Photography, the art of producing pictures by means of the action of light on sensitised surfaces. It is usual to regard the observation by the alchemists of the 16th century that Luna Cornea or Horn Silver (native chloride of silver) is blackened on exposure to light, as the first chemical step in the history of photography, while the foundation of photographic optics was laid by Della Porta in the invention of the camera obscura (1569) at a somewhat earlier period. This property of chloride of silver, and also the darkening of nitrate of silver by light in the presence of organic matter, constitute the leading facts on which the science of photography is based. In 1777 the famous Swedish chemist Scheele found, by experiment, that Horn Silver was blackened quickest at the violet end of the solar spectrum, thus proving that the rays of light are not all alike chemically active. A quarter of a century later Ritter of Jena demonstrated the existence of chemically active non-visible rays beyond the violet rays of the spectrum.
The honour of having been the first to produce pictures by the action of light on a sensitive surface is now very generally conceded to Thomas Wedgwood, an account of whose researches was published in 1802 in the Journal of the Royal Institution, under the title, 'An Account of a Method of copying Paintings upon Glass, and of making Profiles by the agency of Light upon Nitrate of Silver; with Observations by H. Davy.' The misfortune was that no attempts made either by Wedgwood or Davy to prevent the uncoloured portions from being acted on by light (or, as we now say, to fix the picture) were successful.
Nicéphore Niepce of Châlon-sur-Saône was the first to enjoy the satisfaction of producing permanent pictures by the influence of solar radiations. This was accomplished in 1814, and the name chosen to designate his process was Heliography. It consisted in coating a piece of plated silver or glass with asphaltum (bitumen). The plate so prepared was then exposed in the camera obscura for a length of time, varying from four to six hours. Wherever the light acted it rendered the asphaltum insoluble in its usual solvents. Hence, on subsequent treatment with one of these solvents, the shadows of the image dissolved away, and the lights were represented by the insoluble asphaltum remaining on the plate.
Daguerre appears to have begun in 1824 the experiments which eventually led to the discovery of the daguerreotype process. On Daguerre learning that Niepce was working in the same direction as himself, the two formed a partnership in 1829. The discovery of the Daguerreotype (q.v.) was announced in January 1839, but the details of the process were not made public till August of the same year. It consists in exposing a metal plate covered with iodide of silver for a suitable time in a photographic camera, the plate being afterwards transferred to a dark room, and exposed to the vapour of mercury, which develops the latent image, it being afterwards fixed. Although this process has become almost obsolete, it was really the first which was of any practical value, and experts all agree that no other known process renders some subjects—e.g. the human face—with such fidelity and beauty.
W. H. Fox Talbot read a paper to the Royal Society on 'Photogenic Drawings' on 31st January 1839, just six months previous to the publication of Daguerre's process. He produced these in this way: Writing-paper was steeped in a solution of common salt (chloride of sodium), and dried. It was then dipped in a solution of nitrate of silver, which is changed by the action of the common salt into the chloride of silver, some of the nitrate, however, remaining unaltered. Paper so treated is sensitive to light, so that when a fern-leaf, for example, is placed close down upon it between two plates of glass, and daylight is allowed to act on the prepared surface, the paper blackens except where it is covered, and thus a reversed picture of the leaf on a black ground is obtained. This was then placed over another sheet of paper, prepared in the same way, and the light allowed to act through it. Another picture of the leaf was thus produced, but this time with its lights and shades the same as in nature. The white image on a dark ground was called by Fox Talbot a negative, and the print from it he called a positive. These terms are still current in photography; the negative image being produced in the camera, as the first and leading operation—any number of positives being obtainable from it on paper, glass, or any other material capable of forming a support for a photographic image. The Calotype process was patented by Fox Talbot in 1841. In this process Talbot produced his negative by preparing paper on the surface with iodide of silver, subsequently washing it over with a mixture of nitrate of silver, with gallic and acetic acids, and then exposing it in the camera to the object he wished to copy. The invisible image or picture thus obtained was developed by aceto-nitrate of silver and gallic acid. The paper negative was then rendered translucent with wax, and used for the production of many positives in the way described above. The introduction of collodion by Archer marked the next great step in photographic progress, and this, known as the wet-plate process, has been almost eclipsed by the gelatine dry plate now almost universally used.
