motion should be given to it to pre- vent the collodion setting in ridges ; but the precau- tion should be taken not to grind the edges against the bottle, otherwise particles of glass may appear on subsequent plates. In hot weather the collodion does not take so long to set as in cold. The state of the film can be always ascertained by cautiously touching the left- hand bottom corner with the finger. When no longer tacky, the plate is ready for immersion in the bath. The collodion should be filtered if necessary, or it may be decanted from a stock bottle by one of the ordinary syphon arrangements. Sensitising the Plates. 79 Fig. 13- Sensitising the Plates. The film of collodion having set, the plate is immersed in the sensitising solution contained in a vertical or horizontal bath, the former being recom- mended for small plates, though the latter is essential for large sizes. A 'travelhng bath' is perhaps the best form of bath holder, as it is useful for indoor and also for outdoor work. It is of the form given in the figure. The top of the glass solution- holder B, which is held in a case A, is closed by a water- tight india-rubber top d, screwed down by the screws c as shown. The ' dipper ' employed for carrying the plate into the solution during Fig. 14. Fig. is- the operation of sensitising may be conveniently made of PURE silver wire, of the accompanying shape. It is usually, however, made of ebonite or glass. When the plate is covered with the solution by a steady downward motion of the dipper into the bath, it is moved slightly up and down 8o Collodion Processes — Wei Plate. in the fluid to wash off the ether from the surface of the film, and, when all greasy appearance has vanished, it may be left quietly at rest for from one to five or six minutes, according to the temperature and amount of bromide ' present. The motion of the plate in the bath at first is im- portant ; as, if neglected, streaky negatives are apt to result, especially in summer weather. It need scarcely be said that the silver nitrate solution should be free from all sediment before a plate is immersed in it, and it should be kept in order as shown at p. 62. After being very slowly withdrawn from the bath, capillary attraction will be exercised by the solution in the bath on that left on the film, and there will be but a slight quantity left on the plate, quite insufficient to cause the necessity of long drain- ing. On the other hand quick withdrawal necessitates long draining on a pad of blotting-paper ; the edge that occupied the lowest position in the dark slide should be pressed against it. When the surface appears free from excess of moisture, the plate is placed in the dark slide, taking care that the edge that is to occupy the top place in the camera is kept in the same relative position. The slide is closed after the back of the plate has been dried with a piece of rag or blotting-paper. It is here presumed that the camera is in position, and that the view has been focussed, follow- ing out the rules given in Chap. XXXIII., p. 240, and that the exposure is given also in accordance with the remarks to be found on p. 258. De%)elopment. Having decided which developer is to be employed, making the decision after a careful study of the picture, and noting its peculiarities, the plate is removed from the dark slide, the same precaution of keeping uppermost the edge which ' The greater the amount of soluble bromide in the collodion the longer it takes to sensitise fully. Intensifying the Negative. occupied the top in the camera being taken as before. If this were neglected the bath sokition which might have accumu- lated at the bottom edge might flow back over the surface, and thus inevitably cause irregular development if nothing worse. The developing solution having been placed in a clean cup, it is swept with an even motion, without being allowed to stop, over the plate, which is held by a pneumatic holder, or by the fingers, as in coating it with collodion. Littie or none of the solution should be allowed to leave the film, unless it be feared that too much density will be given to the resulting image, in which case it is an ad- vantage to let it wash off a portion of silver nitrate. As the picture appears, the developer is caused to work round to every corner in succession, thus securing an evenness which would otherwise be wanting. An over-exposed pic- ture will flash out at once, and, unless the plate be imme- diately washed, a veil or fog will inevitably be deposited on the surface, caused by the too rapid reduction of the first particles of silver, and the consequently rapid reduction of the remainder. An under-exposed picture will develop very slowly, and will always be wanting in detail by transmitted light, though it may appear fully out when looked at by reflected light, if held over a black background such as the coat-sleeve. A properly exposed picture should develop gradually and evenly, and should take at least half a minute in warm weather to come fully out in every part. When no further action is manifest, the developing solution should be thoroughly washed away, and the next operation should be proceeded with. Litensifying the Negative. This operation is one in which great judgment is required by the manipulator. Too great an opacity will spoil the negative, giving a black and white picture when printed j G 82 Collodion Processes — Wet Plate. whilst, on the other hand, one not sufficiently opaque will yield a grey print, which is unsatisfactory. The opacity must be judged of by the colour oi the deposit as well as by the density, though the former need not be taken into account when the iron developer has been used, as the silver deposit caused by it is of a blackish grey. If a pyro- gallic acid developer be employed the colour is of a de- cidedly reddish tint, and proportionally non-actinic, hence great judgment is necessary to ensure a really good result. When intensity is procured by using the pyrogallic solution the same remarks hold good, though the colour is never so marked as when arising from development. Whatever course be decided upon, it should be borne in mind that the general character of the finished negative will always bear an exact relation to that given by the primary development. Thus a flat-looking developed image will yield a flat-looking pic- ture, whilst one fuU of gradation will yield one similarly- graduated. Should intensification be necessary, the operator must determine whether it would be more advantageous to conduct it before fixing the image, or afterwards. Should over-expo- sure have been given the latter will be advisable, whilst, if un- due exposure, it should certainly take place before fixing. The intensifier should be poured over the plate, and, whilst so remaining, a few drops of the silver nitrate solution (p. 71) should be dropped into the cup, and then the intensifier poured back. The solution is again swept over the plate, and the required density is obtained by removal of the silver. It has been a point causing some discussion as to whether a developed picture may see light before being intensified. The answer to this seems simple. With an iodised film, which has been well washed after develop- ment, it may be exposed to tolerably bright light with- out any danger of producing a veil by the action of the intensifier, since silver iodide is almost insensitive to light, except in the presence of an iodine absorbent. With a Fixing the Negative. 83 bromo-iodised film more caution is required, though the writer has never found that a short exposure in a moderately strong light is hurtful. With a bromised film the less ex- posure given between the two operations the better. When intensifying after fixing, it is customary to flow a little iodine (see p. 72} over the film, then to expose it to light, and afterwards to use the pyrogallic solution. This is nearly useless unless a litde free silver nitrate be present, or all excess of iodine be washed out, any trace of which would render the exposure inoperative. The writer recommends a little bromine water instead of the iodine, for reasons which will be apparent on reading the chapter on emulsions. In mtensifying after fixing, there is a danger of staining the shadows with a reddish stain. This seems to be more due to a pyrogallic stain than to deposited silver, and can usually be got rid of by a little acetic acid diluted with an equal bulk of water. For landscape or portrait negatives it is seldom wise to resort to any method of intensification, except that with silver, as there is great risk of making the half-tones too opaque. The iodide of mercury formula (p. 72) is perhaps the best, if anything more be necessary. Fixing the Negative. This operation calls for little remark. The plate may be immersed in a vertical or horizontal bath if the sodium hypo- sulphite solution be employed, or it may be applied by flow- ing it over the plate : this should always be done with the cya- nide solution. Attention should be paid to see that all the iodide, bromide, or both be dissolved away. This can be ascertained by reversing the plate and noting if the yellow- ish-green colour due to them be absent. Finally the plates should be well washed and drained. A neat contrivance for holding the plates when drained is shown in rig. 16. As it folds up it is suitable for field work, though a draining 84 Collodion Processes— Wet Plate. box, is usually carried, made as in the accompanying sketch Varnishing the Negative. The plate may be allowed to dry spontaneously or by the aid of heat ; the latter method gives a slightly denser image, and therefore a negative should never be heated ^vhen parts of it are dried by ordinary evaporation. Before apply- ing the varnish the plate imist be warmed (see p. 75), to cause the varnish to flow, and also to prevent it drying matt. The varnish is applied like collodion, the same procedure being followed exactly. When all that will has run back into the bottle, any excess that may have collected ait the corner end may be removed by pressing the glass on :a pad of blotting-paper. The plate must again be warmed. The sources of heat are various. In India, or in other hot climates it will be found that exposure to the sun's rays imparts sufficient warmth to the glass. In temperate climates the neatest way of attaining the proper temperature is by placing the plate in a hot-air bath, as used in chemical operations (see fig. 26) ; failing which, a clear fire, a B unsen rose burner, or a paraffin lamp maybe broughtinto requisition. A naked spirit-lamp is dangerous without great care., and the solvents of the varnish, being highly inflammable, r eadily catch fire from any naked flame. Defects in Negatives, 85 CHAPTER XII, DEFECTS IN NEGATIVES. A MERE mention of some of the defects that are to be met with in negatives will suggest a cure, whilst for others, which are a little recondite, explanations will be offered and reme- dies suggested. ' Fog ' on a negative may be due to several causes:— i, it may be due to a dirty plate ; 2, to over-exposure ; 3, to an alkaline bath solution ; 4, to want of acid in the developer ; 5, to improper exposure to actinic light, either in the camera or dark room ; 6, to vapour in the developing room or tent. A minute examination of the condition of the negative and the state of the dark room or tent, will generally show the cause of the defect, which has only to be known to be rectified. A weak image may be due — i, to an unsuitable collodion, a weak sensitising bath ; 2, a bath charged with organic matter ; 3, bad lighting of the subject due to dull weather or a yellow light ; or, 4, an over- strong developer. Pin-holes in the negative may be caused by — i, dust on the plate ; 2, the bath being over or under iodised. Black specks on the picture are usually due to — i, dust in the camera 5 2, slide ; 3, dark room ; or, 4, dust in the collodion. Comet-like spots are almost always due to undissolved particles of pyroxyline in the collodion. Transparent spots, as distinguished from pin-holes, are usually due to dust in the collodion. A scum on the film is usually found when a plate has been kept for a long period out of the bath, or when a too strong development has been used. A plate which is to be 86 Collodion Processes — Wet Plate. kept for a long time before development should be sensitised (or finally dipped) in a weak bath, and only immersed in it sufficiently long to cause all repulsion between the surface of the plate and the solution to be overcome. A collodion containing a larger than usual proportion of bromide is also recommended to secure freedom from stains. The usual explanations given as to the cause of markings like watered silk, is that the collodion contains too much iodide, is too alcoholic, or that the pyroxylin is too strong. The remedies have already been indicated. Black stains at the corners of the plate are often caused by the bath solution flowing back over the sensitised surface, after having been in contact with the wood of the dark slide. Transparent markings are much more common in cold than in hot weather. They generally arise from unequal sensitising of the film in the bath, .and from the developer refusing to flow. A want of sharpness in a picture may be due, to in- accurate focussing, to a want of achromatism in the lens, or to the camera being shaken accidentally by the wind, or by the sinking of the camera legs during the exposure. If the lens be in fault there is no help for it but by ascertaining how much further backwards or forwards the ground glass of the camera ought to be shifted in order to get the sharpest result possible. This can easily be found by actual trial, and when noted the ground glass may be permanently placed in such a position relatively to the glass plate in the dark slide, that when the picture is visually in focus the position of the sensitive plate shall be chemically in focus. A blurring of the image can easily be accounted for ; though, perhaps, there has been more controversy on the subject than on any other photographic phenomenon. It is usually ascribed to geometrical reflections of the incident rays coming through the lens from the back surface of the glass, and no doubt, in some cases, this is absolutely true, though in others it requires a more complete explanation. Irradiation. 37 It must be borne in mind that the rays of light do not strike the surface of the plate perpendicularly except at its centre. The accompanying diagram shows a glass plate, a a, of exag- gerated section, with rays of light passing througli the optical centre, c, of the lens, b, coming from a bright line, a b. The ray ^ c D is perpendicular, and the ray ^ c e makes an angle with the perpendicular. This last ray, after passing through the collodion film (which for the time we may con- sider transparent) would be bent inwards to f, and a Fig. 18. portion would be reflected from the back surface of the plate, and strike the thin collodion film again at G. P'rom G, a portion might be reflected again, and so on. Evidently, in this case, a blurring might take place, but always outwards from the centre of the plate. If, however, the ray of light ^ c E proceeded from the extremity, of the dotted line, b d, which may be supposed to represent a bright line of light, then no blurring would be apparent, as the blur from it would be covered by the image e m, of the bright line. Now in practice blurring is usually most intense when a dark object, such as a tree, is opposed to a bright object, such as the sky ; in which case we may suppose b d tohe a section of the sky, and a b the tree, which we may suppose to.be a dark line in section. Here the blurring is evidently 88 Collodion Processes — Wet Plate. not due to reflections of the incident rays from the glass. To account for it, we must look to another feature of the sensi- tive surface. If a sensitised fihn be examined under the microscope it will be found to consist of minute grains of silver bromide, iodide, or bromo-iodide, and each of these grains individually must reflect more or less light from its surface. A beam of light, therefore, must be dispersed in every direction, and, as has been shown,' the light striking at any point of the film is scattered and reaches the back surface of the plate as a disc, with intensity gradually diminishing from the centre. The reflection from that surface becomes most noticeable when the critical angle of the glass is reached. The direction that the rays take in striking the particles is not of any great moment, as the difference in intensity of the reflections in any one direction is very slight when the angle does not differ very largely from a right angle. Hence it is seen that blurring really takes place from this cause in all parts of a picture taken on a glass plate, but that it is naturally most apparent when a bright light is opposed to a deep shade. There is still another point in this particular scattering of the rays to take into account, and that is, the lateral scattering. Supposing the intensity of the light in the lateral direction to be only ytitt ^'^^ in the perpendicular, the penetration into the film would still be considerable, and a blurring would result on this account. In photographing fine lines close together this kind of blurr- ing is often most apparent, a black line being often filled up, or rendered too fine. It has been argued that blurring is also due to the lens, but a serious consideration of the matter will show that such an effect is hardly possible if it be toler- ably achromatic. The blurring caused by the reflection of the scattered rays from the plate can be destroyed by using an opaque body, on which the collodion shall rest, or it can be partially eliminated by placing a backing of some black or non- ^ See London, Edinburgh, and DiibJin Phil. Mag. January 1 875. Positive Collodion Pictures. 89 actinic colour in optical contact with the back surface. The lateral blurring is a more difficult enemy to face. It can be avoided by dyeing the film with a scarlet or yellow dye ; but this is at the expense of sensitiveness. It may be eliminated also by making the film as transparent as possible (that is, by reducing the particular reflection). This is only feasible in dry-plate processes, and then is only occasionally success- ful. A daguerreotype plate should be free from all blurring, except the very minute amount which may be due to the last-named cause. CHAPTER XIII. POSITIVE PICTURES BY THE WET PROCESS. There is no very distinctive difference between the mani- pulations or the theory of the negative and positive processes. Any distinction between the two may be summed up in this : the metallic image must be of as white a nature as possible, whilst the background must be as dark as pos- sible. The image should also be as transparent as circum- stances will allow, for it must be remembered that all half- tone is dependent on this quaUty, and that in the half-tone consists the value of the picture. After a microscopic exa- mination of a film it will be manifest that the coarser the grain, the more colour should be capable of being seen through the image ; hence it is not amiss to have the deve- loper of such a nature as will produce this effect, and also to cause the silver deposit to assume as white a character as possible. The bath is usually made as follows : — Silver nitrate . . . .65 grammes Water i litre The silver iodide is added, as for the negative bath (p. 61). It should be slightly acidified with nitric acid. 90 Collodion Processes — Wet Plate. If a pyrogallic acid developer be employed, that given at p. 71 is perhaps as good as any. An iron developer may be advantageously made with a large proportion of ferrous nitrate, in order to secure the white deposit. The following is a formula usually adopted. Ferrous nitrate I'errous sulphate Nitric acid, I "45 Alcohol Water 7 grammes 3 grammes I -25 cc. Quant, suf. I litre. Should the deposit formed by this developer be too granular, a little more ferrous sulphate must be added. The pyroxyline for the collodion should be prepared with weak acids, about equal parts of the sulphuric and nitric acids being employed, with as much water as they will bear without dissolving the cotton wool when the tempera- ture is lowered 5° below that given for preparing negative pyroxyline. The collodion should contain more ether than alcohol when such pyroxyline is employed. The writer has found that the following gives satisfactory results : — Ether, 725 400 cc. Alcohol, '812 .... 300 cc. Pyroxyline . . . . .10 grammes. The following may be added to this quantity of plain collodion — Ammonium iodide . . .7 grammes Cadmium bromide . . • i'5 grammes. As in iodising negative collodion, it may be found ad- visable to omit 150 cc. of alcohol from the collodion and to dissolve the iodide and bromide in it, and subsequently to make the addition when the collodion is required for use. A little tincture of iodine, enough to give a sherry colour 10 the collodion, is usually necessary to secure sufficiently Dry -Plate or Alkaline Development. 91 dense pictures. The development should not be carried to such an extent as in the negative process. A picture on a glass support, when viewed by transmitted hght, should, in fact, look under-exposed. If the support for the collodion be a ferrotype plate, the image may be developed very readily so as to give the best effect, or the black background can be used to assist the judgment. It need scarcely be said that when glass is employed the back of the plate should have black velvet or black varnish in contact with it. Bates's black varnish is recommended for backing the plate. The fixing solution is the cyanide solution given at p. 75. CHAPTER XIV. DRY-PLATE OR ALKALINE DEVELOPMENT. As we proceed to the practical part of dry-plate processes, it will be found that the sensitive salt which is principally employed is the bromide of silver. Sometimes the silver bromide has a large proportion of iodide in combination with it and sometimes chloride, but it may be taken as an axiom that the really effective salt of silver is the bromide, though the conditions of sensitiveness may be altered by the presence of either or both of the other two salts. In the development of dry plates by what is known as the alkaline method, instead of the image being built up from a silver salt external to the film, it is built up from the solid silver salt in the film itself The iodide is very feebly amenable to this treatment, but the bromide and the chloride are readily acted upon by the developing agents. These agents are in reality compounds which have a strong affinity for oxygen. Pyrogallic acid and its congeners, in combination with an alkali, are those most usually employed, though in some quarters, more particularly on the Continent, the organic ferrous salts are in equal favour. The behaviour 92 Experiments in Alkaline Developmeni. of the alkaline pyrogallic acid is almost precisely similar to that of the latter salts ; and any argument or experiment that is applicable to the one is also applicable to the others. The following experiments should be made by the student, that he may become acquainted with all the phenomena connected with this class of development. I. Precipitate pure bromide of silver, say 4 grammes, and wash thoroughly ; then place it at the bottom of a test- tube, and cover it with a solution of pyrogallic acid (about •3 gramme to the 100 cc), to within a short dis- tance of the top. Having drawn out from \ inch tubing a fine funnel, let him place the end drawn out just above the bromide, and then pour into the funnel 3 or 4 drops of strong ammonia. It will be seen almost immediately that a black layer forms above the bromide, that the silver is reduced, and that the action continues for a certain time, and then stops. The blackening of the liquid will be found to be due to the alteration in the pyrogallic acid, consequent on the absorption by the alkali of the bromine abstracted from the bromide. 2. Repeat the experiment, replacing the py- rogallic acid by dilute ammonia (say, "880 sp. gr. diluted with 10 times its bulk of water), and drop into the funnel a small quantity of strong pyro- gallic solution. The phenomena presented will be slightly different. A cloud will instantly form in the ammonia, and if the surface of the bromide has been protected by a small diaphragm of paper, the whole of the solution may be poured off, and the surface of the bromide will be found almost unchanged. The diiference is caused by the solubility of silver bromide in ammonium hydrate ; and the portion held in solution is consequently more readily reduced than that remaining in the solid state. 3 and 4. Repeat these two experiments, substituting Revteiu of Experiments 93 potassium hydrdxide (caustic potash) for the ammonium hydrate. The phenomena presented in both cases will be identical ; the silver bromide will be reduced from the solid state. This is because silver bromide is ijisoluble in the potassium compound. 5. The next experiment should be to dissolve a small quantity of silver bromide in ammonia, and then drop into the solution a small quantity of potassium bromide solu- tion. It will be found that a precipitate is immediately caused, which, on analysing, will be found to be silver bromide combined with ammonia. 6. Repeat the foregoing experiments with silver iodide, first with weak solutions, and then with concentrated. It will be found that the concentrated will act upon the iodide, whilst the weaker will not. The iodide not being soluble in ammonia, the phenomena described in experiment 2 will not be observable. Reviewing these experiments, we are led to the following conclusions : — that the alkaline pyrogallates have such an affinity for the halogens that they are capable of breaking the bond existing between the silver and the bromide ; that the portion which is soluble in the alkali is most readily acted upon ; and that the addition of a soluble bromide diminishes the amount capable of being dissolved. Con- sequently, if a surface of silver bromide were treated with ammonium pyrogallate, we should expect that the undis- solved bromide was less readily reduced than that portion which is dissolved, and that less immediate reduction would take place when a soluble bromide, such as that of potas- sium, were present. At page 22 we have said that the photographic image is probably a subsalt of silver. Thus that formed on the chloride will be subchloride (AgoCl). With the bromide the same holds good. The action of light is to isolate the sub- bromide, bromine being liberated. Thus : — Silver Bromide becomes Silver Sub-Bromide + Bromine AgoBrj = Ag.,Br + Bi 94 Dry -Plate or Alkaline Development. Suppose we have a mixture of bromide and sub-bromide of silver, it is very easy to theoretically determine which compound would be first reduced to the metallic state by the action of the alkaline reagent. It is merely a question as to which requires the least work to be done on it as to which will be first reduced. In the case of the bromide of silver two atoms of bromine have to be removed before the metallic silver results, and in the other only one atom. It is evident, then, that in such a mixture as we have assumed, t*he sub-bromide would be the first to be reduced, and not till that had been effected would the bromide be attacked. In the case of a photographic image on a plate before development, the amount of sub-bromide is infinitesimally small ; but, infinitesimal as it is, that must be reduced to the metallic state before the neighbouring molecules of bromide can be attacked. It might be thought, as this must be so rapidly done, that the bromide also would be at once reduced. But here another phenomenon comes into play. It appears that the bromide of silver and freshly reduced metallic silver cannot exist separately in contact, but that immediately new sub-brom.ide of silver is formed. At any rate, an action of somewhat this description takes place in the film. .\n interesting experiment is confirmatory of this. Take an ordinary dry plate, such as an albumen- beer plate, and expose it in the camera. Coat half of it with a bromide emulsion, and develop it by the alkaline method. That part coated with the bromised collodion will be found to acquire density. When dry, remove the film from off the glass plate with gelatinised paper, and also cause the adhesion of a similarly prepared gelatine papei to the surface primarily next the plate, When nearly dry, the exposed film can be spht off from the bromised film ; Thus — Freshly reduced Metallic Silver Ag., I and Silver Bromide become Silver Sub-Bromide. + Ag,Br2 = 2 Ag,Br. Gallic Acid in Development. 95 and on examination it will be found that there is an image on both films. If the sensitive salt in the collodion film exposed in the camera be iodide, an image may be de- veloped, though it will be weak. The fact remains, then, that this action takes place, even though the films be separated by a very thin layer of albumen. It will also be apparent that the image will be stronger when developed with ammonia than with potash, for with the former the silver can be deposited from the solution. The writer has been able to intensify images from alkaline or neutral solutions of sodium hyposulphite and potassium cyanide, in which have been dissolved silver chloride, by this action of pyrogallic acid. The restraining action of soluble bromide (potassium, &c.) is most probably due to the fact that these salts form double bromides with the silver, and not with the sub-bromide, as might be expected from chemical considerations. To reduce the double bromide to metallic silver evidently entails more work than the reduction of the silver bromide by itself, and the freshly precipitated silver has also to loosen the bond between the double bromide before it can combine with the silver bromide to form the fresh sub- bromide, hence the reduction is slower. In practice this is the case. A developer in which a full dose of bromide is present develops more slowly than one in which it is present in small quantities only. It should be noted that the same treatment of the bro- mide is effective when gallic acid is employed instead of pyrogallic, the power of reduction of the former being smaller than that of the latter. This fact proves that with a weakly formed invisible image a strong reducing agent should be used. In the silver bromide emulsion plates, for which this development is particularly adapted, it will be noticed in subsequent pages that in order to obtain pictures which are free from veil or fog one of three conditions is necessary — 96 Dry-Plate or Alkaline Development. either there must be a little soluble bromide in the film, or else if there be an excess of silver nitrate over that neces- sary to combine with the bromide there must be some free mineral acid or the excess must be converted into silver chloride by the decomposition of some metallic chloride present. These conditions are apparently conducive to the for- mation of bright images. It will be profitable, however, to consider the probable cause of this. Bromide of silver is usually formed by the double decomposition of silver nitrate and soluble bromide, such as a proportion of potassium with that of cadmium. Cadmium and other dyad metals may form two bromides, the ordinary bromide and a sub-bromide. In ordinary circumstances the latter compound is found in very small quantities, but when it comes in contact with silver nitrate, a sub-bromide of silver, or, otherwise, bromide of silver, with unattached atoms of metallic silver, is formed. When the soluble bromide is in excess, these molecules — supposing them to be half molecules of silver bromide together with the attached atoms of silver — readily attract the excess of soluble bromide ; and, when these atoms of excluded silver lose their nascent power of attraction, they become in- capable of causing a reduction of the neighbouring bromide when a developing solution is apphed. If, however, these sub-bromide molecules remain as such, the action of the reducing agents is to attack these first, and the reduced silver exerts its power in determining the reduction of the neighbouring molecules ; in other words, causes reduction where light has not acted. Sometimes the metallic or other soluble bromides are contaminated with minute traces of oxides ; when this is the case, nitric acid converts these into nitrates. This same acid also will act upon the sub-bromide, though not on the sub-chloride. Thus Ag^Br is decomposed into AgBr and AgNOg ; or if nitric acid be applied to the soluble bromide, such as of cadmium, before its contact with the silver nitrate, Alkaline Development. 97- we probably have a precisely similar reaction. The addi- tion of certain metallic chlorides produces a similar result, preventing the formation of AggBr, but some acid is almost necessary when any oxide is present. As touching the strengths of the solutions to be employed, ist. The stronger the ammonia, the greater the amount of the silver bromide that is dissolved. If the amount dis- solved be in excess of that necessary to supply a gradual aggregation of silver on those parts on which a deposit has already taken place, the result must be a veil on the whole surface. 2nd. The stronger the pyrogallic acid solution in the presence of sufficient alkali, the more rapid will be the re- duction of the silver bromide ; hence a strong solution is very hkely to cause fog, even if sufficient soluble bromide be present. 3rd. The strength of the bromide solution must be dependent on the rate at which a picture has to be de- veloped. A sensitive salt which has been exposed to the prolonged action of light will bear a very strong solution of bromide, the reason being that it is necessary that the double bromide of the alkali and silver should be formed simultaneously with the reduction of the sub-bromide. If this be not effected, some portion of the silver bromide will be reduced before it is protected by the formation of the double salt. (It must not be forgotten that silver bromide is slightly soluble in the bromides of the alkalis.) There is another form of restraint to the energy of reduction by the alkalme developer which is most important in the present day— viz. a physical restraint. At page 66 we have shown that the action of viscous solutions is to cause the slow deposit of metallic silver from a solution of silver. In a similar way colloidal substances, when moist or wet prevent the alkaline developer acting so rapidly as when free. Thus, instead of a soluble bromide, it is quite feasible to use glycerine and albumen to get the sub-bromide reduced alone H 98 Dry -plate or Alkaline Development. before the bromide itself is attacked. Time is in reality the factor we have to pay every attention to. If we can extend the time of reduction, we can make certain that the salt acted upon by light is alone reduced in the first instance, and allows the secondary action of the formation of new sub-bromide to take place before the bromide is attacked. Whether the restraint be physical or chemical, of one thing we may be certain, that such restraint implies a less energy capable of being brought into play in a cer- tain time ; and if the time be sufficiently long, the energy at any instant will be insufficient to reduce the bromide, though it may suffice to reduce the sub-bromide. If we compare the above method of development with that given in Chapter IX., we shall see that the latter is the only one which is feasible for the wet process, and the present one for dry processes. In the wet process there is a solution of silver nitrate on and in the film ; the unbonded atom of silver in the subsalt is in a state possessing the greatest attractive activity, and development must take place shortly after exposure. If the second method were adopted, all the silver nitrate would have to be eliminated from the film by prolonged washing : in practice it is found that the resulting image is Hable to be of unequal density. Again, the proportion of bromide to iodide in the collodion is small, and as the iodide is only affected by intensely strong alkaline developers, the chance of veiling the image through the reduction of the bromide unacted upon by light is increased, if sufficiently strong solutions are employed to cause the reduction of the sub- iodide. With dry processes the advantages rest with the a'.kaline method. The development takes place hours, or even months, after exposure ; consequently, if the method in Chapter IX. be adopted, it is quite possible that the free atom of silver in the sub-iodide or sub-bromide has partially lost its nascent energy, and that when the free silver uitrate. Alkaline Development. 99 together with the developing solution, is applied to the surface of the film, the intensity of molecular attraction is diminished. The same amount of attractive power may be obtained by increasing the number of molecules acted upon by light ; hence what in the wet process would be a sufficient exposure, for this development, in the dry may be totally inadequate. Besides this, in the most recent process, in which gelatine forms the vehicle in which the sensitive salt is embedded, the addition of silver nitrate is imprac ticable, since it would only induce stains over the whole surface and obliterate the image, even if the image could be readily developed. Coming to the second method, however, the result differs. Even if the silver atom of the sub-bromide be inert, the agents employed will still naturally first attack the re- mainder of the molecule, by taking up the bromine and liberating the other atom of silver. Now this liberation takes place in close contact with another atom, and as the attraction varies inversely as the square, or more, of the distance, a much less attractive force is necessary in order to draw the liberated atom to its partially saturated neighbour. The atom once in situ attracts the. other depositing atoms, and an image is rapidly built up. As a matter of fact, in those processes in which both methods of development are possible it is found that alkaline development causes a gain of from 4 to 50 times in the matter of shortening exposure, more particularly when a large quantity of silver bromide is present from which the atmosphere is excluded by its being embedded in a comparatively thick film, as in the gelatino- bromide process. The organic salts of iron (ferrous) have been alluded to as acting in the same way as the pyrogallic acid and alkali. It is more easy to give the chemical reactions which take place with the former than with the latter. The typical salt may be taken to be ferrous oxalate dissolved in a solution of neutral potassium oxalate. The decomposition of the sub- H 2 lOO Dry -plate Processes with the Bath. bromide entails an alteration in the ferrous salt employed as follows : — Ferrous Oxalate and Silver Sub-Bromide give Ferric Oxalate and 3(Fe,C.,OJ + 2Ag.,Br = Fe, (C„0 J3 + P'errous Bromide and Silver. Fe.Br^ + 4Ag. In some processes the citrate and tartrate of iron are em- ployed. The same form of equation applies also to them. CHAPTER XV. DRY-PLATE PROCESSES WITH THE BATH. It is not proposed to enter into details of many dry-plate processes with the silver bath, as they can be ascertained by the consultation of various manuals. At the same time it is thought advisable to enter more fully than usual into the theory of the subject. The course usually adopted for these processes is as follows : — The plate is coated with a preliminary substratum of gela- tine, albumen, or india-rubber, or else is given an edging with one of them. The collodion is then applied, and sensitising takes place in the usual manner. The silver nitrate solution is next thoroughly washed off in distilled or rain water, and what is known as a preservative is flowed over the surface of the plate. The preservative may be partially washed off, or it may be allowed to dry on it in undiminished strength. The plate is now in a state ready for exposure. The preliminary coating or edging of albumen is given to the plate in order to secure the adhesion of the collodion film. It is found in practice, if this be omitted, that the film, on being wetted, becomes non adherent, and floats off. The substratum, as it is technically called, must al was be of such a nature as not to injure the bath solution, and, to guard against all risk, it is advisable that every portion of Substrata. lOI it should be covered with collodion. The following are formulae for it : Albumen ..... i part Water 50 to lOO parts Ammonium hydrate — Sufficient to give it a smell of ammonia. Or, Sheet gelatine .... 3 grammes Ammonium hydrate . . . 15 cc. Water 1500 cc. The gelatine should be soaked in half the quantity of water, and the remainder added boiling. When cool, the ammonium hydrate should be dropped into the fluid. This solution will not keep, hence it is advisable to make it up only just when wanted. When it is required td cover the entire plate with either of these substrata, it is usual to wet the plate with distilled water, and flow it over, and drain. It frequently occurs, however, that this method produces markings on the negative. A simpler and more effective plan is to cover the end of a glass plate (the breadth of the plate to be covered) with a piece of fine flannel or swan's-down calico, to moisten it with the fluid, and then to squeeze out all excess. This brush, known as Blanchard's brush, is drawn down the plate with an even stroke ; it gives the very finest coating possible. A washed and unpolished plate seems to take more kindly to the colloid body than if the cleaning be finished off with the silk hand- kerchief or chamois leather. In lieu of either of the above solutions the following may be flowed over the plate hke collodion, and be allowed to dry spontaneously : — India-rubber . , . . i gramme Benzine 500 cc. The collodion should be of such a character as to give great density to the developed image. It should also give a porous film, for it must be remembered that when the pyroxy- line parts with its water of hydration it becomes extremely I02 Dry-plate Processes with the Bath. impermeable to all solutions ; so much so that a horny col- lodion will often refuse to develop. It has been the practice with many practical photographers to keep the iodised col- lodion till it is thoroughly aged, and has a ruby tint from the elimination of iodine from the iodides of the alkaUs, and the consequent combination between the alkali and the pyroxyline. This effect is precisely similar to that obtained by a modification of the ratio of the acids to the water in the manufacture of the pyroxyline. Collodion may also be rendered porous by adding water to a portion of it to such a degree that it gives a reticulated film, and by then adding the remainder unwatered. A slight opalescence of the film is not objectionable, and it may even dry almost matt, so long as the necessary disintegration of. the pyroxyline is secured, since it is found that varnishing takes away the dead appearance to a great degree. The sensitising bath should be such as to give a good dense film after the plate has been immersed some time. The solution employed for the wet negative process is of proper strength, unless the collodion be highly bromised, in which case the amount of the silver nitrate may be increased to half as much again, or even to twice the quantity. After sensitising, it is usual to wash the plates to such an extent as to free them from all silver nitrate solution. The first washings are usually made in distilled or filtered water. Rain water is often recommended, but the operator should beware of it, unless it be very clean and at least be twice filtered. Perhaps more bad dry plates are produced by the use of impure washing water than by anything else. When the delicacy of the eff"ect that is produced by the ethereal waves of light on the pure products is taken into considera- tion, it will be apparent that every extraneous force which will overthrow the equilibrium of the particles should be avoided. Thus we might expect that hydrogen sulphide might cause that overthrow, and it inevitably produces fog. A moderate proportion of iron in the water would Applying the Preservative. 103 also produce like results. When the first excess of the silver solution is washed away the danger of the use of impure water diminishes rapidly, and almost any ordinarily pure water may be brought into requisition, but in any case a final rinse of distilled water is to be strongly recommended. Care should be taken that the surface of the plate after with- drawal from the bath is covered by the water without stop- page. The first washings may be well performed in a dish, and an even covering of the surface can be attained after a few attempts. Washing in distilled water should continue till all repellent action due to the alcohol and ether contained in the bath solution disappears. Applying the Preservative. The preservative is usually applied by floating it on the surface of the film for about a minute. Care should be taken that its strength is not diminished by too much water being left on the plate. In some cases it may be applied by immersing the plate in a flat dish or dipping-bath con- taining it. There are some objections to this mode of application however. It will be convenient here to discuss the ends to be obtained by the use of a preservative, i. It must be an iodine or bromine absorbent, for without this quality the film manifestly might be insensitive. 2. It must be capable of filling up the minute pores of the collodion, so that on re-wetting after drying it may give access to the developing solution. 3. It must act as a protective varnish against the atmospheric influences. Regarding the first point there is not much difificulty, as nearly every organic animal or vegetable compound is capable of combining with iodine. Under the head of absorbents we may rank tannin, pyro- gallol, gallic acid, gums, gelatine, albumen, caffeine, theine, and other like bodies. The second requirement may be met by the employment of some of the above, or by the addition to them of sugar in various forms. The last requirement is I04 Dry -plate Processes with the Bath. more difficult to meet, and is very often neglected, as it en- tails that the body should not be hygroscopic. The draw- back to any processes, for instance, in which the preservatives contain gum arable is that moisture is attracted, and the sen- sitiveness of various parts of the plate is affected. No better varnish is known than albumen, though this has its disad- vantages as regards rapidity, unless the greater proportion of it be removed previous to desiccation, or unless it itself becomes a vehicle for holding the sensitive salts, as in the collodio-albumen process. In the writer's opinion an un- exceptionable preservative has yet to be found. It appears dubious whether it will not become advantageous to dis- pense with it altogether, when tTie balance between the pyroxyline and sensitive salts is properly adjusted, as in the case of emulsion plates. It must, then, be borne in mind that the word ' preservative ' is only employed for want of a better. Drying the Plate. Ordinarily speaking, the film is allowed to dry spontane- ously, for which purpose a cupboard or box should be fitted up in the manner described in the various handbooks. Another plan that may be adopted by the student, if the plate be not too large, is the use of the hot-air bath, employed in chemical laboratories. The author has found that up to 8^ X 6i inches this method is useful. It is found convenient to allow the doors to be left open till the surface moisture has disappeared, after which they may be closed and the plates be allowed to dry at the higher temperature. Half a dozen plates may be dried by this means in half an hour, Gum-Gallic Process. Of dry-plate processes, only two will be described in detail; descriptions of others can be found in various practical works on the subject. The first that will be described is the gum-gallic process, as introduced by Mr. R. Manners Gum gallic Process. 105 Gordon. His directions are given, and if carefully attended to will give negatives of unequalled harmony. To any ordinary collodion, 2 per cent, of cadmium bromide is added. The plate having been given a sub- stratum, as shown at p. ioi,is coated with this collodion and immersed in the sensitising bath, and allowed to remain in it from 7 to 10 minutes, according to the temperature. This length of contact with the solution is sufficient to allow most of the bromide to be converted into the silver compound. The washing should be of a thorough nature; the longer the plates have to bekept,"the longer it should be contmued. The preservative made as under is next applied, by floating it over the surface of the plate. 1. Gum arabic .... 7 grammes Sugar candy . . . • 1 75 grammes Water 120 cc. 2. Gallic acid . . . . i gramme Water . . . . 40 cc. These two solutions are mixed in the above proportions. No. I is best prepared by the aid of the heat of a water- bath. The following arrangement will be found useful in its preparation, as well as in numerous other cases : — c is a water-bath two-thirds filled with water ; on the top are rings of varying diameter, fitting into one another, in one of which the flask a, containing the gum-water and sugar-candy, is placed. A small funnel (b) is dropped into the neck of the flask, in the conical part of which a portion of the steam condenses and runs back into the flask. This prevents too great a diminution of the liquid whilst the gum is in the act of dissolving. The mixed solutions should be filtered, but in this operation great difficulty is often found. The most ready method of effecting it is by the aid of a Bunsen water-pump ; by an aspirator of the usual form ; or by an exhausting syringe. The arrangement adopted will be seen from the accom- io6 Dry-plate Processes with the Bath. panying sketch. The pump, or other exhausting appara- tus, &c., is attached to india-rubber tubing. It is preferable Fig. 20. Fig. 21. Fig. 22. to filter the solution whilst warm ; when cold the pores of the filter-paper are rapidly filled up, and the solution re- fuses to pas.s. It may be necessary to fix into the funnel a platinum foil cone, made by cutting a piece of platinum foil in a circle, and cutting off a sector, as indicated in the annexed figure. In any case the filter-paper should be thin and free from iron.' The preservative solution is floated over the plate, and, after remaining on about a minute, is allowed to dry. If the surface appear dull, it should be dried by artificial heat previous to exposure in the camera. The exposure varies according to the developer em- ployed. In developing a plate by any method, it is usual to apply a narrow edging of india-rubber solution round the • This may be detected by moistening it with hydrochloric acid, and letting fall on it a drop of potassium ferro-cyanide. Acid Development. 