Photographie Apparatus.—The most important piece of apparatus used by the photographer is a form of the Camera Obscura (q.v.), generally called simply a camera, with its attached lens that throws the image on a ground glass screen placed at the back, to enable it to be sharply focused. A thin flat box with a shutter, together called a dark slide or 'back,' contains the sensitised plate. When the picture is focused the screen is withdrawn and the 'back' inserted in its place; the shutter is then drawn out, and the sensitised plate, which exactly occupies the place of the glass screen, being now exposed, receives the picture. In a brief time, which nowadays varies from a fraction of one to several seconds in a good light, the shutter is closed, and the slide returned to a room illuminated by a light not chemically active, generally red, orange or yellow green, where the plate is taken out and developed.
The introduction of dry plates for photography, which may be used in the camera a long time after their preparation, has had a great influence in modifying apparatus; and more especially is this true of the photographic camera. Under the older system (wet-plate photography) the plate had to be used immediately after it had been coated and furnished with its sensitive film, or it became useless. One dark slide or back, to hold the plate during exposure was therefore all that was necessary, for only one plate could be prepared and used at a time, a dark room or tent being necessary for the operations. But now that the plates will keep almost indefinitely between preparation and use, any convenient number can be made ready for insertion in the camera, to be exposed to the action of the light one after another. For this purpose what are called double dark backs are employed, each holding two plates—one on each side, and each side being furnished with a light-tight shutter which is drawn out so as to allow the lens to cast the image on the plate inside as soon as the back is fixed on the camera.
Much ingenuity has been applied to camera construction of late years, but, although many new modes of carrying the plates and bringing them under the influence of the lens have been devised, the double-back system, as just described, is the one most generally adopted when glass plates are employed. Various changing-boxes have been devised, which contain a dozen or more plates, and dispense with all but one dark slide, that is constructed to receive and discharge any plate of the series at will. Hare's changing-box is the one most generally known. This has a special form of dark back, which can be charged with one plate from the box at a time, and is then inserted in the camera for exposure. Recently, for small-sized photographs, a device has been largely adopted whereby the camera itself becomes a storage for the plates, a simple mechanical arrangement permitting the exposed plate to fall to the bottom while another takes its place.
But the most recent change in photographic apparatus is due to the introduction, or rather the revival, of sensitive films supported, not on glass, but on a flexible material. We have already seen that Fox Talbot employed paper for his negatives; and, although paper was superseded by glass when the collodion process came into existence, photographers were quick to recognise that such a brittle material had serious disadvantages. Many experimenters endeavoured to produce or find some material which, while possessing the transparency of glass, should be of a less brittle nature, and among these Mr Woodbury in 1876 produced such a compound from collodion, castor-oil, and Canada balsam. This mixture, after being allowed to dry on a sheet of glass, was coated with an emulsion sensitive to light, and after again being dried was stripped from its support and cut into suitable sizes for the camera. It is noteworthy also that the inventor at the same time proposed that such films, being perfectly flexible, might be rolled and unrolled (panorama fashion), so that successive lengths might be submitted to the luminous image, and thus the whole business of changing plates be accomplished by the turn of a handle outside the apparatus. The same idea was taken up by Warnerke a few years later, and his patented roller-slide became obtainable commercially. Warnerke also made a sensitive dry collodion film for use in his apparatus, but its cost—which was at the rate of a penny per square inch—limited its use to a few.