107 Fig. 23. plate by means of a piece of stick, or by a piece of blotting paper held in the fingers and run round. A prettier piece of apparatus which is sometimes employed to give the edging is the following : — A camel's-hair brush, B, is held in position by a couple of wire loops, c c, inserted in a stick, a, and so arranged that the brush may be lowered into the solution without wetting A, The bottom edge of the stick is brought against the edge of the plate, so that the brush rests on the film, and having drawn round the plate, a neat edging is given. The strength of the india- rubber solution may be five times that given on p. 10 1. If the first developer be used the exposure may be from four to twenty times that required to give an ordinary wet plate, whilst if the alkaline developer be employed it will develop well with the exposure of a wet plate. Acid Iron Developer. 1. Ferrous sulphate . Water 2. Gelatine Glacial acetic acid Water 2 grammes 30 cc. 4 grammes 60 cc. 400 cc. The gelatine may be dissolved in the water, and the glacial acetic acid added to it. This is quite as effective, dissolving the gelatine first in the acetic acid, and the solution is much more quickly made. Three parts, by measure, of No. I should be mixed with i part, by measure, of No 2, and after filtering the developer is ready for use. It is better to mix them only a short time before they are required, as a slight precipitation takes place if they be kept long together. To every 4 cc. of the developer i drop of a 60 per cent, solution of silver nitrate should be added, and the apphca- tion be immediately made to the plate. The following is the alkaline developer which may be employed : — io8 Diy- plate Processes with the Bath. Or, 1. Ammonium carbonate . Water . . , . Ammonium hydrate, -880 . Water .... 2. Potassium bromide Water .... 3. Pyrogallic acid , Alcohol .... With this the following proportions No. I solution .... >> 2 ,, . . .2 parts 3 5 grammes 30 cc. I part 16 parts. I gramme 35 cc. I gramme 5 cc 4 parts I part in cold weather) I part. These are well mixed immediately before use, and after the plate has been moistened by water are flowed over it. If the exposure has been of right duration the image should immediately appear, in which case the solution should be flowed back into the developing cup, and the detail he allowed to ' come up ' by the small quantity remaining in the film. When this is secured another part of No. 3 may be added, and density will gradually be attained. The writer prefers not to give the full density by the alkahne solution, but rather to gain it by the appHcation of the pyrogallic acid intensifier with the silver nitrate solution (see p. 71). If this procedure be adopted, the development should be stopped immediately all the detail is visible by reflected light, and the surface should be flooded with a I per cent, solution of acetic acid in water. The intensifica- tion next proceeds as in the ordinary bath dry-plate processes. With an under-exposed picture, if the detail does not appear with the above proportions of the alkaline developing solution, a new mixture is made, nearly all of No. 2 solution being omitted. Unless the exposure be very much under- timed, this is usually efficacious. With an over-exposed picture the image will flash out ; the developer must at once be washed off, and a double amount of No. 2 added, or resort may at once be made to the acid intensification. Ferrous Oxalate Developer. 109 The following is the preparation known as the ferrous oxalate developer : — 1. Ferrous sulphate . . . . IC grammes Water 30 cc. 2. Potassium oxalate . . .10 grammes Water . . . . . 30 cc. One part of i is mixed with 3 parts of 2 in order to form the ferrous oxalate mixture. The ferrous sulphate is decomposed by a portion of the potassium oxalate, and the remainder holds the ferrous oxalate thus formed in solution. To make up a developer .suitable for developing the dry plates in question the follow- ing should be used : — Ferrous oxalate solution . . 4 parts Potassium bromide solution in water, 10 per cent. . . . \ part. These are well mixed and after washing the plate are to be applied. As a rule good density will be attained with it, but the image may require intensification as before. The ferrous oxalate solution may be made by dissolving dry ferrous oxalate in a saturated solution of potassium oxalate in water, but as the solution thus made rapidly deteriorates it is recommended that the above preparation should be employed, since it is then always used in a fresh condition. In order to develop the image the plate should be immersed in water of not less than 16° C. If the iron developer be employed the time of immersion should be two or three minutes, whilst if it be the alkaline developer a mere moistening is sufificient. The method to be adopted with the former is that already described. When the detail comes well out, the intensification of the image is given by the ordinary pyrogalhc acid solution (p. 71). These plates are excellent when employed in a fairly dry chmate, but they are disappointing when much atmospheric moisture is present ; the gum softens and absorbs water, giving rise to spotty pictures. This seems to be due to a fungoid growth upon the gum, and there is no apparent remedy for it. 1 10 Dry-plate Processes with the Bath. The negatives taken by the gum-gallic process, under favourable circumstances, are everything that can be wished for, being delicate, full of detail, having the well-known bloom, and being fairly sensitive even with the iron de- veloper. In the hands of Manners Gordon it has proved the most trustworthy of any bath dry-plate process (except one) with which that eminent photographer has worked. Albumen Beer Process. This process has been fully described in another work by the writer,' and is given as therein described. It was intro- duced by him for solar photography, and was employed by the English Transit of Venus expedition. It is, however, equally adapted for landscape work, and is very certain in its results. The collodion employed can be that described at p, 53, though for more rapid work the following is better : — The relative proportions of ether and alcohol are ad justed according to the temperature in which the plates have to be prepared. With the ordinary samples of collodion the usual silver nitrate bath (p. 6i) can be used, but with the collodion made as above it is advisable to use a bath containing i6 per cent, of silver nitrate. In both cases rapidity is increased by the addition of 2 per cent, of uranium nitrate. It has been found advantageous to dip the plates at first in the weaker bath, allowing them to remain in it for a ccupie of minutes, and then to transfer them to the stronger for ten minutes more. This mode of procedure gives verv sensi- tive and opaque films, the greater part of the actinic rays being thus utilised. The sensitiveness, however, greatly depends upon the porosity of the film, and every effort ' Lisiruction in Photography . Piper and Carter. Alcohol, -825 . Ether . Pyroxyline Ammonium iodide Cadmium bromide • 450 to 350 cc. . 350 to 450 cc. 12 grammes • 3 '5 grammes 9*o grammes. Preparation of Albumen. 1 1 1 should be made to attain the maximum of this quality without injuring the texture of the film. The addition of the largest practicable amount of water to the collodion tends to give this desired porosity. After sensitising the plate is slightly washed, and the first preservative applied, which is — Albumen ..... loocc. ' Water loo cc. Ammonium hydrate . . . 12 cc. This is beaten up into froth (or is mixed by pounding it in a mortar with silica), and, when settled, the clear liquid is decanted off. The solution is mixed, immediately ^ before use, with an equal quantity of ordinary beer or stout, and floated over the plate. When botded beer is used, it is ad- visable to drive off all the carbonic acid by a gentle heat. The excess is drained off, and the film thoroughly washed under the tap for a couple of minutes, and is finally rinsed with a solution of plain beer, to which i per cent, of pyro- gallic acid has been added. The plate is dried in the ordinary manner. The exposure with well-prepared dense plates is often as short as that necessary for wet plates, but great latitude is admissible. With 20 times the minimum exposure necessary to secure a good negative there need be no danger of veil. The development need not be effected for at least a month after exposure. The following solutions are those employed : — I. Pyrogallic acid . I gramme Water .... . 40 cc. 2. Ammonium hydrate, -880 . I part Water .... . 4 parts 3- Citric acid .... . 4 grammes Acetic acid (glacial) . . 2 cc. Water .... . 30 cc. 4- Silver nitrate I gramme Water .... . 20 cc. ' Dried albumen — 5 grammes to 100 cc. of water— may be substi- tuted for the 100 cc. of albumen. 2 This precaution is necessary, otherwise the tannin of the beer is precipitated by the albumen. 112 Dry-plate Processes with the Bath. The description of the development is taken from the work already referred to. ' The washing water before development should be of a temperature not less than 15° C. When the plate is washed, the following developer is employed :— To each 50 cc. of No. I are added 10 drops of No. 2, and after well mixing with a stirring rod the solution is floated over the plate. 'Almost immediately the image begins to appear, and after a few seconds' interval the detail can be seen by reflected light to gradually develop. Another 7 drops of No. 2 are again added to the solution, which is once more floated over the plate. Twenty drops of No. 3 are next poured into the developing cup, and the solution from the plate poured into it. Again the plate is rinsed, this time by the acidified pyro- gallic solution, and intensification given by the use of it with a few drops of No. 4. It is not advisable to allow too much detail to come out with the alkaline solution, but to allow a portion of it to be brought out by the subsequent treatment with pyrogallic acid and silver. The alkaline developer re- duces the bromide salt, and leaves the iodide to be attacked by the silver solution. It will be remarked that no restrainer such as bromide is.employed; the albumen dissolved by the ammonium hydrate plays the part of a retarder, but not as a destroyer of the latent image When the image appears sufficiently dense, it is fixed by either sodium hyposulphite or by potassium cyanide ' One point in the preparation of these plates cannot be too strictly attended to viz. to keep the fingers away from all contact with the film during preparation. A touch, however shght, will cause a stain, and unsightly markings extending across the plate have been traced to the same cause, Collodion-emulsion Processes. 113 CHAPTER XVI. COLLODION-EMULSION PROCESSES. We now come to a class of dry-plate processes which differ from those which have been described, inasmuch as in them the sensitive salt of silver is held in suspension in the collodion. When such an emulsified collodion is poured upon the plate, we obtain a film capable of receiving an invisible impression. The emulsion is principally formed with silver bromide, though certain other additions are some- times necessary in order to ensure clearness in working. The silver bromide is introduced into the collodion by dis- solving some soluble bromide in it, and then gradually adding an alcoholic solution of silver nitrate, the amount of which may be either in defect or excess of that necessary for the complete conversion of the whole of the soluble bromide. It is found practically that silver bromide is most sensitive when exposed in presence of an excess of silver nitrate, but most prone to give veiled images, whereas if the soluble bromide be in excess, a developable image is formed less rapidly, but greater freedom from fog is secured. Some of the probable reasons for this may be gathered from Chapter XIV. In preparing an emulsion, it is rarely possible to hit the exact proportions which shall give neither excess nor defect in one or other of the emulsion-forming constituents, and so to secure great sensitiveness with clearness ; hence it is always better so to arrange the proportions that one or other shall be in known excess. The following experiments may be made with advantage, in order to see what will be the result of having excess of the soluble bromide or of silver nitrate. Prepare bromised collodion as given for the first pro- cess (described p. 116) and to 100 cc. add, in the first case, 6 grammes of silver nitrate; and to another 100 cc. the quantities given. After leaving for twenty-four hours, 1 14 Collodion-emulsion Processes. coat plates with these two specimens respectively, exposing before the collodion has become dried. Note their be- haviour with the alkaline developer (see p. 108). It will be found that with the plate in which there is excess of bromide there will be no developable image, whilst with that prepared with excess of silver nitrate there will be a fog over the image. Next take plates prepared with the same collodions and wash thoroughly under the tap. Both now will give good developable pictures, but that having an excess of bromide will require a longer exposure to give a good nega- tive. Next, take similarly prepared plates, and, after wash- ing, flow over them a solution of tannin, and the images will be found to be more readily developable. Again, pre- pare an emulsion as before, using a soluble metallic chloride instead of the hydrochloric acid, and, having divided it into two portions, add an excess and defect of silver to them respectively. Prepare plates as above, and notice the behaviour. It will be found that with the slightest excess of silver there will be inevitable fog, whilst with the defect the behaviour will be the same as that given above. Per- haps the most sensitive emulsion may be prepared by having a slight excess of silver nitrate and nitric acid, omitting the chloride altogether. The use of silver chloride in the emulsion secures density ; it does not of necessity secure freedom from fog, but, being more soluble in ammonia than the bromide, the ammonium pyrogallate readily disolves it, and immediaitely precipitates it on the parts acted upon by hght. It is believed that this simple explanation is capable of render- ing clear the use of it, as recommended by various writers. The following rules may be laid down : — ist. That nothing but silver bromide is necessary to give a good /image, if the soluble bromide be in excess. 2nd. That if there be an excess of silver nitrate, the emulsion must be acidified with nitric or other mineral acid, or be neutralised by certain metallic chlorides, to secure freedom from fog. Experiments. With regard to the first part of this second rule, it will be remarked that the same necessity arises in the bath processes where much bromide is present in the collodion. It must also be borne in mind that if the first rule be followed, the density of the developed image will be strong; whilst if the latter (unless chloride be present), it may be weak, unless some density-giving body, such as silver nitrite, glucose, &c., be added to the emulsion, in which case good density can be obtained. Having made an emulsion as described, it will be well to make another simple experiment. Coat a plate with it, and allow it to dry. On drying, it will be found that the soluble salts have crystallised on the surface of the plate, rendering the development of an image almost impossible. Here we have evidence that it is necessary to remove these salts. There are two ways of accomplishing this, either by washing the plate after being coated with the collodion, or by washing the whole bulk of the emulsion after allowing it to gelatinise by evaporation of the solvents. In the last method the washed emulsified collodion is dried, and the resulting pellicle is again dissolved in ether and alcohol. It has certain advantages about it which cannot be gain- said ; thus, the sole manipulation in getting ready a dozen dry plates is to coat them with the emulsion, and then allow them to dry. It also has drawbacks; one of the principal of which is the liability to spots on the negative, a point which is difficult to understand, since they probably will be entirely absent on plates prepared with the same emulsion unwashed. It is not proposed to enter into details of all the different varieties of the emulsion processes : two distinct varia- tions will be given, one of which will be typical of an emulsion where the coated plate alone is washed, and the other of a washed emulsion. Both these will be of the simplest character, and have succeeded in the hands of the writer. I 2 Ii6 Unwashed Emulsions, Unwashed Emulsions. Canon Beechefs Process. The following are Canon Beechey's directions, which if followed will give tolerably certain results. Take Cadmium bromide (dried) . . .90 grammes Alcohol, -805 I litre, and allow the mixture to stand, and then decant from it any quantity that may be required. To each 100 cc. of it add I "6 cc. of strong hydrochloric acid. Of the above solution take . . 50 cc. Ether (720) . . , . no cc. Pyroxyline 2 to 2 5 grammes. The pyroxyline should be that prepared at high tempera- ture, and may contain nitro-glucose if thought advisable (see p. 48). It will be found necessary that it should stand at least a day before being ; used filtering through tow only partially frees it from small particles of undissolved cotton. If much of the emulsion is likely to be required, one of the tall graduated glasses, as in fig. 24, will be found con- venient ; any quantity can then be syphoned or decanted off. When the collodion is clear it is ready for sensitising : that part which is to be emulsified should be poured into a glass beaker. For every 100 cc. of the above, take 5-6 grammes of silver nitrate and powder it carefully in an agate mortar, or by means of a glass stopper on a thick sheet of glass. Place it in a test-tube, with just sufficient water to dissolve it, and add to it 30 cc. of alcohol, -805. This alcoholic solution of silver nitrate should be added to the collodion (fig. 25) drop by drop, and the emulsion should be stirred continually whilst the additions are made ; finally, the test-tube should be rinsed out with another 30 cc. of alcohol, and added to the collodion. After the final addition the emulsion should be very smooth and rather thick, though when poured upon a strip Beechey^s Process. 117 of glass it will appear of a transparent nature. After keeping twenty-four hours, however, it will be creamy, and appear of an orange colour when a candle flame is viewed through small layers of the liquid. This colour is indicative of a proper preparation of the emulsion, showing that the silver bromide is in a very minute state of division. It is possible to cause an emulsion to have a decidedly bluish-green tint, in which case the particles seem to be in a different state of aggregation to that when the ruddy tinge Fig. 24. is seen. When further improvements in the emulsion are made, it may be that this blue tint will be the mark of a more sensitive preparation. This emulsion should be used soon after the creamy state is attained, as otherwise it will again become thin, and the silver bromide will rapidly pre- cipitate on the bottom of the bottle. The plate having been coated with a substratum or edging (see p. loi), the collodion is applied in exactly the same way as is the unemulsified collodion for the wet process. It is necessary that the emulsion should be rendered homo- 1 1 8 Collodion-emulsion Processes. geneous, otherwise the film will appear granular. This is effected by shaking it in the bottle half an hour before it is applied to the plate. When the collodion is set, it is im- mersed in a dish of distilled water or filtered rain water till all the repellent action between the solvents and the water is eliminated, and till the great excess of silver nitrate and the other soluble salts is washed out. It may then be passed through another dish of water if found necessary, and finally allowed to rest in a dish containing beer, to every litre of which 2 grammes of pyrogallic acid has been added. The best kind of beer is that known as sweet ale, the saccharine and gummy matter being more abundant in it than in that known as bitter ale. Any trace of silver which may remain in the film combines with the organic matter, and the danger of veil is thus reduced. The drying is conducted in the usual manner, care being taken not to disturb the plates till they are thoroughly desiccated. If the calculation as to the amount of silver nitrate neces- sary to combine with bromide and hydrochloric acid be made, it will be found that there is a considerable excess of silver nitrate in the above emulsion. The organic matter of the preservative is present to give intensity during develop- ment. The development will be carried out by the alkaline method given at page io8, or the ferrous oxalate at page 109, the whole of the descriptions applying to the process under consideration. CHAPTER XVIL WASHED COLLODION EMULSIONS. We now come to the class of washed emulsions. There are almost endless varieties of preparation, but experience seems to show that the simpler the formulae are kept the more certain are the results. The following is a mode Washed Collodion E7nulsions. 1 19 of preparation which has almost invariably given rapid and excellent results, and the writer strongly recom- mends it. The plain collodion is prepared as follows : — P2ther, 730 50 cc. Alcohol, -820 to -830 . . -25 cc. Zinc Bromide .... 5 grammes Pyroxyline . . . . ' . 2-5 to 3-5 grammes. The variation in the amount of pyroxyhne is given, as on its quality largely depends the amount which is essential. With ordinary pyroxyline the smaller amount will suffice, whereas, if it be of a short pulverulent class, the larger quantity will be necessary. The writer recommends the ordinary tough pyroxyline, prepared from ordinary cotton previously boiled in strong alkali, and in the strength of acids given at p. 45. The zinc bromide may be dissolved in the alcohol, with a small amount of water in addition, if found necessary. To the above quantity of zinc bromide should be added about 30 drops of nitric acid, or i small drop of bromine. The reason for either of these additions has already been given. If the bromine be employed, care should be taken to estimate the quantity of silver nitrate with which it will combine. This may be conveniently executed by dropping, say, 3 drops into 50 cc. of water, and precipitating with a standard solution of silver nitrate, or by taking care to have an excess of silver filtering, washing the precipitate, and gently igniting it, in order to convert the silver brcmate into silver bromide, and then weighing it. When the bromised collodion is perfectly clear from all floating particles, which can be secured by allowing them to settle, or by filtering them through cotton which has been previously well washed and rinsed with alcohol, it is ready for the addition of the silver nitrate. It is well to allow an excess at least of \ per cent, of the silver salt if great sensitiveness is required, otherwise the bromide may be allowed to be slightly in excess. To convert the above 1 20 Washed Collodion Emulsions. amount of zinc bromide into silver bromide would theoreti- cally require 7-56 grammes, but in practice it is found that this amount cannot be depended upon. When nitric acid is used with the zinc bromide it will be found that 8-5 grammes suffice. When the bromine is used the amount required must be subject to experiment. The student may find it convenient to add the silver nitrate solution httle by little till he hits the point where an excess commences, and then to add h per cent, more of silver nitrate. To ascertain when the excess occurs, a drop of the emulsion, from time to time between the additions of the silver nitrate solution, should be poured on to a glass plate, and a little potassium chromate dropped on to it ; a red coloration due to silver chromate shows the slightest excess. The silver nitrate is dissolved up as in the last process, and poured in as already described. It may be as well to note that finer-grained emulsion is sometimes made by keeping out half the collodion, adding the whole of the silver little by little, and then stirring in the other half of the collodion. In order to obtain a maximum sensitiveness, the emul- sion should be left for from 24 hours to 60 hours, the time depending much on the kind of pyroxyline employed. If a large batch of emulsion is to be made up, it may be advisable to prepare 50 cc. first, and at the expiration of 24 hours, 48 hours, 60 hours, to wash it as directed below, and test its qualities ; and a note should be made of the period at which it seems most sensitive, and at the same time free from fog. It is noteworthy that the washed emul- sion usually appears to possess the same qualities as the unwashed. If therefore, this process of testing be con- sidered too tedious, the emulsion may be tested at intervals in the unwashed state, or, to speak more correctly, after it has been washed after coating the plate. When the emul- sion is in a proper state, it should be poured out into a flat dish, and be allowed to set. A gentle agitation with a glass Preparatioti of Emulsion. 121 rod causes more surface to be exposed, and the evaporation consequently takes place more rapidly. In a moderately warm room half an hour will generally suffice to render it in a condition fit for the subsequent washing. This may be known by the glass rod separating the gelatinous mass in flaky pieces, which retain their shape after a minute's interval. The emulsion is next covered with distilled or pure water, and allowed to soak for 2 or 3 minutes, when the liquid is drained off, and the emulsion transferred into a jar, and again covered with water. After stirring well, and allowing a time to elapse (say 15 minutes) in order that the water may penetrate into the interior of the mass, it is again drained off and replaced by fresh water. This wash- ing is continued till the wash water,, treated with hydro- chloric acid if an excess of silver have been employed, or with silver nitrate solution if excess of bromide have been employed, shows only a slight opalescence. It is important that the washing be done rapidly, as long soaking of the gelatinous emulsion is greatly detrimental to securing den- sity of the image when alkaline development is employed. The reason of this lack of density seems to be due to the fact that some small portion of the precipitated pyroxyline is soluble in water, and that it is this organic substance which causes the silver to be reduced in the film to such a state of subdivision as to cause opacity. When the washing is complete, the emulsion pellicle is pressed between blottmg-paper in order to get rid of the greatest part of the water which is occluded in it, and is allowed to dry spontaneously or by the aid of heat. The first mode of drying is the safer, as it sometimes occurs that the heat of even a water oven, fig. 26, is sufficient to shrivel up the pellicle to such an extent that it becomes very insoluble in a mixture of ether and alcohol. The pro- cedure that the writer recommends is to allow it to become nearly diXy in the water oven, and then to allow the last remains of water to evaporate spontaneously, A slight quantity of 122 Washed Collodion Emulsions. Fig. 26. water is not hurtful to the emulsion, and if, after hard pres- sure with a spatula on a square of the pellicle, there is not sufficient moisture to damp blotting-paper, it may at once be transferred to a bottle to re-dissolve. The bottle em- ployed should be capable of holding twice the amount of solvents that will be used, as space is required for shaking; The solvents em- ployed are equal parts of pure ether and ab- solute alcohol, 100 cc. being employed for every i| gramme of pyroxyline employed. With some pyroxy- line the resulting images are deficient in vigour. To correct this, to the first wash water a strong solution of tannin, or salicine, &c., may be added. A modification of the above emulsion may be prepared by emulsi- fying with an excess (say 5 grammes) of silver nitrate, after 15 to 20 drops of strong nitric acid added to each 100 cc. of the col- lodion. After the addi- tion of the exces.s of silver nitrate a sufficient quantity of some metallic chloride, such as of cobalt, may be added, in order completely to neutralise that excess of silver, and to leave a slight excess of the soluble chloride. This method is due to Mr. New ton, and in his hands appears to work satisfactorily. The presence Drying the Plate. 123 of a free chloride is not so destructive of sensitiveness as the free bromide, hence the preference that is given to the former over the latter for neutralising any excess of silver nitrate. It is often useful to keep the pores of the col- lodion open by a little resinous matter, such as gum am- moniacum. This gum is very insoluble, and, if employed, a saturated solution of it in alcohol should be prepared, and the resulting varnish should replace the alcohol em- ployed for re-dissolving the emulsion pellicle. With all the washed emulsion processes the plate is coated as with ordinary collodion and allowed to dry, no preservative being necessary. The dark heat which is radiated from a slab of ^ iron placed over a spirit lamp or Bunsen burner is recom- i;mended by Mr. Woodbury, to cause the rapid evaporation of j the solvents from the coated plate. The plate must not be exposed to the naked flame from these sources, as the blue colour is sufficiently intense to cauSe a veil to spread over its surface on development. Washed emulsion plates will keep indefinitely both before and after exposure, as will the emulsion if all excess of silver nitrate be washed away. The exposure necessary is largely dependent on the presence of soluble bromide or chloride and on their quantity. As a rule the plates require half as much exposure again as a wet plate. The development is conducted as laid down for the pre- vious process, and calls for no especial remark. CHAPTER XVIIL THE GELATINO-BROMIDE PROCESS. It would not be advisable to enter into all the various modifications which could be described regarding this pro- cess. It is thought sufficient to describe but two methods of preparation of the plates, both of which give results which cannot readily be beaten, either for rapidity or for brightness in the resulting negatives. This process is the most modern one, and its advent opened out a new era in 124 The Gelatino -bromide Process. photography, since sensitive surfaces can now be prepared which are 60 times more rapid than the older wet process which has been described. It must, however, be said that tnis gain in speed does not at the same time signify gain in beauty of result. For giving beautiful prints we beUeve that the collodion processes surpass this newer one, as the picture is so much more under control in the matter of giving intensity. The gelatino-bromide process, as its name signifies, is one in which gelatine acts as the vehicle in which the sensi- tive salts are embedded, and it very much depends on the quality of the gelatine as to the rapidity and freedom from disfigurements which the plates prepared with it possess. The gelatine employed should be of the purest, and it should not set at too low a temperature when it is liquefied. It may be stated, as a rule, the good gelatine will absorb enough cold water to dissolve it, if the temperature is raised above 33° C. If only one quahty of gelatine is to be used, Heinrich's medium hard will be found very suitable, but we prefer to mix this with Nelson's No. i photographic gela- tine, as we shall presently describe. One fact must alVays be borne in mind, that frequent reheating of gelatine spoils its setting qualities ; hence care must be taken as to this by keeping back a certain quantity of it during the prepara- tion of an emulsion which requires much heating. The sensitive salt or salts are emulsified in the gelatine very similarly in maimer to their emulsification in the collodion emulsion processes described in the last two chapters. The general outline of the process is as follows : — Adding gelatine to the emulsion. Setting of the emulsion. Washing the emulsion. Draining the emulsion. Cleaning the gla-ss plates. Coating the plates. Drying the plates. Packing the plates. Dissolving the soluble haloids. Dissolving the silver nitrate in a solution of gelatine. Emulsification of the silver haloids. Increasing the sensitiveness by the application of heat or other- wise. Outline of Operations. We shall deal with each of these operations in the order given. First of all the following should be weighed out : — 1. Potassium iodide .... '3 gramme. 2. Potassium bromide . . . ■ 8*7 grammes. 3. Nelson's No. i photographic gelatine 2 grammes. 4. Silver nitrate . . . .11-4 grammes, f Heinrich's gelatine ... 10 grammes. ' Nelson's No. i gelatine ... 6 grammes. Nos. 3 and 5 are covered with water, in separate vessels, ' and stirred to get rid of all adhering dust. Nos. i and 2 are dissolved in 3-5 cc. and 40 cc. respectively. No. 3 is swelled in 30 cc. of water, and then dissolved by the aid of heat. No. 4 is dissolved in 15 cc. and heated to 50° C. A drop of strong hydrochloric acid is added to the solution of No. 2. In the dark room (the lighting of which will be described in a subsequent chapter) No. 3 is added to No. 4, and shaken up in a bottle, preferably an orange- coloured hock bottle, till the mixture between the two solu- tions is complete. About | of the solution of No. 2 is then added drop by drop to the mixture in the bottle, and thoroughly shaken between each addition, and then to the remaining portion of No. 2 No. i is added, and the mix- ture added drop by drop into the bottle. If the equivalent of the silver and haloids be calculated, it will be found that the latter are in excess. It follows from this that no com- bination between the silver nitrate and the gelatine can occur, since the affinity of the haloids for the silver is far greater than is that of the organic substance. When the emulsification is complete, if a drop of the fluid be poured upon a glass plate and a candle flame be examined through it, the flame should appear ruby coloured, showing that each particle of silver is in very fine state of division. The next process is that of heating the emulsion to give sensitiveness. In the state in which it is before so doing plates prepared with it are slow and give thin images, 126 TJie Gelatino -bromide Process. but after boiling great density of image is secured together with rapidity. The boiling is analogous to the ripening in the collodion emulsion processes. It is convenient to empty the emulsion out of the bottle into a flask capable of holding at least 200 cc. of liquid, and this is placed in a saucepan of water, heated over a Bunsen burner. The flask should be placed in a jacket to prevent the access of light to the sensitive material it contains. A tin canister answers very well, if a clean cork be loosely placed in neck of the flask. The water should be brought up to boiUng- point, and kept in a state of ebullition for about 40 minutes. There seems to be a time when maximum sensitiveness is arrived at, after which there is a gradual decrease in sensi- tiveness, and the emulsion is apt to become granular. The boiling makes the particles coarser than when the emulsion was first mixed, and on examining a drop of it on a strip of glass the transmitted colour will be found to be changed. Instead of the ruddy tint, it will appear to be bluish, with perhaps a tinge of orange in it. Whilst the boiling is taking place No. 5 should be swelled in 65 cc. of water and dissolved by the aid of heat. It and the emulsion are then cooled down by allowing cold water to run over the flasks containing them till a temperature of about 23° C. is reached. The emulsion and the gelatine are then mixed, and well stirred up together, so that the mass is approxi- mately homogeneous. A white jam-pot is an excellent receptacle for the mixture, since it will then set in a solid lump and be ready for the next operation. As far as this point the two methods are dissimilar, but henceforward the manipulations for the two are the same. We therefore shall proceed to give the alternative method of preparing the emulsion. The same quantities of the different materials are weighed out as before, and the numbers we refer to are the same as those previously employed. No. I is dissolved in 3 cc. of water, No. 2 in 40 cc. of Boiling the Emulsion. 127 water. No. 3 is soaked, swollen, and dissolved in the same water in which No. 2 is dissolved. No. 4 is dissolved in 30 cc. of cold water, and ammonia ('880), diluted to half- strength, is added drop by drop till the oxide of silver first formed is redissolved. No. 5 is dissolved in about 60 cc. of water. The solution of gelatine and bromide (Nos. 3 and 2) is allowed to cool to about 20° C, and then the ammonio- nitrate of silver solution is added drop by drop, with constant shaking or stirring. The iodide solution (No. i) is finally dropped in. If the solution of No. 5 be at once added a fairly rapid emulsion will be formed, but if the emulsion be allowed to stand for from 18 to 24 hours before the addition is made, a much more rapid plate can be pre- pared from it. The emulsion will then be grey by trans- mitted light. The emulsification takes place in a cool solution, and therefore there is less liability for the gelatine to be acted on injuriously by the ammonia than if heat be applied. In warm weather as much as half of No. 5 may be at once added to the emulsion to prevent its becoming granular. The emulsion is then, as in the first method, transferred to a jam-pot and allowed to set. In hot weather it is as well to let the jam-pot and its contents stand in iced water to enable the emulsion to set rapidly and firmly. A certain amount of firmness is a desideratum, as in the subsequent w^ashing if only slightly set there will be too much adherent water. The washing has next to be effected, and this can only be carried out by causing the set emulsion to be divided into very fine shreds. Squeezing the emulsion through coarse canvas, such as is used for Berlin wool work, or, better still, through mosquito netting, breaks the emulsion up to a proper state of subdivision.^ The contents of the jam-pot are placed in a square piece of such netting, twisted up in it, and squeezed through into a jar of water, the canvas being held beneath the surface to prevent the shreds rejoin- ing. When all is squeezed through, the particles of gelatine 128 The Gelatiiio-broniide Process, may be transferred to the canvas once more, and be freely doused with water from a water-tap or by hand, and then left to soak in water for half an hour. The squeezing operation is again performed, and after another half-hour's washing the emulsion may be drained. This may be carried out by placing it on a hair sieve, or over the mosquito netting. A couple of hours' rest should render it sufficiently free from all adherent water. The emulsion may now be retransferred to the jam-pot, which should be carefully cleaned, and then surrounded with hot water of about 30° C. till it is well dissolved. The addition of "03 gramme of chrome alum is a safety to the emulsion. It may be dissolved in a small bulk of water and then dropped in. Next 14 cc. of absolute alcohol are stirred into the viscid mass. If extreme rapidity be required, to the amount of emulsion here supposed to be made 10 drops of ammonia diluted with 100 of water may be stirred in, the temperature of the liquid being kept up to 33° C. for a couple of hours. After filtering, the emulsion is now ready for coating the plates. This may be carried out by allowing it to percolate through a double thickness of swan's-down calico. The plates are cleaned with nitric acid and water, and then well washed in clean water. They are then dried with a clean diaper duster, and placed in a heap in the dark room for the next operation, after removing all adherent dust or fluff by a soft brush. The emulsion, which should now be in a flask, must be kept in water of about 44° C, and, supposing a whole plate (8^ in. x6^ in.) is to be coated, about 70 cc. of it put into a glass measure which has a lip, and a pool of the warm emulsion is poured in the centre. In fact, the plate is coated as with collodion (seep. 10 1), except that it is not drained so completely, a fair amount being left on the plate. The plate is next placed carefully on a perfectly level shelf of slate or thick glass, and kept there till the gelatine is; set, Mixing Gelatijie Emulsion. 1 29 Fig. 27. when it is transferred to a rack such as shown in fig. 27. When the rack is full it is placed in a drying cupboard, of which there are many varieties. If a large number of plates are to be -dried, a room which is dark, warmed, and well ventilated will answer, otherwise the cupboard must be re- . sorted to. The best form of cupboard that the writer has tried is shown in fig. 28, a description of which is taken from 'Instruction in Photography.' ' B is a zinc boiler, from which are taken two pipes, d and H, leading to the coil of pipes, c c c c. A supply tank, t. Fig. 28. is fastened against the side of the cupboard, and a supply pipe joins the coil pipe at h. From d another pipe, a, is ' Piper and Carter, 5 Furnival Street, Holborn, K 130 TJie Gelatino-bromide Process. led, terminated with a tap, which allows any air to be got rid of, which would otherwise stop the flow into c c. At h is a tap, which allows the whole apparatus to be emptied at pleasure, k is a hot-air shaft, being some four feet above the box. It is terminated by a bend in two directions, and can be fitted with a cap, if required, in which are pierced ori- fices. Beneath are a couple of ventilating inlet pipes, likewise bent in two directions, l l l is a false bottom, pierced with holes, on which the drying racks are placed, f is a gas jet, which heats the water. (The cupboard is shown with only one door.) Each door is made light-tight by means of fillets, which need not be described. The hinges are pianoforte hinges. The piping is made of composition gas-pipe, though perhaps iron would be better ; still, as they are, they answer perfectly. The interior of the cupboard must be maintained at a constant temperature of about 25° C, and the gas must be regulated accordingly. A gas regulator is an excellent piece of apparatus to have to keep the cupboard up to this temperature. It must be recollected that the great desideratum in drying the plates is a constant circulation of air rather than any great heat. The plates should be dry in at least 24 hours, but it is well to keep them as if drying in the cupboard for 48 hours, to ensure perfect desiccation. The plates have next to be stowed away, and there is no better plan than to place them face to face in pairs, and then to make up bundles of these pairs, six plates in each, and tie them firmly together. Perfectly clean and dry pink blotting-paper may also be placed between each pair of plates, and the bundles made up as before. Exposure of Gelatino-bromide Plates. 1 3 1 CHAPTER XIX. EXPOSURE AND DEVELOPMENT OF GELATINO-BROMIDE PLATES. There are one or two contrivances in the market which serve to indicate the speed— the relative sensitiveness — of a plate. Such a contrivance is Warnerke's sensitometer, which is a most useful piece of apparatus. A very fair judgment of the rapidity of a plate may, however, be arrived at by exposing different portions for 5, 10, 15, &c. seconds to the light of a candle placed 1 2 feet off the plate. It will require a very rapid plate to show any exposure having been given to the portion which has really been exposed 5 seconds. The plate after preparation should never see any light which is injurious to it, and great care should be taken that in the preparation the light which must perforce fall on it is of the least hurtful colour and of a very feeble character. The same remark applies to placing the plates in the slides, and in their subsequent development. As to exposure something must be said. The plates pre- pared as described in the previous chapter are exquisitely sensitive to white light. With a rapid rectilinear lens by Dallmeyer, using the smallest stop (No. 5), an open land- scape on a bright spring day is impressed in half a second, always supposing there are no very near deep shadows, which will require a slightly longer exposure to bring out detail in them. With these particular plates, when they are covered fairly thickly with emulsion, the exposure may be prolonged to 3 seconds under the same circumstances without any detriment to the resulting negative. This is what is techni- cally called latitude of exposure. A negative with such a prolonged exposure requires care in development, but all K 2 1 3 2 The Gelatino-bromide Process. details in every par-t of the picture can be brought out without difficulty if proper precautions are used. With a larger stop, say No. 3, which is usually small enough to place the whole of a picture in good focus, only \ of the atove least exposure need be given, which is about ^th of a second. For such exposures ' uncapping the lens ' is impracticable, and resort must be had to some sort of mechanical shutter, a de- scription of some of which will be found in a subsequent chapter, as will be the choosing of the picture. When the slides containing the plates are carried to the camera they should be shielded from light, and the front board of the slide should only be withdrawn when the focussing cloth is thrown over the camera, also to protect it from light. Light usually enters a lens through the slots which are cut in the brass mounts for the diaphragms or stops. This orifice should be covered over with an india-rubber band. It also frequently happens that the light coming through the lens is reflected on to the sides of the camera. This may be overcome by placing an oblong diaphragm behind the lens just large enough to cut off all light which does not fall on the plate. The inside of the camera should be of a dead black, and, in fine, too much care cannot be taken to ensure the absence of all reflections. For developing these plates there are almost innumerable formulas, but the writer gives two for alkaline development which he has found work really well with him, and also one for ferrous oxalate. The first formula stands thus : — 1. Pyrogallic acid 2. Potassium bromide Water . 3. Ammonia (-880) Water . 4. Sodium sulphite dry. 40 grammes I litre. I part . 9 parts. A saturated solution in water. Development. 133 For developing a plate we usually commence by taking the following proportions : — No. 3 4 cc. No. 2 . . 4 cc. No. 4 . . . . . . . 4 cc. No. I . . . . . . . '2 gramme. The amount of No. i is not necessarily very exact, provided there be sufficient of it present, and it can be approximately judged after a few trials. A useful piece of apparatus is a bone or horn spoon to act as a dry measure for it. The above are mixed together, and then made with water to 60 cc. This quantity will be found sufficient with which to develop a ' whole plate.' The plate having been taken out of the slide, should be carefully dusted with a broad soft brush and laid in a dish very slightly larger than the plate. The developing solution should then, with an even sweep, be made to flow over the plate, care being taken that no bubbles form on the surface. Should a bubble appear it must be broken at once \vith the finger, or with a brush kept for the purpose. The appearance of the image should be watched. If it begin to appear in 10 seconds, the de- veloper may be kept in motion over the plate, and the image be brought out with the strength of developer given above, until the highest lights appear as blackish patches at the back of the plate when viewed by reflected light. Should the image not appear in 20 seconds, 2 more cc. of No. 3 should be added, when, if the exposure be anything ap- proaching that necessary, the image should be rapidly brought out and gain density. Should it appear that the density necessary to give the highest light will be arrived at before the whole of the detail in the shadows is out, it is an excellent plan to pour back the developer into the cup, and allow the image to come out under the m- fluence of the developer which is absorbed in the film. 134 The Gelatino-broviide Process. The rationale of this is that the developer soon exhausts itself in the parts of greatest density, whilst it is almost unaltered in the parts where no detail has put in an appear- ance. When the detail has been brought out in this manner, the image should be intensified by a fresh appH- cation of the developer. In the case of so-called instantaneous pictures, when exposure has been excessively short, it is a good plan to soak the plate in 3 cc. of No. 3 and 60 cc. of water, and after a couple of minutes to add to the developing cup the pro- portions above indicated of Nos. i, 2, and 4. This will cause what would otherwise be an under-exposed picture to develop as if it had received a longer exposure. Another developer which the writer can recommend from personal experience is a follows : — 1. Potassium carbonate, pure . . .3 parts Water , .8 parts 2. Potassium bronude ..... i part Water 25 parts. 3. Pyrogallic acid ...... dry. 4. A saturated solution of sulphite of soda in water. 3i CC. of No. I, -5 cc. of No. 2, -2 gramme of No. 3. and4cc. of No. 4 are mixed, and made up with water to 70 cc. and applied. This is a normal developer. The writer prefers to commence with 2 cc. of Nq. i, with the same amounts of Nos. 2, 3, and 4, and to add more of No. i, if required. For instantaneous pictures the plate may first be soaked in 3^ cc. of No, I and 60 cc. of water, and the bromide, sulphite, and pyrogallic acid subsequently added. Negatives de- veloped with this developer are delicate, but the density may be increased by a final addition of 2 drops of ammonia, •880. The ferrous oxalate developer may also be used as prepared at p. 109. The writer recommends that it should be used of half-strength {i.e. diluted with water) till all the detail appears, after which a fresh quantity of the strong developer is applied, to which i cc. of a 2-5 percent, solution Washing the Negative. 135 of potassium bromide is added. This will bring up the negative to the necessary density. In the case of instantaneous pictures the developer should, as before, be used half-strength, but an addition to every 50 CO. of 20 drops of a 2-5 per cent solution of hyposulphite of soda in water should be made. This addition gives mar- vellous developing properties to the ferrous oxalate, increas- ing its power three- or four-fold. The plate after development must be well washed, and should be fixed in a bath of hyposulphite of soda, as given at p. 75. Films containing much iodide take somewhat long to fix, especially if the emulsion has been prepared by the ammonia process. The plate should be thoroughly washed for a couple of hours in constant changes of water. When the film is tender, and has a tenaency to leave the plate, between the developing and fixing operations it should be immersed for five minutes in a saturated solution of alum, the film being washed both before and after the immersion. When finally drying the negative care should be taken that no drops of water are left on the film, as they are apt to cause marks. Our advice is to sponge the surface of the negative with a very soft sponge before it is dried. Over- exposed pictures, and pictures taken on plates on which the layer of emulsion is small (as in some cheap commercial plates), will be found to lack density, and they must be in- tensified. Unfortunately the surface of gelatine almost precludes the intensification being carried out as for wet plates, owing to the gelatine staining. Resort is therefore had to mercury intensification, and it is believed that the two following are permanent. The following solutions are prepared 1. Mercuric chloride . . .6 grammes Potassium bromide . . .6 grammes Water 300 cc. 2. Silver nitrate .... 