To Messrs Morgan & Kidd of Richmond belong the honour of having first applied a gelatine emulsion to paper. This paper is now made by many dealers, and is commonly called bromide paper. Its principal use is for enlarging, but at the time of its introduction it was used for negative work, the paper being rendered semi-transparent by an after-operation. It must also be noted, too, that for this work the inventors employed a roller-slide of the kind suggested by Woodbury. The Eastman Company next took up the matter, introduced a roller-slide, together with a paper film of very reliable quality. This paper is sold in spools, ready wound, so that the buyer had merely to take a spool from its case, insert it in the roller-slide, and he immediately had material ready for reeling off forty or fifty negatives, to be subsequently separated by cutting, developed, and rendered transparent with a preparation of vaseline. These films were objected to by some on account of the trace of grain from the paper which was left on the picture printed from it, and a 'stripping film' was next adopted as previously proposed by the Rev. W. Palmer. By this modification the surface bearing the image could be stripped from the original paper and transferred to a stiff sheet of insoluble gelatine, which became its final support. Film photography has recently been brought to still greater perfection by the employment of transparent and flexible celluloid in sheet form. It is curious to note that this substance, invented by Parkes about 1855, was long ago proposed as an efficient substance for use in photography, and it would doubtless have been so used if collodion, when applied to it, had not had a solvent action. But as it is quite insoluble in water, it forms a perfect support for a gelatine film, and, now that it can be manufactured nearly as clear as glass, it represents the best thing yet introduced. Its general use is limited by its cost, which is greatly in excess of glass, but it presents so many advantages that it is very largely employed. The material is now made thin enough to be wound on spools and used in roll-holders. Beyond the advantage of lightness, portability, freedom from breakage, reduced cost of carriage, &c., which the films undoubtedly possess, there is one gain in their employment, of a technical nature, which is important enough to receive recognition here. They are free from halation. Halation is apparent in negatives taken upon glass plates as an encroachment of the light parts upon the dark portions, and is seen in its most aggravated form in the blurring out of windows in interior views. It is caused chiefly by reflection from the back surface of the glass.
The great rapidity of modern dry plates allows a photograph to be taken in such a mere fraction of a second that the camera can be held in the hand during the operation. Various hand-cameras are now made, and meet with extensive employment, especially by tourists, and are more or less disguised as despatch-boxes; but they are becoming so common that the disguise is more apparent than real.
The extreme sensitiveness of modern dry plates has also given rise to what is known as flash-light pictures, which are photographs taken by the almost instantaneous flash produced by scattering powdered magnesium into a lamp-flame. Many ingenious forms of lamps have been devised for this purpose. They mostly consist of a spirit-lamp in conjunction with a receptacle for the magnesium powder, with a pneumatic ball and tube attached. Pressure of the ball urges a puff of air into the powder, and carries it into the flame. This system is much used for taking groups and portraits at night in private and public rooms. The best workers employ a branch tube from the ball, which exposes the lens at the moment of maximum light.
Lenses.—The quality as well as the kind of lens used is of great importance. An explanation of the different forms and properties of Lenses is given under that head, but it is necessary to say a few words here about the kinds used in photography. They may be divided into two classes—portrait lenses, and view lenses. The former are of large aperture, but give a small image; while the latter have a small aperture, but give an image which covers a far larger surface. In the portrait lens rapidity of action has been the chief thing considered, for it is used in a studio where the amount of light available is always more or less limited. A portrait lens unless of the 'doublet' form is not suitable for view purposes; but a view lens can well be used for portraiture under certain conditions, and is one of the best lenses to use for groups. Under the head of view lenses come a large number which have fanciful names attached to them that are apt to mislead the tyro, but which are all more or less alike. In the early days of photography telescopic objectives were made to do duty in the camera, but they gave a very small field, and were in other respects unsuited to the purpose. Then came (1841) the portrait lens designed by Petzval, and made by Voigtlander of Vienna, an invention which marks an era in photographic progress. The single view lens, which is the cheapest and for pure landscape is still unequalled, had its first improvement in the patent aplanatic lens made by Grubb in 1857. Although called a single lens, it consists of a combination of crown glass of concavo-convex form, cemented to a flint divergent meniscus. The single lens was modified later on by Dallmeyer, who subsequently, in 1888, introduced a new form of view lens which, possessing the usual advantages, had the quality, hitherto unknown in a single landscape lens, of giving an image free from curvilinear distortion. The same feature had been secured by Dallmeyer in his well-known triplet lens, which was invented in 1860, and which, as its name implies, consisted of three combinations. This lens was serviceable for copying, architectural subjects, as well as for landscape, and was a great favourite with photographers. It has now been superseded by the doublet form of lens, which, under the name of rectilinear, symmetrical, &c., is the most commonly used form of photographic objective. It generally consists of two combinations of similar construction placed with their concave surfaces facing one another, the necessary stops or diaphragms for increasing definition and reducing spherical aberration being inserted in a slit in the brass mounting tube midway between them. These lenses are constructed to take different angles of view, according to the require- ments of a given case: for taking subjects in a confined situation, such as a narrow street, or for depicting the façade of a large building at very limited range, a 'wide-angle' lens is necessary. For the convenience of those carrying a number of lenses, Ross has devised a series of 'symmetrical' lenses that all fit one flange. Every lens is sold with a set of diaphragms or stops; and, in the case of the lens last mentioned, these usually take the form of a rotating plate, pierced with the necessary apertures. But in other lenses the diaphragms are in the form of separate plates, and are kept in a small leather case by themselves, the operator removing any of the series, and inserting it in the lens when wanted. These diaphragms are marked with what is known as their focal value, which means that the number marked represents the fraction which the aperture of the diaphragm is of the focal length of the lens. To make this plain, let us suppose that the focal length of the lens is 6 inches, and that the diaphragm in question has an aperture of a quarter of an inch. As there are in 6 inches 24 of such quarter inches, the figure 24 will represent its value, and this will be expressed thus: . In like manner, a half-inch diaphragm would be marked , and a one-eighth inch diaphragm . There is, however, another method of marking lens stops, which has been adopted by the Photographic Society of Great Britain, and which is called the 'Uniform Standard,' or U.S. for short. In this system is called No 1, and the U.S. number for any other size is found by dividing the focal length of the lens by the diameter of the opening as already described, squaring the result, and then dividing by 16. Of late years a remarkable improvement, which, however, had for some time been applied to microscopes, has been made in photographic lens diaphragms, in a contrivance which, from its resemblance to the natural diaphragm of the eye, which expands or contracts automatically according to the amount of light to which the organ is subjected, is called the iris diaphragm. This consists of a number of flat blades or tongues of thin blackened metal, which are fastened to a ring in the lens mount. By a turn of this ring the blades expand or contract, so that the aperture can be enlarged or diminished as required, while a scale marked on the outside of the lens mount, and a travelling pointer, indicate the focal value of every change of aperture. We may mention here one more important change in lens manufacture—the introduction of aluminium instead of brass for the necessary metal part, by which the weight of the instrument is greatly reduced. The same useful metal, only now rendered available by its cheaper manufacture, is also coming into use as a substitute for brass in the other metal fittings of cameras. A new kind of glass, known as Jena optical glass, is now being employed in the construction of photographic lenses. By its use a larger field can be covered with a given aperture than was the case under former conditions.
Wet-collodion Process.—Collodion (q.v.) is the name given to the solution of pyroxylene, a kind of gum-cotton, in a mixture of ether and alcohol. When this is flowed over a glass plate it gradually dries into a transparent film. It was first introduced for photographic purposes in 1851 by Mr Scott Archer, and has been of great and important service. For fully a quarter of a century the wet-collodion process was almost exclusively practised by photographers—in the earlier years for the production of positives on glass, and occasionally on leather or other non-fragile materials; latterly by modifications the process was more extensively employed for the production of negatives. Dry-collodion processes have also been in use, although on a much more limited scale. These are the stages in the wet-collodion process: (1) A glass plate made perfectly clean is coated with collodion, to which the bromide of cadmium and either iodide of potassium or iodide of ammonium have been added. (2) The collodionised plate is 'sensitised' by immersion in a bath of nitrate of silver, containing 35 grains to every ounce of distilled water. (3) Production of latent image by exposing the sensitised plate in the camera after the object has been focused. (4) Development of latent into visible image by flooding the plate with a solution of sulphate of iron (ferrous sulphate), or of pyrogallol acid, to either of which some acetic or citric acid is added. (5) Fixing of the permanent image by immersion of the plate in some solvent of those parts of the sensitive surface upon which the light has not acted. This solvent for wet plates is cyanide of potassium, but for more modern processes hyposulphite of soda is employed.