6 grammes Water . . . . . 300 cc. 136 The Gelatino-bromide Process. To No. 2 is added a 20 per cent, solution of potassium cyanide in water, till the precipitate of cyanide of silver first thrown down is very nearly redissolved. The solutions are filtered. To intensify an image, the plate when dry is immersed in water for five minutes, and placed in a dish containing No. I. Here it is allowed to bleach thoroughly. It is then washed in running water (or constant changes of water) for a quarter of an hour, when it is placed in a dish containing No. 2. The image immediately takes a black colour and becomes dense. The film must be next thoroughly washed. It may happen that the density is too great, in which case it can be reduced by immersion in a 4 per cent, solution of sodium hyposulphite. Instead of No. 2, a saturated solution of sulphite of soda may be employed, and the same mode of reduction pursued if the intensity is too great. Before the negative is taken into regular use for printing it should be varnished. The varnishes given at p. 76 will answer, and the manipulations given at p. 84 should be followed. Defects in Gelatine Negatives. The worst defect of all js the' tendency of the film to leave the plate. This is usually cured by immersion in the alum bath between fixing and developing, as it generally happens that this ' frilling ' of the plate takes place in the hyposulphite of soda solution. A badly cleaned plate is also conducive of frilling, for which there is no cure. Blisters are usually followed by frilling, and what applies to it applies to them. Green fog is seen when alkaline de- velopment is employed; the emulsion from which the plates are prepared is then at fault. General fog is usually due to the emulsion, but when it is not very bad the plates may be used by giving slightly prolonged exposure and adding treble, the amount of Paper Negatives. 137 potassium bromide solution prescribed for the alkaline de- veloper. Yellow or brown stain all over the negative is often seen after alkaline development. It may be got rid of by soaking the plate in a i per cent, solution of hydro- chloric acid in water. It is safer instead of the water to use a saturated solution of alum, and to add the hydrochloric acid to that. This prevents any chance of blistering or frilling. Opaque spots are usually due to dust on the plate, which settles during drying or before exposure. Transparent spots are due to the use of gelatine in which there is a small amount of greasy matter. Transparent pinholes on the negative after fixing may be due to dust on the plate, probably entering the dark slides. This can be avoided by rubbing the dark slides with a little glycerine. The glycerine acts as a trap to the dust. Dark lines on the negative are often due to the scratch- ing of the film by some rough substance before develop- ment. The scratching seems to decompose the silver salt in a similar way to light. CHAPTER XX. PAPER NEGATIVES. Had the historical order of photographic processes been followed, the calotype process would have been described immediately after the Daguerreotype process, but it seemed more likely that the details of the former would be better understood after a study of the collodion wet and dry processes. The original process which Fox Talbot intro- duced has been but little improved, and it is therefore given nearly as he described it, modifications being suggested where necessary. 1^8 Paper Negatives. The paper employed should be as tough and grainless as possible, capable, however, of holding sufficient of the sensitive compound to give a body to the image. Good English paper of the consistency of medium Saxe answers every purpose. The great drawback to all papers of the present day seems to be the chance of transparent spots appearing during development, and a consequent damage to the image. What is the chemical nature of these spots is not known, but they can generally be got rid of by brush- ing a dilute solution of hydrochloric acid over the surface of the paper, and then thoroughly washing off all excess of acid. When dry the paper is ready for impregnating with silver iodide. This last is formed by taking — No. I. Silver nitrate .... 3 grammes Distilled water . . . . 20 cc. ,, 2. Potassium iodide .. . .3 grammes No. 2 is poured into the solution of No. i with constant stirring, and a. precipitate of silver iodide is formed. The potassium iodide being in shght excess, a certain quantity of the silver iodide is held in solution. The precipitate is allowed to settle at the bottom of the glass measure (in which we will suppose the two solutions to have been mixed), and the supernatant liquid is poured off; water is again added, and after stirring it is again poured off. This operation of washing is continued some three or four times, or until the soluble potassium nitrate is nearly eliminated. The silver iodide is next dissolved in a solution of potas- sium iodide. This is poured on the silver iodide and well stirred. As this quantity will not effect complete solution, crystals of the potassium salt must be added till after much stirring the solution is semi-transparent or milky. Distilled water 20 cc. Potassium iodide Water 30 grammes 60 cc. Preparation of Sensitive Paper. 139 The paper is next cut to a convenient size, and is pinned on a flat board. The solution is applied by a brush of cotton-wool, a good form adapted for the pur- pose being given in the figure, a is a glass tube . of about 20 centimetres long, and above i centi- metre diam.eter. A loop of string, B, passes through the tube, across which is placed a thin tuft of cotton- wool, c. The loop is then pulled up into the tube, a sufficiency of cotton-wool being allowed to remain externally to form the brush. It is advisable first to wash the wool in a weak solution of alkali in water, taking care, however, that none of the alkali re- mains in the fibre, and that it is thoroughly dried before being used as a brush. The solution is brushed up and down and across the paper, till the whole surface has received a uniform coating of the dissolved iodide. When partially dry the paper is immersed in a dish of distilled water, all air-bubbles being carefully removed from the surface. After soaking for a couple of minutes it is removed to a second dish, and sub- sequently to a third dish. The water removes the potas- sium iodide, and leaves a primrose-coloured silver iodide on the surface of the paper. After the washing has been con- tinued two or three hours, the paper is hung up and dried. In this state it is nearly insensitive to light (though not quite so, as the iodide is in the presence of organic matter), and can be stored away between the clean miprinted leaves of a book. When required for use, the paper is pinned on to the board as before, and a mixture of the following solutions is brushed over it : — No. I. Silver nitrate .... 5 grammes Glacial acetic acid . . , 8 cc. Water 50 cc. No. 2. Saturated solution of gallic acid in distilled water. To every • cc. of No. i add 60 cc. of distilled water, next I cc. of No. 2, and finally 30 cc. of distilled water. 140 Paper Negatives. If the temperature be high, the water must be increased to such an extent that immediate reduction of the silver nitrate may not take place. After well mixing, the solu- tion is appUed lightly, but plentifully, to the iodised paper with the cotton-wool brush already described, and all excess blotted off on filtering-paper of the purest descrip- tion. Two sheets are then placed back to back with blot- ting-paper between them. The paper is most sensitive in its moist state, but it is also capable of giving pictures when dry, or until the surface of the paper becomes discoloured by a reduction of the gallate of silver. For exposure in the camera a sheet may be placed between two pieces of glass, or the corners may be gummed on to a sheet of glass, the paper taking the position of the collodion film of the ordinary pro- cesses. The exposure varies considerably according to the preparation of the paper ; and it should always be sufficiently prolonged to give a trace of the sky-Hne on the undeveloped paper. To develop the picture, the paper must be pinned on the board as before, and equal parts of No. i and No. 2 applied, with similar quantities of water as already indicated. This is applied with the brush, and is continued till the developing action begins to flag. When this is the case, the gallic acid solution. No. 2, is applied very hghtly, until the deep shadows begin to dim by transmitted hght. The development must now immediately be arrested, otherwise the picture will be veiled. For an under-exposed picture more of No. i should be used than that given, and if over-exposure be feared (in- dicated by the picture being fairly visible), No. 2 should be in excess. A little consideration of these points will show how development may be equalised in dark parts. An artist in the production of these pictures will be able to produce a picture by a little attention to the above details, whilst a mere manipulator would probably produce nothing but an image wanting in delicacy and gradation. Fixing and Waxing the Paper Negative. 141 The negative is fixed by immersion in a sodium hypo- sulphite solution. Sodium hyposulphite . . .60 grammes Water i litre. The fixing being complete, which may be known by a total disappearance of the yellow of the iodide when the paper is viewed by transmitted light, the picture is washed in abundant changes of water, until all the hyposulphite is thoroughly eliminated. This may be known by applying the test given at p. 161. The washing should take at least three or four hours even in running water. It may be advisable to call attention to the necessity of the addition of acetic acid to the sensitising solution. This is always advisable in warm climates, as it greatly restrains the reduction of the silver nitrate; the less added, however, the more sensitive the paper will be. When the - paper negative is dry it is ready for waxing. A flat-iron (preferably a box-iron) is heated to such a tem- perature that it will readily melt white wax. A cake of this substance is brought in contact with the iron whilst the latter traverses the paper. The whole of the picture, except the sky, should be rendered translucent by it. The superfluous wax is absorbed by blotting-paper placed upon the negative, over which the hot iron is passed. It is a mistake in this last operation to heat the iron too much; over-heating causes the blotting-paper to take up too much of the wax, and leaves the grain of the paper visible. It sometimes happens that yellow spots occur in the whites of the picture. These are generally removable by the applica- tion of a dilute solution of hydrochloric acid. It need scarcely be remarked that this entails a thorough washing. The acid must never be applied till all the sodium hypo- sulphite is thoroughly eliminated, for if any remain in the paper it is decomposed by the acid, and the inevitable result will be a fading of the picture. Further remarks on this subject will be found in the chapter on silver printing. 142 Paper Negatives There are various processes for the production of paper negatives extant, amongst which may be mentioned those of Le Gray, Blanquart-Evrard, and Prichard. That "due to Le Gray was at one time a great favourite, its distinguishing feature being that the paper is waxed before being sensitised. The waxed paper is immersed in a solution of potassium iodide and bromide, together with sugar of milk, and after drying is treated with a solution of silver nitrate, acidified with glacial acetic acid. The development is carried on much in the same way as that indicated in the above process, the paper being submerged in the fluid. This last process, perhaps, is better adapted to careless .manipulation than that described above, as all danger of staining the back of the picture is avoided. There are several negative and positive papers in the market at the present time, some of which are very excel- lent. Eastman's negative paper is prepared with a gelatine emulsion, such as was described in Chapter XVIII., and can be developed by any of the developers given in Chapter XIX. The method of holding these papers in situ in the camera will be given in the chapter devoted to apparatus. After using the ferrous oxalate developer before washing, we usually pass the paper through a bath of water which has been rendered slightly acid by hydrochloric or sulphuric acids. This prevents the formation of any oxide of iron in the water, and removes the oxalates. The nega- tive after well washing is fixed and again washed. The paper is rendered translucent by waxing, or by applying crude paraffin to the back. The Eastman Company also issue what they call stripping films. These, after fixing, are transferred on to a collodionised plate, placed under pressure for a quarter of an hour, and then treated like a carbon print (see page 177), being immersed in hot water. The paper support strips off and leaves the film bearing the negative image on the glass plate. A thin sheet of gelatine is then soaked in water and applied to the film. The two Silver Printing. 143 are dried together, and when thoroughly desiccated the negative is stripped off the glass. This thin film can be printed from either side. Mr. Warnerke has introduced a negative tissue which is remarkable for the fact that it consists of a paper support, rendered almost transparent by some preparation of varnish, and coated on both sides with a gelatine emulsion. Such tissue is developed by the alkaline or ferrous oxalate de- veloper, and is printable at once without further operations being gone through. Mr. Vegara has taken up a patent of the late Mr. W. Woodbury for the production of a tissue of very much the same character. The fault in all paper negatives is the presence of a certain amount of grain, which is very objec tionable for some purposes, more particularly in small pictures. In the fourth chapter the results of the action of light on silver chloride and organic compounds of silver were shown. In this part it is proposed to treat the subject rather more fully, as silver printing entirely depends upon it. The student would do well to make the following experi- ments for himself, as. by so doing the rationale of the varia- tions in the processes will become famihar to him, and many failures will be avoided by a study of the theory. Take any ordinary paper which contains size of some description, and immerse it in a solution of sodium chloride. Hang it up and allow it to dry, and in non-actinic light (adopting the manipulations which will be presently de- CHAPTER XXI. SILVER PRINTING. Sodium chloride . Water. I gramme 50 cc. 144 Silver Printing. scribed) float several pieces of convenient dimensions on a solution of silver nitrate for three minutes. When dry to the touch, place one of these pieces under a negative in a printing frame, and expose it to the action of the sunlight ; after a few seconds open the frame in a subdued light, and note the result. The parts acted upon by light will have a violet tint, and if ammonia be applied to a portion of the darkened paper it will be found that the image almost entirely disappears. For reasons already given this will indicate that the silver chloride is dissolved. Allow further play of sunlight, say for a couple of minutes, and again note the result. It will be found that the image is much redder in colour, and that ammonia fails to remove all the colora- tion. From this we infer that the organic compound formed by the size of the paper and the silver nitrate is acted upon. Next take another sheet of the same paper and wash out all excess of silver nitrate, and expose under a negative, and examine the print at the same intervals as before. It will be found that the short exposure produces hardly any perceptible darkening, whilst with the longer it is much less than in the previous experiment. From the results of experiments already detailed in the fourth chapter, it will be seen that the absence of silver nitrate prevents the darkening of the silver chloride, and that the organic com- pound is the more impressionable. A minute examination of the image will also show that there is a spotted irregular appearance in the darkest parts. There is an easy theoretical explanation of this. The chlorine liberated from the darkening silver chloride is taken up by the organic compound, bleaching it to a certain extent, forming white chloride of silver, which in its turn is capable of being acted upon by light. Experiments with similarly washed Silver nitrate Water . 5 grammes 50 cc. Experiments on Printing. 145 paper will show that, though the first darkening is much slower than in the unwashed paper, yet the action of the former approaches more nearly the rapidity of the latter, as the organic silver oxide, which is greedy of chlorine, is formed. Next, fix these paper images by immersion in the bath of sodium hyposulphite, as given at a subsequent portion of this chapter. Both prints will assume a foxy-red colour that containing no free silver nitrate losing least in depth. The reason will be apparent. The silver subchloride, AgjCl, formed is soluble as far as one atom of silver and one atom of chlorine is concerned (AgCl), leaving behind one atom of metallic silver. Since it is only the surface of a particle of silver chloride that is blackened, the darkening of the compound is in itself a protection from the penetra- tion of light into it. It can be shown that the depth to which such penetration can take place with ordinary ex- posure is very superficial, hence the metallic silver left behind must be exceedingly minute ; so small is it indeed that the most delicate balances are too coarse to weigh it. It may be of interest to note an experiment which was carried out to test this. One thousand square centi- metres of a glass plate were coated with a layer of silver chloride held in situ by inert collodion, and exposed to sunlight in the presence of an excess of silver for five minutes. The original amount of chloride was 102 centi- grammes, and after fixing in the bath the metaUic deposit was dissolved off in nitric acid, and estimated volumetrically, and found to give only 55 milligrammes of metallic silver. The organic substances employed in printing may now briefly be considered. Albumen undoubtedly comes first, owing to the properties it possesses of giving a good tone when converted into albuminate of silver if in contact with silver chloride and excess of silver nitrate, and also on account of its insolubility after coagulation. The formula for albumen is taken as C72H,osN,sSOo2, though it can scarcely be said to be established with any L 146 Silver Printmg. certainty. Albumen coagulates in the presence of nitric acid, and also at 65° C. It is precipitated, but not coagu- lated by alcohol. It combines with the metals, prominent amongst which is the compound it forms with silver. Silver albuminate is white, turning a dark-red brick colour in the presence of white or other actinic lights. The change is speedily effected, and, like other organic compounds of silver, is not dissolved by ammonia after darkening, though the addition of an alkali speedily dissolves the white albumi- nate. This alone prevents the adoption of an alkaline solution of silver, such as the ammonio-nitrate of silver, for sensitising paper coated with this and some soluble chloride, as the effect would be simply to dissolve it. Gelatine, which is the size used in some papers, combines with silver, and forms a red tint on exposure to light ; owing to its colour, and the greater difficulty of toning, it is not usually employed in print- ing operations. Starch (C9H10O5) forms a compound with silver, which on exposure to light darkens to a more violet colour than either of the preceding. It is largely used in sizing paper, and it is consequently necessary to note this colour. From the foregoing remarks it will be seen that all the bodies which are employed in sizing ordinary paper will combine with silver, but there are other reasons why albumen is that which is usually employed. Its adoption is due to the fact that it remains on the surface of paper, forming a smooth and thin layer, which is capable of holding z;z stiu the different chlorides, and that on the application of silver nitrate this delicate film is converted into an organic salt of silver together with the silver chloride. In all printing operations one point is a desideratum, viz. that the image should be on the surface of the paper, and not sunk into it. The importance of this may be tested by sensitising albu- menised paper on the reverse side, and endeavouring to obain a print on the albumen surface in the ordinary manner. It will be found that the image will appear feeble Theory of Toniiig. ^47 by reflected light, though by transmitted light it will appear well-defined and dense When albumen is used fresh, and in a slightly alkaline condition, the resulting print possesses greater stability than any of the foregoing substances ; as re- gards delicacy of image it cannot be surpassed. Unfortun- ately albumen is most easily applied to the surface of paper when slightly acid, the acidity being due to decomposition, and the resulting compounds formed are more liable to change. It may be asked, why not print in pure silver chloride alone, held m situ by some vehicle, such as collodion ? This is not impossible though impracticable, as the reduction of the silver chloride image by the fixing solutions is so great that the print would be wanting in vigour. With the addition of some organic compound, however, it becomes quite feasible ; but then, be it remembered, the depth obtained is due to that organic substance. Thanks to the discovery of Mr. Wharton Simpson, that silver chloride and some organic compounds of silver (amongst which we may name citrate of silver) will emulsify in collodion, prints are readily obtained by the coUodio-chloride, and possess a beauty which cannot be sur- passed. In the next chapter this process will be described ; merely mentioning en passant that in this process, as in any other in which printing on silver chloride takes place, an excess of silver nitrate is beneficial. The colour of prints obtained is always objectionable if fixed directly after taking out of the printing frame ; and resort is had to an operation called toning to render it more pleasing. This toning may consist of gilding the silver image, platinising it, or substituting some other metal for it. The colour of the silver print when appearing through this other metal may give a pleasing tint, or it may fail to do so, according to the extent to which the operation is carried. It will be seen, in the practical instructions in printing, that the picture is more or less washed in water before toning. By immersion in water the violet-coloured image becomes of a L 2 148 Silver Printing. red colour : to what this change is due is rather uncertain. It has been held that it is due to the water dissolving a cer- tain part of the silver oxide. It may be due to a different compound being formed by the combination of water with the altered compound, but this is doubtful. In order to tone the picture, certain solutions of gold, platinum, or other metals are made, and the print immersed in them ; the first of these metals, in the shape of gold trichloride, is that usually employed. It is, therefore, proposed chiefly to con- fine the remarks on toning to that process in which that metal is principally employed. The gold trichloride has the formula AuClg, which is a fairly stable compound. If its temperature be raised to 170" C. it becomes decomposed, a pale yellow and insoluble powder, gold chloride, AuCl, re- sulting, chlorine being evolved. When the former salt of gold is mixed with a solution of silver nitrate, the chlorine leaves the gold to form silver chloride, and the latter salt of gold is formed. Acetates, as also the carbonates of the alkalis, are capable of precipitating gold from a neutral solution in the presence of any disturbing cause — such as organic matter. It will be seen from the formulae given for toning solu- tions, p. 158, that one contains chloride of lime, and as an example of one kind of toning this one will be considered. If a print so thoroughly washed that all excess of silver nitrate is elimmated be immersed in this solution, it will be found that the gold deposits very slowly, and that the image becomes feeble and spotted in appearance, whereas with a print in which the excess of silver nitrate has been but partially removed, the toning or gilding action takes place much more readily. Chloride of hme is a mixture of calcium chloride (CaClv) with calcium hypochlorite (CaCl20.2), being made by passing chlorine over calcium hydroxide, or common slaked lime. Calcium hydrate + Chlorine = Water + Calcium chloride + ^pochl^i^e 2Cali,0 + 2 CL =2H,0-t- CaCL + CaCLO, Theory of Toning. 149 Now gold trichloride, when uncombined with an alkali, is generally in an acid condition, due to the presence of hydrochloric acid, and in order to neutralise this, calcium carbonate forms part of the toning bath. On immersing the silver print the equilibrium is disturbed, and the gold begins to deposit, and, consequently, chlorine is liberated. In the directions for use of the toning bath it is stated that the solution should be made with hot water if required for im- mediate use, whilst if made with cold it must stand twenty- four hours. This causes a certain quantity of the hypo- chlorous acid to be evolved, and leaves a small portion of calcium hydrate in solution. In the case of the thoroughly washed print, the chlorine attacks the silver subchloride or the organic silver oxide, and forms the white silver chloride ; for, be it remarked, the gold which is situated nearest the printed surface is first reduced, and the image is therefore in close proximity to the chlorine, and readily attackable by it. At the same time the silver oxide and sub-chloride are in proxi- mity to the chloride of lime, which reduces them also to the state of chloride, particularly if it be rich in hypochlor- ous acid. In the case of the slightly washed print the same action takes place, but we have a new element to deal with. In this case the solution or excess of the silver nitrate held in the pores of the paper gradually becomes diffused to the surface, combines at once with the liberated chlorine, and forms chloride of silver, but not at the expense of the image. It will be noticed in toning operations that this chloride is absolutely formed on the surface of the print, and can be removed by a slight rub with the finger. On fixing the print, the silver chloride is in both cases removed ; in the one from the image itself, in the other from the paper. This accounts for the spotted appearance in the one case, and its absence in the other. To test this theory, the following experiments were under- taken by the writer. A print was thoroughly washed, then Silver Printing. immersed in a solution of lead nitrate, and again slightly washed. On applying the chloride of lime toning bath, the print quickly changed to a rich brown colour, and, after fixing, had all the qualities of a properly toned print. In this case the lead combined with the chlorine, and acted like the silver nitrate. Another toning bath, consisting of lime water and gold trichloride, was prepared. Two well- washed prints were immersed, and left respectively for three minutes and fifteen minutes ; on the latter a slight deposit of gold was visible, and also a diminution in the depth of the print after fixing. With the former the print was less affected. Prints in which silver nitrate and lead nitrate were present both toned admirably, but rather too rapidly for safety. On examination a trace of hypochlorous acid was found in the toning solution. A slight addition of chloride of lime was next made, and prints in which silver and lead nitrate were present were immersed in the solution : they toned gradually and regularly. This last experiment, which was confirmed by others, showed that the calcium hypochlorite contained in the chloride of lime acted as a retarder to the toning operation, as the chlorine contained in hypochlorous acid combined with the silver nitrate equally with that evolved from the precipitating gold. This manifestly would check the deposition of the gold. Another toning solution used is one made with sodium acetate and gold. In practice it is found that toning takes place most regularly when the print has been previously well washed. On adding a solution of sodium acetate to silver nitrate, a sparingly soluble silver acetate and sodium nitrate are formed by double decomposition. If, then, the silver nitrate be present in the print, the greatest portion of the adjacent sodium acetate is decomposed, and sodium nitrate left in its place. The sub-chloride and oxide of silver both seem to be as readily attacked by chlorine as the silver acetate. Hence the chlorine, having nothing at hand to absorb it (sodium nitrate not being Theory of Toning, 151 able to do so), attacks the silver of the print and produces the bleaching action already referred to. When all the free silver nitrate, however, is washed away, the conditions are changed ; the sodium acetate will absorb chlorine, and form a chloracetate and hydrochloric acid, as indicated in the following equation : — Gold , Sodium \^ , Sodium trichlor- Hydrochloric . • • I + It = Gold + ^ ^ > + -J trichloride acetate acetate acid 2AUCI3 +NaC2H302= 2Au + NaC^ClaO. + 3 HCl. Eventually an evidence of this reaction may be traced in the fact that the solution becomes acid, and refuses to tone. The foregoing experiments exemplify the following laws : — (i.) That a neutral solution of the gold toning bath is necessary. (2.) That some active soluble chlorine absorbent must be present, either in the print or in the solution. (3.) That when the affinity of the absorbent for chlorine is violent its action must be retarded. In considering any toning solution, these three qualifica- tions must be taken into account, and if one of them be violated a perfect print must not be expected. The theory of fixing prints is the same as already in- dicated at page 74, and need scarcely be touched upon. The reason why potassium cyanide cannot be usefully em- ployed as a fixing agent has been already shown to be due to the fact that the organic oxide of silver is soluble in its solu- tion. It must be strongly impressed upon the student that two forms of double hyposulphites of silver and sodium are formed, one of which is soluble and the other insoluble (see p. 74). The soluble form undergoes a change in light which renders it insoluble ; hence fixing the print in daylight should be avoided. When prints are immersed in a solution of sodium hypo- sulphite, a certain portion of this salt is combined with the 152 Silver Printing. double salt of silver formed. Every print immersed there- fore leaves a smaller quantity of the uncombined sodium hyposulphite in solution ; and since the double salt of silver and sodium is soluble in the uncombined hyposulphite, it follows that care must be taken not to fix too large an area of print in the same solution, otherwise the in - soluble salt will be formed in the pictures. The effect of this is seen in the fading of prints. No amount of washing will eUminate this insoluble form. The acid vapours to be found in the air will decompose it, and cause a liberation of sulphur compounds, which gradually bleach the black portions of the image, and give the whites a jaundiced appearance. Even where the soluble double hyposulphite has been formed, washing the prints in a thorough manner is essential for permanency, for any trace of it will decom- pose in a similar manner, as will also the sodium hypo- sulphite itself In the writer's opinion the prints should be immersed in two separate solutions of the sodium hypo- sulphite ; the first will form the necessary soluble salt, and the latter will cause it almost entirely to disappear, all traces being subsequently eliminated by the washing water. Some American writers have proposed to shorten the washing of the print by a final immersion in a solution of iodine, tetrathionate being formed ; the reaction would be as follows : — Sodium hyposulphite + Iodine = Sodium tetrathionate + Sodium iodide 2 Na^SoOg -t- = NaSjOe + 2 Nal. Both these salts are soluble in water, but less so than the sodium hyposulphite, and the tetrathionate appears to be more readily decomposed. If a silver compound be pre- sent with the hyposulphite, the same reaction apparently takes place, though an exact analysis of it has not yet been undertaken. The quality of the washing water is important — it should if possible be rain water, otherwise pure spring Manipulations in Silver Printing. 1 5 3 water, to which a Httle alkah should be added for the first washing. This renders the eUmination of the soluble salts more complete. The causes of instability in a print are as yet imperfectly recognised ; and, owing to the want of chemical knowledge on the part of some photographers, the fading has assumed a more mysterious aspect than is warranted. Take the case of the acetate toning solution. It has been shown that it be- comes acid, and the prints are often taken direct from this bath to the hyposulphite solution. An acid immediately commences to decompose the latter, and fading necessarily results. Washing between each operation should be in- sisted upon, and then the chances of fading are reduced to a minimum. It may be mentioned here that the writer has for the last year fixed his silver prints with a 20 per cent, solution of sulphite of soda. In this body there is no liability for sul- phur to be liberated during the .fixing operation, all that can be liberated is sulphurous acid. Silver chloride and organic compounds of silver are soluble in the sulphite, but only to the extent of of that to which they are soluble in the hyposulphite. Silver sulphite is formed, which is solu- ble in sodium sulphite, and this is readily soluble in water. It is too early as yet to stare the permanency of prints so pre- pared, but there is every reason to believe them to be more stable than prints fixed in the ordinary manner. CHAPTER XXIL MANIPULATIONS IN SILVER PRINTING. The papers employed in silver printing are known as Saxe and Rive, the former being suitable for large pictures, whilst the latter are preferable for smaller sizes. The fol- 1 54 Manipulations in Silver Printing. lowing formula may be used for the albumen solution, with which to coat the paper :— Ammonium chloride , . .10 grammes Spirits of wine . . . . 15 cc. Water ..... 135 cc. Albumen ..... 450 cc. The first three are thoroughly mixed, and the albumen, derived from the whites of eggs, is gradually added to the solution. Perhaps the simplest way of effecting solution and perfect mixture is to half fill a bottle with the albumen, and then to add a fair supply of roughly powdered glass. Shaking the bottle will cause the flocculent matter to be Fig. 30. broken up, and leave it in a state ready for filtering through sponge or well-washed tow. The paper, having been cut into sheets of convenient size, is floated on the fluid, con- tained in a dish, the hands grasping its two opposite corners. The convex surface of the paper thus formed is first brought in contact with the solution. As the hands are drawn apart, the paper pushes out all air-bubbles before it, and at length lies in perfect contact wdth the solution. It is a wise precaution to take, however, to raise the paper from one corner, to make certain of the absence of all air-bubbles, and then to allow it to remain at rest on the solution for a minute. It is next removed, and hung on a line to dry, being held by a couple of American clips, or thrown over a stretched cord. This last plan is apt to cause markings, though it is probably necessary when large sheets of paper are manipu- Sensitising Bath. 155 lated, owing to their tendency to tear if only susi)ended by two corners. When dry the paper will not be flat, and should therefore be rolled and put away between flat boards. When a print having a dull surface is required, the following formula is smietimes used : — Ammonium chloride . . 6 grammes Gelatine '6 gramme Water 300 cc. The gelatine is first dissolved in hot water, and then the remaining salt added. The paper is floated for three minutes on this solution. Another mode of producing a dull surface, and which is very effective, is to use resinised paper. The annexed formula is workable, and is due to Mr. H. Cooper, jun. : — Frankincense . . . . i gramme Mastic '8 gramme Calcium chloride . from -5 to I gramme Alcohol 45 cc. Good Rive paper is immersed in this solution for half a minute, after which it is ready for floating on a moderately strong sensitising bath. The Sensitising Bath. When a paper is weakly salted, say, having half the amount of chloride given in the formula for albumenising paper, a weak sensitising bath is usually employed, whereas with paper strongly salted, or for the resinised paper, one somewhat stronger is necessary. The following formulae will show what the extreme strengths of solution should be :- Silver nitrate . . , . 6 grammes Water 100 cc. and Silver nitrate . . . .15 grammes Water 100 cc. The paper is floated on either of these solutions in the manner given for albumenising paper, the time of contact 156 Manipulations in Silver Printing. varying from three minutes in hot to five minutes in cold weather. It should be removed slowly from the sensitising bath to prevent waste of solution, and when hung up to dry by an American clip in the dark room, the dramings should be collected by attaching a slip of blotting-paper to the bottom corner. It is always advisable to have one corner lower than the others, as the sensitising solution thus drains more equally away. In order to preserve sensitised paper from coloration due to the decomposition of the organic salt of silver, it may be placed between sheets of blotting-paper impregnated with sodium carbonic. Ready sensitised paper is sold in the market, the discoloration being prevented, as a rule, by the addition of nitric acid in some form. Printing the Picture. Printing operations are rarely carried on in the same tem- perature and state of atmospheric moisture as those in which the paper is dried ; hence it is advisable to allow the paper to assume the conditions of the former before rigidly con- fining in the frame. The fact that paper expands in moist air at once shows that the dimensions of a photograph can never be relied upon as being accurate. Measurements have shown that a drawing, for instance, will vary as much as i per cent, in certain conditions of the atmosphere. The same remark applies to plans printed in the ordinary lithographic press ; the scale is never correct except under fixed con- ditions. The negative should be placed in the printing frame with the varnished side next the paper. A convenient form of frame, and one which is usually employed by photo- graphers, is shown in the diagram, b is a sheet of thick plate-glass, which rests in a frame, A. The negative is placed on the glass, the sheet of paper over it, then a smooth felt pad, and over this a back, c, hinged in the TJie Printing Frame. 157 centre. Two cross-bars, d d, to which are affixed springs, cause the back to press on the pad, and are held in position as shown by E e. The use of the hinged back is to allow the print to be examined when required. During such an examination one of the catches e is loosened, and the por- tion of the picture beneath one half of the back can be inspected without any danger of the relative position of the paper and the negative being changed. The depth of the print is an important point to attend to. It must be re- FiG. 31. membered that much of the apparent vigour is lost in the subsequent operations of toning and fixing, and due allow- ance must be made for this. It requires considerable practice to judge correctly of the proper depth and no fiixed rule can be given, so much depending on the relative pro- portions of the chloride to the organic compound of silver, and on the nature of the toning bath. Much might be said about the artistic manipulation of prints, but it hardly enters into the scope of this work, though some hints will be given in the chapter on the picture. 158 ]\IanipiLlations in Silver Printing. To?iing. The following toning baths may be considered as standards '. — The water should be boiling if the solution be required to be used at once, otherwise it should stand in an uncorked bottle for twenty-four hours. II. Gold trichloride . . . • "25 gramme Sodium acetate .... 7 grammes Water . . . . . .X litre This should be mixed a day before being used. A very excellent toning bath for ready sensitised paper is as follows : I. Borax . . . . .24 grammes The borax should be dissolved in the water with the aid of heat. Nos. i and 2 should be mixed in equal parts im- mediately before use to form the toning bath. Before toning, the solutions should be filtered in a clean dish, slightly warmed if the weather be cold. The prints are placed in water of about 15° C, and the washing continued as indi- cated in the last chapter, according to the toning bath employed. They are then immersed in the toning solution three or four at a time, and the dish is kept in constant motion, so as to allow an equal toning action throughout. It is likewise essential that no two prints should stick together, for the same reason. According to the colour of the print desired, so must the continuation of the toning action be regulated. If a rich chestnut brown be required, I. Gold trichloride Chloride of lime Chalk (precipitated) . Water •25 gramme •25 gramme I teaspoonful I litre. Water 2. Gold trichloride Water I litre. •25 gramme I litre. Toning and Fixing the Print. 1 59 but very little apparent change in the colour of the print is necessary, whereas if an engraving black tone is sought, the action must be continued till the image is decidedly blue. It is not to be inferred that these rules are absolute in every case ; so much depends on the sizing of the paper and on the amount of chloride present that they are not applicable in all cases, but with Saxe paper, prepared as given in the foregoing formulae, they will hold good. Fixing the Print. The fixing solution is made up as follows : — Sodium hyposulphite . . . 200 grammes Water i litre. Between toning and fixing it is essential that the prints should be well washed. The necessity of this may be under- stood by referring to p. 15 1. It has been sometimes recom- mended to acidify the washing water, but the proposer of this plan can have had no thought of the danger to the per- manency of the prints which he thereby introduced ; an acid at once begins the decomposition of the hyposulphite. The writer strongly urges the necessity of a strongly alkaline condition of this bath, and in practice he adds 50 cc. of strong ammonium hydrate to it when fixing prints. Mr. J. Spiller was the first to point out the use of ammonium carbonate in the solution; he showed that it dissolved out a certain compound left in the whites of the picture, which otherwise was insoluble, and which readily decomposed under atmospheric action. The pictures should be im- mersed in the solution for ten or fifteen minutes, the time varying according to the thickness of the paper ; they should then be washed (unless they be placed in a second solution of hyposulphite, as already suggested), rapidly at first and afterwards more slowly. Perhaps the best way of elimmat- ing the greater part of the hyposulphite is to place the prints in a large tub of water, which is kept m motion, and after five minutes' washing to place them in a smaller i6o Manipulations in Silver Printing. quantity of water. After this they may be removed to a washing trough, where the water will be changed several times an hour. The accompanying idea of a washing trough may prove useful. It is one which was designed and is employed by Mr. England, and has answered well the purpose for which it is intended, a is a trough, at the side of which is a syphon, s, the inside leg reaching to within 2 or 3 millimetres of the bottom, the bend of which is a little below the top of the trough, b is a cradle, pivoted on a rod, E, which passes through the sides of a, as shown, c is a water-wheel attached to the wall, on to which a gentle stream of water from the tap, f, plays, g is a small arm at- FlG. 32. tached to the axle of the wheel, having a rod suspended from it, which is attached to the cradle, b. As the wheel slowly turns the rod is raised, and the prints are caused to move about in the water. The water runs into a trough through the pipe h, and when it reaches the top of syphon pipe the trough gradually empties itself, leaving the prints on the gutta-percha strips which form the bottom of the cradle. It will be seen that the supply of water must always be rather less than that which the syphon is capable of carry- ing away. A useful addition to the trough is a horizontal pipe attached to the well of the wheel, moving from side to Testing for Hyposulphite. i6i side by the motion of the wheel, and thus distributing the entering water over the surface of the prints by means of a fine rose. This prevents the chance of any of the prints getting surface-dry, which sometimes happens. In a trough of this description twelve hours suffice to ensure the total removal of the hyposulphite. Should this mode of washing be napplicable, the prints may be placed in dishes, changing the water every quarter of an hour for the first hour, and every haJf-hour subsequently for six hours. If during this time they are well sponged twice or three times with a soft sponge, it will be found on apply- ing one of the following tests that the hyposulphite is eliminated. The first test is based on the reaction of iodine with sodium hyposulphite, shown at p. 152. Tajce a small piece of starch the size of a pea, powder it and boil it in 10 cc. of water till a clear solution is obtained ; add 5 cc. of a saturated solution of iodine in alcohol to the clear Hquid, A dark blue colour due to starch iodide will now be apparent. Drop 2 drops of the solution into two clean test-tubes, and fill up one with distilled water, and the other with the water to be tested. A faint blue colour should be perceptible in the first test-tube, whilst the presence of hyposulphite in the other will be shown by the total absence of colour. The contents of the two solutions in the test-tubes can be best compared by placing a piece of white paper behind them and examinmg them by reflected light. The sodium hyposulphite may not be found m the washing water, yet a trace may remain in the prints. If a very weak solution of iodine be brushed across the back of a print, the absence of all colour will indicate the presence of the hyposulphite. One selected out of a batch of prints may thus be tested, though it is rarely necessary if the water indicates that the washing has been thoroughly effected. The dishes that are used for holding the fixing solutions must in no case be employed for any other purpose. The M 1 62 Collodio- and G elatino-cJilortde Processes. material of which they are made should be, if possible, glass or porcelain, and never tin or zinc. The defects in prints due to defective manipulation, and not to want of artistic skill, are but few in number. Red marks that repel the toning solution can usually be traced to contact with hot and moist fingers. A red tone after fixing is due to an insufficient deposit of gold, and a blue tone to an excessive deposit. The whites may appear yellow through the fixing solution being of insufficient strength, or through paper being used when the sensitive surface shows signs of discolouring through too long keep- ing. The general cause of the fading of prints has already been detailed. CHAPTER XXIII. COLLODIO- AND GELATINO-CITRO-CHLORIDE PROCESSES. This process is intended to be employed for printmg on glass or paper, and for permanent silver prmts nothing better can be desired. The following is a formula which is taken from the published process of Mr. Wharton Simpson : — No. I. Silver nitrate Water No. 2. Strontium chloride Alcohol No. 3. Soda Alcohol No. 4, Citric acid Alcohol 4 grammes 4 cc. 4 grammes 60 cc. 1-5 gramme 30 cc. 3 grammes 30 cc. To every 50 cc. of plain collodion i cc. of No. i is added, being previously mixed with 2 cc. of alcohol, in order to prevent precipitation of the pyroxyline. Next 2 cc. of No. 2 are added with constant shaking, and -5 cc. of Nos. 3 and 4. In a quarter of an hour it is fit for use. Making Emulsion. 163 It will be noted that there is a large excess of silver nitrate present. The amount necessary to combine with the strontium chloride is only "29 cc. and with the sodium citrate "iS cc. of silver nitrate ; there is, therefore, present more than double the amount of silver nitrate necessary to combine with them. As already shown, this excess is necessary. In practice, particularly when printing on glass, it has been found very difficult to prevent the salts crystal- lising in the film whilst drying ; and in order to overcome this source of annoyance, a method analogous to that of the washed bromide emulsion process may be employed. The above proportions of strontium chloride and sodium citrate may be kept, but the silver nitrate should be reduced to one-half. The plain collodion is made up with half the solvents usually employed to dissolve the pyroxyline, and consequently only half the above quantity is used in mixing the coUodio-chloride. After the emulsion is formed it is poured into a dish, allowed to set, well washed, dried, and then dissolved up in the proper proportions of solvents, in the alcohol of which \ cc. of the silver nitrate solution has previously been added. In this state the coUodio- chloride contains the same necessary excess of silver nitrate, but the strontium and sodium nitrates are absent. This diminishes the risk of crystallisation taking place in the film, and with a certain class of pyroxyline this is entirely avoided. The silver citrate supplies the necessary organic matter by which a vigorous image is obtained. If a glass plate has to be coated with the emulsion, the same directions as those given for coating emulsion plates should be followed, with the addition that it is well to dry the film before a fire, and to print whilst it is still warm. When a paper has to be coated more difficulty is found. The paper must be strongly sized; ordinary paper allows the collodion to penetrate through its pores, and a mealy appearance is sometimes the result. Arrowroot paper, sup- plied by most dealers in photographic materials, is perhaps iM 2 1 64 Collodio- and Gelatino-cJiloride Processes. the best kind. Obernetter, of Munich, uses an enamel paper as a support. A similar paper is prepared by . coating ordinary paper with a strong solution of gelatine, in which barium sulphate, known as ' Mountain snow,' is mixed. When dry, this gives an impervious skin to the surface of the paper. The paper is pinned on to a board, the edges being turned up 2 or 3 millimetres, and at one corner a spout is formed, from which the collodion is poured off. The emulsion is now applied as if 10 a glass plate. Some operators find that by fuming the film with the vapour of ammonia, after thorough drying, increased vigour is imparted to the print. In any case this end may be attained by applying a solution of gallic acid and acetate of lead, together with a few drops of a solution of silver nitrate. The print may be toned in any of the ordinary toning baths. Ammonium sulpho-cyanate and gold have been recom- mended, but the tones thus obtained vary greatly in richness- For printing on glass a special printing frame has been designed, but this is not required if the precaution be taken to gum a strip of paper along . the corresponding edges of the sensitive plate and of the negative. They may then be separated one from the other with the certainty that they will fall into their original position. The prints are fixed in sodium hyposulphite, made as under : — Sodium hyposulphite . • • 33 grammes An immersion of eight minutes in this solution is sufficient. A gelatino-citro-chloride emulsion, which was intro- duced by the author, may be made as follows :— Water I litre. I. Sodium chloride Potassium citrate Water 3'7 grammes 1-8 gramme 50 cc. 2. Silver nitrate Water 1 1 grammes 50 cc. 3. Gelatine . Water 15 grammes 17s cc. Printing with Iron and Uranium Compounds. 