Dry-plate Processes.—It is hardly necessary to do more than name a few of the earlier dry-plate processes, since, except for experimental purposes, they have all been beaten out of the field by the recent one known as the gelatino-bromide process. Several advantages arise, especially for field-work, from using dry sensitive plates. With the wet process the operator, when away from his studio, must take with him a dark tent, collodion, a silver bath, and developing agents, besides a supply of water for washing purposes; but these impedimenta are not required with dry plates. Dry-plate processes are of two principal kinds: (1) Those in which the collodion is applied to the glass plate, and afterwards sensitised in the silver bath, as in the wet way, but with a 'preservative' such as albumen 'flowed' over the surface, and the plate allowed to dry. (2) Emulsion processes, in which the sensitive silver salt is held in suspension in the collodion or gelatine. A good emulsion can be prepared by adding some soluble bromide, such as bromide of cadmium, to the collodion, and afterwards an alcoholic solution of nitrate of silver. The glass plates are simply coated with this emulsion, washed in water to remove the soluble salts, and set aside to dry, when they are ready for use. The collodion emulsion process is still employed to a limited extent, chiefly for the production of transparencies; recent experiments indicate that it may yet compete with gelatine emulsion in sensitiveness.
The earliest form of the gelatino-bromide process, at present so universally employed, appears to be due to Dr Maddox, who published the details of a workable emulsion of this nature in 1871. The process was improved in 1873 by Mr Kennett, and again in 1878 by Mr Charles Bennett. It was found that if the emulsion was kept at a temperature of 90° F. for some days, or boiled for half an hour, the sensitiveness of the plate coated by it was so greatly increased that a view which could only be taken formerly in 30 seconds could now be taken in one. A very sensitive gelatine emulsion can also be formed by using ammonia along with the bromide of silver. Dry plates produced by some form of the gelatino-bromide process are now manufactured on a large scale. When properly made they will keep good for years, and they can be developed months after having been exposed. But it is generally admitted that the best results are obtained when the plates are not old, and when development follows exposure without undue delay.
PHOTOGRAPHIC PRINTING. Silver Prints.—The details of the well-known silver-printing process are briefly as follow:
(1) Suitable paper is coated on the surface with a smooth thin layer of albumen, to which chloride of sodium or of ammonium has been added. (Originally the paper was salted only—i.e. the albumen was dispensed with—and this old method is being largely revived.)
(2) Silvering of the paper by floating it on a solution of nitrate of silver, from 30 to 60 grains of this substance being used for every ounce of water, according to the amount and kind of chloride in the paper. It is afterwards dried.
(3) Exposure to light. The silvered paper is exposed beneath the negative in a printing frame, the time of exposure varying according to the brightness of the light and the character of the negative.
(4) Toning. In order to give a pleasing colour to the print it is usual to tone it in a solution of chloride of gold. Quite recently the metal platinum has been used for toning silver prints on plain paper with very fine results. One method has been introduced by Blanchard, and another by Lyonel Clark, the latter employing the same salt, the chloro-platinite of potassium, which is used for the platinum printing process to be presently described.
(5) Fixing. The print, when taken from the toning bath, is steeped in a solution of hyposulphite of soda, which removes the undarkened silver salt that is still sensitive to light, and so fixes the image.
(6) Washing. Silver prints require to be washed quickly and thoroughly after treatment with the hyposulphite of soda. Prolonged soaking is harmful, and imperfectly washed prints soon spot and fade. Gelatine is now largely coming into use as a substitute for albumen: it is more suitable for the rendering of delicate detail, and the prints with due care exercised in their production ought to be more permanent. The papers known as Aristotype, Argentotype, and Celerotype are gelatine emulsions of chloride of silver spread on paper.
Printing in Salts of Iron.—The metal iron in some of its chemical preparations is now very largely employed in a number of photographic printing processes—e.g. cyanotype, chrysotype, kallitype, platinotype, &c.—and is capable of producing results with all the fidelity and delicacy of the silver process, in some cases at considerably less cost, while the manipulations are greatly simplified. There are certain preparations of iron, known as ferric salts, that are not affected in appearance when certain other chemicals are brought in contact with them; other preparations of iron, known as ferrous salts, produce highly-coloured pigments when combined with these same chemicals.
The action of light can change ferric into ferrous salts; hence, if a piece of paper be coated with a ferric salt, dried, and placed under a fern leaf, a piece of lace, or a photographic negative, and exposed to light, an image in a ferrous salt is produced which is capable of being developed into a highly-coloured image when acted on by a suitable reagent; and not only so, but, as this reagent has no action on the ferric salt, they may be mixed together in the first instance and thus applied to the paper, when the action of light will develop the highly-coloured image, a simple wash in water completing the operation. This is in outline what is known as the Cyanotype or Blue printing process, first published by Sir J. F. W. Herschel in 1842, which in detail is as follows:
Two solutions are made up, one containing sixty-four grains of ammonio-citrate of iron to the ounce of water, the other forty-eight grains of ferricyanide of potassium to the ounce of water. Mix equal quantities of these solutions, and with a soft sponge or flat camel-hair brush quickly and evenly cover one side of good smooth white paper. This is best done by gas or candle light; then place to dry where it will not be affected by daylight. The paper so prepared is chiefly used for copying plans and drawings on tracing cloth: the tracing, which should be in a good opaque black ink, is placed on the top of the paper, and both are covered with a glass plate to keep them in perfect contact. Ten minutes in a very bright light will suffice for exposure. The print is now washed, when the lines of the drawing will appear white on a blue ground. The same kind of paper can be exposed beneath a photographic negative in order to secure a rough proof of the picture, but in this case the time of exposure is much increased. A disadvantage of the above process is that the original black drawing on white paper appears as a white drawing on a dark-blue ground.
The following modification, known as the Peliet process, produces blue prints on white paper: Gum arabic, 25 parts; common salt, 3 parts; perchloride of iron, 8 parts; tartaric acid, 4 parts; and water to make up to 100 parts. Well-sized paper is coated with the above and treated as in the preceding; it is very sensitive to light. A good tracing in bright sunlight is sufficiently printed in from ten to fifteen seconds. The print is developed by immersion in a saturated solution of ferrocyanide of potassium (yellow prussiate), and the design immediately appears in blue. The print is now rinsed in cold water, and then transferred to a 10 per cent. solution of hydrochloric acid; another rinse in cold water completes the operation.
Chrysotype.—This is a modification of cyanotype, also published by Sir J. Herschel. The paper is merely coated with the ferric ammonium citrate, and may be developed after exposure with a neutral solution of gold chloride, washed with water and dried. The resulting print is in metallic gold in a finely-divided state, and is of a fine purple colour. A dilute solution of nitrate of silver may be substituted for the gold when the image is in metallic silver.
Kallitype.—In this process paper is washed with a strong solution of neutral ferric oxalate. After printing in the usual way, it is developed by the following solution: Nitrate of silver, 50 grains; citrate of potash, 800 grains; bichromate of potassium, 1 to 2 grains; rain water, 10 ounces. The precipitate formed is next dissolved by the addition of ammonia (strength, '880)—about a drachm will be sufficient. After filtering add 35 drops of strong nitric acid, and the developer is ready. This solution is very cheap and easily prepared. The resulting prints possess a fine rich black colour.