165 Nos. 3 and 2 are mixed together, and then an emulsion formed in the usual way by adding No. i. Sometimes it is found easier to emulsify if the sodium chloride and potas- sium citrate are kept separate, each in 25 cc. of water. In this case the chloride emulsion is made first, and then the citrate added. The emulsion is squeezed into cold water, and slightly washed for a quarter of an hour. It is then drained and dissolved (see pages 128 and 129) with the addition of 15 cc. of alcohol. Should the emulsion appear granular, it is heated for 10 minutes on a water-bath. Glass plates may be coated in the usual manner, or paper may be floated on it as in coating albumenised paper. Prints on such a paper possess great vigour. The image prints of a violet tint by reflected light, and by transmitted light is of a deep chocolate. If fixed at once, the tint by reflected light is burnt sienna colour. Prints may be toned by any of the ordinary toning baths, though with somewhat of a difficulty. The following toning bath is said to yield good tones :— Ammonium sulphocyanate . . 7 grammes Water i life Gold trichloride . . • • -12 gramme. The print should after toning be transferred to another solution of ammonium sulphocyanate (12 grammes to I litre of water), where it should remain 5 or 10 minutes. After this it is washed, and placed in a solution of sodium hyposulphite (see page 159). It is finally washed and dried. CHAPTER XXIV. PRINTING WITH IRON AND URANIUM COMPOUNDS. The majority of these processes are not very generally employed for the production of prints, but stiU they are useful for certain purposes, such as copying maps, plans, i66 Printing ivitJi Iron and Uraninni Compounds. &c., by contact. An exception, however, is in the platino- type process, which is now very generally employed, and to which a separate chapter is allotted. Printing Processes with Salts of Iron. Sir John Herschel investigated the relative sensitive- ness of the different salts of iron, and came to the con- clusion that the double citrate of iron and ammonia was more readily acted upon by light than any other, whilst after it came the double oxalate of iron and potassium. To produce the former salts, take a weighed quantity of ferrous sulphate, dissolve in water, and boil with nitric acid till it is thoroughly oxidised and in the ferric state : next precipitate with ammonium hydrate, and wash the ferric oxide in warm water to get "rid of all the soluble salts. Transfer the washed oxide into a glass beaker and gradually add a solution of citric acid, and warm. When a small trace of ferric oxide remains undissolved, the addition of the citric acid should be stopped. Take the same amount of citric acid already added to the ferric oxide, and care- fully neutralise it with ammonium hydrate, testing the operation with litmus-paper. Then mix the two solutions together and evaporate to dryness over the water-bath, and when sufficiently concentrated allow the crystals of the double citrate of iron and ammonium to separate out. After carefully drymg between blotting-paper they are ready for use. The double oxalate of iron and potassium may be prepared in a similar manner. When required to render paper sensitive the following proportions should be taken : — Double citrate of iron and ammonium . lo grammes Water (distilled) lOO cc. This is applied to the paper with a brush, or else the paper may be floated on it. When dry it is exposed beneath a negative from a minute in bright sunshine to a quarter of an hour in diffused light, when it is ready for development, Her sellers Processes. 167 though the image will be barely visible. If a blue picture be required, all that is necessary is that the print should be immersed in a solution of potassium ferri-cyanide. After a few seconds the image will be found perfectly developed. A copious washing in water (in which a little citric acid has first been dissolved for the first washing) will dissolve out ail the soluble salts, and leave the blue image unchanged. The theory of this reaction has already been explained in Chapter IV., and need not again be discussed. When pic- tures were developed by this method the process was called cyanotype by Sir J. Herschell. Instead of developing with the potassium ferri-cyanide, the exposed paper may be immersed in a dilute and neutral solution of gold trichloride. The gold gradually deposits on the exposed portions and gives a purple image. This method of producing pictures on an iron salt has been called the chrysotype. The reduction of the gold follows from the fact that the ferrous salts are capable of reducing salts of gold to the metallic state when coming in contact with them in solution. In the case of pictures taken by means of the double oxalate of iron and ammonium, it is well to add to the gold solution a little neutral ammomum oxalate. The development in this case takes place very rapidly. To fix the pictures they should be immersed in water slightly acidified with hydrochloric acid, and then be thoroughly washed. An exposed paper prepared with any double s-alt of iron and ammonium may be developed by floating it on a solution of silver nitrate to which a trace of gallic acid and acetic acid have been added ; the ferrous salt reduces the silver nitrate, and causes the metallic silver to deposit where the ferrous salt existed. The gaUic acid subsequently causes a further reduction of the silver nitrate, and the first deposit of silver attracts the following. An image is thus built up. Founded on the same reaction pictures may be obtained by means of platinum tetrachloride, mercuric chloride, and 1 68 Printing zvitJi Iron and Uranium Compounds. potassium dichromate, &c., though greater exposure with these is necessary. Another modification of the iron process described is the production of a positive from a positive. It is founded on the fact that potassium ferrocyanide forms an insoluble compound with a ferric salt and not with a ferrous salt. If, then, a ferric salt be acted upon by light it gets reduced to the ferrous state, and if the paper be floated on potassium ferrocyanide, on the part unacted upon by light a blue pre- cipitate is formed, and on the part acted upon a slight stain of lighter blue. To prepare a paper which shall give clean prints it is usual to mix the iron salt with a solution of gum, and to develop with a mixture of ferro- and ferri-cyanides of potassium. Both the exposed and non-exposed parts of the paper are tinted blue, but the gum dissolves off the portion of the paper exposed to light, carrying that portion of the blue colouring matter with it which was caused by the iron which was reduced to the ferrous state and which had combined with the potassium ferricyanide ; thus leaving the part unacted upon by light behind as blue on a white ground. This process is useful for copying tracings, but the i)aper in this case must be placed in contact with the back of the tracings. In engineers' drawing-offices paper of this description has come into very general employment, as it can now be purchased ofexcellent quality. About 1857 Salmon and Garnier brought out a process dependent on the fact that the ferrous salt resulting from ferric citrate is more hygroscopic than the ferric citrate itself. Paper coated with the ferric citrate is exposed, and then covered over by an impalpable powder, such as plumbago. The surface is then gently breathed upon, and more or less of the powder adheres, approximately in the inverse ratio of the amount of actinic light that has been allowed to fall on it. When sufficient intensity is secured the non-adherent powder is removed by a soft brush. The unaltered citrate is easily washed out of the film, leaving the powder image on the Poitevin's Process. 169 surface of the paper. Better results are obtained when sugar of milk or loaf sugar is mixed with the citrate. The process is not perfect, being defective where half-tones are required ; for the reproduction of engravings, however, it is excellent. It will be noticed that, to produce a positive picture, a transparent positive reversed as regards right and left must be employed. Foitevi?i's Process with Ferric Chloride and Tartaric Acid. In Poitevin's process another property of a ferric salt is brought to bear, viz. the fact that it makes gelatine insoluble. A 6 per cent, solution of gelatine in water is prepared, with which is mixed any suitable pigment. Paper is floated on it whilst still warm. The paper now presents a uniformly coloured surface. To sensitise the paper it is immersed in a solution of Ferric chloride . , . . .10 parts Tartaric acid 3 parts Water 100 parts, and after drying in the dark it is ready for exposure. When exposed to light, the gelatine, which is now insoluble, be- comes soluble in hot water. If, therefore, the paper be exposed beneath a positive (reversed as regards right and left), an image may be developed by simple immersion in hot water. The parts which are insoluble remain next the paper, hence a perfect image may be developed with care. The student should compare this process with the autotype process (p. 1 74), and note the comparative advantages and disadvantages of the two. It seems to .the writer that there is a possibility of a great future development of this process. Printing with Uranium Salts. The usual salt of uranium employed for printing pro- cesses is the uranic nitrate, and, as has been indicated, this is reduced to the uranous state by the action of light in the I/O Printing wWl Iron and Uranium Compounds. presence of organic matter. The following may be taken as a good strength of solution : — Uranic nitrate 40 grammes Distilled water ..... 250 cc. The paper should be floated about eight minutes, as for sensitising paper in the silver bath. When dry it is ready for exposure, which is somewhat long. To produce a brown picture, float the exposed surface on the following : — Potassium ferricyanide . . . i gramme Nitric acid ...... 2 drops Water . . . . . . 2 50 cc. In about five minutes the whole of the detail will be visible. After thoroughly washing in slightly acidulated water the image will be fixed. To produce a grey picture, the exposed paper should be floated on Silver nitrate . . . . .2 grammes Water . . . . . . 40 cc. Acetic acid . . . . . 3 or 4 drops. The image appears very rapidly, and attains full intensity if the exposure have been sufficiently long. If it be weak, a few drops of a saturated solution of gallic acid added to the above will produce the desired effect. Washing in water will fix the picture, though care should be taken that no chlorides or carbonates are present in it. If any doubt exist as to their presence, sodium hyposulphite must be resorted to. The picture may be toned with gold,, platinum, or other salts, as may be desired. Uranium will also reduce the soluble salts of gold to the metallic state ; hence a picture may be developed with these. A pleasing variety in these prints can be made by mixing with the uranic solution some ferric salt, and de- veloping with the potassium ferricyanide. The resulting tone is richer and quite as permanent. Various other modifications have from time to time been made for the production of different shades of colour in the print. The Platinotype Process. 171 CHAPTER XXV. THE PLATINOTYPE PROCESS. The beautiful platinotype process, which is rapidly gaining favour, is dependent on the reduction of a ferric salt to the ferrous state. Mr. Willis, to whom the discovery of this process is due, found that when a platinous salt, the chloro-platinite of potassium, was mixed with a ferric oxalate and then floated on a hot solution of neutral potassium oxalate, that where light had acted, there the platinum salt was reduced to the metallic state. The following seems to be the reaction: — Ferric oxalate becomes Ferrous oxalate and Carbon dioxide. Fe,{C,0,)3 = 2Fe(C„04) + 2 CO, When the hot solution of neutral potassium oxalate is applied, the ferrous oxalate is decomposed and the following reaction takes place : — The image formed is in one of the most stable sub- stances known ; being unattacked by ordinary atmospheric influence, the pictures by this process may be considered to be permanent. The paper used has to be sized to prevent the image sinking into the surface^ Gelatine or arrowroot are the sizings which are most usually employed. For the latter size the following preparation answers: — 10 grammes of arrow- root are rubbed up in a mortar with a little water, and gently poured into a litre of water which has been brought up to boiling-point. After the liquid has boiled a short time Ferrous oxalate and 6 Fe(C,0^) become Ferric oxalate, = 2 Fe,(C,0,), 172 The Platinotype Process. 200 cc. methylated spirit are added, and the solution is filtered. The solution should be poured into a dish slightly larger than the paper to be employed. The sheets are drawn mto the solution, taking care that no air-bells form. They should be left in it 2 or 3 minutes, when they are taken out and hung up in chps to dry. The following solutions are prepared according to the directions given by PizzighelU and Hubl :— No r. Ferric oxalate . . • • 8 grammes Oxalic acid .... "5 gramme Water 26 cc. This solution must be kept absolutely in the dark. No 2. Chloro-platinite of potassium . 8 grammes Water . . . . . 5° cc. When preparing the sensitising solution the following is the mixture : — No. I 22 parts No. 2 24 parts Water 4 parls. The two experimenters we have quoted add a certain proportion of a solution of chlorate of potash to this to give more or less deep blacks, but the above may be taken as the normal solution. The coating of the paper takes place in a feeble light. Yellow light is the best, but it is hard to see the colour of the solution. The paper should be pinned by the corners on a smooth board, and the sensitising solution applied with a piece of flannel enclosing a pledget of cotton-wool. For 1,000 c. square about 3 cc. of solution is required. This quantity should be poured in the middle of the sheet of paper, and be immediately spread over with a circular mo- tion. The rubbing should be very gentle, and should be continued until the coating is as uniform as possible. The paper must be dried in from 5 to 10 minutes, and it is of the Development of Platirium Prints. 173 utmost importance that it should be thoroughly dried. If the paper take too long in drying, the image will appear sunken, and if too short a time the plantinum will wash off during development. When the paper is placed on a negative it should be backed with vulcanised indiarubber, or well-waxed paper, to prevent the access of any moisture to it during printing. The time of exposure varies upwards from two-thirds of that necessary to give to a silver print of the same subject : it depends on the sensitiveness of the material and the mode of development. The process works best with negatives of good density and gradation. To develop the print the following solution should be made : — Potassium oxalate (neutral) . 300 grammes Water ..... i litre. The solution should be brought to a temperature of be- tween 77° C. and 85° C, and is conveniently kept heated in an enamelled iron dish over a source of heat. The develop- ing solutions may be used over and over again, decanting from any gum crystals which may appear on cooling. The development takes place by floating the paper on the solu- tion, or it may be dragged over its surface. The print is fixed and washed by passing the print into a solution of hydrochloric acid (i part to 60 of water), immediately after developing, where it should remain, face downwards, for 10 minutes. It should then be passed into another bath of the same strength for the same time, and finally be washed for a quarter of an hour in three or four changes of water. With a feeble negative the temperature of the develop- ing bath may with advantage be lowered to 60°, or if a print be much over-exposed the same artifice may be resorted to. Platinum prints in a wet state appear more brilliant and lighter than they do when dry, and this must be taken into account. 174 Printing with Chromium Saits. The Platinotype Company issue paper which gives tones approaching sepia, with which they issue special instructions. It is believed that this tone is dependent on the use of a mercury salt in combination with the platinum. Platinum paper, before and after printing, should be stored in boxes containing calcium chloride. Such boxes, of a very convenient form, are supplied by the Platinotype Company. The chloride may be dried from time to time over a Bunsen burner. CHAPTER XXVI. PRINTING WITH CHROMIUM SALTS. As already pointed out in Chapter IV., p. 32, the dichro- mates are acted upon by light in the presence of organic matter, and the result is to render such organic matter insoluble in, and non-absorbent of, water. The following experiments may be undertaken. I St. Let albumenised paper be prepared such as is de- scribed at p. 154, preferably omitting the chlorides, &c., and employing only the albumen, and float it on a 6 per cent, solution of potassium dichromate. If the student exposes one of these pieces of paper in a dried state beneath a negative or an engraving, he will find that on soaking it in cold water all the albumen that has been acted upon by light will remain insoluble, whilst that protected will readily dissolve. Three or four small pieces of gelatinised paper may next be prepared by brushing over the paper a viscous solution of gelatine, in which is dissolved the above pro- portion of the dichromate. When dry, they may be fully exposed to light beneath negatives of line engravings. On immersing one of them in cold water, it will be noticed that the protected parts immediately begin to swell, through the Swan's Process. 175 absorption of water, whilst those portions unprotected remain unchanged. On immersing another sheet in hot water, the protected gelatine will dissolve away entirely, whilst the rest will remain firmly attached to the surface of the paper. Another sheet of exposed gelatinised paper may next be brushed over with thin, greasy, lithographic ink, and after soaking in cold water, a wet sponge may be applied to remove all the ink that will come away. It will be found that the non-absorbent parts retain the ink, whilst the latter reject it. If portions have been only partially protected, as in the case of what is called a half-tone negative, the ink will be found to adhere thinly on them, owing to the gelatine having become only partially non-absorbent. One of the earliest processes in which a dichromate was used was that due to Salmon and Gamier, and is similar in principle to the powder processes which are to be described. Poitevin and Talbot were, however, first in the field with a practicable application of it. Swan's process was the first commercially successful, and a brief outhne of it may not be uninteresting, as it is still worked by Braun of Dornach. The organic matter employed is gelatine, and it is applied to the surface of paper, after having been coloured with some unalterable pig- ment, such as lampblack, and sensitised with ammonium dichromate. The prepared paper is next exposed beneath a negative till it is judged sufficiently printed. The student must now try to realise the work that the light has been performing. Those parts of the gelatine next the negative will have become insoluble to a depth correspond- ing to the intensity of light entering, and as there will be but little of the negative which will not allow some light to pass through, it may be considered that the whole of the exterior surface of the gelatine has become insoluble, whilst the soluble portions remain enclosed between the insoluble layer and the surface of the paper. If such a print were immersed in hot water to dissolve away the unaltered gela- 176 Printi7ig ivitJi CJiromium Salts. tine, the viscid solution would remain imprisoned, and no development of the image would be possible. This difificulty Swan overcame by cementing the insoluble surface to paper by a solution of india-rubber. On immersion in hot water the original paper easily strips off, leaving the water free access to the soluble gelatine. When this is com- pletely dissolved away, an image in pigmented gelatine remains on the india-rubbered paper,' though reversed as regards left and right. This defect, again, was overcome in one of two ways — either by using a negative reversed as regards left and right, or by the following procedure. Another piece of paper, coated with starch or gelatine, was applied to the image, and allowed to dry in contact. The india-rubbered paper was then moistened with benzine or some other india-rubber solvent, and detached. It will be well to draw the student's attention to the reason why the portions of the film of gelatine become insoluble to depths corresponding to the intensity of light, instead of becoming only partially insoluble through their whole depth. The light that is chiefly effective in causing the reduction of the dichromate is the blue. Now, since the dichromate is of an orange colour, it is evident that an absorption of the blue will take place, and experiment has shown that a small thickness of gelatine coloured by it will prevent any effective ray being transmitted. In order to cause the reduction of the chromium compound, the amplitude, multiplied by the number of the waves, must be of a certain constant numerical value ; if the product falls short of this constant no change will be effected. On this assumption it will be readily seen that insolubility will take place only to certain depths, depending on the length of ex- posure and intensity of the light. At the same time it will be seen that it does not necessarily follow that the whole of the gelatine or other organic body becomes insoluble to that depth, but that the ratio of soluble to insoluble matter in- creases as the depth becomes greater. This last point is The Temporary Support. 177 important, for it seems that the photo-mechanical printing processes are really dependent on it. In order to obtain perfection in prints formed in gelatine, the image should be dark-coloured and tranf.parent or translucent, in order that the minutest difference in shades may be observable ; in other words, the white ground of the picture must play its part in this as in silver printing. J. R. Johnson was the first to improve upon Swan's process ; he found that the insoluble gelatine could be caused by atmospheric pressure to adhere to any impervious surface. This he effected in the following ingenious manner. The undeveloped picture, printed in the usual manner on gelatinised paper, was immersed in cold water, and allowed to absorb a certain quantity of the fluid, causing the unaltered gelatine to swell slightly. Immediately that the curling of the paper in the water showed that sufficient fluid had been imbibed, he brought the surface of the gelatine and a metal plate nearly in contact, a thin layer of water being allowed to separate them. This water he squeezed out, and, the gelatine continuing to swell owing to the fluid remaining in the pores of the paper, a partiail vacuum was created between the two surfaces, and the insoluble gelatine was found to adhere firmly to the metal plate. When the picture, so held, was immersed in hot water the paper backing could be stripped off, and development took place on the temporary metallic support. Gelatinised paper applied to the image could then be employed as a final support. In practice it was found that this method of development was liable to cause a loss of sharpness in the image, owing to the tension of the gelatine. To overcome this, Mr. Sawyer, of the Autotype Company, prepares an insoluble gelatinised paper support, to which the gelatinised paper is caused to adhere by the same means. In this case the support will expand with the image, and the want of sharpness is thus overcome. It would be impracticable to recount all the various N 178 Printing zvith Chromium Salts. suggestions that have been made for the improvement of the gelatine process. Much ingenuity has been brought to bear on it, and it seems now to have arrived at a state bordering on perfection. Many of the improve- ments in it have been patented, and thus the working of the process has been in a measure restricted to the Ucensees of the Autotype Company, to whom most of these patents have been assigned. The manipulation of the autotype process will be described, as it is that which has gained the greatest success. In regard to the insolubility of dichromated gelatine after exposure to light, a remarkable fact was noticed by the writer. It was found that where the insolubility of the gelatine caused by light had once commenced, it continued in the dark, and that the action was further increased by exposure to what would ordinarily be non-actinic light. This remarkable property has been utilised in the auto- type process to diminish exposure ; and Marion, of Paris, Kkewise took advantage of it in a process known as Mario- type. An outline of this process is as follows : — A paper is coated with gelatine, rendered insoluble by alum, and sensitive by potassium dichromate. It is exposed beneath a negative and a sheet of gelatinised and pigmented paper, which has also been impregnated with potassium dichromate, is brought in contact with it in an unexposed state. The two are kept beneath pressure in the dark for eight or ten hours, and are then withdrawn. The action set up in the impregnated paper by the light is communicated to the other coloured gelatine, and, as it starts from the bottom surface of this towards the top, the soluble portions are exposed to the solvent action of water, when the paper support is removed. The development takes place in the ordinary manner, and the image is not reversed as regards right and left. The process is not practis-ed to any extent, but is a curious example of a catalectic action started by the impact of light. Autotype Process. 179 All gelatine which has been long in contact with a dichromate, when dried becomes insoluble after a time without any exposure to light having taken place. The probable cause of this has been shown at p. 32. In hot climates the drawbacks to the use of gelatine in any form are that the ordinary temperature of the water is such as to render it liable not to set, but to remain in solution, and if dried it rapidly becomes insoluble. With care, of course, the want of setting power may be avoided, but there is no doubt that the difficulties of working this process in the tropics are far greater than in a temperate climate such as that of England. The paper when coated with gelatine and pigment is technically termed carbon tissue, and as such it will be referred to. Since the original patent of Swan many improvements in the manufacture of the tissue have been made, and the different substances added to the gelatine are only partially known to the public. The Autotype Company, who possess Swan's patent, together with all others which are essential to the right working of the process, supply the tissue at a reasonable rate, and an amateur cannot do better than procure the needful supply from them in preference to making it himself. Should he determine to make it himself, however, the following solution should be pre- pared : — Nelson's No. 2 flake gelatine . . 100 grammes Sugar (brown) 10 grammes Honey soap 10 grammes Glycerine 20 cc. Water . . . • ■ . 49° cc. Pigment of a permanent nature is finely ground, and in- corporated with a little warm gelatine and glycerine, and then mixed into the above. Aniline dyes may be employed, though some are apt to render the film insoluble, as are also certain kinds of pigments, There are two ways of N 2 1 80 Printing with Chromium Salts. applying this gelatine solution to paper. A fixed quantity- may be taken in a measure and applied to paper which has previously been soaked in warm water, all excess of mois- ture being blotted off on blotting-paper. The paper in this case is placed on a carefully levelled glass plate, and the proper quantity of fluid poured on and distributed evenly over the surface by means of a glass-rod. In cold weather the gelatine will set almost at once, and when firm the paper is hung up to dry. In warm weather iced water may be caused to come in contact with the bottom surface of the glass plate, which will cause the setting to take place rapidly. Rapidity in setting and drying is conducive to sensitiveness, and hence must not be overlooked. The next method is simpler, perhaps, and almost as effective. A porcelain or other dish, a, is placed on a hot-water tin, B, the water being kept at boiling point by a lamp or Bunsen burner. Over the dish is placed a level table, d, at one end of which is a roller, g, that is on a level with the top surface of a glass, e, placed on the table, d. The paper, f, is floated on the warm gelatine solution contained in the dish, drawn through it, seized by the hands and drawn over the roller on to the plate, e, where it is allowed to remain till the gelatine is well set, after which it is hung up by clips to dry. The dish has to be removed each time that paper is floated ; if b be lengthened, the dish can be run backwards and forwards in a very simple manner. In making the tissue a great point is the selection of the paper. It will be found advantageous to use rather a porous kind, not over-sized. A wash of ammonium hydrate im- proves it, as all grease is thereby removed. Another point to be attended to is the temperature of the gelatine solution. If raised too high, the coating given to the paper becomes uneven. Much practice is required before paper can be evenly coated, and it will even then probably be found inferior to that obtained from the manu facturers. Air-bubbles are a constant source of annoyance. Manufacture of Tissue i8l and are with difficulty avoided ; the surface should be well skimmed from them before paper is floated. The tissue when dry is improved by being rolled in a copperplate press, though it is not essential if the glycerine added have been sufficient to cause it to remain somewhat limp. Once coating is generally sufficient, though if the white of the paper show through the gelatine and pigment it will be necessary to give it a second coating in the same manner as before. Fig. 33. Should it be required to sensitise the gelatine at once, 40 grammes of potassium dichromate should be added to the above. Whether the tissue be home-made, or be supplied by the Autotype Company, it can be sensitised in a solution of Potassium dichromate . . - So grammes Water i litre. This solution should not contain free acid, as if it does the tissue is liable to become insoluble. This fact has been utilised by Dr. Monckhoven in his method of preparing 1 82 Printing with Chromium Salts. carbon tissue. In order to float the tissue on the above solution, a dish somewhat larger than the piece to be sensitised is used ; and it is coiled up in a small roll, with the ge- latinised surface outside. The extreme end of the tissue forming the roll is turned up for a centimetre. In this form the tissue acts as a spring, and will unroll itself if allowed to do so. Advantage is taken of this. The turned-up end is brought to one side of the dish and dropped on the solu- tion. The hand grasping the roll is gradually unloosed, and the tissue, uncoiling itself, pushes the end which first touched the solution to the farther side of the dish, and lies flat on the solution, all chance of air-bubbles cUnging to it being thus avoided. Fig. 34. After floating for three minutes, the turned-up end is pinned to a lath, by which it is hung up to dry. The dry- ing-room should be well ventilated, and have a constant current of dry air circulating through it in order to cause rapid drying, which is so favourable for sensitiveness. When quite dry the paper is exposed under a negative in the ordinary manner, taking the precaution, however, to leave a small portion at the external edges of the tissue not ex- posed to hght, since this gives greater certainty of adhesion to the metal plate, or other impervious surface, in the subse- quent operations. A mask of brown paper placed over the negative effects this. Owing to the colour of the pigment no change of appearance in the tissue will be noticed if examined after exposure. It is therefore necessary to resort to an Necessary Exposure to Light. 183 actinometer in order to judge of the exposure. The simplest form is chloride of silver paper exposed in the same hght as the tissue through a small aperture surrounded by a medium tint of the same hue as that which the chloride takes after moderate exposure. Two to three such tints may be required- By adopting the plan of under-exposing, and leaving in the dark, or in non-actinic light, as explained at p. 178, the exposure, of course, can be materially reduced. When in a developable state a shallow tin or other dish is filled with water, and a finely-mulled zinc plate is placed at the bottom of it. The plate must have been previously treated with what is known as waxing composition, made as follows : — Beeswax . . . . ... 3 parts Yellow resin ..... 3 parts Oil of turpentine . . . .160 parts. These proportions are not absolute, as the composition of the beeswax varies. ' The resin must be added to the beeswax in such proportions that the gelatine film will remain on the plate without cracking or peeling, even when dried in a hot room, but at the same time will leave the plate readily (when the applied transfer paper has become dry) without the application of any force.' The plate is first rubbed with a piece of flannel, on which has been smeared ai small quantity of the fatty body. All excess of wax, except a very fine layer, which persistently adheres, must be removed by polishing. It is not necessary to wax the plate each time a print is reiTioved, but this must be done whenever the gelatine image shows a tendency to stick to the zinc plate during transference to the permanent support. The plates are freed from dirt and greasy matter by the application of a little turpentine, ammonia, or potash. To attach the gelatine surface to the zinc plate the tissue is immersed face downwards in the water in the dish, and as soon as it begins to curl tipwards, the zinc plate is lifted out of the water, carrying between it and the surface of the 1 84 Printing with Chromium Salts. gelatine a layer of water. The plate is then placed on a small low stool (slightly smaller than the zinc plate), and the excess of water squeezed out by means of'a squeegee. The squeegee is shown in the annexed figure. It consists of a flat bar of wood, into which is let a piece of india-rubber about \ centimetre thick and 2 centi- metres broad. When all the su- perfluous water is thus expelled, ' the gelatine fiim is allowed two are three minutes to expand, and is then placed in warm water of a temperature of about 40° C. The annexed figure shows the developing trough as supplied by the Autotype Company, and it certainly is very Fig. 36. convenient for the purpose, a is a trough, fitting into a ' A certain amount of dexterity is required to prevent the paper cockling at the edges ; the india-rubber of the squeegee must be brought to bear with considerable pressure on to the surface of the paper, and the strokes made with it should commence from the centre and finish towards the ends. Development of the Print. 185 case B, leaving a space d below, in which a Bunsen burner can be placed, in order to heat the water in a ; along the sides of A are grooves, c c, into which the zinc plates slide. After half a minute the paper can be removed from the gelatine, and after the water has had free access to it the image begins to develop rapidly, particularly if the plates be moved vertically in the trough. When all the soluble portion is dissolved away, the picture is washed in cold water, and dipped for a second or two in a weak solution of alum and water, and put aside in a rack to dry. Some operators prefer to use a deep dish in which to develop, and doubtless the development is equally readily executed by so doing, but only pictures of a total area less than that of the dish can be developed at one time, whereas with the trough arrangement as many as a dozen pictures can be put in hand at once. For an amateur this is not a matter of great importance. When the gelatine image is dried, a piece of transfer paper (which is paper coated with gelatine subsequently rendered insoluble in water by alum or other such body) is placed in water of about 60° C, and softened. The picture on the zinc plate is placed in a dish of cold water, and the softened transfer paper is applied to it in the same manner as was adopted for causing the undeveloped gelatine image to adhere to the zinc plate. After drying, the picture will peel off from the plate and adhere to the transfer paper. The carbon print is then complete. The same manipulations, with a few evident modifica- tions, are necessary when the temporary support is pliable. When a reversed negative is employed, the image may be developed on the final support. Willis's Aniline Process. CHAPTER XXVII. Willis's aniline process. Willis's aniline process may next be briefly described. It is dependent on the action of dichromates on organic mat- ter, though the printed image is given colour by means of aniline. Sized paper is floated in potassium dichromate, to which a little phosphoric acid has been added. It is then exposed beneath a transparent or translucent positive, such as a plan or map, and when the image is perfectly visible, it is exposed to the action of aniline vapour. Aniline salts have the property of striking a green, black, or reddish colour when brought in contact with acidified di- chromates ; hence those parts which have not been exposed to light, or have been shielded from it (as is the case with the lines of the positive print), are deeply coloured, the rest of the paper remaining of the faint colour due to the reduced chromium oxide. In developing these prints, ani- line is dissolved in spirits of wine, and the mixed vapours are allowed to come in contact with the print. It will at once be evident what an extremely valuable process this is for copying engravings, plans, and tracings. All that is required is a sensitising solution, a sheet of glass to place over the plan, &c. (which, when exposed, should have its back in contact with the sensitive paper), to keep them in contact, and the sensitised paper. A rough box with a lid, on which can be stretched the printed paper, a basin to contain the aniline solution, and a spirit lamp to warm it, complete the outfit. The prints can be washed, and are then tolerably per- manent. This process was patented by the inventor, Mr. W. Willis, the father of the inventor of the platinotype process. The Powder Process. 187 Whether in the face of the many other processes to effect the same object the aniline process will be worked is a matter of conjecture. There are various modifications of this method of printing by using coloured aniline dyes, such as rosaniline. For some purposes they are useful, but as a rule, they are better for the reproduction of subjects executed in hne than for half-tone negatives. THE POWDER PROCESS. Reference has already been made to Poitevin's process, in which orginally salts of iron were employed to sensitise gelatine, the development being effected by the application of plumbago, or other impalpable powder. The dichro- mates subsequently were found to answer better than the ferric salts, the development of the prints being somewhat more easy with them. A mixture of gum-arabic, sugar, and a little glycerine, together with a sensitising solution of potassium dichromate, is prepared and poured over a glass plate, or other impervious surface, and allowed to dry in a warm temperature. The plate thus prepared is at once exposed for a few minutes beneath a transparent positive and withdrawn. Those parts acted upon by light will be found to be hygroscopic in the ratio of the time of expo- sure and intensity of the light. Any impalpable powder brushed over the plate will now be found to adhere to these hygroscopic parts in proportion to the moisture which they hold. Hence a positive, reversed as regards left and right, will result. When the image is developed it is coated with collodion, and can then be transferred to paper, &c., in an unreversed position. The soluble dichromate will be washed out during the process of transferring. This process is sometimes employed for obtaining images which can be burnt in on glass or enamels. For those who wish to try the process the following formula for the sensitive com- Woodburytype. pound will be found efificient. It is due to Obernetter, of Munich : — Dextrine ...... 4 parts White sugar ..... 5 parts It is sometimes recommended to give the glass plate a preliminary coating of plain collodion. The powder must be very gently applied with a cotton-wool brush or finecamel's- hair brush. The Woodburytype process is an exceedingly ingenious method of obtaining a mould of a gelatine print, from which other prints may be obtained. A rather thick film of sensi- tive gelatine is prepared, resting on a tough film of collo- dion. This is placed beneath a negative, the collodion side being next the image. It is then exposed to light pro- ceeding from a point, or to sun-light, arranged in such a manner that it always receives the rays in one direction. Uncontrolled diffused light will not do, as, owing to the thickness of the gelatine, the image on development would be ill-defined. When sullficiently printed, the gelatine picture is developed as if it were an autotype print, pre- senting the image in considerable relief. When dried, the gelatine picture is placed on a perfectly flat metal plate, and a sheet of soft metal (lead, for instance) is pressed on it by means of an hydraulic press. This latter presenting an exact mould of the former, is then placed in a press made as in accompanying figure. Gelatine is next dissolved in hot water and fine pigment or permanent dye added to it, and the viscous solution thus prepared is poured on to the mould. Paper of a very even texture, and which has been strongly sized, is placed on the top of the pool of liquid gelatine, and the top plate of the press, hinged as Ammonium dichromaie Glycerine . Water 2 parts 2 to 8 drops for every 100 cc. of water . q6 parts. WOODBURYTYPE. Production of Prints by Woodbury type. 189 shown in fig. 37, is brought down on to the mould and firmly locked by the catch, also shown in the same figure squeezing out the superfluous gelatine. When it is judged that the gelatine has set (which it soon does, owing to its contiguity to a mass of cold metal), the top is raised and the paper, which now bears the picture, is detached. The print is immersed in a solution of alum to render the pic- ture insoluble. The top plate, which is of glass, must be a perfect plane, otherwise the liquid gelatine will not be squeezed out from the portions that are to re- main white, and the print will be uneven and mottled image is necessary in order to give sufficient intensity to the reproduction, for it must be recollected that the gelatine solution, filling even the greatest depths of the mould, will present but a thin layer on drying. If the mould were obtained from an ordinary gelatine print there would not be sufficient depth of colour properly to represent the various gradations of shade. The pictures produced by this pro- cess are presumably permanent, and can be produced at a cost but httle in excess of the gelatine solution and the paper employed. As can be understood, there are difficul- ties in the way of producing any large surface which should be represented by pure white, since, however homogeneous a paper may be, it is invariably slightly thicker in some parts than others, and this prevents the glass plate attached to the lid of the press from fulfilling its functions in an absolutely perfect manner. The great relief of the original Photo -lithographic Transfers. CHAPTER XXVIII. PHOTO-LITHOGRAPHIC TRANSFERS. Another process, to which reference must be made, is that perfected by Colonel De C. Scott, R.E., and Sir Henry James, late Director of the Ordnance Survey of Great Britain. It also is dependent on the insolubility of gelatine when treated with a dichromate and exposed to light. It will be described in detail, as it is capable of producing prints in printer's ink, as well as in ink suitable to give a transfer on to zinc or stone. From such transferred prints the original drawing can be reproduced by ordinary surface printing. It may be well to notice the requisites for these transferrable prints. First, the image should be made in an ink which is readily held by a lithographic stone or mulled zinc plate. Secondly, it must be capable of a fair amount of resistance to pressure ; that is, it must be tolerably hard and cohesive, otherwise the act of passing a paper holding the image through a lithographic press would cause a spreading of the ink, and a consequent want of sharpness in all the impressions taken from the stone. Thirdly, the ink must be of such a quality that a very thin coating is sufficient to leave a sharp and firm impression on the stone or zinc plate. Fourthly, the paper on which the image is developed must be tough, and not easily torn or stretched. These requisites are fulfilled if the following directions are attended to. The best paper to select is that known as bank post paper, which is not highly sized. If it be, the sizing should be removed by immersion in boiling water, previous to coating it with the gelatine solution. The solution is prepared according to this formula : — Potassium dichromate . . .44 grammes Gelatine 44 to 66 grammes Glycerine 2cc. Water I litre. The varying quantity of gelatine is due to the fact that Preparation of Gelatinised Paper. 19 1 some gelatines give much more body to the solution than others. Thus, if fine-cut gelatine be employed it has been found in the writer's experience that the larger quantity will be necessary, whilst if the harder qualities of gelatine be em- ployed the smaller quantity will usually suffice. The gelatine is of course thoroughly softened in half the above quantity of water, and then the remaining half, in which the dichromate has been dissolved, is added in a boiling state. The solu- tion is poured into a dish, and placed over the hot-water tin, as described at p. 181. A sheet of paper of the proper size is floated on it for three minutes, and then hung up by two corners to dry. This causes the coating to be thicker at the bottom corners than the top, to avoid which resort may be had to the artifice shown in the figure, p. 181. In any case a second coating is required, and this is given in a similar manner. If the paper have been hung up to dry previous to the setting of the gelatine, the opposite corners to those by which the sheet was first suspended should be hung lowest. This secures a fairly even coating. The paper in this condition, even when damp, is slightly sensitive, and therefore it should be dried in a room which only ad- mits non-actinic light. It is exposed in the ordinary manner beneath, a negative, which should be of a line engraving, and not in half tint.^ When the lines appear of a well- defined fawn colour on a yellow ground, the paper should be removed to the dark room for subsequent treatment. If the object be to make a print to transfer to stone or zinc, the following ink should be prepared (though any lithographic ink will answer fairly well) : — Lithographic printing ink . . .16 parts by weight Middle linseed varnish ... 8 parts Burgundy pitch .... 6 parts Palm oil . . . . . .1 part White wax . . . . .1 part Bitumen 2 parts. ' 'Partial success has been obtained by Sir Henry James in render- ing even this latter class of work. 192 Photo-lithographic Transfers. The ink and varnish are first mulled together with a muUer, the Burgundy pitch and bitumen are next melted over a clear fire till all water is driven off, the wax next melted, and finally the palm oil. When properly melted they should readily catch fire, which shows that certain gases are being liberated. The ink and varnish are now well stirred into it, and the mixture run into the pots for storage. Should it be desired only to make a single print, the best ordinary chalk lithographic ink may be employed. Where a lithographic press is available, a fine and even coating of one of these inks is usually given to a stone by means of a lithographic roller, the paper bearing the picture is then placed face downwards on it, and pulled through the press, by which plan a thin coating of ink is given to the entire sheet of paper. In the absence of a press the ink may be rendered liquid with turpentine, and an even film of ink may be given with a fine sponge. To develop the picture the print is floated hack down- wards on a dish of water, having a temperature of about 50° C, and is allowed to remain on it till the lines are seen as depressions. It is then removed on to a sloping board, and a stream of warm water, of about 70° C, is poured over the surface ; the soluble gelatine being in a hydrated condition, is carried away together with the ink that covered it, and the image is left, formed of ink resting on slightly- raised ridges of insoluble gelatine. A very soft sponge dipped in the hot water and applied to the surface aids the development, in fact it can rarely be accomplished without it ; but the most delicate touch is required for this part of the operation, as the ink on the fine lines is very liable to be carried away. The developed print is next washed in cold water, and then hung up to dry. In this state it is ready for transfer to stone or zinc, if transfer ink have been employed. It is beyond the scope of this book to describe the transferring operations : these are described in other Lithographic Press. 193 194 Photo-lithographic Transfers. works. 1 A very convenient lithographic press, suitable for amateurs, has lately come under the notice of the writer ; the figure on the preceding page will give some idea of its form. It is cheap and well adapted for this process, as well as for certain photo-mechanical printing processes. A A is a cast-iron carriage of the form shown, b is the bed of the press, which is caused to move in the carriage by means of a roller, to which is attached the arm h. d d are drawers containing the necessary plant, c is a litho- graphic stone, shown in position, and held firm by means of the cross pieces of angle-iron fitting into the slots as shown. F is a substitute for the usual scraper ; it consists of a roller, round which, as well as round a smaller roller, is passed a band of flannel. A downward pre.ssure can be given to the roller by an ingeniously devised screw-motion, F, which, whilst giving the necessary pressure, yet causes it to take the natural bearing of the stone or plate, k k are the clamps by which it can be attached to a table, and r is the roller supplied for inking it. With this machine the impressions pulled are excellent, and it is very portable. Plates made of composition similar to solder are supplied by the manufacturer. They are excessively sensitive to greasy ink, and a number of impressions can be pulled off without clogging the work. To clean these plates all that is required ' is to wash out the old work with a solution of caustic potash, and then to scour the surface with fine emery powder. A dilute solution of acid is poured over the plate, and after washing under the tap, and gently warming, it is ready to receive a transfer. Should only one copy of the picture be required, the print, which should in that case have been printed in litho- graphic ink, is placed in a copying, lithographic, or typo- graphic press, face up, and a slightly damped piece of white or other paper placed over it. When the pressure is brought ' Instruction in PJiotography , published by Messrs. Piper & Carter; or in Sir Henry James's work, published by Messrs. Longmans. P J loto -lithography in Half-tone. 195 to bear, the ink is retained by the latter, and a good im- pression is thus obtained. This method has been named by Sir H. James as the papyrograph. It must not be mistaken for another process, used for copying letters or circulars, and known by the same name. Various modifications of this process have from time to time been proposed, such as coating the gelatine with albu- men, but in the writer's experience, when a picture is to be obtained by dissolving away the gelatine, no better process than the above can be used. Another process which differs from the foregoing is one brought out by the writer under the name of papyrotype. It is based on the fact that when light has acted on a gelatinized surface then water is not absorbed. Thus by exposing a bichromated paper, prepared as in the above process, to the action of light behind a negative, the lines, which are represented by transparent glass in the negative, are printed through and the gelatine becomes non-absorbent, whereas the parts which have not been acted upon remain absorbent of water. If such an exposed paper be placed in water, and be then made surface-dry, and a roller covered with a fine layer of greasy ink be passed over it, those parts which are non-absorbent will 'take' the greasy ink whilst those which have taken up water will repel it. It does not need much intelligence to see that in this way a print in greasy ink is obtained which can be laid dow^n to stone or zinc as desired. PHOTO-LITHOGRAPHY IN HALF-TONE. It will be noticed that both these processes are best fitted for reproducing subjects which are represented by lines, and it is a different matter to produce prints from the lithogra- phic stone in half-tone. It is easy to reproduce on the printed bichromated gelatine a print of a half-tone subject in different blacknesses of greasy ink, but when such are 196 Photo-engraving and Relief Processes. laid down to stone or zinc the parts which should be half-tints become deep black and the prints from the press become smudges of black interspersed here and there with patches of white. If the whole surface be grained we have a different result. Suppose for instance that embedded in the gelatine are very fine crystals which will dissolve in water, manifestly those parts which are most strongly acted upon by light will have the finest grain, the surrounding gelatine being unable to expand, whilst the parts which air has strongly acted upon will swell to a certain extent and leave a more pronounced grain in those parts. Such is the artifice that Quartermaster-Sergeant Hus- band, R.E., has adopted in his process called papyrotint. With his gelatine and bichromate he mixes common salt and other such crystalline bodies, and coats paper with the mixture. When dry he exposes it beneath a half-tone negative and soaks it in water, and then passes a roller con- taining greasy ink over it. The lights and sliades are in this way produced by greater or less fineness of grain, and as a result he is able to produce excellent photo-lithographs in half-tone. There are several processes in the market to obtain the same result, but they are all more or less secrets ; though it is believed that they all depend more or less on the grain given to the transfer which has to be laid down to stone. CHAPTER XXIX. PHOTO-ENGRAVING AND RELIEF PROCESSES. Niepce's process, it will be recollected, was founded on the fact that a bitumen of Judaea, when exposed to light, became insoluble in its ordmary solvents if partially saturated. Silver plates were coated with bitumen, and Ni^pces Process. 197 after exposure the unaltered portions were dissolved away and iodine applied. The remaining bitumen was then re- moved, and the image was consequently formed of metallic silver on a ground of silver iodide. Had Niepce removed the iodide by any proper solvent, he would have obtained a plate slightly engraved. Most of the present processes for photographically obtaining relief blocks, and also engraved plates, are based on the same principle as Niepce's ; in fact, there is very little departure from his mode of working until the biting-in commences. The student must distinguish between a relief and an. engraved plate. The former is in- tended to be printed in the ordinary printing press, the portions representing the lines of the sketch being raised as in a wood-cut, whilst with the latter they are in depres- sion. An outline of a successful process for the production of either a relief block or an engraved plate in line will now be given. A plate is coated with a thin film of asphaltum or bitumen of Judaea, dissolved in chloroform or other con- venient solvent, and after drying it is ready for exposure beneath a subject. If an engraved plate be required, the parts that have to be bitten in are the lines ; hence those portions must be protected from the action of light, since in order to lay the surface of the metal bare they should be covered with the soluble asphaltum. In taking a print from an engraved plate, the latter is reversed as regards left and right, therefore it is evident that a reversed positive should be employed, from which to print on the metal plate. For the production of a relief block, by similar reasoning it will be found that an ordinary unreversed negative picture is required, as it is from its nature reversed as regards right and left. Whether a positive or a negative be employed, the opacity should be extreme on those portions which are to protect the sensitive layer from light. Such a positive or negative is placed in contact with the plate, and exposure given till it is judged that sufficient insolubility is given to 1 98 Photo -engraving and Relief Processes. the exposed portions. The soluble portions are then dis- solved away by a solvent which is nearly saturated with the asphaltum. If the manipulations have succeeded, the metal should be petfectly bare in parts. Steel, copper, or zinc plates may be employed for this work ; the two former are more especially suitable for engraving. The mordant usually employed for these may be a mixture of hydrochloric acid with potassium chlorate, which causes an evolution of chlorine. For zinc, hydrochloric acid alone may be em- ployed, though it is well previously to dip the plate in a solution of copper sulphate. For an engraving the biting-in need be but very shght, though much of course must depend on the nature of the work as shown by the thick- ness of the lines. The thicker the lines the deeper must be the biting-in. For a relief block the biting-in has to be carried to a far greater extent ; in fact, as deeply as seen in an ordinary wood-cut. This involves very tedious manipu- lation after the first biting. The plate has to be warmed, dusted with resin ; again heated to slightly melt the bitumen, so as to allow it to flow down the sides of the bitten-in lines. This process has to be repeated till a sufficient depth is attained. When there are larger spaces of white in the print, the metal is usually removed by a fine saw, or a graver. Relief-block making is essentially difficult in almost every stage, and rarely repays an amateur the labour he may bestow upon it. Ehrard, of Paris, has another method of producing en- gravings, which is also dependent on biting in. He prepares a transfer, as for zincography, and, after going through the usual manipulations to transfer it to a copper-plate, he plunges it into an electro-plating bath for a few minutes, thus covering the copper with a thin film of silver, the lines of the engraving being protected by the greasy ink. After a rinse in dilute acid the plate is transferred to a bath of mercuric chloride, where the silver is converted into the double chloride. After washing, the ink is removed, and the biting process Talbofs Process. 199 allowed to proceed. The details of this process are a secret, but what is stated above gives a general idea of the process. The analogy that exists between this and Fox Talbot's process of engraving a daguerreotype plate is obvious. Another process for obtaining the same results, various modifications of which have from time to time been an- nounced, is due to Talbot. It consists of printing the nega- tive on a gelatine fijm, washing away the unaltered gelatine, and making an electrotype from it. In the trade there are several firms who practise either photo-engraving or relief- block making, but it is not known which methods they adopt, as the several processes are kept secret. x\mongst these may be named Goupil, Gillot, and Dujardin, of Paris; Dallas, and Leitch & Co., of London. Scamoni, of St. Petersburg, also makes very beautiful reproductions of engravings, &c. His method seems to be based on the building up of a relief on the negative itself, and then taking an electrotype. Fig. 39 is a print from a photo-relief plate by Warnerke, produced by a process of which the details are not as yet published. The processes above described are adapted to the re- production of line work in contradistinction to half-tone drawings or photographs from nature. The- production of photo-engraving or photo-etchings in half-tone is also much p-actised ; and there are two methods by which they can be done. One is by first graining a plate with some grain, and then transferring to such a plate a gelatine cast in half-tone, and biting this with some etching chemical. Such a process was that which Fox Talbot v/orked in the early days of photography. He gave a grain to a smooth copper plate by pouring on to it a solution in ether of camphor and resin. This gave a thin layer of resin and camphor, and a gentle heat expelled the latter and left the former on the plate in .small granules. A stronger application of heat caused the resin to adhere to the copper surface. On such PJioio-engraving and Relief Processes. Fig. 39, Photo-collotype Processes. 2or a plate he poured gelatine and bichromate of potash, and exposed it beneath a positive to the action of light. Where the light acted strongly it became more impervious to water, and where least, was quite pervious. When printed he placed the plate (waxed on the back) in a solution of ferric chloride, the solution bit into the copper surface almost proportionally to the intensity of the light that the different portions had received. The parts of the plate where the grain of resin rested remained more or less unacted upon: the deeper the biting, the more the grain was ' under-bitten.' Thus the grain was least in the deepest portions, which represent the deep shades of the picture, and greatest where the biting was least. Several of Fox Talbot's plates produced in this manner gave the promise of the great things which have been effected by a slight modification of his process. It is believed that several processes in the market are really Fox Talbot's original one improved in detail ; and this method of pro- ducing half-tone plates is by preparing a gelatine mould, the surface being grained, and taking an electrotype from it. It is believed that Goupil's process and others also are based on this principle. We cannot quit this subject without remarking that some very beautiful half-tone typographical blocks were produced by Pretsch as early as 1858, his process being based on that of Talbot, already mentioned. CHAPTER XXX. PHOTO-COLLOTYPE PROCESSES. By a photo-collotype process is meant a ' surface printing ' process, by which prints are obtained from the surface of a film of gelatine, or other kindred substance. The general methods by which such surfaces are formed are based upon 202 Photo-collotype Processes. the one fact already pointed out at p. 175, that gelatine, like other similar bodies, when impregnated with potassium dichromate, becomes incapable of absorbing moisture after full exposure to light ; and that where light has partially acted, there it becomes only partially absorbent, when com- pared with the amount it will absorb when entirely guarded from light. Suppose we prepare a film of gelatine with which has been mixed some potassium dichromate, by floating a warm solution of the mixture over a smooth surface, such as a thick glass plate, and when dry expose it beneath a negative in which we have different degrees of light and shadow, as in a landscape or a portrait negative ; on immersing the film in cold water, we shall have a picture impressed in which the different degrees of shadow are represented by different degrees of relief. If the back of a similarly treated gelatine film be exposed to hght previously to its immersion, the relief afterwards will be found to be much slighter. This is evidently a necessary consequence. If over either of these surfaces, when all superfluous moisture has been removed, a smooth soft roller carrying a fine layer of greasy ink be passed, it will be found that the greasy ink will adhere to the parts exposed to light in nearly exact proportion to the intensity of light which has acted on them. With the film in which the relief is high the ink will take less readily, because the roller, even when tolerably soft, will fail to come in contact with the exposed parts. With the film having but small relief the difficulty will not be found. If such a film as the latter be now placed in a printing press, an impression from it may be obtained, but it will be found that as regards right and left the pictures will be reversed. A reversed negative is therefore necessary. Theo- retically the number of impressions which can be pulled from the surface is not limited, if the surface be kept damp, and if a fresh application of ink be given by the roller. It will be found, however, that after each pull there is a tendency of Preparation of Printing Surface. 203 the unexposed gelatine to adhere to the paper, and thus to spoil the printing surface. In order to prevent this it has become customary to introduce into the gelatine some substance which will harden it. Certain gum resins, alum, chrome alum, and kindred substances effect this hardening, and one or other of them is to be found in the formulae given for most of these processes. Albert, of Munich, may be said to have first discovered a thoroughly workable process, based on the above principles, and we shall briefly give an outline of the method he adopted as being a typical one, and unencumbered with any of the large number of modifications introduced at various times by other experi- menters. A piece of plate glass some 2 centimetres in thickness is coated with a gelatine mixture made as follows : — I. Good glue ..... 10 grammes. Water 80 cc. II. Potassium dichromate ... 3 grammes. Water . . . . . . 40 cc. These are dissolved separately and mixed warm. The plate is then coated and dried by heat, 5 or 6 hours' exposure to a temperature of about 60° C. being sufficient to effect desiccation. The plates are now exposed back uppermost to light for about a quarter of an hour, the gelatine films resting on a smooth black surface, after which they re- ceive over the first a second coating made as follows : — I. Gelatine ...... 8 grammes. Water ...... 100 cc. II. Potassium dichromate ... 3 grammes. Water . . . . . . 40 cc. To No. I is added 60 cc. of white of egg, and afier 204 PJioto-collotype Processes. heating to 6o°C., No. 2 is mixed with it, and the solu- tion is filtered through cotton-wool. This coating is dried, and the plate is ready for printing. The expo- sure depends upon the quality of the light ; it must be continued till the whole of the detajls are visible on the gelatine, and much of the success depends upon the depth to which it is carried. When judged suffi- ciently printed, the back of the plate is again exposed to fight to such a degree that the resulting relief when the film is wetted will be small. The film, is now washed to re- move all excess of the dichromate, and is again allowed to dry. The dried plate is next placed for 5 minutes face uppermost in a dish containing a 25 per cent, solution of glycerine in water. The back is then embedded on the bed of a lithographic press by means of plaster of Paris, and is lightly rubbed over with linseed oil, and again shghtly damped with water, A soft roller, charged with greasy ink, IS then passed over the surface, when it is found that a perfect print appears on the surface. The plate, the surface of which is in contact with a piece of paper, is now passed beneath the press, and an impression pulled. Such a press as that in fig. 38 may be employed. Mr. Ernest Edwards introduced an important modifica- tion of the above by mixing chrome alum with the gelatine to harden the gelatine film. He only uses one coating to the glass plate, and when dried strips it from the glass surface, and prints it in this condition. He retransfers the film to a pewter or other metal plate, and. pulls an im- pression from it when thus supported. By this device the danger of destroying the printing surface, owing to the possible breakage of the glass plate, is overcome, and in consequence the cost of production is diminished. For a full description of the process, which is named * Heliotype ' by the inventor, the student is referred to another work.' Instriictiott in Photography. Piper & Carter. Elementary Photographic Optics. 205 It is scarcely possible to enumerate all the different col- lotype mechanical processes. We may mention in England the autotype, Pumphrey's, the heliotype, and the photo-tint processes as being amongst the most successful. CHAPTER XXXI. ELEMENTARY PHOTOGRAPHIC OPTICS. Fig. Without entering into any discussion as to the theory of light, it will suffice to glance at the more general laws of geo- metrical optics, such being sufficient to show the principles on which photographic lenses have been designed. A ray of light, whilst passing through a medium of uniform density, travels in straight lines, and when a ray of light passes from any medium to one more dense, at any angle less than a right angle to the tangent of the common surface, the direction of the ray of light is bent in towards the normal of the common surface ; and if the rays pass from a medium to one less dense, it is bent away from the normal, Fig. 40 explains what is meant by the above. Let g g represent the section of a thick sheet of glass with parallel surfaces. Let a ray of light, a b, strike the top surface of the glass at B. Glass being a denser medium than air, the ray will be bent in towards the normal, N n, of the surface, and strike the lower surface of the glass at c ; on the ray of light emerging from c to the air it will be again bent away from the normal n' n', and move in the direction c d, which is parallel to a b, since the surfaces of the glass plate are supposed to be parallel. N' In n' 2o6 Elementajy Photographic Optics. It is found experimentally that the sines of the angles which the ray makes with the normal at the surface of the two media have a fixed ratio to one another, and that this coefficient is dependent on the media through which the ray passes. Thus from air to ordinary flint-glass the coefficient is about 1-5, and from the flint glass to air the reciprocal about or -66. Applying plane trigonometry to this expe- rimental fact, it will be found that there is a limit to the angle at which a ray of light can pass from any medium to one less dense, since the limit of the sine of an angle is unity. When the ray strikes the surface at this particular angle or at a greater the rays are reflected back, and the limiting angle itself is called the critical angle, or angle of total reflec- tion, for these two media. A reference to this is made in a subsequent chapter. Instead of the surfaces of the glass being parallel we may have them inclined at an angle to one another, and in this case the refraction at each surface will follow the same law. An object which is really at k, fig 41, will apparently occupy the position k', which is Fig. 41. equivalent to saying that A , K 2. ray of (monochromatic) ^ \ light projected in the di- ^- — ^-t '^^ rection ka would have a .^.^-^^^^^ ^^^^ direction c E after passing ^ A k. through the prism. If the projected beam of light in the direction k a be white, it will be found, as already noted in the second chapter, that on emerging from c it is split up into rays of the different rainbow tints. If we take any three distinctive rays in the red, yellow, and blue, we shall find that the red is least refracted and falls in a direction r, fig. 42, the blue most and falls at B, and that the yellow occupies the intermediate position. This difference in the index or co- efficient of the refractive power of the media for different coloured rays gives the phenomenon known as dispersion. Dispersion. 207 It is found by experiment that the angles formed by the directions of the different rays of light vary ac- cording to the composition of the glass employed for the prism ; that with one specimen, for instance, the angle foimed by r and y does not bear the same ratio to the angle Fig. 42. formed by y and b that it does when another specimen is employed. It is owing to this difference in dispersive power of various glasses, that it has been found pos- sible to cause the component rays of white light to be nearly equally refracted, and yet to show no appreciable colour, due to dispersion. It will be seen in fig. 43 that, by employing opposing prisms of different composition, the dispersion may be almost entirely overcome. Thus it may happen that by placing a prism B of the dimensions, and in the position shown, the rays originally forming Fig. 43. white light, and which were decomposed by the prism A, might be so bent, owing to the difference in the dis- persive power of two media, that they emerge from b parallel to each other, instead of each ray forming a defi- nite angle with its neighbour, and that still the original ray 2o8 Elementary Photographic Optics. may be refracted. Supposing b and a to be of the same homogeneous medium, it is evident that the same result would not be obtained. If the distance between b and a were diminished till the adjacent surfaces touched, the paral- lelism of the rays emerging from b would still be obtained, and, owing to the small dispersion of the rays in a, an inci- dent ray of white hght would emerge as white light. The two media we have been supposing to be employed are only hypothetical. Unfortunately up to the present date no two media have been found whose dispersive power can be utilised so as absolutely to correct one another. Supposing we have a series of prisms and their frusta joined together as shown, it is evident that the surfaces may Fig. 44. be worked at such angles that the rays of light proceeding from an object at any distance from them may cut in one point and form an image of that object. In the figure 44 we have supposed the luminous object to be infinitely distant and to form one single image. By rounding off the angles the same result may still be obtained and will form a lens. The curve that a glass would take, to give such theoretically perfect results, would be practically unsuitable, owing to the difficulty of grinding it ; and also because it would only be correct for a particular distance and direction of object. In practice lenses are worked to spherical surfaces, as being most convenient, and being capable of approximate accuracy. We will first glance at the inaccuracy that the spherical The Use of the Diaphragm. 209 Fig. 45. surface may cause when uncorrected by other means. If the rays of light, striking the lens obliquely, or along its axis, be reflected from any distant object, they will be prac- tically parallel rays, and if different annuli of the lens be covered up, it will be found that the point of intersection of the rays will vary, the intersection of the marginal rays will be nearer to the lens than that of the central rays, fig. 45 ; thus when the whole of the lens is utilised the object will appear wanting in definition owing to what is called ' spherical aberration.' This defect is over- come in a great mea- sure by placing a diaphragm in front of the lens, fig. 46, the oblique rays and the central rays passing through which can be brought approxi- mately in focus on a plane at right angles to the axis of the lens. Again, a diaphragm has a further advantage in that it allows the foci of a distant and a near object to lie on one plane. Fig. 47. Fig. 46. The nearer an object from which the rays proceed is to the lens, the longer will be the focus after they pass through the lens. Let the rays issue from a distant and a near object. From fig. 47 it is apparent that if the whole lens be p 2 1 o Elementary Photographic Optics used (supposing spherical aberration eliminated) there would be no plane, x x, on which the two objects would appear at all defined. The effect of a diaphragm, or ' stop,' as it is technically called, is to narrow both pencils of light so that neither of them is much out of focus at any point intermediate between the foci of the extreme rays. See Fig. 48. fig. 48. This will be entered into further on in this chapter. Supposing that the rays from the near object formed an angle with the axis of the lens, and those from the distant object coincided with it, a larger diaphragm might be em- ployed if the plane on which the images of the objects have to be received makes an angle with the axis of the lens, fig. 49. It will be seen that the swing-back of a camera serves this purpose. Fig. 49. / It is to be observed that nearly the same results can be obtained by placing the diaphragm behind the lens instead of in front, fig. 50 ; and also that the size of the diaphragm determines the brightness of the image, for only a portion of the lens is utilised. The Use of the Diaphragm. 211 With a lens such as shown there is a difference in the resulting images when the diaphragm is placed in front or behind the lens. In both cases we have distortion, but the distortion in one case is the reverse of that in the other. When the diaphragm is in front of the lens the image of a square would be barrel-shaped. When it is behind the curvature would be reversed, fig. 51. It would be useless in either case to take an architectural subject with such a lens unless the building occupied but a small propordon of the picture. The reason of this distortion will be apparent when it is remembered that the margin of the lens, its sur- faces being portions of spheres, will cause greater refraction than the central portion. When the diaphragm is in front of the lens it is the margin of the lens which gives the image of the corner of the square. The image of the centre of each side is formed by a portion of the lens which is more central, and therefore is less proportionally bent. When the diaphragm is behind the lens different portions of the lens are used to form the image, and consequently the distortion is reversed. By placing a lens on each side of the diaphragm it is evident that distortion due to this cause Fig. so. Fig. 51. 212 Elementary Photographic Optics. may be entirely overcome, and thus -we get what is called a doublet lens. It will be found that with certain lenses, if we attempt to obtain a sharp focus of horizontal and vertical lines near the margin of the focussing screen, we shall fail : either the one or the other will be indistinct. This is due Xo. astigmatism^ a defect also caused by the spherical form given to the surfaces of lenses. Lenses have various shapes given to them ; the following are the different forms : — M is the double convex ; n, a piano- convex ; o, a con- cavo-convex ; p, a double concave ; q, a plano-concave ; r, a meniscus. Lenses in which the concavity is greater than the convexity can have no actual but only a virtual focus, Fig. 52. as may be seen by making a diagram. All such, when combined with other lenses, in which the convexity prepon- derates, ^ will either increase the focal length or give a virtual focus to the combination. In photographic lenses the chief use of concave lenses is, by making them of suitable glass, to secure achromatism . The principal focus of a lens is the point where rays which enter parallel meet on emergence. As an example we may refer to fig. 47. The optical centre of a lens is that point in the axis of the lens through which lines joining any points in an object and their images would intersect. ' The material in which the lenses are worked must be taken into consideration in determining this. Focal Lengtli. 213 Any point in any object and the image of that point are said to be the conjugate foci of the lens ; and the conjugate focal distances are said to be the distances of the optical centre of the lens from these two points. The equivalent focus of a lens is a term applied to a compound lens. It is the focus of parallel rays entering the lens. It is termed equivalent from being compared with a single lens that would produce the same sized image at the same distance from the object. To find the practically focal length of a combination of lenses, measure a distance of say 50 metres away from some fixed point, and place a rod at the extremity. From this rod measure a line of say 10 metres in length, exactly at right Fig. 53. angles to the first line, and place a rod over this point. Now place the front of the camera exacdy over the starting-point of the first line and level it, the lens being in the direction of the first line. Having marked a central vertical line in the ground-glass with a pencil, focus the first rod accurately, so that it falls on the pencil line in the ground-glass. Take a picture of the two rods in the ordinary manner, and measure back as accurately as practicable the distance of the centre of the ground-glass from the starting-point, and also on the negative the distance apart, at their base, of the images of the two rods. Suppose the first measured hne — A B to be 50 metres ; b d, the 2nd line (the distance apart of the rods), to be 10 metres ; a c to be 30 centimetres ; and 214 Eleinentary PJiotographic Optics. EC, the distance apart of the images of the bases of the two rods, to be 6 centimetres. Then bd + ce:cb::ce:cf, which is the equivalent focal distance. • oii. (50+ *3) '06 . . CI- = -dL = 30 centimetres. io-o6 It is, therefore, this distance along its axis from the ground-glass of the camera to the optical centre of the lens. The student will readily devise the means of setting off the distance thus found on the brasswork. It would be out of the scope of this work to touch on the higher mathematics of optics, and the following formulte are only true when the thickness of a lens may be neglected. The relation of the conjugate foci to one another is ex- pressed by the following formula : — -i = L _ ^ V f u Where v is the distance of the optical centre of the lens from the ground-glass, u is the distance of the optical centre of the lens from the object to be photographed,/is the equiva- lent focal distance. P>om this it will be seen that if u is very great, then i is so small that it may be neglected, and there remains v =/ That is, the image of an object at a great distance will be at the equivalent focal distance. Applying the above formula, suppose we have a lens where/= 30 centimetres and « = 40 centimetres :— I = J - -1 - -i. V 30 40 ~ 120" That is T20 centimetres, or the distance of the ground- glass from the centre of the lens must be 120 centimetres to bring it into focus. Let it be required that u should be n times greater than Choice of a Lens. 215 V, which is the same as saying that the image must be ^ n the size of the object. Then— I ^ £_ ^ _ I . f V nv nv ' or- Suppose, as before, /= 30 centimetres, and it is required to diminish the image of an object to 5 of the size of the original : — ^ _ 30 (4 + i) _ 27 centimetres, 4 Ti = nv = X 37 '5 = 150 centimetres, or the ground-glass must be 375 centimetres and the object 150 centimetres from the lens. By similar reasoning, if the object is to be enlarged 4 times, it will be found that the above distances must be reversed. In choosing a photographic lens the purpose for which ic is required must be kept in view, for it will be evident that the requirements necessary may be different. In a lens for taking portraits we have, for instance, certain properties which are not essential, and even might be detri- mental in a lens for taking landscapes. With the former the objects to be photographed are generally within a few feet of it, and there are a variety of points situated in dif- ferent planes which ought to be impressed with sharpness on the photographic plate, and that without any distortion, The last desideratum puts the employment of a single lens out of the question unless a small stop be used, and it is evident that a double lens must be used. Starting with this, it is quite evident that the curves of the surfaces of portrait lenses must vary from those for landscape work, and must be so designed as to be capable of delineating points in different planes not far from the lens itself It will be found that this 2 1 6 Elementary Photographic Optics. Fig. 54. can be secured by combining lenses of the same or different focal lengths, separating the pairs by a long interval. This limits the extent of field and necessitates the employment of object glasses of wide diameter in order to cover a sufficient area. In practice the lenses are so far separated that the amount of surface of the photographic plate which can be utilised for some purposes, scarcely exceeds the diameter of the lens itself Again, rapid- ity is an essential quahty of a good portrait lens, and the curves of the surfaces of the lenses, and their separation, must be so adapted that, with- out the use of any diaphragm, they shall give a fairly sharp image of a figure or part of a figure when placed at a reasonable distance. Spherical aberration is a positive ad- vantage for some of these requisites. Fig. 54 gives an idea of the curves and also the amount of separation which is given to the lenses of a Petzval portrait combination, on the pattern of which many of the modern ones are still constructed. The dark shaded portions show the crown glass, and the light shaded por- tions the flint glass lenses. In one of the beautiful por- trait lenses introduced by Dall- meyer we have a decided varia- tion from this model. The advantage of this lens, fig. 55, is that two components of the back combination are capable of being slightly separated, giving a greater depth (though a more diffused) focus than ordinarily obtainable. Fig. 55. Landscape Lenses. 217 For landscape lenses it is not so necessary that points lying on different planes near the lens should be brought in focus on to the photographic plate, but that objects at a dis- tance from the camera, though lying in far different planes, should be sharply defined, and also that objects lying at a considerable angle from the axis of the lens should be in good focus. This latter requisite does not exist to nearly so large an extent in a portrait combination ; hence, evidently, the curvatures of the lenses must be different, as also the amount of separation between the two lenses, when a double combination is employed. For ordinary landscape work there is nothing to prevent the adoption of a single lens, since the distortion produced by it would pass unnoticed, though, as already pointed out, architectural subjects de- mand freedom from all distortion, and, therefore, a com- bination of lenses has to be resorted to. All single lenses, for certain optical reasons, have the meniscus form given to them, and fig. 56 gives an idea of the forms adopted by some of the best makers. As already pointed out, the lenses are rendered achrom- atic, the achromatism being adapted ^ig. 56. for the actinic rays more than for ^ ^ 3 the visual rays. Fig. No. i shows a meniscus flint lens cemented to two crown concavo-convex lenses. No. 2 has a crown double convex cemented to a double concave flint lens, whilst No. 3 shows a crown concavo-convex lens cemented to a meniscus flint lens. Of a combination of lenses for architectural work we show three examples. The first is of the 'rapid recti- linear' type, as made by Dallmeyer, fig. 57. It is formed by a symmetrical pair Of lenses of flint and crown ; the concave surfaces of the lenses face each other. If we call the focal lengths of the combination io'5 in., the focal lengths of each lens will be found to be about 20, 21! Elementary Photographic Optics. Fig. 57. and the separation between the two lenses to be about 2 inches. It may be useful to give a rule for ascertaining approx- imately the focal length of any pair of lenses when combined. Multiply the focal length of one lens by that of the other, and divide by the sum of their focal lengths less the distance of separation. In the above case we have — /- 20 X 20 400 , f= = !—• = 10-526. 40-2 38 ^ The diaphragms for this combination occupy a posi- tion half-way between the symmetrical lenses, and there- fore give no distortion. This lens covers an angle of about 60°, The next lens, fig. 58, is what is known as a ' wide angle ' doublet, in which the separation between the lenses is very small, and their foci considerably shorter, in proportion to the area of the circle that it is to cover. Some of these combinations are made so as to cover a circle whose diameter sub- tends an angle of 90° from the optical centre. The objection to these lenses is the unequal illumination and the small stop that is obliged to be employed with them, and their consequent slowness. The following diagram (fig. 59) shows a section of the 'triplet lens,' in which the place ordinarily occupied by the dia- phragm is replaced by a 3rd compound meniscus lens. There were certain advantages connected with this lens at the time when it was introduced, but, since the manufacture of non- distorting doublets giving Fig. 58. Flare Spots. 219 Fig. 59. a fairly flat field has been perfected, they are compara- tively obsolete. It is, however, a good illustration of the ingenuity with which opticians aimed to meet the requirements of photographers. In the doublet lens the posi- tion of the diaphragm is important, otherwise — as can well be under- stood—the second lens will not correct the distortion of the first. In the case of a doublet in which both lenses are symmetrical, the diaphragm should naturally occupy a position half-way between them. If the focal length of the front lens be different from that of the back, the diaphragm must occupy a position propor- tional to the focal length of the lenses. With certain classes of doublet lenses as formerly con- structed there was formed a fogged central patch on the exposed plate. This was due to what is called a 'flare spot,' which is a circular patch of light seen on the ground-glass immediately in a line with the axis of the lens. It is, in reality, an image of the opening in the dia- phragm. If glass were perfectly transparent, such a defect could not exist ; but, owing to its reflecting hght from its surfaces, it has a reality which is often very troublesome. The surface of the lens reflects the aperture in the diaphragm and forms a distinct image of it, and if this image happen to coincide with the focal distance of the lens, the flare spot is sure to make its appearance. By slightly altering the position of the stop this defect is overcome. But as will have been noticed before, the position of the diaphragm in a doublet lens is of importance for eliminating distortion ; hence by curing this defect distortion might be introduced. By previously altering the distance of the separation of the two lenses, both evils may be avoided. At the best it 220 Elemeyitary Photographic Optics. seems, however, that the flare spot is really only distributed over the entire area which the lens covers. This reflection from the surface seems to account in a measure for the veil on negatives, which is often apparent when using certain slow lenses where bright objects have been photographed, and the exposure prolonged to enable the details in dark shadow to be capable of development. The veil is prob- ably the photograph of the illuminated lens. We must again revert to the diaphragm, or ' stop,' in order to give some further idea of its use, and also of the necessity which may exist for using one of large or small aperture. In the case of a single lens we have already shown that the position of a stop affects the shape of the distortion, depending whether it be placed in front or rear of the lens. It may now be stated — and the reason will be apparent on examining the previous figures — that on the distance of the diaphragm from the lens is dependent the amount of distortion, as is also the size of the picture which the lens is capable of defining ; whilst at the same time the flatness of the field is also in a great measure due to a large distance being maintained between them. In constructing a lens, then, an optician has to hit a mean in order to give a satisfactory result. From these remarks it will be evident that a lens which embraces a wide angle should give least distortion, because the diaphragm must be necessarily closer to the lens than when the angle is curtailed. It is for this reason that the employment of a wide angle lens, with a plate of a size larger than that it was constructed to cover, is found to yield more satisfactory pictures than if a lens capable of embracing a less angle be employed. Thus a wide angle landscape lens intended to be used for a 40 x 30 centimetre plate, gives more accurate pictures on a 20 X 16 centimetre plate than does a lens embracing a more moderate angle when used for the same sized plate. Illuminating Power. 221 When a diaphragm is used, with the ordinary landscape lens or a double combination of lenses, there is a certain inequality of illumination of the field. The aperture of the diaphragm is for obvious reasons circular, and when the rays of light strike this in any direction but axially, it is evi- dent that the admitted light must be diminished, varying in fact as the cosine of the angle the rays make with the axis of the lens. Thus the margins of the picture will on this account have less illumination than the centre. Another cause of the falling off of illumination is this :— If we have two equally bright and equidistant objects, so placed that the image of one falls on the margin of the plate and of the other at the centre, the area occupied by the first image will be greater than that occupied by the second, and consequently the marginal illumination will be less. Mr. Dallmeyer states in the first of two articles ' which he has written on this subject, that 'the diminution of light from the centre towards the margins of the pictures from both these causes increases rapidly with any increase of angle of view beyond 40°. At this obliquity the extreme margins only re- ceive 80 per cent, of the light falling upon the centre., at 50° it is reduced to 70 per cent., at 60° to 55 percent., at 70° to 45 per cent., or less than one half. Therefore the larger the angle included in the picture the more apparent becomes the defect.' In the same article Mr. Dallmeyer insists that the aperture of a diaphragm should always be expressed in terms of the focal length. Thus an aperture of 5 centimetres when used with a lens of 50 centimetre focus, should be called yV aperture, which is a means of expressing the intensity of a lens. The aperture of the diaphragm also determines the amount of depth of focus, and this increases as the diameter of the aperture diminishes. Any point which is out of focus is re- presented by a disc of confusion, and when such a disc does not exceed a certain diameter, the eye is unable to distinguish > Year Book of Photography, \Z^(i13.■:i^\Z^^. 222 Elementary Photographic Optics. it from a point. In practice i minute of arc is taken as the limit. When the diameter of this disc, as viewed from an ordinary distance for examining a picture (40 to 50 centimetres) subtends more than a minute of arc, the object will appear to be out of focus, whilst if less it will be in focus. Hence we may argue that the smaller the aperture of the diaphragm the greater the depth of focus there will be, since the foci of nearer objects and distant ones may all be made to fall within this limiting angle by diminishing it. A reference to fig. 48 will aid the student in comprehending this. Taking a disc of -25 millimetre diameter, which is about a minute of arc as seen from a distance of 50 centimetres, as the greatest admissible diameter of disc of confusion, a table is readily constructed of the nearest point which will be in focus when any aperture of diaphragm is employed. Suppose we know the equivalent focus of the lens in ques- tion to be 25 centimetre focus, and that we are to use an aperture of 2-5 centimetres : Taking the formula which will give an approximately true value — _ = i _ ^ V f when the distance is in focus, the nearest part of the fore- ground which can be considered sharp will have a focus which is longer than the equivalent focus by -25 centimetre, for — Cent. Cent. Millimetre. 2-5 : 25 :: -025 : x X = -25 centimetre ; = J. _ J ^ -25 V 25 25 + -25 25x25-25 _ 1 2525 .*. V = 25*25 metres. That is to say, all parts of the picture lying beyond 25 Advantage of Short Foci. 22j metres will appear to the eye to be in focus. The following table has been constructed on that basis : — Intensity, or Aperture Ratio Relative Exposures Focal Length of Lenses in Centimetres lO IS 20 25 30 40 Distance of nearest distinct Objects in Metres 1 10 1 15 1 20 1 30 1 40 I 2-25 4 9 i6 4-1 2 7 2-1 1-4 9-1 6-1 4-6 3-1 2-4 i6-2 IO-8 8-2 5-5 4-2 25-2 16-9 127 8-6 6-5 36-3 24-3 i8-3 123 9-3 64-4 43-1 32-4 217 16-4 TOO-5 67-2 50-5 33-8 25-5 The annexed formula will approximately give the nearest point p which will appear in focus when the distance is accu- rately focussed, supposing the admissible disc of confusion to be "025 centimetre : — ^= •41 y. p y. a, when/ = the focal length of the lens in centimetre', a = the ratio of the aperture to the focal length. The result is in metres. In the application of the foregoing formula the stu- dent should note the advantage of using a lens of short focus in lieu of one of long focus, viz., that more of the foreground can be placed in the picture without any detriment to it through ' fuzziness.' It can also be shown that an enlargement from a small negative is better than a picture of the same size taken direct as regards sharp- ness of detail. Suppose, for instance, we wish to compare for sharpness a picture taken with a lens 50 centimetres focus with an enlargement of the same size, from an original negative taken with a lens of only 10 centimetres focus, both having the same aperture ratio, say The negative in the last case would be only ^ the size (linear) of the former. To compare the two the disc of confusion in this 224 Elementary Photographic Optics. latter should only be '005 centimetre diameter, and this should give the distance of the nearest distinct object, since, when enlarged 5 times it will give a disc of "025 centimetre diameter, which we have already taken as the limit of dis- tinctness. Calculating as before for the lens of smaller focal length, •5 : 10 : : -005 : x X = '\ centimetre 11 I _ -I V lo ~ 10+ -I •loio .*. V =^ lO'i metres, that is, after enlarging a picture to the size given by a lens of 50 centimetres focal length, an object io"i metres will still appear in focus. In looking at the table, it will be found that with the direct picture of the same size the nearest object in focus will be at 50*5 metres distance. Calculation shows that the gain in an enlargement compared with a direct negative is inversely proportional to the focal lengths of the lenses. This, of course, refers only to an aplanatic lens, and care must be taken to distinguish between the advantages to be gained in enlargement by the use of a smaller lens, with the disadvantages that ensue from the deterioration in the relative values of light and shade. The student should remark that in doublet lenses the apertures in the diaphragms do not show accurately the available aperture of the lens. In order to ascertain their correct value, a distant object should be focussed in the camera, in order that the focussing screen may be at the equivalent focus of the lens ; this screen is then removed and replaced by a glass over which is pasted any opaque paper. A candle is brought near the centre of the opaque screen in which a small hole has been punctured. The front com- bination of the lens is illuminated by the rays of light coming through the orifice, and the diameter of the disc of light seen on the front of the lens gives the available aperture of the lens when used with that diaphragm. A pparatus. 225 CHAPTER XXXII. APPARATUS. It will be unnecessary to describe much of the apparatus that is in daily use by the photographer, as some have already been described in various chapters, and some must be left to individual taste. In those kinds of apparatus which are to be described it must be borne in mind that the recom- mendations made are merely the results of the writer's indi- vidual experience, and it is not improbable that something better may be known to others. Cameras. — It should be considerea an essential in every camera, excepting one used for copying and enlarging, that it should have a back that at least will swing at an angle away from the vertical plane, and it is a great comfort when a movement in a horizontal plane can also be given to it. Technically, the backs which can move thus are termed ' swing-backs.' The accompanying figure will give an idea of a do.uble swing-back, and also of the kind of camera which for landscape work seems everything to be desired, a is the front of the camera into which screws the lens l. The lens can be caused to occupy a position out of the centre of the camera by the double move- ment shown in fig. 61. a x's, the board to which the lens is attached by means of its flange sliding in the grooves b b, which are fixed on to the main movable front c c. This front also slides in grooves d d, attached to the body of the camera. These fronts are fixed at any point required by means of the screws Q 226 Apparatus. e and/ which run in the slots as shown. Reverting to fig. 60 it will be seen that the camera has what is known Fig. 61. as the ' bellows ' form, the bellows T 1 ' 0 ' V C B being attached to a and also to the swinging framework d. e is connected with r by means of a rod, passing through the side of the framework, and terminated by a clamping screw k. r can be made to approach or recede from A by means of a slow-motion screw turned by the handle x. d is connected with m by pivots which work in the brass plates h, and since c is fixed as regards the vertical plane, it is evident that d can move through any small angle about H, without in any way interfer- ing with the other movements of the camera, and the angle can be maintained by clamping the screw, which works in a slot as shown. Thus, then, a swing away from the vertical plane is secured. The motion of d in a horizontal plane is secured by pivoting the frame m on to R. If the clamping screw k be loosened, m, and therefore d, can be moved through any small angle in a horizontal plane, and can be fixed in that position by tightening k. The double swing motions are therefore secured, f is a bar with a long slot cut in it, so arranged that clamping screws in c and a can fix it and give additional rigidity to the camera. When r has been moved along the tail-board q, so that c touches a where the clamping screws m and K are loosened, the latter is free to turn up against the ground glass g. When a small pin at s is withdrawn from p, this board, being hinged as shown, folds round the turned-up tail-board and q is kept in position by means of a small snap spring fixed to the bottom of the camera. The camera itself can be attached to the stand by the tail-board q, in which position the greatest length of the picture is horizontal, or by e when the height of the picture Reversing Back. 227 Fig. 62. has to be longer than its breadth. A camera 21 x 16 centi- metres of this form, when packed in a leather case, weighs about 6 kilogrammes, or 14 lbs. P'or work in the studio where the diminution of weight is no object, a rigid form of camera can be adopted. Such a form we give in fig. 62. This is a camera adapted for taking cartes de visite, and it will be noticed that the alteration in focus is secured by a different ar- rangement to that in the last. The front part, which ^ carries the lens, slides outside the back part, the move- ment being effected by a pair of racks fastened on the base board, on which a long pinion works. Some photographers prefer this motion to that given by the screw, since the hands do not interfere with the position of the body whilst viewing the image on the screen. It will also be no- ticed that there is a long carrier for the dark slides, and that the dark slide is more than double the length necessary to secure one picture. The object of this is to be able to give two exposures on the same plate, and thus to economise time. It is very convenient to have attached to the camera what is known as a 'reversing back.' For this adjunct it 0.2 228 Apparatus. is necessary that the camera should be square in section. Fig. 63 gives an idea of it. It will be seen that the slides fit into the back, which can be placed with the ground glass and plate having their greatest length horizontal or vertical. By this plan the camera can be utilised for taking pictures of either shape without any alteration except of the back itself. For some classes of views a panoramic camera is a very useful piece of apparatus to employ. For instance, where the view embraces 120° any lens would be incapable of giving the whole picture. Fig. 64.. 1,1 unless at least two views were taken from the same spot and afterwards joined. The fault in such a picture would be that there would be two or more fixed points of sight, which must inevitably give a more or less untruthful com- plexion to it. In a panoramic ca- mera the eye is supposed to travel round the view, the point of sight altering at each movement of the eye. There is some- thing to be said for this kind of perspective, since the angle the eye sees distinctly at one time is very small in comparison with what is delineated with an ordinary lens. Some of the magnificent views in Switzerland by Braun, of Dornach, were taken by such a species of camera, and they certainly are more pleasing than they would have been had the point of sight been abruptly altered. The accompanying figure (64) gives an idea of Liesang's panoramic camera. In all cameras of this description it is necessary that the rotation should take The Pantascopic Camera. 229 place about the optical centre of the lens, as by the move- ment of the lens round that point there will be no displace- ment of any object near the axis of the lens. The student will remark that a doublet lens giving straight lines is a de- sideratum, as a single lens distorts, and must of necessity displace objects slightly whilst it is moved round its optical centre. Had it been practicable to have used a curved plate with the radius of curvature of exactly the focal length of the lens, the lens alone might revolve. As such plates are expensive the following device is employed to enable a flat plate to be used. By means of the cord and pulle}', b, shown, the dark slide a, which carries a long plate, is made to roll on the circle (of which the focal length of the lens is the radius), touching it at that point in which a vertical plane, passing through the axis of the lens, cuts it ; and since the only part of the image which can fall on the plate is through a slit, the middle of which lies in the same vertical plane as the axis of the lens, it is manifest that, pro- vided the slit be narrow enough {i.e. that the arc does not differ much from the tangent of the angle formed at the optical centre of the lens by the planes passing through the centre and the side of the slit) the picture will not suffer in definition. The exposure is given by turning the winch, c, which causes the rotation of the lens and body of the camera whilst giving the necessary motion to the plate. A band passing round the back of the plate holder and attached to the slit prevents the ingress of light to the sensitive surface, excepting on that portion opposite the slit and passing through the lens. For the outdoor part of dry-plate work the apparatus is comparatively small, and consists of a camera, focussing- cloth, camera-stand, a set of double-back slides, or a single back adapted to a changing box, or some form of Warnerke roller slide if sensitive paper be employed. A double back is of very simple construction. The slide is divided into two parts, hinged so as to fold one against 230 Apparatus. the other, one portion carrying a thin blackened and hinged iron or tin plate. A sensitive plate is put in each half of the slide, the sensitised surface being outwards. The blackened plate prevents the passage of light from one to the other. The plates are placed in the camera as usual, and opened for exposure as with the ordinary slide. It will be seen that for every couple of plates one double back is required, and it will seldom be convenient to carry more than three of these, on account of their weight. If it be decided to use a changing box, there is none better than that manufactured by Hare. In order to use it, ^ _ ^jW^^" is longer than the top of the box, and is capable of folding, and in it is a slot through which a plate can pass in or out of the box. The box itself is fitted with grooves, the positions of which are marked on the outside of the lid by an ivory scale. In order to fill the slide, its end is slipped into a groove which borders the slot, and when it is home a spring is forced on one side, and this opens the end of the dark sUde, and the slot in the shutter is uncovered. The lid of the box is now moved till the slot in it is over a plate as registered by the scale. The box is now gently in- verted and the plate passes into the dark slide. This latter cannot be removed till the shutter once more covers the slot, and the act of removing it closes the shutter over the opening. This is a very simple method of changing a plate even in bright sunshine, and is always successful provided the plates Fig. 65. it is necessary to have a dark slide especially constructed, the pecu- liarity of which consists in its having a movable end-piece through which the plate passes into the holder. The plate box W itself has a lid, which Warnerkes Roller Slide. 231 are carefully cut to the proper size. The method of re- turning an exposed plate to the box is self-evident. The weight of a dozen plates and this changing box should not be much more than that of half a dozen plates in double backs. Exposing sensitive paper iti t/ie camera. — AVarnerke's was the first roller slide which was in the market, and as such we describe it. Several other patterns have recently been brought out. Fig. 66. A band of sensitive tissue is rolled round one of the movable rollers a, and after passing over f f, which consists of a couple of round bars and a flat blackened board, is attached to the other roller a. The band can be made to pass from one to the other by turning a thumbscrew placed at d d, and can be fixed at any time by the clamping screws c c. It is thus evident that after an exposure has been given to one part of the tissue, another portion may be brought forward to receive a fresh exposure. The rollers &c. are enclosed in a box B, which answers to the ordinary dark slide, the sensi- tive surface being protected by an ordinary shutter. It may be mentioned that a capital substitute for ground glass may be made by coating an ordinary plate with the following varnish : — Ether . ...... 500 cc. Mastic . . . . . . .30 grammes. Sandarac 30 grammes. 232 Apparatus. After dissolving these resins, benzine is added little by little till the grain becomes sufficiently pronounced. In order to ascertain if the face of the sensitive plate in the dark slide occupies the same position as the ground- glass surface of the focussing screen, a bright object should be focussed accurately on the latter. The slide should next be filled with a piece of ground glass as if it were a plate, or a plate may be used. If the image retains the same defini- tion it may be presumed the focussing screen is correctly placed. In wet plate photography it is well to test the purity of the silver wires which are in the dark slide. The usual contamination is copper ; if a drop of dilute nitric acid be applied to the wire an absence of green colora- tion, due to the formation of copper nitrate, may be deemed conclusive that the silver is tolerably pure. In any case, how- ever, it is a good plan to give the wire a coating of shellac varnish. Another point about the dark slide which should be alluded to is that its front should not be less than | inch away from the surface of the sensitive plate. A nearer approach is apt to cause markings. Cafnera stands. — The camera-stand next requires a few remarks, as the comfort of working Fig. 67. depends much on the form adopted. The essentials of a stand for land- scape work consist of rigidity, light- ness, and compactness when folded up. The annexed diagram gives a form which is convenient, though perhaps rather heavier than is de- sirable. It is on Kennett's principle, with a modification introduced by Lane. The inverted top is shown in the diagram, fig. 67. A is a circular mahogany disc to which is attached a brass hinge b of the form shown. On this hinge works the whole of the brass framework to which the legs are attached, c is a screw which passes through the centre of the frame- Tripod Stand. 233 work, and also through the wooden disc, and it is by this screw that it is attached to the camera. It will be noticed that c has a collar half-way down, and that this can clamp the camera to the disc when required, d d are levehing screws, by which the disc (and con- sequently the camera) can be ac- curately levelled, e e are pins into which the tops of the tripod legs fit. Fig. 68 gives an idea of the appearance of the tripod when ready to receive the camera. cc-2sq brass collars which are fixed to the top half of the tripod, d and / are movable collars which respectively clamp the bottom half b of the top to the top half and the top half to the head to which the camera is attached. When not in use the head is detached, the bottom halves of the legs slide into the top halves ; and they are strapped one against another and form a comparatively compact bundle. The subject of the camera-stand cannot be passed over without mentioning the very ingenious method that has beea made by M. 'Warnerke ^ ^ ^ Fig. 69. for combining ordinary pictures, taken from the same point, to form a panoramic view. In order to secure an accurate junction of two pictures taken from the same point, but in different directions, it is necessary that the lens and camera should revolve about the optical centre of the lens, for the same reasons as were given when describing the pantascopic camera, p. 228. By adopting the accompany- 234 Apparaius. ing device, fig. 69, this can be secured. The camera-stand is screwed to the small hole, and the camera itself is attached by a screw to some point in the slot. When the hole is vertically beneath the optical centre of the lens, and the camera is turned, it moves round the optical centre of the lens. Lenses.— As regards the choice of lenses, it is very diffi- cult to give advice. If the student is confined to the choice of one lens for landscape-work, he should unhesitatingly procure a doublet-lens, which has no perceptible flare-spot, and which embraces an angle of about 50°, since with it he can take both landscapes and architectural subjects. If he can afford another lens, perhaps a landscape wide-angle lens IS next to be recommended, being exceedingly useful in most positions and giving great brilliancy of picture. The most complete battery of lenses would be a rectilinear doublet, embracing an ordinary angle; a wide-angle doublet to embrace about 80°; a couple of single landscape lenses' one embracing about 50° and the other about 70°. For portraiture, the most useful lens is one which will give a ' cabinet ' sized picture, and it may be supplemented by one of those quick-acting lenses, which are familiar to all portraitists of the day, for taking instantaneous pictures of children, &c. It should be noted that many fine por- traits and groups have been taken with the ordinary land- scape doublet-lens, which, though slower than the portrait- lens, yet is sufficiently rapid to be usable. Instantaneous Shutters. — ln connection with lenses it is necessary that a word be said regarding the means of giving rapid exposures. In the market there are many numerous so called instantaneous shutters which can give exposures from the -1^^ second upwards. It may be well to slightly touch upon the rules which should regulate a properly made shutter. If the shutter be attached to the lens, first and foremost every action should be a double action, that is, if one part of the instrument moves up, another part should move down. Drop S/nitter. 235 The reason for this is that the centre of gravity of the whole apparatus (including in this the camera, the shutter, and the legs) should remain as nearly as possible unchanged in position before, during, and after exposure. When the centre of gravity shifts, there is of necessity a vibration set up in the camera, with the result that the picture will not be sharply defined. Again, except in Fig. 70. the fiinal closing of the shutter, no part should be arrested with a jerk, for that will entail vibration. Thus sup- posing we have a pair of shutters opening from the centre, the passage to and from the positions at which they will commence to close again should be as gentle as possible. Another point is that the whole aperture of the lens should be opened as rapidly as possible, and the principal part of the exposure take place when the whole lens is engaged in doing its work. The effective exposure may be taken as the sum of the apertures into the time during which such aperture is utilised. It will be evident, then, that the longer the full aperture is employed the greater will be the effective exposure in a given time. Hence two shutters may be employed which open and close the aperture of the lens in equal times, and yet one may give half as much more exposure to the plate than the other. The cheapest shutter is that known as the drop- shutter, and the maxims above laid down can be brought into play with it. The principle of a drop-shutter is the passing of an elongated aperture cut in a board over the front of the lens. The longer the aperture in the board proportionately to the aperture of the lens, the longer the 236 Apparatus. latter is uncovered. If gravity be held as the moving power for the shutter, no great rapidity of exposure can be given to the plate ; but gravity may be aided by an elastic spring of varying tension, and so the times of exposure may be altered. A shutter of this description should not be attached to the lens, as there is only one part which moves, no counterbalancing movement taking place. In practice it is well to attach the shutter by a velvet bag to the lens, and hold the shutter during exposure. Fig. 71. The dark tent.~i:\\^xt are numerous patterns of dark tents m the market, some very simple and some complicated. These tents are intended for wet plates, but they can also be conveniently used for the development of dry plates either in a room or in a shady place. The tent that will be described is one of Rouch's form which has been slightly modified by the writer. When closed it forms a shallow oblong box. When opened, the lid forms the front of the tent, and the cloth is extended over the breadth of the box by means of movable iron rods. Dark Tint. 237 The developing and other solutions are carried in the small cupboard a, beneath which is a space in which can be carried a box divided centrally into two divisions, one half to carry clean plates, and the other to act as a draining box, similar in principle to that given at page 84. b '\% d. small cupboard in which the developing cups, cotton ^.ool, and sponge are kept, whilst c contains carefully decanted bottles of collodion, d is the window covered with non-actinic cloth, opening inwards, so that when it and the outside door which protects it are opened, white light can be admitted. Across d runs a curtain, which can be removed in dull light. On the top of c and against the window the travel- ling bath is placed when the tent is packed : though when in use it is slid into a well on the left-hand side, and pro- tected by a frame covered with waterproof material, ^, hinged at the far end. The frame fits over a fillet, to prevent any light being admitted, /is a waterproof sink, connected with which is an india-rubber tube to carry off the waste water. At the side of the tent js an india-rubber water bag, the tube from which passes through a light-tight connection in the cloth side of the tent. The tube is terminated by a clip. Over the window is a small shelf, which can fold up against the front, and on it is kept the badger-hair brush for dusting the plate. From underneath the shelf spring two varnished deal battens, which can be turned underneath it, and these are used to support the clean plate when necessary. The plate can be coated inside the tent, and be immersed in the bath with the cloth up in the position shown. The dark slide rests against the space be- neath A, and is then ready to receive the sensitised plate as soon as the tent curtain has been folded round the waist of the operator so as to exclude all light. This exclusion of light is easily maintained by the body lightly touching the front of the lower part of the tent. When the developing operations are proceeded with, the plate is removed from the dark slide, and the latter is placed in the pocket 238 Apparatus. in the top of the tent. The legs of the tent are of the ordinary tripod form, and are fitted on to a square frrame which is secured to the bottom of the tent by thumib- FlG. 72. screws. In order to work in a tent of this descripttion an assistant is necessary, unless all the apparatus can be placed in a light spring truck. This is seldom feasilble. On the Picture. 239 If a well beaten and tolerably smooth road be traversed, an assistant can well carry this tent and legs, whilst the operator can carry the other apparatus, supposing the size of the plate to be used not to be larger than 21 and 16 centimetres. The writer worked with this tent in Egypt for some months and found it very convenient and tolerably cool. This would not have been the case had the cover been made of india-rubber sheeting, as is often recommended. One thickness of black twilled calico, and two of orange tammy, were the materials with which this tent was covered, A white cover might be an improvement. The annexed figure shows a very convenient dark room which was designed by De la Rue for the Transit of Venus expeditions. It was adapted for working dry or wet plates. Of course a small room may be made, but of whatever size it should be capable of efficient ventilation. The fumes of ammonia are pernicious to be inhaled for long, and the hyposulphite bath is apt to give off sulphuretted hydrogen when gelatine plates are fixed in it. The light which should be admitted to a dark room will be discussed in a subsequent chapter. CHAPTER XXXIII. ON THE PICTURE. In a text book of this class it is impracticable to enter into the discussion of all the rules which should govern the composition of a picture. It will suffice to point out a few of the leading ones which should be followed. In com- parison with the painter, the photographer is sometimes under a disadvantage, in that he is unable to choose a point of view to represent some particular feature, in which every- thing that is objectionable to artistic feeling may be left 240 On the Picture. out or modified, or in which some extraneous object may be introduced in order to give proper harmony to the pic- ture. Thus the painter may render a distant landscape in a favourable aspect of light and shade from some particularly suitable spot, though the foreground which may be at hand may be totally unsuitable for pictorial effect. The latter he may discard for one which may be better fitted for his purpose, taking it from any other locahty, providing it is not incongruous. The photographer, on the other hand, is rarely at liberty to use this artifice, unless he resort to the laborious process of printing from two or more negatives, although when the object is attained the result amply repays any labour that may have been expended. It need scarcely to be said, when combination printing is resorted to, that the greatest care is requisite to avoid incongruity, cr an inartistic massing of light and shade. When confined to a single negative, there is nothing for the photographer to do but to make the best of his landscape, including his fore- ground. This usually entails a sacrifice to a certain extent of either one or the other, and it is the possession of the know- ledge as to where the sacrifice is to end that marks the differ- ence between the successful artist and the mere manipulator. Besides focussing the object by the lens, as will be pre- sently described, there is the focussing of the picture as a whole ; that is, the securing of the necessary harmony of light and shade. In a good and artistic photograph the object on which the subject of the picture is to be built should stand prominently out in the print, the eye should instinctively rest upon it without being distracted by other parts. Thus sweeps of shade may lead up to a more highly lighted portion in which should be the principal object, or a sweep of light may lead the eye to a dark object which then should occupy the same prominent position. In a negative as it is developed this may often be unattainable, but by judicious masking of parts during printing this harmony may generally be secured, providing the taste Lines in the Picture. 241 of the operator has been educated. It cannot be expected that an inexperienced photographer can at once form his picture, so as to give the best possible combination to the materials at his hand, until he has attained a thorough practical knowledge of chiaroscuro, and is able to translate the colours he sees on the ground glass of the camera into monochrome. In choosing a point of view for a photograph, then, it is necessary that there should be this instinctive translation of colour into monochrome ; a knowledge of the rules which Fig. 73. 4 i govern the formation of an artistic picture ; and a perception of the masses into which light and shade should group themselves. Supposing that the photographer intends to make the study of an old wrecked boat lying on the sea shore ; the colour is most deceptive, the general tone being of one tint. In fig. 73 we have the example of such a study taken by Manners Gordon, a gentleman whose productions are always artistic. In analysing the work we find that he has obeyed certain rules. Thus he has made the keel to occupy a position about ^ way up the picture, and the R 242 On the Picture. nearest point of the stern occupies about a similar distance of the length of the picture. The landscape being sub- sidiary to the boat, he has caused the horizon hne to be about \ way up the picture, and in order to break uniformity, he has so arranged that the boat should not be sym- metrically placed in regard to the centre of the plate. The lines of boat also make an angle with the. horizon, and these are again balanced by the thwarts, &c. It would have been a very easy matter to have made the picture wanting in harmony by placing the camera more to the rights Fig, 74. i 1 6 t and causing the lines of the boat to run parallel to the horizon, in which case the boundary of the small pool of water in which it is lying would have had the same direction. The keel of. the boat might also have been placed nearer the bottom of the picture, and the general mass of it have occupied a central position. In this case there would have been a symmetrical picture, the general lines running parallel to the horizon and at right angles to it, the result of which would have been that the eye would be partly satiated with it, and there would have been little variety and much monotony. As it is, the picture, which Instability in a Picture. 243 can only be faintly represented by the woodcut, is pleasant to look at. Instead of a boat being the object to be delineated, we may have it as an accessory to a landscape. As an example, we have a view, fig. 74, taken on the Thames by Woodbury, The object of interest is undoubtedly the village beyond, with its church, and middle distance formed by the trees. If the boat were taken away there would have been a large space of Fig-. 75. bare shore, unbroken by any object to relieve its monotony^ The boat, however, happened to be there, and the artist has seized the chance to make a picture. Notice how it is made subsidiary to the general landscape. It does not occupy such a prominent position as in the last example. It is kept about ^ from the edge of the picture, and the keel occupies a little over \ of the distance from the bottom, and the line of the village is placed about \ up. Were the boat brought lower down or more central, it would have appeared to have been the 'motive' of the picture It is evident how such a picture might have been spoilt from a want oi 244 On the Picture. knowledge of art rules ; as it is, it is a beautiful example of artistic photography. A third example of a study of boats is given to show- certain other points which are often neglected. We have here, fig. 75, a specimen of a picture that might have been readily spoilt. It should be noticed how the lines of the masts, sails, and pier are parallel, and were the figure re- moved from the side of the boat, and the small skiff made to lean in the other direction, the effect would have been to give the idea that the boats, &c., were tumbling out of the picture, and a sense of instability would have been created. The opposing line of the mast of the small skiff, the incHna- tion of the figure, and the small post in the foreground, balance the general lines, and no impression of insecurity is left. The general composition, too, of the picture should be noted. The lines forming the extremities of the spars fall on the body of the skiff, while a sense of support to the outer line of the large sail is given by the post. The line forming the top of the post and the top of the pier also approximately passes through the cap of the man and the top of the mast of the skiff. The picture is then built, as it were, on diagonal hnes. A slight change in the position of the camera would have altered all this. Again note that the general mass of light is opposed to the black hull of the boat, intensifying the interest with which the boat, evidently the principal object in the picture, is regarded. The accompanying woodcut, fig, 76, taken from a pho- tograph by Woodbury, well illustrates the treatment of an old water mill. In this case the angle of the wall, that is, the base of the corner of the most prominent piece of ma- sonry, is placed about \ way up the picture. Had it been placed lower it would have been aggressive, whilst if placed higher it would not have given sufficient solidity to the mill. The water-wheel base, the object of interest, is placed nearly centrally in the breadth of the picture, as from the subject there is no danger of symmetry, which A Water Mill. 245 Fig. 76. is always distasteful. The shoot of water occupies a posi- tion central in both directions. Had it been placed much lower, there would have been a sense of a want of falling room. It will be noticed that the fall of the water natu- rally enhances the effect of the compo- sition, and the light on it at once attracts the eye from the dark surroundings. If* the picture be covered from the bot- tom to where the board is thrown across the stream, it will be seen how a slight va- riation in the position of the camera might have altered the ge- neral aspect of the picture. A favourite study with some photo- graphers are forest scenes, and the next two examples shall treat of them. In the first, fig. 77, we have an old oak surrounded by smaller trees, the fore- ground composed of bracken and ferns. The base of the tree is placed about ^ way up the picture, for if lower there would have been a feeling that there was not sufficient ground for it, the principal object, to have taken firm root in, and there would have been a sense of unfitness of position. Had it been placed higher the foreground would have been too prominent, and the first idea might have been that the raison d'etre of the picture was simply the ferns in the foreground, for those at the foot of the 246 On the Picture. picture would have been out of proportion to the oak to play the part of an accessory. The distance, or what might be called the horizon line, is drawn about ^ way up the picture, but being so broken by the shrubs and smaller trees it is invested with no importance, and consequently need not be dwelt upon as following any particular rule. The outside of the trunk of the oak is placed ^ way from the right-hand edge ; had it occupied a more central position the picture would have appeared cut in two by it. As it is, the dark foliage behind it fills in the side of the pic- PlG. 77. 1 1 2 + ture, and there is no feeling that the oak is out of place. Had the foliage been light there would have been a danger that the eye mignt have been offended, but this is one of the cases in which the position of the camera must be made subservient to the operator. The whole force of the picture is given by the light, which breaks against the trunk of the oak ; and as with the trunk, so with the branches, care has been taken to prevent any single bough cutting the picture into two divisions, Notice, too, the stability given by the straight stems of the trees, in the distance. Pictorial Focus. 247 Fig. 78. In the next picture, fig. 78, we have the distance, or perhaps more strictly speaking," the middle distance as the point of interest. The horizon hne is kept in the weakest part, the centrCj of the picture. The trees in the foreground are so grouped that they frame the view with dark masses, relieved by the light foliage of some of thenearer bushes. and shrubs. The foreground finishes at a distance of about \ from the bottom. More of it would take away from the -value of the middle distance, as it would place it in the weakest part of the picture — viz., cen- trally j less of it would have ren- dered the picture bald, and have cut off part of the deeper shades which are so valu- able in giving the effect of distance to the stream be- yond. This picture would have been spoilt had the ca- mera been so placed as to give more top foliage, since the bough which now partially crosses the picture at about | the height, would have caused an ugly division, and also the tops of the distant trees and the sky would have appeared. This latter, in views such as that under criticism, is objectionable, as patches of white give the eye an inclination to wander off towards it, and it would have been an^ insufficient precaution to have printed in 248 On the Picture. clouds from another negative, owing to the difficulty that would exist in subduing at the same time the lights on the leaves of the near trees. As it is, the picture is in pictorial focus. By placing the stream to the right or left, the balance would have been wanting, and its general direction would have been altered to such an extent as to have given a feeling that it was a subsidiary part of the picture instead of an essential. The next example, fig. 79, is intended to show a picture taken on the diagonal ; not on an absolutely straight line, but Fig. 79. one in which the general direction of the picture is on the diagonal. The point of interest is the extreme distance of the stream, and accordingly it is placed in one of the strong- est positions in the picture, viz. -i way in both directions from the margins, and it contains the highest light, as seen in the water. This brilliant light is repeated in the clouds, and more faintly still it is echoed in the rocks, where it takes approximately the same form, though it is repeated in a lower tone and of different dimensions. The picture might easily have been spoilt by placing the distance in a Figures in a Landscape. 249 central position, and by arranging that the dark moss-covered rocks in the foreground should have been shifted to the side Had these dark masses been closely opposed to the highest light, the value of the distance would have been increased, though in their present position they are fairly well placed. In the succeeding photograph we have a capital example by Manners Gordon of a picture built up on purely artistic principles. The principal object of interest is the cottage, the value of which is enhanced by the admirable grouping of the sheep. The middle distance and background may be considered merely as accessories to support the subject of Fig. So. chief interest. The general direction of the picture is on the diagonal, being carried down from the chimney-top of the cottage along the bank-side to the right-hand bottom corner. The value of the composition lies principally in the light on the two sheep in the centre group, which reflects itself as it were in the whitewashed cottage front. This may be seen by imagining the front to be of the same local colour as the gable end of the cottage, or by first hiding the sheep by the finger, and then contrasting the effect produced on the mind with that as shown. It may be remarked that the opposing lines of the clouds balance the lines of the landscape, which would not have been the case had the 25Q On the Pictwe. general contour of the clouds followed in any ' degree the contour of the sky-line. In a woodcut it is impossible to give all the expression that is to be found in the photograph, but the student may gain a fair knowledge of the rules which have been followed. As regards the introduction of figures into a landscape, it may be necessary to say a few words. It should be clearly understood that the one must be made subsidiary to the other; that is, if the portraits of the figures are required they must be made the principal objects, and the whole landscape Fig Si must be made subservient to them. On the other hand, if a landscape is to be photographed, the figures, though pro- minent, yet should occupy such a position as to be subor- dinate to it, though they may enhance and give the ' forte ' points to the picture ; and above all things care must be taken that the figures compose as well with each other as with the landscape. Robinson, in his ' Pictorial Photography,' ' a work which every photographer should possess, says : ' The figure must be eloped). 50. Agl + AgCl in gelatine developed ferrous oxalate {deiieloped) 51. Agl +3 XgCl paper, washed {print). 52. AgI + 3*gCl + AgN0., wet {print). 53, AgH-3\gCl in gelatine, or on paper, developed with ferrous citro-oxalate or acid developer 54. Agl4-3AgCl + AgN03, acid developer (rf^r'(?/<'/^(^'). 55. AgBr exposed to light treated with I exposed to spectrum {print and also developed). Mixture of Iodide and Chloride. 301 gelatine and developed with the same ferrous developer. There is a difference in the curves obtained with collodion and gelatine, but not more than is explainable by the fact that the former is essentially porous and the latter almost continuous. One Equivalent of Iodide to Three of Chloride.—When three equivalents of silver chloride are taken with one of iodide, we have, on printing a washed paper, the curve shown in No. 51 ; exposing the same paper moist in the presence of silver nitrate we have No. 52 ; the reasoning given when the mixture of bromide and iodide was under consideration holds good. Nos. 53 and 54 show the same equivalents of sensitive salts held in paper, the former showing the action of development on washed salts and the latter on the same exposed in the presence of moist silver nitrate. No. 50 shows the effect of the spectrum on equal pro- portions of the iodide and chloride when emulsified in gelatine. Paper and also collodion films containing silver chloride were blackened in the light and treated with a solution of iodine till the darkening was obliterated, washed, and then exposed, with or without sensitisers ; we had nearly the same results on printing and on development as shown in No. 55, hence the curve is not repeated. (The same applies to darkened bromide treated with iodine, exposed to the spectrum and developed.) This appears to be a con- firmation of the view already propounded regarding the formation of a new molecule, in the case of the chloride and iodide the new molecule taking the form of AggClI, as already indicated. ^ From these results we may observe that to obtain a compound sensitive to the green a mixture of iodide and bromide, or iodide and chloride, should be employed, the former in preference to the latter, since it is more sensi- tive. The same sensitiveness to daylight with the former in 302 Destruction of the Photographic Image: gelatine plates can be obtained as when using pure bromide alone, the sensitiveness being preserved by a shift of the maximum to the green. Mixtures of Silver Chloride and Bromide. There is nothing special calling for remark in a mixture of these two sensitive salts. The printed spectrum and the developed spectrum seem to be a combination of the spectra impressed on each individually, a slight prolongation towards the least refrangible end taking place. Mixture of Silver Iodide, Brofutde and Chloride. When these three salts are combined together we have spectra which are very similar to the spectra produced on iodide and chloride, or iodide and bromide, with a pro- longation towards the red. CHAPTER XXXVI. ON THE APPARENT DESTRUCTION OF THE ACTION OF LIGHT ON THE PHOTOGRAPHIC IMAGE. There is a phenomenon which we may now treat of, smce the last chapter has described the ordinary action of the spectrum on the sensitive image. Several years ago^ the writer made experiments on the causes of the apparent destruction of the photographic image, and later on ^ showed how these causes explained the phe- nomenon of solarisation, or reversal of the photographic image. The image on the daguerreotype plate was long known to be destroyed by the action of the vapour of » Philosophical Mas^azine, June 1878. » Ibid., September 1880. Destruction of the Daguerrean Image. 303 iodine, bromine, or chlorine. In fact it was no un- common thing in early days for an operator to resensitise a daguerreotype plate after exposure and before development by the means of one of these agents, the first image being totally destroyed. The action in this case is very simple. We have. Silver sub-iodide and iodine give silver iodide. AG,I + I = Ag,I„ or, Silver sub-iodide and bromine give silver iodide and silver bromide. 2Ag,I + 2Br = Ag^I^ + AgjBr^. There are, however, other agents which the writer found destroyed the photographic image. It was found that all the mineral acids, and bromides and chlorides of certain dyad metals (such as cupric bromide and chloride) were equally effective, and that substances which readily parted with oxygen also destroyed the image. The chemical equations relating to the dyad bromides and chlorides can be well expressed thus : Silver sub-bromide and cupric bromide give silver bromide 2AgjBr -f 2CuBr2 = 2Kg^i^ and cuprous bromide, -f 2CuB. With the mineral acids the chemical reaction is not so easy to explain. The effect is probably due to the con- version of the sub-bromide of silver into bromide by the abstraction of one atom of silver. Thus, Silver sub-bromide Silver bromide and silver. sAg^Br = Ag^Br, -i- 2Ag. The atom of metallic silver is here supposed to be taken up by the acid. If a sensitive film of silver bromide in a collodion emulsion be placed after exposure in solutions of potassium bichromate, potassium permanganate, hydroxyl, &c., it is found that the action of light on a plate is also destroyed. 304 Destruction of the Photographic Image. Experiments showed that this was due to the oxidation of the silver sub-bromide. This silver salt is unsaturated, and besides, metallic silver is capable of attracting other bodies, of which oxygen is one. When it has combined with it, unless such oxygen can be removed, the image becomes undevelopable, as there is no nucleus on which the metallic silver precipitated by the act of development can be de- posited. Ozone has the same effect as the liquid solutions. If a plate which has been exposed and subjected to the action of these oxidising agents be treated with nascent hydrogen, the image becomes developable, showing that when the oxygen has been removed the sub-salt returns to its former activity. These preliminary experiments led the writer to inves- tigate the action of light on films which had been exposed to white light, and again exposed in solutions of many of these destructive agents in the presence of white light and of the spectrum, and also when exposed in presence of the alkaline haloids under similar conditions. The question to be solved was whether the action of light aided this de- struction, and whether it applied also to the visible image. Sir John Herschel has noted in the ' Philosophical Transactions' for 1840 that if paper be prepared with ist, acetate of lead ; 2nd, potassium iodide ; 3rd, nitrate of silver, and be then darkened in the sun, and again be washed over with potassium iodide, on re-exposure to the sun the paper lightens again. The experiment may be tried in a variety of ways. The simplest, , perhaps, is to salt ordinary unglazed paper with a lo-per-cent. solution of common salt, and when dry to float on a solution of silver nitrate of about the same strength, and then to dry and expose it to the daylight to blacken. When the blackening is produced, if the paper be slightly washed and then be treated with a 5-per-cent. solution of potassium iodide (slightly acidified with nitric acid) in the dark, and while still damp be exposed beneath a negative to Caused by liberation of the Halogen. 305 the light, it will be found that those portions beneath the transparent portions will rapidly bleach, and we shall have a negative image instead of a positive, but reversed as regards right and left. The same experiment may be repeated, substitutincr potassium bromide for the iodide, and the same results will be obtamed. It may be asked if any metallic iodide or bromide will be effective ; and to this an affirmative answer may be given ; but the use of acid is not necessary in all cases. Those metals which form two iodides or bromides must be used extremely dilute, or the bleaching will take place in the dark- that is, supposing the highest type of bromide or iodide be employed. Thus a strong solution of zinc iodide may be used and acidified, whilst a very dilute solution of ferric iodide must be used. Here then we have a proof that the visible image can be destroyed by the action of light. In this case the de- struction IS due to the liberation of the halogen from the metal or alkali by the action of light. In the invisible but developable image the action is the same. The following remarks are from the paper by the writer in the ' Philosophical Magazine' : — If a plate be prepared with silver iodide by the ordinary wet process, be briefly exposed to light, and after washin- be treated with a solution of potassium iodide and then be exposed to an image in the camera, after dipping in the silver-bath and developing, a positive image is obtained. It matters not whether the potassium iodide be alkaline, neutral, or acid, the same effect will be noted ; also that there is no difference if, after treatment with the potassium iodide, the plate be washed or not, the reversal of the image will still be shown. In this case the iodine is liberated as before, but the action is increased by the access of oxygen from the air ; in fact, it is a mixture of effects. If potassium bromide or any simple bromide be substi- X 306 Destruction of the Photographic I mage. tuted for the iodide, the same result obtains. Silver iodide, if prepared with an excess of soluble iodide, or if, after preparation with excess of silver, it be treated with a soluble bromide, is insensitive to light ; and the explanation of this perhaps may be found in the fact already stated. It has been usually held that a soluble iodide, such as potassium, can destroy an invisible impression made by radiation ; but this is not the case in the case of monad metals, if it be treated with the iodide in the dark. If, how- ever, any iodide of a dyad, such as cupric or ferric iodide, be employed, which readily liberates an equivalent of iodme, the destruction is accomplished in the dark. The least favourable iodides for such destruction are the monads. If a plate prepared with silver iodide have a prelimmary exposure given it, and then be exposed for a considerable iime to the image' formed in the camera, a reversal of the image will take place as before. If, however, such a plate, after washing, be treated with an aqueous solution of pyro- gallic acid, potassium nitrite, or any other deoxidising agent, such reversal of the image will not be obtained ; nor will it if it be exposed in a cell containing such a substance as benzine, or if exposed in dry hydrogen. From this we learn that, to obtain reversal, oxygen must be present m some form or another, and that, if a substance readily taking up oxygen be in contact with the silver-salt, a reversal cannot be readily obtained. An interesting corroboration of the above statement is to be found in the exposure of an already exposed plate in a cell containing a dilute solution of permanganate of potash, bichromate of potash, or hydroxyl, when it will be found that the reversal takes place with the greatest facility. The same reversals may also be obtained by using any of the mineral acids in a diluted form.' ' It must however, be remembered that the soUitions must be very dilute, or the whole effect of the preliminary exposure will be destroyed, since these oxidising agents are active in the dark, but act more readily in the light. Difference between Gelatine and Collodion. 307 The above experiments show, then, that a reversal may be obtamed by the presence of the iodides or bromides (and in a more feeble manner, by that of the chlorides), and also by oxidismg agents and mineral acids ; whilst the presence of a deoxidismg agent, or the exposure of the plate in a medmm free from oxygen, prevents the occurrence of the phenomenon. With the bromide of silver we have rather different phases of the phenomenon to consider. The development can be earned out with the alkaline or the ferrous oxalate developer, a mode which is more easv to carry out than the development by precipitation of metallic silver from an aqueous solution of silver nitrate. For experimental pur- poses, films contammg silver bromide may be of collodion or of gelatme; but the behaviour of the silver salt in the two vehicles is somewhat different, and has to be considered separately. Collodion is, or should be, a strictly neutral substance; that is, it is merely a medium in the pores of which tne silver-salt is entangled and kept in position, and has no effect on the progress of development or on the action of light, beyond that which may be due to its physical qualities, its chemical constitution remaining unchanged The microscope tells us that a collodion film is essen- tially porous, and free access of the atmosphere to the silver is thus obtained. Gelatine, on the other hand, is a sub- stance readily acted upon by oxidising agents and by the halogens; and consequently it may have an effect on the progress of development and on the action of light its chemical constitution becoming altered. It is a homo- geneous film, and not porous in the ordinary sense of the word, and is a protective agency against the atmosphere to those silver-salts which may be embedded in it. If a film containing silver bromide, whether in gelatine or collodion, have a preliminary exposure given to it, and then be treated with a soluble bromide of an alkali, such as of potassium, and be again exposed to light in the camera, 3o8 Destruction of the Photographic I mage. it will be found that there is not such a rapid reversal of the ima-e as with the iodide, but that longer exposure is required to effect it, the reason being that bromide of silver prepared with a large excess of soluble bromide is still sensitive to liaht. If, therefore, the light decomposes the soluble bro- mide on the plate, liberating enough bromine to form fresh bromide of silver with the sub-bromide formed by the pre- liminary exposure, that freshly formed bromide, being sensi- tive to light, is again reduced to the sub-bromide state by the same rays which formed it. It will be evident, however, that reversal should take place more rapidly with the soluble bromide present than without it ; and such is the case. It is useless to treat a silver bromide film with a soluble iodide, since silver iodide is immediately formed, arid the reactions that take place are similar to those already de- scribed. , If bromide of silver in collodion be exposed to the camera without the presence of any other substance a reversal takes place. Roughly speaking, the reversal takes some sixty times more exposure to the light than is requisite to produce the maximum ordinary effect. To trace the cause of this reversal it is only necessary to treat the film with a 5 per cent, solution of potassium nitrite, when it will be found that the reversal does not take place. The same holds true when the film is treated with any deoxidising solution, or if the plate be immersed in benzene or hydrogen. The cause, then, of the reversal in this case is evidently an oxidation ; and this may be further verified by treating the film, after a preliminary exposure, with bichromate o potash, hydroxyl, &c.; it will then be found that the reversal takes place much more rapidly than when these oxidising agents were absent. The same may be said of the mineral """"^If silver bromide be held in a gelatine film, the action of light is somewhat different. If the plate be exposed in the camera for a short time, say a few seconds, the iraage^ Oxidation of tJie Image. 309 develops in the usual manner and we have a negative image ; if it be prolonged to, say, a minute, the image is reversed on development ; a further exposure causes a negative image to be produced, whilst one much more prolonged causes a positive image again to be formed on development. Here are four distinct phenomena ^ which need explanation. To solve the problem offered, plates should be exposed when saturated with a solution of potassium nitrite as before, when it will be found that the phenomena are absent, a reversal being almost impossible to obtain unless the lenglih of exposure be such as to thoroughly oxidise the nitrite at the expense of gelatine. For ordinary purposes it may be said that a reversal is non-existent under these conditions. If a plate be exposed in benzene, however (a liquid which does not permeate through gelatine), the phenomena are still existent. If a plate be exposed to such an extent that there is a marked image apparent before development, and be then immersed in water, it will be found that where the image appears the gelatine refuses to swell to the same extent that it does when the light has not acted. Taking these two experiments together, it is evident that the gelatine has played some part with the silver bromide. It may therefore be presumed that the three last phenomena are due, the ist to the oxidation of the surface-particles of the bromide and a consequent change in colour, the 2nd to the change in colour of these particles permitting the coloured rays to which it is sensitive to strike a deeper layer, and the 3rd to the oxidation of this layer at the expense of the gelatine. The 3rd and 4th phenomena are so unimportant that they are scarcely Avorth investigating. The presence of organic matter is evidently necessary for their appearance ; at least the writer has never been able to obtain them with collodion films not containing a preservative. As before, the experiment of saturating one of these ' Mr. C. Bennett described these phenomena in the British Journal of Photooraphy in 1878. 310 Destruction of the Photographic Image. gelatine films with bichromate of potash shows that the reversing action is very much increased by the presence of the oxidising agent. Having treated of these reversals of the image in a general way, it now remains to show which radiations are effective in producing them. For testing this, spectrum photography was resorted to, a special dark slide having been constructed capable of holding a cell which would contain the plate, together with the liquid, gas or vapour whose action it might be desired to test. Three flint-glass prisms were used and the lens of the camera had an equiva- lent focus of about 2 feet, the coUimating lens being of the same focus. The time of exposure was, as a rule, three minutes to the sunlight or to that of the electric arc, care being taken in the latter case that an image of the positive pole fell on the slit so as to give a continuous spectrum. The action of potassium iodide on silver iodide will first be described. A plate was exposed after being sensitised, and after washing was immersed in a cell containing a i-per-cent. solution of potassium iodide and exposed to the spectrum. The result is shown in No. 1 ; the same rays which cause an image to be formed in the usual manner likewise caused a reversal (dotted curve, No. i). A plate similarly prepared was exposed in a i-per-cent. solution of potassium bromide for the same length of time, with the result that a reversal was obtained in the blue and likewise in the red, but much less marked in the latter (No. 2). These two experiments tend to prove that, in reality, it is the bromide that is acted upon to some extent, and the effect is not entirely due to the silver-salt. This was particularly manifest in the case of the iodide and bromide slightly acidified with a mineral acid, and was much less marked when the solution was alkaline — in the latter <:a.se, the reversal taking place in the blue, and not in the red regions of the spectrum. Reversal of the Image. 311 To see if the silver-salt had any marked effect on the rapidity of oxidation, a silver-iodide plate was washed, given the same preliminary exposure, and then placed in the spectro-photographic apparatus without any surrounding fluid. A reversal was obtained in the blue, but not to any- FlG. 08. Plate showing reversing actions of iodides, bromides, and oxidising agents on silver iodide. N.B.- The curves below the line show the reversals or positive imaees • the curves above the Ime show the ordinary action, or negative images th^ frdinate^ approximately represent the amount of action. ^ ' thing like such an extent as when placed in soluble iodides or bromides. The reversal, therefore, when the plate is ex- posed in the latter is partially due to the action of radiation 3 1 2 Destruction of the Photographic Image. on the bromide, and partly to that exerted on the silver-salt itself. A silver-iodide plate, treated as before, was next exposed in a weak solution of potassium bichromate, when there was a strong reversal in the red (No. 3), and no action whatever in the blue. Permanganate of potash was next substituted for the bichromate ; and the same reversing action was found, with the addition of a negative image in the blue (No. 4). With hydroxyl the same phenomena were observed as with the permanganate, the reversal taking place a little further into the green (No. 5). Studying the absorption due to these three oxidising agents, it would appear that the reversing action is due to the action of light on the salt of silver, which is changed by the preliminary exposure to light, and not to the action of light on the mediufu in which the plates are placed. With mineral acids a reversal was always obtained in the red and in the blue, a portion of the spectrum in the green and yellow remaining unreversed (No. 6). Now the action of these acids is not a strictly oxidising action, but is pro- bably a removal of the loose atoms of the silver which goes to form the subiodide, and leaving silver iodide behind as the result of the action. The results of the action of acids do not, therefore, vitiate the above deduction. A plate exposed in benzene or in nitrite of potash showed no reversal even with a very prolonged exposure. It should be remarked that the action of permanganate and bichromate of potash when very feeble is sometimes to give feeble negative images in the red and blue in lieu of positive images ; also positive images in the blue, and feeble negative images in the red (see dotted curves in Nos. 4 and 5). But this is to be accounted for by the fact that the dilution of these oxidising agents is so extreme that the reducing action on the unaltered sensitive salt is far greater than the rapidity of the oxidation. Reversal of the Image, 313 Ord iriary bromide of silver in collodion or gelatine may be taken as giving almost identical results under the influ- ence of a soluble bromide. Now ordinary bromide is sensi- Fk;. gg. Plate showing reversing action of bromides and oxidising agents on silver haloids. ^•^•r^The curves below the line show the reversals or positive images • the curves above the line show the ordinary action, or negative images ; the oroinates approximately represent (he amount of action. tive as far as B (see dotted curve, No 7) ; and it might be presumed that this sensitiveness to the rays of lower re- frangibility would cause a modification in the action of the 3 1 4 Destruction of the Photographic Image. soluble bromide. A reference to Nos. 7 and 8 will show that this is the case, but that at the same time the features which are so marked with the action of bromide on silver iodide are present. No. 7 is the curves due to silver bromide in collodion which had received a preliminary exposure and was then exposed in a 5-per-cent. solution of acid potassium bromide. It will be seen that the curves in Nos. 7 and 2 are similar, showing that the principal action is due to light acting on the soluble bromide in the presence of an acid. No. 8 is a similar plate exposed in an alkaline solution of KBr, in which there is a modification of the curve. The last loop is probably due to the silver sub-bromide itself, since the oxidation of this salt by oxidising agents occupies approximately the same position (see No. 10). No. 9 shows the effect of permanganate of potash ; and when it is compared with No. 10, which is the curve due to oxidation by bichromate of potash, it will be manifest that the chief oxidising action lies in the red and ultra red of the spectrum. No. II also shows the effect of bichromate of potash on silver bromide after a preliminary exposure, the plate in this case being a gelatine plate. It will be seen that the bichromate totally arrests all action in the blue, whilst it rapidly causes a reversal in the red. Nu. 12 shows the effect of mineral acids on silver bromide, by which it will be seen that a maximum of reversal takes place in the red and in the blue. As before stated in regard to the iodide, the action. of these acids can scarcely be regarded as an action of oxidation. No. 13 shows the phenomena due to over-exposure of silver bromide, by which it will be seen that reversal takes place in the blue and not in the red. Comparing this with Nos. 7, II, and 12, the effect of extraneous matter in caus- ing a reversal is very marked. Collodion plates exposed in benzene, or in aqueous solutions of pyrogallic acid, potassium nitrite, and -sodium sulphite gave no reversal whatever. OrtJiochromaiic Photography. 315 Gelatine plates exposed in benzene gave the phenomena shown in No. 13, whilst with the other media no reversal at all was obtained. The explanation of the apparent contradiction shown by the behaviour of a gelatine plate exposed in benzene has already been given. The actions of many other liquids and gases ^ have like- wise been tried ; but it was thought that the examples given sufficed, since they all pointed to the same conclusions, which may be summarised as follows : — ist The reversal of an image is due, in the majority of cases, to the oxidation of the subsalt of silver which formed by the first impact of light on the exposed salt of silver. 2nd. The oxidation is due to the action of light, the rays of lower refrangibility being the most powerful accele- rators of oxidation. 3rd. Reversal of an image may be due to the presence of any haloid of an alkali, the reversal in this case being partly due to the action of light on such a haloid, and partly due to the tendency to oxidation of the subsalt of silver. 4th. The presence of a mineral acid tends powerfully to cause a reversal. CHAPTER XXXVII. ORTHOCHROMATIC PHOTOGRAPHY. We now come to the effect of certain dyes on sensitive salts of silver, but as the salt to which the application is most generally made is the bromide, the remarks that will be made will be principally applied to it. In 1873, I^r- H. W. Vogel, of Berlin, found that the ad- dition of certain dyes, such as corallin, magdala red, gave more than one, or changed the position of the, maximum of ' Ozone was most marked in its oxidising properties, and gave a curve very similar to No. 12 both with the iodide and bromide of silver., i6 OrtJiochromatic Photography. sensitiveness in the spectrum, Vogel experimented with the collodion processes, since at that date the newer gelatine pro- cess was not workable. The application to gelatine plates, however, was a natural outcome of this, and who first applied it is doubtful. Vogel, Eder, and the writer all experimented in this direction. Eder, however, carried out a very exten- sive series of experiments with different dyes, and gave a very complete discussion of each. As a matter of fact, the two typical dyes which are at present most employed are eosin and cyanin blue. The following figure^ will give an idea of the relative Fig. loo. No. I is the effect of Hoffman's violet, gentian violet, methyl violet and acid violet. No. 2 shows the effect of benzaldehyde green, ethyl green, meythl green, and other varieties. No. 3 is iodine green. No. 4, cyanin blue. No. 5 is eosin and the members of the eosin group. No d, rose Bengal and ammonia. No. 7, coerulein. No. 8, chrysanilin. No. 9, eosin and silver chloride. No. 10, eosin and cyanin. • Taken from a paper rearl before the Society of Chemical Industry, June 30, 1887, by C. H, Bothamly, F.I.C., F.C.S., and is a repro- ductioa from Eder's paper. Eosin-dyed Plates. 317 sensitiveness to the different parts of the spectrum of a gelatino-bromide plate to which various dyes have been added. When collodion is employed it is easy to employ an excess of silver nitrate, in order that when possible the dye may form a coloured compound with the silver, which is sensitive to light. With the gelatine process this is not so easy, though it can be effected by an artifice (the reason why the silver cannot be in excess in a gelatine emulsion has already been pointed out). Eosin and the variations of eosin, such as erythrosin, will each form a compound with silver nitrate, which in a neutral or acid solution of the for- mer is precipitated by the addition of a solution of the latter. This precipitate can be washed by decantation till all traces of silver nitrate are eliminated. This solid organic com- pound of silver thus isolated dissolves readily in a very dilute solution of ammonia, and can in this manner either be added to the gelatine emulsion, or a prepared plate can be soaked in it and dried. Plates prepared in this manner are exceedingly sensitive, though some writers assert that they are not so sensitive as when prepared in the following manner. In the chapter on the making emulsion by the ammonia process, if dye be added to a small extent to the bromide solution so that it is of a salmon colour, and the precipi- tation takes place in the presence of the dye, it appears that the dye combines with the silver bromide, forming a double compound which gives a film which is very sensitive in the green. Another plan which is very usually adopted to convert an ordinary plate into one which is sensitive to the green and slightly in the yellow, is to make a solution of the dye in ammonia and to bathe the plate in it, and tlien to allow it to dry. The following formula was given by Dr. Mallmann. The plates, having been dusted are placed in Ammonia 2 parts Distilled water 200 parts. 3 1 8 OrtJiochromatic PJiotography. Afterwards the plates are dipped in a bath made as below : Erythrosin or eosin (i : i,ooo of water) . 25 parts Ammonia 4 parts Distilled water . . . . . -175 parts. They are kept in this for i minute to i-]- minute, and are drained on a porous tile and dried in the dark. The addition of a few drops of a saturated solution of cyanin blue to the above carries the sensitiveness of the plate further into the red of the spectrum (see No. 10 for fig. 94), and was the combination first made, he believes, by the writer in 1882. As to the practical use of the addition of these various dyes something must be said. If we take sunlight and pass it through one or two prisms and view it on the screen, we find that the yellow region is by far the brightest, and the blue and violet region the feeblest in luminosity. Yet it is these latter regions which have the principal effect in photography on ordinary plates. The following curve of the luminosity of the spec- FiG. IOI. mm m 1 ■ ■ ■ ■ ■ ■ mm 1 ■ ■ ■ ■ ■ wA SB m I ■ ■n ■■■ ■■■ hh _ '110 10c 90 30 wm 70 r r<\ w 60 % \l 50 J) 40 / V 30 — T f \ 20 1 0 s 1 [ e a _ 1 10 Yiolei Blue 0reen Oran 2^1 trum of sunlight is taken from a paper by the writer which appeared in the ' Philosophical Transactions ' of the Royal Use of Yellow Glass. 319 Society for 1887, and is for a sun of medium altitude. The height of the ordinate measures the luminosity at any particu- lar part of the curve. From this it will be seen that the greatest effect on the eyes is produced by the rays which lie between the green and the yellow. If, therefore, a photograph can be taken with some body which is more sensitive to this part of the spectrum than to any other, manifestly it will give a truer effect of the illumination of the object seen than if the blue rays are the active ones. Thus in a yellow flower or in a sunset sky there is very little blue, but a great deal of yellow, the latter being very luminous. If these two ob- jects be photographed, one with a compound which is most imoressed by the yellow, and again with one which is most sensitive to the blue, it is evident that the former will give a more truthful monochrome print than the latter. For an exact representation in monochrome, perfect truth can only be attained when the curve of sensitiveness of the compound to the spectrum follows the curve of luminosity of the spectrum, and at present such a compound has not been, nor, if an opinion may be expressed, will it ever be, found, but an approximation made be made by an artifice. Suppose we look through yellow glass at objects lighted by sunlight, the blue will be nearly entirely cut off and the violet entirely, but the general effect of a coloured landscape or drawing, so viewed, will have the same general effect of illumination though the colours themselves will all be yellower. If we take the illumination of the spectrum trans- mitted through yellow glass it will be of the character shown in fig. loi in the lowest curve, from which it will be seen that the place of maximum illumination is very little changed, though the violet and blue are absent. If we attempted to take a photograph through such a yellow glass on a wet plate we should get no effect, though if we used a gelatine-bromide plate, which is sensitive to the green, an impression would be made, but the predominating colour value would be 320 Orthochromatic Photography. in the green. If, however, we employ a plate dyed with cyanin, erythrosin (one of the eosin group), or a mixture of the two, a reference to fig. loo will show that a strong efTect will be made by the green and the yellow, so that a photograph may be taken through a yellow glass which will fairly represent in monochrome the illuminative value of the different colours. If a landscape be taken with these dyed plates (which have been named isochromatic or orthochromatic) without yellow glass intervening there is very little difference between the result and that obtained on an ordinary plate, the reason being that the objects reflect a large proportion of white light, and that the blue light in this reflected light is th-at which is most effective (see fig. loo). If, however, a yellow glass be placed in front of the lens, the photograph will differ considerably in appearance. The blue of the sky, for instance, in the resulting print, will be rendered a grey instead of white, whilst a white cloud will be white. The greens of the trees and grass will be more harmonious, and so will any yellow object which may be in the view. Mr. B. J. Edwards has introduced a very neat arrangement for introducing a yellow screen between the object and the plate. With a doublet-lens, where Waterhouse stops are used, he fastens between two thin cards, in which is cut a circular aper- ture, a thin sheet of gelatine dyed with aurine or some other similar colour. The film is so thin and even that no serious deviation takes place in the rays of light passing through the lens, when such a card takes the place of the stop. The writer adopted the plan usually recommended of placing in front of the lens a piece of orange glass, but only one piece out of some twenty was sufficiently true in its sur- face to be capable of being utilised, whereas by employing Mr. Edwards's plan no difficulty was experienced. For copying coloured pictures, as already hinted, these orthochromatic plates are excessively valuable, always taking the precaution of using some yellow medium as indicated Theory of the Effect of Dyes. 321 above. The exposures have to be more prolonged than when a plate without the screen is employed, since the most effective rays are cut off The increased exposure may vary from 6 to 60 times that usually required, the time depending on the sensitiveness of the combmation. The theory of the effect produced by the addition of dyes to sensitive salts of silver may be summed up as follows : — 1. There may be an organic salt of silver formed, either by the combmation of silver nitrate or of silver bromide (or chloride) with the dye ; in the latter case, the formation of the compound is aided by the solubility of the bromide (or chloride) in ammonia. Such an organic salt of silver may be sensitive to rays of the spectrum to which the inorganic salt is insensitive, or it may only exalt the sensi- tiveness of the inorganic salt for some particular part of the spectrum. 2. If the dye is fugitive, i.e. one that rapidly fades in the light (as cyanin does) the faded dye becomes oxidised, and the oxidised body acts as a nucleus on which the silver reduced by the developer is deposited. , Vogel has called some of these dyes optical sensitisers, a:nd explains the term by stating that where the dyes absorb 'the light there apparently a sort of secondary action takes place. This is not an explanation which our present knowledge allows the writer to admit. It is believed that the first given explanations are more consistent with the theory and fact than the latter. The locality of the spectrum in which the dyes may be expected to increase the sensitiveness of the plates can be readily found by examining the absorption spectrum of the dyes or their compounds. Where the light is absorbed, there the sensitiveness may be increased. It has been stated by some writers that there is no exact coin- cidence between the two, but only a general coincidence. By careful examination, however, and by taking proper pre- cautions, the writer has found that the coincidence between V 322 Light for the Dark Room, the localities of absorption and chemical action in the spec- trum is exact, and does not follow a quasi law enunciated by Kundt, which can scarcely, as yet, be fully accepted, and which would have made only a general coincidence between absorption and chemical action possible. Thus, in every case of chemical action taking place by the spectrum, it may be laid down that the body acted upon absorbs in the same localities. This is a law which must hold good, since it is part of the law of the conservation of energy. CHAPTER XXXVIII. LIGHT FOR THE DARK ROOM. The two previous chapters will have paved the way for consideration of the light which is admissible in the so- called dark room. A reference to fig. 94 in Chapter XXXV. will show that a gelatino-bromide plate is sensitive a very long way towards the red, starting from the blue of the spectrum. The accomi)anying diagram taken from another work ' by the author will illustrate the kind of light which comes through glasses of different kinds, together with the light which passes through certain dyes. No. 9 is added, as at one time there was an idea that a solution of quinine cut off all chemically effective light. The diagram at once disposes of this. Looking carefully at Nos. 20 and 21, in fig. 94, it will be seen that the most efficient light is that coming at the extreme red of the spectrum. No. 2, fig. 102, shows that ruby glass would be an excellent light filter were the rays in the blue absent. A combination of ruby glass No. 2 and No. 6 (stained red) accomplishes this and leaves the portion in "the red alone. Stained red by itself is excellent, and • Instruction in Photography. Piper & Carter. Light through Ruby Glass 323 there are very few plates which the light coming through this medium will afiFect if the exposure be not too prolonged. Fig. 102. Stained red glass by itself allows the passage of more light altogether than does the ruby glass. The annexed diagram Will sho^y the proportion of light which passes. The areas 324 Light for the Dark Room. of the curves give the intensities of the lights. That of the naked hght was 2639, that transmitted through orange glass 268, and that through ruby glass only 115. In other words, stained red glass only allows yV the total light to pass, but ruby glass (medium) only allows about -^V- When the light is dull, as in winter, it is evident that such a diminution of light is so great that there will not be sufficient for the proper manipulation of plates during development, and recourse must then be had to artificial light, of which we shall presently speak. Fig. 103. Coloured glass, however, is not an essential ; paper when of a proper colour and stained throughout answers equally well, though it cuts off more of the original light. Thus a combination of chrysoidine-dyed paper and magenta-stained paper, Nos. 3 and 4,' fig. 102, will be efficient. The two dyes should not be mixed, but two sheets of paper, one dyed with the one and the other with the other, should be superimposed. The worst feature in aniline dyes, however, is their ceitainty of fading in white light. The best medium of all is the common orange paper which is used for packing purposes. Two thick- Artificial Light. 2 25 nesses of this are almost absolutely safe, and if three thick- nesses be used when sun-light falls on the window there will be no danger of veiling the plate. If thought desirable the paper may be oiled and rendered more translucent. This of course allows the passage of more light and is as safe. Some photographers use a paper which is known as canary medium. It is paper impregnated with lead chromate and gives a pleasant whitish light. If the paper used be thin, it is not safe, since too much of the green of the spectrum passes, but if stout paper be employed it may be used with safety, since the light penetrating is small. The writer's advice is, however, to use the orange paper in preference to any other. When artificial light is used a lantern should be con- structed or purchased. There are several in the market which are satisfactory. They are usually carefully glazed, but it is no detriment if the glass breaks and has to be removed, since a piece of orange paper above alluded to can be put in its place. A screen made by half cutting through a sheet of large card- board at three equal intervals can be made by the amateur ; the centre portion of each leaf is removed and filled in with orange paper. Such a screen folds round an ordinary chamber candlestick, and if the light striking a white ceiling be too mtense, it can be cut off by placing loosely over the top a sheet of newspaper or a board. The reflection from the ceiling has never been found by the writer to hurt a plate during development, as might be expected if considered theoretically. The illumination from any source varies inversely as the square of the distance ; and if the ceiling is six feet off it is illuminated only of that which a sheet of white paper would be one foot off. The ceiling becomes another source of light. It is again weakened by'' travelling back to operator, say another six feet. Roughly, it may be said that the illumination is ^-^^ of that from the light at a foot off. If one second will blacken a plate when a candle •526 Actinomeiry. is one foot off, of a candle will not visibly affect it in the short time necessary to place the plate in the developing dish and pour over the developer. When the developer has wetted the plate it becomes comparatively insensitive. When electric lighting is available for the dark room the lamp may be surrounded with an orange paper, and is the most charming hght to work with. For plates which are prepared with erythrosin a deep red light alone should be used ; a combination of ruby glass and orange paper may be used in this case. CHAPTER XXXIX. ACTINOMETRY. Amongst the earliest methods of comparing the chemical energy of different lights is that known as Bunsen and Ros- coe's, its value having been originally pointed out by Pro- fessor Draper, of New York. The process is dependent on the fact that combination takes place between hydrogen and chlorine when the mixed gases are exposed to the action of light The two gases may be evolved by the electrolysis of hydrochloric acid, and then they are in the right proportions for recombination. Such a mixture of gases, when exposed to sunlight, combines with explosive violence, though in dif- fused light the recombination takes place gradually, and in proportion to the intensity of the light, and to the time dunng Ahich they are exposed to it. This affords a method of securing a registration of the intensity of the light, for the hydrochloric acid formed may be collected in water, and the amount may be estimated by various chemical means. As for general use this method did not prove altogether satisfactory, and Bunsen and Roscoe abandoned it for one which will be described in deiail. Professor Draper A ction of L ight on Ferric Oxalate. 327 had also pointed out that ferric oxalate when exposed to light gives out carbonic acid, and, in 1859, Mr. H. Draper, of Dublin, turned this fact to practical use by elaborating a system of which the following is an outline. The chemical reaction on which the method is founded is this : — Ferric oxalate = Ferrous oxalate + Carbonic anhydride. Fe,3C,0, = FeC.O, + 2CO,. The light vibrations are able to split up the ferric oxalate molecules, and for each molecule so shaken, one molecule of carbonic anhydride is liberated. Mr. H. Draper's appa- ratus consisted of 500 grains of a standard solution of ferric oxalate held in a glass cistern, rendered opaque by japan- ning, the light being admitted by leaving uncovered one square inch of the cistern. After exposure to the light for any desired time the amount of carbonic anhydride dis- engaged was known by the difference in weight before and after exposure, the loss due to evaporation being checked by comparison with a similar cistern containing distilled water. There are one or two objections to be noted as regards the accuracy of this method. The ferric oxalate being a coloured soktion, it is uncertain to what depth the light penetrates into it, and it has yet to be proved that with equal intensities of light acting for the same time through the smie aperture, double the same amount of chemical decomposition is produced after passing through two units of thickness, as is produced after passing through one. In all apparatus of this kind, too, the surface reflection has to be taken into consideration, as also the material of which the transparent parts of the cistern is constructed. If we could be assured that the value of the ultra-violet rays in- creased in the same ratio as the blue rays, the apparatus would suffice, but we have reason to think that this is not the case. Hence this construction must be taken as yield- ing an approximation to the true results rather than as the 328 Actinomeiry. true results themselves. We consider that the best results would probably be attained by throwing certain definite portions of the spectrum on some medium, and noticing the results of each. This would give a true idea of the relative amounts of photographic energy existing in each portion. It cannot be too deeply impressed on the student that these processes do not measure the actual energy in the impinging rays of light ; this is altogether a different matter, into which We cannot enter here. Professors Bunsen and Roscoe conceived the first practicable method of measuring the actinism of day- light and sunlight, by the exposure of sensitive silver chloride paper to their action for certain lengths of time. After an elaborate investigation, they came to the conclu- sion ' that equal quantities of the intensity of light into the time of insolation (exposure) correspond, within very wide limits, to equal shades of darkness produced on chloride of silver paper of uniform sensitiveness.' Starting with this idea they carried out a laborious research into the prepara- tion of a paper that should be uniformly sensitive, and which might, therefore, be considered as a paper of standard sensitiveness. The following is a short resume of their work : Choosing sodium chloride as the soluble chloride, they found, I St, tnat a paper will not give uniform results when simply floated on the solution ; but that it must be im- mersed in it. 2nd. That the stronger the solution the greater sensitiveness would be given to the paper. It was, therefore, necessary to fix some reasonable limit to the strength, and this they fixed at a 3 per cent, solution. 3rd, that using the sensitising solution of silver nitrate above a greater strength than 6 per cent, gave no difference in the results as regards sensitiveness, but that below that strength it rapidly diminished. 4th. That the presence of the salt resulting from the decomposition of the sodium chloride and silver nitrate had no effect on the sensitiveness, and that at ordinary temperature and moisture the sensitised Standard Chloride Paper. 329 paper would keep at least 15 hours unaltered. 5th. That the thickness of the paper employed had no material in- fluence on the result— an important point, as in comparing the darkening due to the light with a standard scale of gra- duated tints it was necessary that the paper should be suffi- ciently opaque to cut ofif all shade from the tint beneath, which might shine through the paper. 6th. It was found that it was possible to impregnate 5 square metres of paper in a solution containing 60 granules of sodium chloride without any danger of reducing the strength of the solution to such a degree as to cause any variation in sensitiveness. 7th...The time of sensitising, betwe^a 15- seconds to 8 minutes, also caused no alteration in the readings.: It will be noticed that the above results give great latitude in preparing a paper, the only conditions abso- lutely necessary being that the paper shall be uniformly soaked with a 3 per cent, solution, that the sensitising bath of silver nitrate shall not be suffered to drop below a 6 pei cent, solution, and that the sensitising shall not be less than 15 seconds of time. With such a preparation we have then a standard paper which will give uniform results — that is, we have a paper, which, when exposed for a certain time to a certain intensity and quality of hght, will always produce the same amount of darkening. The most perfect method of measuring intensity of daylight, then, would be to cause a strip of sensitive paper to pass gradually before an opening of a certain size, such opening to be ex- posed to the zenith. Unfortunately, this method is impracti- cable ; for, as we shall find shortly, the darkening action in strong hght, such as we have in sunshine when the sun is in the meridian, is exceedingly rapid; whilst in weak hght such as we have on a cloudy day, the darkening is exceedingly slovr. It might be suggested that the opening should be wedge- shaped, so that the shade itself might be graduated and therefore be readable at some part, and at a very early date in the history of photography this plan was proposed by 330 Actinometry. Mr. Jordan ; but practically it is found that the opening necessary for the production of a readable tint in full sun- shine, with paper passing slowly before it, is so small that there are mechanical difficulties in the way of securing accuracy ; and when it is considered that one end of the aperture would have to be at least loo times the width of the other, the impossibility of obtaining a proper gradation with a slip of paper of reasonable width is apparent. To meet this difficulty a most ingenious instrument was devised by Professor Roscoe, in which exposure takes place for certain regulated times, at fixed intervals, during every hour. The registration is effected by the following contrivance A long strip of paper is rolled upon f, and fastened to d. In the latter is a clockwork arrangement, the escapement, b, being placed as shown. To move the clockwork, the arma- ture of an electro-magnet takes the place of a pendulum, and every time it is attracted and released by the magnet, a tooth of the wheel is released, and the paper is moved a small piece forward across the weak spring, E, which is seen on the top of D. The use of the spring is to cause the paper to be in contact with a circular aperture of about \ (fig. 104): Fig. 104. Standard Tint. 331 inch in diameter, left in the cover of the instrument, and through which the exposure is given. The result of the exposure is thus to leave circles of more or less blackness (the blackness varying according to the intensity of the light and the time of exposure) at intervals along the strip of paper. The different exposures were given by a toothed wheel annexed to a clock, which gave the necessary contacts for fixed times to move the armature in fig. 104. This instrument has been superseded by a form in which the paper is sensitised in a sheet and wrapped round a cylinder attached to clockwork. The cylinder moves round at intervals every hour by the clockwork, and when a com- plete revolution has taken place it shifts laterally, so that a fresh piece of cylinder is brought under the aperture. The following is the standard tint of grey adopted by Fig. 105. Roscoe. He took 1,000 parts of zinc oxide and 1 part of lamp-black, and ground them thoroughly together to such a point that no further grinding altered the tint. This he found the most convenient tint for com.parison ; and, when carefully gummed on to paper, it was unaltered in shade. This mixture then gave the shade from which all his measurements were made, all other tints being referred to it. 332 Actinometry. To obtain a graduated shade he applied what is known as the pendulum apparatus, which in general outline con- sists of a pendulum swinging in front of sensitised paper in such a manner as to give a gradation of exposure to it, and a consequent variation in. tint. At each point of the paper the time of exposure was known, and the point was then found answering to the standard tint, and the relative values of the other portions of the gradations calculated. It may perhaps be found necessary hereafter to apply a correction to those readings taken in sunlight, as it may be found that the different integrations of the spectra formed by sky, cloud, and sunlight produce slightly different effects. Another method of securing uniformity in measurement has been employed by the writer. It consists of a rapidly revolving cylinder, or drum, B, on which is attached a series Qf black and white sectors, as in the diagram. A convenient length for this drum has been found to be 6 inches. To the cylinder, b, is fixed a small pulley firmly attached to one end, over which is passed a cord communicating with the wheel, A. These are of such relative dimensions that the cylinder rotates at least 15 times in a second, when a is caused to rotate but once. Along the top, and nearly touching the cylinder, is a blackened brass support, c, with a slot in it, on each side of which is a scale of inches, dividing the length of 6 inches into 120 parts, that is, each inch into 20 parts. Monochromatic light is thrown vertically downwards, on the scale, and any tint to be compared is brought on to the scale, and moved till an exactly identical shade is found on the rotating cylinder. A series of 6 read- ings is taken, beginning by moving the tint from white to black, and next from black to white. It will nearly always be found that this is necessary, as the readings in one case would be as much too high as in the other they would be too low. A mean of the six gives very nearly the truth. The accompanying diagram gives the results of the reading of a strip of paper which had been exposed beneath an Gradation in Photagraphs, 333 apparatus, giving an arithmetical progression of exposure for each unit of length : — Fig. io6. The ordinates measure the amount of whice, I'o being white, and o black. The abscissae show the exposure, with a fixed intensity of light. From a curve of this kind, when the tint is compared and known, and the time of exposure is also known, the intensity of the actinism can be judged. Roscoe's standard unit has an ordinate of 76, that is, it is a visual combination of 24 parts of black with 76 of white. This method of examining the gradations caused by dif- ferent intensities is well worthy of more complete study, as it throws much light on the false effects which are produced m photographs. If we prepare a rotating wheel with black spokes so cut that when rotating in front of a uniformly lighted surface they give the exposure along the length of the spokes in arithmetical progression (thus : if the length of spoke be 10 inches, the exposure at that distance is i, at 5 inches \, at 2% inches \, and so on), we shall find, on exposing a plate to the image of this rotating wheel as formed by a lens, or by causing the wheel to pass close to it, that the gradation of the negative obtained, when viewed by transmitted light, will not coincide for any length with 334 Actinometry . the gradation as seen by the eye. The following figure shows the results of measurements obtained by the dia- phanometer/ from plates to which different exposures have been given behind the rotating wheel, a, b, and c are the curves from ordinary wet-plate negatives. The ordinates measure the amount of transparency, i being total trans- parency, and o total opacity ; the abscissa denote the rela- tive times of exposure, or what is approximately equivalent to it, the relative intensities of light acting on the negative, supposing RosGoe's law referred to above to hold good. It Fig. 107. will be seen that the curves have very nearly, that is, within the limits of error of observation, the same form. Thus, taking, the exposure of a equal to -5 and i, or i to 2, the corresponding transparencies are 77 and .37-. Taking the same transparencies of b, the times of exposure are -34 to •66, or very nearly i to 2. The same will be found with c ; the dotted curves, e and D, show a portion of the negatives, B and c, intensified in the ordinary manner, and the same relation to exposure still holds good. This is an important point, as it shows that the same relative intensities of light are maintained in a negative as the opacity is increased. ^ See London, Edinburgh, and Dublin Phil. Mag. Sept. 1874. spurge's Sensitometcr. 335 The ordinates to the chain-dotted straight Hnes show the transparency that should result if photography gave perfect gradations. It will be seen that the tendency in all negatives is to cause a loss of gradation in the deep shadows as well as in the lights. This accounts for the loss of detail that is always seen in the extreme tints of a photograph. It is also worthy of remark that a thin negative seems to give a better gradation than one intensified. It must be distinctly understood that the above curves apply to negatives only under one class of development. Under others the curves would show considerable variations. Amongst the most striking would be the form they take when near the parts in which total transparency is repre- sented. A more detailed account of these will be found in the * Photographic News ' for July and August, 1877. Fig. io8. An excellent instrument for making investigations on the density of deposit of negatives and for the darken- ing of different papers is what is known as Spurge's Sensi- tometer. In fig. 108 c represents the appearance of the top and b that of the bottom of the instrument. Each of the various holes shown in c has a fixed ratio of i to 2* with the J 3 6 ' ^ dinometry. next hole. Thus the amount of light admitted to the plate or paper is halved every third hole. By employing such an instrument almost every variation in the action of light may be studied. For estimating the different optical densities of deposit, the writer has adopted the following plan, which gives very great facihty of measurement. Fig. 109. The light, whatever it may be, is placed at a ; a lens at distance of its equivalent focus, in this case 9 inches. The negative, n, the varying thickness of deposit in which it is desired to measure, is placed in front of this lens, and another lens, throws the image on the screen s, in front of which is a rod, R, whose shadow is cast by the light coming through i.,,. This is the ordinary optical lantern form of apparatus. At one side, at a convenient distance, a mirror, m, is placed, with the angle so adjusted in azimuth that it reflects the light from A over the patch illuminated by the lens \.,,. This naturally throws another shadow of the rod alongside the first shadow. The screen, s, may be transparent or opaque, whichever is deemed best. Where the shadows of the rod fall, a square mask is cut out to enable the two shades to be viewed without distraction to the eyes by glare from adjacent parts. It will be seen that, as the light and the reflected beam are stationary, the method of varying distance cannot be adopted to vary the intensity of the light. To obtain the necessary variation, revolving sectors are employed. Density of Deposit. '337 These sectors are attached to a small electro-motor which works with four Grove's cells. The aperture of the sectors can be mcreased or diminished during motion by a simple arrangement. This is an admirable plan of graduat- ing light, and answers for all purposes of the sort. Any variation in the light used will not cause any error, since the same light is also the comparison light. The following figure and table give an idea of the value Fig. PROPbRTION OF TRKNSlMITTED LIGHT 2 3 4 5 6 7 9 10 II 12 13 14. 15 16 17 The figures at foot correspond with number of note in sensitometer. Naked light Glass and gelatine film No. I sauare Angular Aperture. 177 I sq 2 3 4 5 6 7 124 I2li 117" III q6 83 60.V No. 9 square 10 II 12 13 14 15 16 17 18 Angular Aperture. 5'i 40f 29if 17 12 n 4 338 Actmometry. of a gelatine plate exposed behind Spurge's sensitometer and developed with ferrous oxalate and measured in this instru- ment, r. T U(. Putting this table another way, it means that the light in passing through the various parts is diminished by so many lyyths, thus— No. I square allows of the total light to pass ; ^ 177 No. 2 177 The film and glass iM, or 7 of the light nearly ; 177 and so on through the whole scale. The following figure shows the curve of density of the same negative for intensity of light increasing in arithmetical progression. ^ ° Fig. III. iRElATIjVE J;RANS PAR£NCiYl_ if Dt POSIT I 0 2 4 6 INTENSirV ACtriNP 10 12 I* 16 20 22 24 26 28 30 32 Sometimes it is convenient to form a screen of varying thickness, such as Warnerke's, and this may be done by C elestial Photography. 339 taking a uniformly coated plate and exposing adjacent por- tions of It through a square aperture to a feeble light say of a lo-candle gas flame 10 feet off, commencing with 10 seconds' exposure, and going up next to 12! then to 16 next to 20 next to 25, next to 32, then to 40, and so on,' doubling the exposure every third hole. The development should take place with ferrous oxalate carefully neutralised to which are added 2 or 3 drops per ounce of a 4 per cent solution of potassium bromide. The development should be earned sufficiently far to render the most exposed square nearly opaque, and then it should be optically measured as described above. It has been found that the deposit given by ferrous-oxalate development is practically black, and that It cuts off the visual and the photographic rays in exactly the same proportion. Such an instrument is excessively handy when Spurge's sensitometer is absent, and the results can be equally re- hed upon. ^ CHAPTER XL. CELESTIAL PHOTOGRAPHY. Physicists have turned photography to account in their study of the heavenly bodies, most of which, in one way or another, have been made to impress their image on sensitive plates. The student who may take a landscape with the sun shining direct into the lens will soon satisfy himself that the exposure necessary to obtain a good photograph of our luminary, when unclouded, is very small, so short, indeed, that solarisation is frequently induced, though the landscape Itself may be capable of proper development. Taken with the ordinary camera and lens a photograph of the sun is prac- tically useless, since a lens of short focus is only capable of giving a very small image, and one on which none of the Celestial Photography. 340 markings which characterise his surface can be seen even ^ith the aid of a magnifier. Since the prime object of solar photography is to enable the surface of the sun to be studied, it is evident that other means must be adopted m order that it may be delineated on a sufficiently large scale. A lens or object-glass gives an image of the sun of a Fig. 112. diameter of about A of an inch to each foot of its focal lensth. It Is, therefore, evident that in order to secure a photograph of rt of 4 inches (about .o cen.nnetres) d,a- meter the lens employed must have about 40 feet ocal ngth. No,v, 4 mches has been proved by exper.ence .0 be about the least diameter for a solar image m wh.ch sun.poB can be effectually studied Before the mtroducfo. of Siderostat. 341 Foucault's siderostat a telescope would have had to be mounted equatorially, and a clock motion would probably have been necessary, since the motion of the earth, even with the short exposure necessary, would have marred the definition to a certain extent. Since siderostats have been classed amongst available instruments, the difficulty atten- dant on the mounting of such an enormous length of telescope has disappeared, and a lens of great length can be employed, mounted on a less heavy tube, placed in any convenient position, and supported in its length, if necessary, along the ground. A siderostat belonging to ihe Royal Society, made by Cooke on Foucault's model, is given in the accompanying fig. 112. Its principle is the same as that of the heliostat, already described at p. 268, and shown at fig. 89. A is a mirror, silvered on the external surface, which has been worked to a perfect plane. It is suspended on two axes, x x, working a U-piece, s s, pivoted at the base, and therefore capable cf moving the mirror so as to face any given direction, p is the polar axis, set so as to point to the pole of the heavens ; the inclination being regulated by a movement along an arc, affixed to the prin- cipal supporting pillar of the instrument. Attached to the polar axis is the declination circle, e, to which the ordinary movement is given by the clockwork, g, which communi- cates its motion by the connecting rod, f. To the lower extremity of the polar axis is attached a movable arm, which can be clamped, so as to form any angle with it. At the bottom of this arm is a socket joint, pivoted at B, in which c, a rigid and perfectly true rod, is capable of sliding. When using the siderostat, it should be set with the polar axis in the meridian. The beam of light can then be caused to be projected in any given horizontal direction by the motion of s s, whilst its vertical direction is adjusted by the movement of the arm b. k k are cords which can turn f, and consequently E, and hence the motion -of a in the horizontal plane can be adjusted without interfering with 342 Celestial Photography. the movement of the clock, h h are cords working on the movable arm, to which b is attached ; a vertical adjustment can therefore be given to the reflected beam. The following method can be employed for obtaining a solar image with the lenses of very long focus by the aid of the siderostat. The lens with its tube is placed in a position such that the direction of their axes cuts approximately the centre of the mirror. Since the mirror is supposed to be a perfect plane, it is manifest that an image of the sun should be formed at the principal focus of the lens, as perfect as if the axis of the lens itself were pointed to the luminary. It is needless to describe the camera, which, in fact, instead of being attached to the tube, may remain detached so long as the plane of the sensitive plate is kept accurately perpendi- cular to the axis of the lens, and so long as all light, except that admitted through the lens, be excluded. This is, perhaps, better than rigidly attaching it to the body of the tube, as it gives facilities for exposing the plate very close to the principal focus. It has been considered most important that such a position for the exposure should be obtained. The reason of this will be evident when it is remembered that the only means of giving the exposure is by causing an opaque screen, in which a slit is cut, to pass across the beam of light. Were such a screen passed in front of the lens, or at any part of the telescope other than the principal focus, the impression of the image might continue during the entire exposure. When the exposure, however, takes place at the principal focus of the lens, during each portion of the exposure a definite portion of the image alone is impressed. To secure good detail in the representation of the sun's surface such a method of impressing the image is necessary, smce, however excellent may be the workman- ship of an instrument, there is always some small tremor in the movements, and consequently a risk of an imperfec- tion in the "image. There is much to be said in favour of this method of solar photography, and something to be Photo-heliograph. 343 said against it, and it seems a point which has yet to be decided as to whether this or the plan next to be described is hkely to give the most accurate results. A less unwieldy instrument which was first adopted for solar photography was one designed by De la Rue, and known as the Photo-heliograph. The accompanying figure shows the latest pattern, and is taken from one of those which was lately employed by the expeditions for observing the transit of Venus. At a is a lens of about 4 feet focus, having a cell on which is cut a very fine screw, so fine and accurate, indeed, that the lens can be caused to advance or recede from B by the yoWh part of an inch by turning the cell through a portion of a turn. About /is the principal focus of the lens, at which point are placed cross wires or a ruled grating ; the focus of which can be accurately obtained by a slow-motion screw turned by the handle, h. This moves an inner tube in which the diaphragm holding the wires is inserted. Immediately in front of / and running in a pair of grooves, is the exposing screen, in which there is an ad- justable opening or slit. At ^ is a spiral spring, which tends to keep the slit below the point where the image is formed, whilst at (? is a little pulley, over which runs a thread attached to the top of the exposing diaphragm, and termi- nating by a loop. The preliminaries to exposure are to draw the diaphragm up to e by the thread, and then to place the loop over a pin (not shown in the figure) ; this brings the slit above the place where the image is formed. The expo- sure is given by cutting the thread ; the spring, g, pulls the diaphragm towards it, and the slit traverses the image. The duration of exposure can be regulated between -o^th and Ys^th part of a second, a margin sufficiently wide to suit the sun as seen through almost any condition of the atmosphere. Below / is placed a magnifying lens, which takes the form known as ' the rapid rectilinear.' Its function is the same as that of an eye-piece in a telescope, and by altering the distance between its optical centre and the focus of the Celestial Photography, Fig. 113. Process for Celestial Photography. 345 object-glass any size of image can be produced. In the instrument under consideration the diameter of the sun's image has been fixed approximately at 4 inches, and conse- quently the adjustments of the secondary lens are made so 1 . '"''^"^ ^^n^^:^on from those dimensions. B is the holder in which the slide carrying the sensitive plate IS placed. Some of the means of adjustment have already been pointed out ; a further one is that of the secondary mag- nifier, which by a slow-motion screw can be caused to recede or advance along the axis of the telescope. It will be seen hat every means of securing a sharp image of the sun together with that of the cross-wires or ruled gratings is to be found in the instrument. The telescope is mounted equatorially, d being the polar axis, c and e the declination and right ascension circles, and f the clock movement By means of g a motion can be given to the tube in right ascension, and by a corresponding handle attached to the tube (and not shown in the figure), a motion in declination. The greatest danger to the accuracy of this instrument is distortion, through the multiplication of lenses, and the risk that exists of these not being properly centred. When attention has been paid to this, as it has been by the eminent optician who has constructed them, they leave little to be desired. It is quite possible to get photographic images of the sun with an ordinary telescope, substituting a photographic lens for the eyepiece and fixing a camera behind to receive the image. A telescope of 5 feet focal length and a stereo- scopic doublet will give a very fair image up to 4 or 5 inches in diameter. The main difficulty is in procuring a rapid exposing apparatus which, as has been said, should be at the focus of the telescope objective and should move with- out jar. The best process for solar photography cannot as yet be said to be determined. Some observers state that the wet plate answers best, whilst others say that some one of the dry processes gives the finest results. 346 Celestial Photography. It seems that in the earliest days of the discovery of photography by Daguerre impressions of the solar image were made, and it would require a somewhat long list to record the names of those who have successfully adapted the art to astronomical purposes. For the registration of the phenomena connected with the total eclipses of the sun the same difficulties as to the names of the workers would arise. The first recorded endeavour to employ photography for this work dates back to 185 1, when Berkowsky obtained a daguerreotype of the solar prominences during the total eclipse. From that date nearly every total solar eclipse, the observation of which was possible to European observers, has been studied by its aid, and has tended to the solution of some of the problems which arose concerning the solar physics. In i860 the first regularly planned attack on the problem by means of photography was made by De la Rue and Secchi, and in subsequent echpses it has been con- tinued. As regards photographing the corona the general opinion seems to be that it is better to employ an ordinary photo- graphic lens of a focal length of some 180 centimetres with the camera mounted equatorially, than to employ the ordi- nary telescopic objective. The coronal light during the eclipse is faint, and in order to get full effect it was necessary that the ratio of the aperture to the focal length should be as great as possible. Since the advent of the new sensitive processes it is now possible to use an enlarged image, such as given by the photoheliograph, though up to the present this instrument has not been employed with perfect success during echpses. In the future, however, much may be hoped for by its employment. Lunar photography has occupied the attention of various physicists from time to time, and when Daguerre's process was first enunciated, Arago proposed that the lunar surface should be studied by means of the photographically pro- duced images. In 1840, Dr. Draper succeeded in impress- De la Rue's Lunar Photographs. 347 ing a daguerreotype plate with a lunar image, by the aid of a 5-mch telescope. The earliest lunar photographs, how- ever, shown in England were due to Professor Bond of the United States. These he exhibited at the Great Exhibition of 185 1. Dancer, the optician, of Manchester, was, perhaps, the first Englishman who secured lunar images, but they were of small size. After these might be mentioned many names, but it is unnecessary to refer to any before that of Crookes, who took the next step in the matter The instrument that Crookes employed was an 8-inch refractor belonging to the Liverpool Observatory, which had a focal length of about 12! feet. The diameter of the moon was therefore about 5 centimetres. Crookes affixed a small camera to the telescope and focussed the actinic rays by trial, there being found a great deviation between their focus and that of the visual rays. The motion of the moon not being capable of being followed in the telescope by means of the ordinary equatorial arrangement driven by clock- work, the necessary accuracy was obtained by mechanically following It by means of the slow-motion screws attached to the declination and right ascension circles. The cross wires in the finder were kept on one point of the image of the lunar surface, a highly magnifying power being used in the eyepiece. Crookes found that with different telescopes the necessary exposure varied between 4 seconds and 6 minutes. In 1852, De la Rue began experimenting in lunar photography. He employed a reflector of some 10 feet focal length, and about 13 inches diameter. An abstract of a paper read before the British Association appeared in the British Journal of Photography.' In it is given a very complete account of the methods he adopted In the first part of the paper De la Rue points out that If the image of a bright star is allowed to traverse a photo- graphic plate, the result is not, as one would expect a straight line, but one which is broken up and disturbed and 348 Celestial Photography. which consists of an immense number of points crowded together in some parts, and scattered in others. These disturbances being due to our atmosphere, it follows that if the telescope be made to follow the motion of a heavenly body, an exposure other than instantaneous must, to a greater or less extent, render every point of it a confused disc, and that, therefore, a photographic image will never be so perfect as the optical image given by the same telescope until instantaneity be secured. 'Notwithstanding, however, the disadvantages under which a photographer labours, I have obtained pictures of celestial objects showing details which occupy a space less than two seconds in each dimension. I might, I think, say even one second. Now i second = of an inch on the collodion plate, a second on the lunar surface, at the moon's mean distance, being about i mile. The lunar picture m the focus of my telescope is about i inch diameter, but this varies of course with the distance of our satellite from the earth ' De la Rue then stated that he considered a magnifying arrangement attached to the telescope as impracticable to secure good pictures, owing to the increase of exposure that would be necessary, and the consequent defects due to atmospheric disturbances. He considered that the enlarge- ment ought to take place after the negative is taken. He then describes the adjustment of the motion of his telescope to the lunar motion, which he effected by altering the length of a conical pendulum or friction governor, which altered the time of its rotation (or double beat). He pro- posed to effect the same alteration by another plan, which he subsequently adopted. _ De la Rue at first obtained his lunar pictures in his 13-inch reflector, by placing the sensitised plate at the side of the tube opposite the diagonal reflector, the light being thus twice reflected. Subsequently he obtained pictures directly at the focus of the mirror, which did not give him De la Rue on Lunar Photography. 349 that increased rapidity of exposure which he had conjec- tured would result. He states: 'I am inclined to infer that Steinheil's result, 35 to the loss by reflection of the luminous ray, does not hold good as regards the actinic ray.' He next compares the advantages of the reflector oxer the refractor, the principal one being that the foci of the actinic and visual rays are coincident. ' The time occupied in takmg lunar pictures varies con- siderably. It depends on the sensitiveness of the collodion, on the altitude of the moon, and the phase. I have recently produced an instantaneous picture of the full moon, and usually get strong pictures of the full moon in from 2 to 5 seconds The moon, as a crescent, under like circumstances, would require about 20 to 30 seconds in order to obtain a picture of all the parts visible at the dark hmb.' ' Portraits of the moon equally bright optically, are by no means equally bright chemically ; hence the light and shade in the photograph do not correspond with the light and shade in the picture ; and hence the photograph frequently renders visible details which escape optically. Those portions of the moon near the dark limb are copied photographically with great difficulty, and it frequently required an exposure 5 or 6 times as long to bring out those portions illuminated by a very oblique ray, as others apparently not more bright when more favourably illu- minated.' In the practical instruction for the photography, De la Rue lays down that the silver bath must be as nearly neutral as possible, that cadmium iodide is the best iodiser to use with the collodion, and that the pyrogallic acid developer should be employed. For lunar photographs there can be no doubt that if they are required to be enlarged, iron de- velopment should not be attempted, since the deposit becomes too granular ; but the wriier is inclined to think that the 350 Celestial Photography. rapid bromide emulsion plates developed by the alkaline method will furnish pictures which are equal to those pro- duced by the wet method as described above, and certainly give a great decrease in exposure. Mr. Rutherfurd at a later date having tried an iii-inch refractor of the ordinary type, and also a 13-inch reflector, finally constructed a refracting telescope in which correc- tion was made only for the chemical rays, and with this in- strument he has produced some of the finest pictures of the moon which have ever been taken. With the great Mel- bourne reflector, however, photographs which are nearly perfection have been obtained, and there seems even yet to be a balance of opinion in favour of the reflector as against the refractor for this kind of work. Undoubtedly, where absolute coincidence of foci of all rays can be secured, all other conditions being the same, the best photographs ought to be obtained. In lunar photography an unfavour- able condition of the atmosphere is undoubtedly the greatest difficulty to be encountered. In a climate like England the air is rarely steady enough for the purpose. In countries which are more favourably situated as regards hygrometric conditions the difficulty is much reduced. In 1874-5, whilst the writer was in Egypt, Sir A. Campbell and him- self had an opportunity of taking some lunar photographs with a refracting telescope of 7-inch aperture belonging to Mr. W. Spottiswoode. On the nights that the experi- ments were made really excellent negatives were obtained, which bore enlarging to 12-inch diameter. The apparatus employed was extemporised, and therefore of a rather rude description, but quite sufficiently true to give an idea of the excellent pictures that might be taken in such a climate with the appliances usually adopted for such work. The photography of the planets has not as yet yielded anything of great value. Mr. Common and others, however, have obtained excellent pictures, though necessarily small, of Jupiter, Saturn and some others. No doubt before long Photographs of NebiilcE. 351 there will be a stride taken in this branch of photography which will render its universal adoption by astronomers a necessity. In the earlier editions of this work it was stated, ' Photo- graphing the stars is more a feat of photography than of practical utility in the present state of our knowledge, though at some future time it may be possible to map the heavens more thoroughly by its aid than has at present been done.' The prophecy which was then made is being amply fulfilled. The Brothers Henri of the Paris Observatory in France, Mr. Common and others in England, and Mr. Gill at the Cape of Good Hope have not only shown that stellar photography is something more than a feat ; that it is the most ready means of mapping the heavens, and of ascer- taining the star magnitudes in a way which was before impossible. There is a divergence of opinion as to whether a reflector or a refractor is the better instrument to employ for this purpose ; our own opinion tends to the use of the former. By means of photography stars which the eye cannot see in the telescope have been made to impress their images on a sensitive plate, since, with photography, time is a function which is as important as the brightness of a star. Mr. Isaac Roberts has used photographs of the midnight sky to discover the position of minor planets. During the exposure of a plate the telescope accurately follows the motion of the stars. The apparent motion of the minor planets differs slightly from that of the stars, hence, while the latter leave small points on the plate on development, the former leave a linear track. This observer, too, has photographed nebulae with which astronomers were un- acquainted, showing that our knowledge in this direction may be increased by the use of the sensitive plate. Regarding the photography of nebulae there is a classic series of photographs by Mr. Common of the great nebulae in Orion, a series which is not only interesting but important, 352 PhotograpJiy with the Microscope. as it will be a standard by which to compare others taken in future years. Comets have been photographed and even meteorites, so that in the scientific study of the heavens it cannot be doubted that photography will play an even more important part in the future than the eye has done in the past. CHAPTER XLL PHOTOGRAPHY WITH THE MICROSCOPE. Photography from a very early period of its existence has been utilised for securing accurate drawings in mono- chrome of what the eye can see in the microscope. This branch of the art is excessively fascinating, and can be worked in any leisure moments, either by day or night, when the enlargement is limited to say 50 diameters; but in order to secure images of greater dimensions it is always advisable to employ sun-light. The apparatus required is not very extensive. An ordinary microscope with say finch and i-inch objectives and an A eyepiece is all that is necessary as far as the instrument itself is con- cerned. If the objects to be photographed are mounted on a slide, and not merely placed in a cell for examination, any ordinary camera may be attached to the microscope, as the tube can then be brought into a horizontal position. It has often been recommended to employ a camera as much as 6 feet in length, in order to secure great increase in the size of the object, but in the writer's experience it is unwise to go beyond 18 inches, a length just sufficient to enable the operator to grasp the slow-motion focussing screw, whilst his eye can be directed to the focussing-screen. When the longer camera is employed, the operation of focussing has to Powers to be Used. be conducted by an assistant, and, however intelligent the latter may be, it will always be found that greater accuracy will be obtained by the operator's own hand, for it must be recollected that the difference of Wtt^ of an inch m length of focus may determine whether the definition is good or bad. The camera and the microscope should not be attached rigidly to one another. It is far better that each should be free to move independently, though care should be taken when an accurate focus is obtained, that each shall occupy a perfectly unalterable position during exposure. Perhaps the rnost simple way of attaining this object is to substitute for the ordinary photographic lens used with the camera a short brass tube, which screws into a flange. A piece of velvet should then be formed into a cylindrical bag, open at both ends, and a little longer than the brass tube above referred to If each opemng of the bag is provided with an elastic band* a perfectly light-tight junction between the tube and the body of the microscope may be made. Some operators prefer to use the eyepiece as a magni- fier ; It seems better, however, simply to employ the objec- tive. If the objective only be used, it is wise .to unscrew the tube of the microscope, in order to secure a larger field which otherwise the diameter of the tube would limit li must not be inferred that the use of the eyepiece as a magnifier will cause indifferent pictures in every case In the instrument used by the writer the definition given bv it was certainly bad. ^ The student is recommended to commence with a comparatively low-power objective. The i-inch will be suitable in every way; and whenever he has obtained mastery over the manipulations with it, he may venture on the ^ or l-mch. A higher power than these can seldom be recommended; probably the^ inch is the highest power which can be worked with ease. The tube of the microscope should be placed in an accurately horizontal position as A A 354 Photography zvith the Microscope. should also the camera ; and care should be taken that the axis of the tube fixed in the latter should be in the exact continuation of the axis of the lens. This can only be effected by very careful arrangements. As a rule it will be found that when the body of the microscope is in a horizontal position the friction on the axis on which it turns is sufficient to cause it to remain in the position in which it is placed ; if not, obvious precautions must be taken to prevent any movement between the time of focussing and exposing the plate. Supposing that sunlight is to be em- ployed for the purpose of illuminating the object, the next operation is to throw the image of the sun by a con- denser on the object, in such a manner that the axis of the condenser and that of the objective may be m a line with one another. This may readily be ascertained by noticing the illumination when no object intervenes between the rays emerging from the condenser. It is advisable, first of all, however, to place the heliostat (the one described at p. 268 answers the purpose) in position. This can be done with sufficient accuracy by rough obser- vation with the eye, and noting that the centre of the mirror is about the same height, and in the same horizontal line as the tube of the microscope. The condenser is then brought into the reflected rays, and an image of the sun brought to a focus on the object. In some cases the heat rays have to be cut off, otherwise injury to it ensues. A alass cell with parallel sides containing well-boiled distilled water is found to subdue the heat sufficiently when placed in the path of the beam. The focussing is now proceeded with, and is best performed by removing the ordinary ground glass, and substituting for it a plate of ordinary patent piate, viewing the image by a focussing glass, as described in photo-spectroscopy, page 266. The portion of the object to be photographed should be brought into the centre of the field, and when nearly in position the sUde should be clipped on to the stage by a couple of wire Monochromatic Light. jre springs, and the adjustment effected in tl,e usual manner The absolute focussing should next be taken in hand A rough approximation is first obtained by the rack and pmton motmn, and the final focus obtained by The s ow nrotton screw, which is attached to every good t^icroscope When wewmg the image through the focussing glas t wm be found that m no position is the object quftf free from colour. In one focus it will appear shiply defined thou " surrounded by a red band, whilst the difinition wHl appe^ aTlu Zo Th" ^■'^ -™»ded by km in tf ■ K ''"^ '° ^ of ^chromat .sm tn the objective, and the >^;,., position should be re iraltlr *? ""^^^ ^ ™- 4 are as a rule the most active in causing the photoeranhic image to be formed, it is evident, if the^tter ^ocu" Xh i^irxe^rf ^''^'^^^■^^^^--''■--■'"■"^ Monochromatic Li^ht.-I^,^ fact that coloured fringes are sure o border the image shows a. once that the objectke s not properly corrected, and there would evidently be an advantage were it possible to work with monochromadc iLt Ihis can be accomphshed in the following manner. I„fi, 114 RR are rays coming from a heliostat or other source of hgh. and an image of the source (or in the case of an art fida ight, an miage of the condenser) is formed on the si t s of he CO limator c by the lens The parallel rays produced by the lens l, pass through the two prisms p, and pf a„d are focussed by a lens l, on to the screen b, in Uich i 'a slit T formmg a spectrum on d. The rays which pass through s' t f s"ec™d"cr 7 '"^ ^""^ ™ -^-geof the sufL^ 01 the second prism focussed on the object on the stase of the rn.croscope. This gives monochromatic light, and accord ing to the kmd of plate used for photographing the ob,e« Ich t Vlrre'-'l ? TL" Vh~ '° mches foca, length, andt. c^s-iLri^holter^ "^te"^ 3 56 Photography with tlie Microscope- no need to have these lenses achromatic ; ordinary spectacle lenses may be made to answer. Fig. 114. ^' 'Ah' The photoi^raphic delineation of opaque objects is much found to increase rapidly if any endeavour be made to use a hisherpowerthanafirrch. The same arrangement a hat Indicated may be employed, causing ho»ever ,0 fall on the object. This gives ^'T' ' tv if f«u S and with great magnifyitrg power the ^''^ ° /Xukv is excessive. When once a focus is obtamed all difficulty vanlh and by the use of dry plates any amount of expo- lu ma; be given without any deterioration o the nnage. The annexed figure shows an arrangement by wh ch a mic~m;beLp.oyedinitsordinaryvertical^^^^^^^ The instrument was in the Loan Collect,°n f Sc.^^^^^^^ Instruments at South Kensington, and is of German inake. Thttom of the camera might advantageously be altered to a : Artificial Light for Illiimination. 357 bellows shape, a is the microscope : b a right-angled prism reflecting the rays of light which pass through the objective into c, a brass tube fixed to the camera d. The length of the camera is supported by two iron rails g. It will be noticed that the same method of illumination can be applied in this pofskion of the microscope as when it is used hori- zontally, for the rays of light can be reflected by the mirror f. Fig 115, Hitherto we have only supposed the sttident to be work- ing with sunlight, but as may already have been inferred, artificial light may be employed. The great desideratum with the latter is that it should be steady, and should proceed as nearly as possible from a point. The light from a magnesium lamp has been recommended, but in the writer's experience it is most unsatisfactory. The electric light is good, but is somewhat diflicult to manage, though, owing to the intensity of the illumination, the time necessary to keep the points in proper adjustment is not very great. The lime-light, perhaps, possesses the greatest advantage over all lights, since it is so perfectly steady. A gas flame, a paraffin lamp or even a candle may be used where 358 Photography with the Microscope. very great enlargement is not required, now that plates are obtained which are so sensitive as those prepared by the gelatine process ; and no doubt the majority of micro- scopic objects are at present photographed with one of these sources of illumination. . The use of photography with the microscope has not as yet been fully developed, but there can be no doubt that as more workers enter into this field the greater will be the advances made. APPENDIX, LIST OF ELEMENTS. Symbols. . Al . . Sh . . As . . Ba . . Bi . . B . . Br . . Cd . . Cs . . Ca . . C . . Ce . . CI . . Cr . . Co . . Cu . . D . . E . . F . . Gl . . Au . . H . . In . . I . . Ir . . Fe . . La . . Pb . . Li . • Mg. . Mn. - Hy. . Mo . . N i . . Nb . . N . . Os . . O . . Pd . 36o Appendix. Names. Phosphorus Platinum Potassium Rhodium Rubidium Ruthenium Selenium Silicon . Silver . Sodium Strontium Sulphur Tantalum Tellurium Thallium Thorium Tin Titanium Tungsten Uranium Vanadium Yttrium Zinc Zirconium List of Elements — coittinucd. Symbols. P . Pt . K . Rh . Rb . Ku . Se . Si . Ag . . Na . S . Ta . Te . Tl . Th . Sri . Ti . W . U . V . Y . Zn . Zr . Atomic Weight. 30- 96 196-7 39 '04 104-1 85-2 103-5 78 28 107-66 22-99 87-2 31- 98 182 128 203-6 231-5 117-8 48 184 240 51-2 93 64-9 90-0 COMPARISON OF THE METRICAL WITH THE ; COMMON MEASURES. From Dr. Warren de la Rue's Tables. MEASURES OF LfiNGTH. In English Inches In English Feet = 12 Inches Millimetre . Centimetre . ... Decimetre .... Metre . . . • Decametre . Hectometre Kilometre . . Myriometre 0-03937 0-39371 3-93708 39-37079 39370790 3937 -07900 39370-79000 393707-90000 0-0032809 0-0328090 0-3280899 3-2808992 32-8089920 328-0899200 3280-8992000 32808 9920000 I Inch = 2-539954 Centimetres. i Yard = 0-91438348 Metre. I Foot = 3-0479449 Decimetres. i Mjle = 1-6093149 Kilometre. Appendix. 35 j MEASURES OF SURFACE. In English Square Feet In English Sq. Yards = 9 Square Fee Centiare, or sq. metre . Are, or loo sq. metres Hectare, or 10,000 sq. metres 10-7642993 1076-4299342 107642-9934183 I -1960333 119-6033260 1 1960-3326020 I Square Inch =6-4513669 Square Centimetres. I Square Foot =9-2899683 Square Decimetres. I Square Yard =0-83609715 Square Metre, or Centiare. I Acre =0-404671021 Hectare. MEASURES OF CAPACITY. In Cubic Inciies In Gallons = 8 Pints = 277-27384 Cubic Inches Milliiitre, or cubic centimetre Ceniilitre, or 10 cubic cents. Decilitre, or 100 cubic cents. Litre, or cubic decimetre Decalitre, or centistere Hectolitre, or decistere Kilolitre, or stere, or cubic metre .... Myriolitre, or decastere 0-061027 0 610271 6-102705 61 -027052 610-270515 6102-705152 61027-051519 610270-515194 0 -00022010 0-00220097 0-02200967 0-22009668 2-20096677 22-00966767 220-09667675 2200-96676750 I Cubic Inch =16-3861759 Cubic Centimetres I Cubic Foot =28-3153119 Cubic Decimetres. I Gallon =4-543457969 Litres. MEASURES OF WEIGHT. In English Grains In Troy Ounces = 480 grains Milligramme Centigramme Decigramme Gramme .... Decagramme Hectogramme Kilogramme Myriogramme 0-015432 0- 154323 1- 543235 15-432349 154-323488 1543-234880 15432-348800 154323-488000 0-000032 0-000322 0-003215 0-032151 0-321507 3-215073 32-150727 321-507267 I grain = 0-064798950 Gramme. iTroyoz. = 31 -103496 Grammes. I lb. Avd. = 0-45359265 Kilogramme. I cwt. = 50-80237689 Kilogrammes. INDEX. ABE ABERRATION, spherical, 209 Absorbents, use of, for chlorine, &c., 22, 25 Absorption of light and consequent work, 10 — spectra of dyes and glasses, 323 Acetic acid in the developer, 67-69 — necessity of, in the calotype sensi- tising solution, 141 Achromatism, 207 Acid, hypochlorous, formation of, by action of light on silver chloride, 22 — hypobromous, formation of, by action of light op silver bromide, 26 — pyrogallic, employment of, in the alka- line dev loper, 02 — gallic, employment of, in the alkaline developer, 95 Actinometers, Bunsen and Roscoe's, :!26 328 — Draper's, 326, 327 Actinometry, 326 Action of solvents employed in collodion, 45 Albert's process, 203 Albumen, use of, for paper positives, 145 — beer process, no plates, defects in, 112 — substratum, 100 Albuminising paper, 154 Alcohol in the developer, 69 Alkali, effect of, on glass, 57 , — on grease, 57 Alkaline developers, strengths of. 97 • formulae for, 108 — development, 91-100 • comparison of the, with the acid method, 98 — ,~ effect of, in shortening exposure, 99 Alkalinity, effect of, on collodion, 52 Anihne process, 186 Antimoniure'ted hydrogen and sulphur, action of light on, 34 Aperture of a lens, 221 — doublet lens, method of finding the correct, 224 CHT. Apparatus, 225 Archer's employment of collodion, 5 Ascertaining the adjustments of the dark slide, 232 Astigmatism, 212 Autotype process, 179 BACKING for positives on glass, 91 — to prevent blurring, 88 Bath, sensitising, for negatives, 58 dry plates, 102 positives, 89 Bath-holder, travelling, 79 Beechey's process, 116 Bitumen of Judasa, action of light on, 2 3, tilurring, or irradiation, 86 CALCIUM chloride in the toning bath, 148 Calotype process,, 137 brush for use in the, 139 paper to be used in the, 138 sensitising the paper in the, i^o Camera, 225 I — carte-de-visite, 227 — exposing sensitive paper in the, 231 — focussing the picture in the, 255 — for the microscope, 352 — front, 225, 226 — Meagher's, 265 — pantascopic, 228 revolution of the, round the optical centre of the lens, 228 — reversing back, 227 — spectroscopic, 265 Carbon print, development of the, 183, 184 — process, single transfer, 185 — tissue, manufacture of, 179 sensitising, i8j Celestial photograph}', 339 Changing box (Hare's) for dry process, 230 Charles's claims to Wedgwood process, 2 Chloride in an emulsion, 114 Chlorine and hydrogen, action of light on a mixture of, 33 364 Index. CHR ENE Chromic acid and alcohol, 32 Chromium salts, action of light on, 31 printing with, 174 Cleaning glass plates, 55, 77 _ Coating the plate with collodion, 78 CoUodio-chloride process, 162 emulsion, washed, 163 fuming, with ammonia, 164 fixing, 164 Collodion, 42 ^ action of solvents employed, in, 49 — and india-rubber as a support, 35 — coating the plate with, 78 — considered as a vehicle for sensitive salts, 37 — discussion as to iodisers in, 51 — effect of alkalis in, 52 — for albumen beer prDcess, 110 — for dry plates, lot — for positives, loi — formula for plain, 50 — formulae for Ijromised. 119 — formulae for negative iodised, 53, 54 — limpid and viscous, 55 — reticulation in, 55 — testing plain, 54 . , . , Colloidal bodies, addition of, to develop- ers, 67 Colour and density of the deposit, 82 _ Copper bromide intensifying solution, theory of the, 70 — sulphate in developers, 67 Corona, photographing the, 346 Cyanide, potassium, as a fixing agent, 74 DAGUERRE'S discoveries, 3 Daguerrean image, development of the, 40 intensifying the, 41 fixing, 41 Daguerreotype, 38 — etching by Grove's method, 42 — plate, manipulations in the preparation of the, 39 , • r 1 1 Daguerreotypes, reproduction ot, by elec- trotypy, 41 _ Dallmeyer, his portrait lens, 216 Dark-room, 239 Dark-tent, 236 Defects in gum gallic plates, 109 silver prints, 162 Defects in negatives : Black specks, 85 Blurring, 86 Dark lines, 137 ' Fog,; 85, 136 ' Frilling,' 136 Markings like watered silk, 86 Pin-holes, 85, 137 Scum on the film, 85 Spots, 85, 137 . Transparent markings, 86 Want of sharpness in the image, 86 Defects in negatives : Weak images, 85 Yellow or brown stain, 137 De la Rue's lunar photographs, 348 Density and colour oi a deposit, 82 Deposition of siiver by the developer, 64 Detergents, formula for, 57 Developer, explanation of the term, 20 — ferrous oxalate, 109, 339 Developers absorb oxygen, 64 — alkaline, formulae for, 108 strength of, 97 — copper sulphate in, 67 — ferrous salts in, 99 — formulae for dry plate, 109, iii — for negative pictures, formulae for, 67 — for positive pictures, formulae for, 90 — nitric acid in, 96 . — restraining action of soluble bromide in, — ^t^heory of strong and weak, 65 ■ — viscous, 66 Development, alkaline, 91 -100 agents employed in, 99 effect of, in shortening exposure, 99 — methods of, explained, 18 — of albumen beer process, 11 1 — of dry plates, 91-100 — of gelatino-bromide plates, 131 — of gum gallic plates, 109 — of iron prints, 167 — of the calotype picture, 140 — of the carbon print, 183 — of the photographic image, 63 — of wet plates, 80 — strength of solutions for dxy plates, 07 Diamond's, Dr. Hugh, connection with the collodion process, 5 Diaphragms, uses of, 209, 219, 220 Dichromates and organic matter, 31 Diffraction gratings, photography with, 276 — spectrum, 276 Dippers, 79 Disc of confusion, 272 Dispersion of light by prisms, 287 Distortion caused by single lenses 211 Draining rack, 84 — box, 84 Dry-plate or alkaline development, gi-iOD — processes with the bath, 100-112 Drying cupboard, 129 Drying the plate, 104 — washed emulsion, 121 plates, 123 Dyed plates, 320 EDGING, or substratum for dry plates, I TOO Emulsion, experiments with bromide, 113 — forming an, 116, 125, 126 — gelatino-citro-chloride, 164 — processes, 11 3-128, 165, 289 Emulsions, washed, 118 Index. ENE Energy, transference of, lo — expended, not necessarily shown by number of atoms decomposed, 12 Enlarged photographs, comparison of, with originals, 223 Experiments as to the cause of reversal of the image, 305 — exemplifying the danger of using dirty glass plates, 56 — illustrating the desirability of washing an emulsion, 115 — illustrating the action of alkaline develop- ment, 114 — on organic salts of silver, 27 — showing the radiations causing reversal of the image, 312 — with chromium salts, 174 light, 6 sensitised paper, 143, 144 silver chloride, 27 Explosives compared with photographi- cally sensitive compounds, 13-17 Exposure and development of gelatino- bromide plates, 131 — rule for, 258 FADING of silver prints, 153 Ferric citrate, 166 — oxalate, 166 action of light on, 327 Ferric salts, action of light on, 29 experiments with, 29 Ferrous sulphate as a developer, 64 " — instability of, 68 Figures in a landscape, introduction of, 250 Film, disintegration of a collodion, by var- nish, 75 Filter paper, detection of iron in, 106 Fixing coUodio-chloride prints, 164 — silver prints, 153, 159 — solutions, 75, 164 for calotype pictures, 141 — the image, 74 theory of, 74 Flare spots caused by lenses, 219 ^ how to obviate, 219, 220 Foci, conjugate, of lenses, 213 Focus, artistic, of a picture, 240 — of a lens, 212 7 to find the equivalent, 213 Focussing the picture, 255 Fog on a negative, and its causes, 85 Frames, printing, 156 GELATINE as size In paper, 146 — negatives, defects in, 136 — substratum, 100 Gelatino-bromide plates, exposure and de- velopment of, 131 — process, 123-137 applying the developer, 133 formulae for developers, 132, 134 INT ■ Gelatino-bromide process, manipulations in, 125-130 methods of preparing plates, 123 most suitable gelatine for, 124 outline of, 124 Gelatino-citro-chloride emulsion, 164 Glass, coloured, quality and amount of light passing through, 322 — yellow, effect of placing in front of lens, 320 — substitute for ground, 231 Gray (Le), his waxed paper process, 142 Greasy matter, effect of alkalis on, 57 Groups, posing of, 251 Grove (Sir W.), his method of etching daguerreotype plates, 42 Gum, gallic process, 104 plates, development of, 109 — solutions, filtration of, 105 HARE, his changing box for drj' plates, 230 Heat for varnishing, sources of, 84 Heliostat, 261, 268, 355 Herschel (Sir John), his first photograph on glass, 5 — his experiments in photospectroscopy, 260 Historical sketch of the discovery and pro- gress of photography, i Hyposulphite sodium as a fixing agent, 74 danger of, in fixing prints, 151 and iodine, 152 testing for, 161 IMAGEj Daguerrean, destruction of, 303 — fixing the, 74 — \ explanation of the term, 20 — intensifying the, an explanation of term, 20 — methods of giving intensity to an, 69 — reversal of, 305-315 agents causing the, 305 radiations producing the, 310 — the visible and invisible (latent), 18 — visible, destruction of, by the action of light, 305 India-rubber substratum or edging, 100 Ink for photographic transfers, 191 Instability, molecular, of sensitive com- pounds, II Instantaneous pictures, development of, 134, 13s — shutters, 234 Intensifier, ammonium sulphide, 73 — copper bromide, 73 — ferrous sulphate, 71 — mercuric chloride, 72, 135 — potassium permanganate, 72 — pyrogallic acid, 71 — with Schlippe's salt, 73 Intensifying an image, explanation of term, 20 366 Index. INT Intensifying the image, 69-81 — solutions, formulae for, 71-73 — stains due to, 83 Intensity given by varnish, 76 Iodine and sodium hyposulphite, 152 Jodisers to be used in collodion, discussion as to the, 51 Iodising solutions for the calotype process, 138 Iron, printing with salts of, 166 Irradiation, 87 JOHNSON, his improvements in the carbon process, 177 LATENT image, explanation of the term, 18 Le Gray, his waxed paper process, 142 Lens, aperture of a, 221 — Dallmeyer's portrait, 216 — intensity of a, 221 — Petzval's portrait, 216 — rapid rectilinear, 217 — triplet, 218 — wide angle rectilinear, 218 Lenses, forms of, 211, 212 landscape, 217, 234 — on the choice of, 234 Light, action of, on various compounds, 21 antimoniuretted hydrogen and sulphur, 34 chlorine and hydrogen, 33 chromium salts, 31 ferric oxalate, 327 '■ ferric salts, 29 organic bodies, 33 salts of silver, 27 • — silver bromide and sub-bromide, 26, 93, 94, 284 ■ chloride, 21, 287 • — iodide, 23, 277 uranic salts, 30 vanadic salts, 30 — apparent destruction of the action of, on the photographic image, 302-315 — artificial, 357 . , , — kind and proportion of, passing through glass of various colours, 322 — passing through coloured papers, 324 — reflection of, from polished metal sur- faces, 275 — suitable for the dark-room, 322-326 Lithoaraphic press, 192 Lunar photography, 347 MAGNIFYING lenses, use of, for focussing, 26O Manipulations in gelatino-bromide process, 125-130 — . in the preparation of photographic transfers, 190-195 PHO Manipulations in wet plate photography, 77 Mercuric chloride intensifier, 70, 72, 135 theory of the, 70 Metals and alloys, light reflected from polished surfaces of, 275 Methods of development explained, 18 in alcohol, 75 Metrical measures, equivalents of, in common measures, 360 Microscope, camera for the, 352 — photography with the, 352 Microscopic examination of a developed image, 66 Monochromatic light for the microscope, 355 Moon, photographs of the, 347 N' EBUL/E, photographs of, 351 Negative, intensifying the, 69, 81 Negatives, over-exposed and under-ex- posed, 81 — defects in, 85 Niepce (Nicephore de), his process, 2 Nitric acid, action of anhydrous, on organic matter, 43 ON the picture, 239 Optical centre of a lens, 212 Orthochromatic photography, 315-322 — plates, 320 Oscillation, effect of well-timed application of force on an, 17 — upon another oscillation, effect of one, 14 Oxygen absorbers as developers, 64 PAPER, albumlnising, 154 — as a support and substratum, 35 — for callotype process, 138, 142 — for photo-lithographic transfers, 190 — negatives, 137 — plain salted, 155 — resinised, 155 — sensitive, exposing in the camera, 231 Papers, coloured, amount and quality of light {jassing through, 324 Papyrotint process, 196 Papyrotype process, 195 Photo-collotype processes, 201 process by Albert, 203 Photo-engraving in half-tone, 199 and relief processes, 196 Photogenic drawings, 4 Photo-heliograph, 343, 346 Photo-lithographic transfers, 190 Photo-lithography in half-tone, 195 Photo-spectroscopic arrangements, 261 Photo-spectroscopy, 259 — absorption of spectrum rays by different transparent solids, 274 — camera for, 265 — diffraction spectrum, 276 Index. 367 PHO Photo-spectroscopy, gratings for, 276 — - Herschel's experiments in, 260, 261 — materials for prisms, and lenses 274 Picture, printing the, 156 — on the, 239 Planets, photography of the, 351 Plate, coating the, with collodion, 78 — drying the, 104, 129 — glass, cleaning the, 57, 77 Plate-cleaning solution, formula for, 57 Plates, drying cupboard for, 129 — dyed, 320 — formula for making sensitive to yellow and green, 317 — for platinotype process, 171, 174 — method of stowing, 130 — orthochromatic, 320 Platinotype process, 171 developing the print, 173 method of sizing the paper, 171 paper for, 174 — -- sensitising solutions, 172 Points, the nearest, of a landscape in focus, 223 Poitevin, his improvements on Ponton's process, 6 process, 169 Ponton (Mungo), his dichromate process, 5 Positive and negative, example of a, pic- ture, 4 — pictures and their support, 35 by the wet process, 89 — production of, from a positive, 168 Potassium cyanide as a fixing agent, 74 and nitric acid, dangerous effects of, 57 Powder process, 187 Preservative, definition of, 103 — applying the, 103 Printing, manipulations in silver, 153 — frames, 156 — sensitising baths for silver, 155 — the picture, 156 Prints, silver, fixing, 159 — washing silver, 160 Prismatic dispersion of light, 206-208 Prisms, method of placing, at the angle of minimum deviation, 263 J'ure water for washing dry plates, the necessity of, 102 Pyroxiline, manufacture of, 45 — effects of high temperatures in the manufacture of, 45 — effect of diluted acids in the manufac- ture of, 44 — formula for preparation of, 45 — Warnerke, his formula for preparation of, 48 RACK for dryine plates, 129 Reade, his discovery of the use of gallic acid, 5 Red end of the spectrum, photographs of the, 267 SOD Reflectors, coincidence of visual and chemi- cal rays, 350 Refraction, laws of, 205 Registration of tints, 331, 332 \ apparatus for, 332 Relief blocks by photography, 199 Resinised paper, 155 Reticulation in collodion, 55 Ritter, his researches, i Room, the dark, 239 Roscoe, his actinometer, 326 Rouch, his dark tent, 236 Rules to be observed in choosing a land scape subject, 254 Rutherford, his lunar photographs, 350 SALMON and Garnier's process, 168 Scheele, his researches, i Sensitised paper, preservation of, 156 Sensitising bath, 58-62, 89, 102, 155, 169, 172, 181 for dry plates, 102 formula; for, 61 keeping in order the, 62 over-iodised, 62 — the paper for the calotype process, 139 Sensitive compounds, theory of, 11 Sensitiveness of salts of silver prepared from double decomposit on of various metallic salts, 152 Sensitometer, Spurge's, 353 — Warnerke's, 131 Siderostat, 269, 341 Silver acetate in the sensitising bath, 62 bromide, action of light on, 26, 93 sensitiveness of to spectrum, 284, 292-302 " chloride, action of light on, 21 experiments with, 27 — — sensitiveness of, to spectrum, 287, 299-302 Silver haloids, influence of spectrum on, 277-302 — iodide, action of light on 23 dissolved by silver nitrate, 60 sensitiveness of, when exposed to the spectrum, 277, 292-302 — nitrate, impracticability of using, with gelatine vehicle, 99 purity of, 60 silver iodide dissolved by, 60 — organic salts of, action of light on, 27 experiments with, 27 — printing, 143 _ manipulations in, 153 sensitising baths for, 155 — salts of, effect of certain dyes on, 315, Slide, adjustment of the dark, 232 — dry plate (double back), 229 Sodium acetate in the toning bath, 150 — hyposulphite and iodine, 152 as a fixing agent, 74 Index. SOD Sodium hyposulphite, testing for, i6i — tetrathionate, its formation, 152 Solar photography, 339 Solvents, action of the, employed for collo- dion, 49 Spectra, absorption, of dyes and glasses, 323 — photographic, 277-302 Spectrum, influence .of, on the haloid salts of silver, 277-302 — focussing the, 265, 266 — luminosity of, 31b — photography, experiments showing the radiations causing reversal of the image, 310 — rays, absorption of, by various trans- parent solids, 274 — sensitiveness of different parts of, to various dyes, 317 — simple means of forming a, 8 Spurge's sensitometer, 353 Stains due to intensifying, 83 Starch as size for paper, 146 Stellar photography, 351 Stripping films, 142 Substitute for ground glass, 231 Substratum or edging for dry plates, 100 Sulphite of soda solution, fixing silver prints with, 153 Sulphur and antimoniuretted hydrogen, action of light on, 34 Sulphuric acid, action of, on organic matter, 42 Support and substratum, 35 Swan, his double transfer carbon process, 175 Swing back, uses of the, 210, 257 the natural position of, 258 TABLE of elements and their combining weights, 359 Table of metrical and common weights and measures compared, 360 Talbot, his discoveries, 5, 199 Tent, dark, Rouch's, 236 Testing for sodium hyposulphite, 161 WOO ' Testing plain collodion, 54 Toning baths, 153, 165 Toning prints, theory of, 147 — silver prints, 158 Transfer, double, of carbon prints, 1623, 163 Transfers, photo-lithographic, i<,o URANIC salts, action of light on, 30 printing with, 165 V'ANADIC salts, action o light oni, 30 Varnish, formula for, 76 — intensity given by, 76 Varnishing the film, 75, 84 Vegara's negative tissue, 142 j Vehicle, collodion as a, 38 Vehicles for holding sensitive salts in siln, 36 '■ - — defects of gelatine and albumen as,, loi Vertical position of the swing-back, 257 Viscidity in developers, 66 WARNERKE'S negative tissue, 142 — roller slide, 231 — sensitometer, 131 Washing an emulsion, 121 -127 — dry plates, 102 — trough, 160 Water, distilling, 58 — • impurities in, 59 ■ — wash for dry plates 102 Waves of light and heat, 8 _ — co-existence of shorter with longer, 16 Waxed paper, Le Gray's process, 142 Waxing composition for zinc plates used in the autotype process, 183 — paper negatives, 141 Wedgwood, his process, 2 Wet plate photography, manipulation in, 77. , Willis's aniline process, 186 — platinotype process, 171 Woodburytype, 188 PRINTED BY SPOTTISWOODE AND CO., NEW-STREET SQUARE LONDON