Platinotype, or Platinum Printing Process.—The metal platinum can be deposited from some of its chemical preparations in an extremely fine black powder when brought in contact with one of the iron salts altered by light. Herschel explained how to get prints in platinum, but the process now employed is that discovered by Willis. Captain Abney, F.R.S., thus describes the chemical action upon which the process is based: 'Mr W. Willis, jun., found that he could obtain an image in platinum black, by means of development, if he sensitised his paper with ferric oxalate, with which was mixed a solution of chloro-platinite of potassium. The action of light on this paper is to reduce the ferric salt to the ferrous state, and when the ferrous salt is in solution the platinous salt is reduced by it. By floating the exposed paper on a solution of neutral potassium oxalate, which is a solvent of the ferrous oxalate, the platinum salt in contact with it is immediately reduced to the metallic state, and an image is thus built up. To fix the prints they are immersed in dilute hydrochloric acid, which dissolves away any ferric oxalate there may be, and also gets rid of any oxalate of lime.'
Paper prepared as above described is supplied com- merically. And after being exposed to light beneath a negative in a printing-frame for about one-third of the time necessary in the case of a print on silvered paper, its lemon-yellow tint is found to change, where the light has reached it, to a pale, dirty-gray colour. Development is conducted in an iron enamelled tray, beneath which a spirit-lamp or bunsen burner is placed, so as to keep the oxalate solution at a temperature of about F. Underexposed prints will benefit by this temperature being exceeded, whilst those which have received more light than they should have had can be advantageously treated with a much colder developer. The print is floated on this warm bath, which turns the faint image of the picture to a dense black, and fixation follows by placing the picture in a series of water baths made slightly acid with hydrochloric acid; these remove the iron from the paper; a simple rinse in plain water completes the operation. The developer can be mixed in bulk, for it keeps well, and the same quantity will develop a large number of prints one after the other. The platinum prepared paper will keep well if damp be excluded. For this reason it is sold in tin tubes, which have at one end a small quantity of calcium chloride, a salt which is so greedy of moisture that it will absorb all in its neighbourhood. The favour with which this process has been received, because of its permanence and its quick results, is well indicated at photographic exhibitions, where a large proportion of the pictures shown are invariably platinotypes. Mr Willis, the inventor, has introduced a cold bath platinum process, in which the metallic salt is contained in the developer, and this modified method is said to present many advantages. The developer thus prepared is not of a lasting kind, and only enough must be mixed to meet the existing demand. Another modification of the platinum process, made known by Willis, but more generally associated with the name of Captain Pizzighelli, yields a dark image in the printing-frame. The only necessary after-treatment is a bath of weak acid, followed by washing in plain water. There is every reason to suppose that platinum will be the printing process of the future, but unfortunately the price of the metal, which in 1890 went up more than 100 per cent., is calculated for the present to limit its use.
Bichromated Gelatine Process.—So far back as 1839 Mungo Ponton announced that paper steeped in bichromate of potash and dried changed its colour when exposed to light. It was subsequently discovered that light not only alters the composition of the bichromate, but also oxidises the size (gelatine) of the paper. Gum, starch, and albumen were also found to become, like gelatine, insoluble when exposed in contact with the bichromate of potash or ammonia to the action of light. If ordinary gelatine be soaked in cold water, it absorbs the water and swells, and then if heated, or if hot water be poured on it, the gelatine melts. If some bichromate of potassium or ammonia had been added to the cold water, the gelatine would absorb the chemical along with the water. If now the gelatine be dried and exposed to light until the stain imparted by the bichromate is altered in colour, it will no longer swell in cold water, neither will it dissolve in hot water; the action of light has made the bichromated gelatine insoluble. It is to gelatine thus chemically modified that we owe the 'autotype' or 'carbon'—more correctly 'pigmented gelatine'—process. The Collotype, Woodburytype, and some forms of photoinc engraving and photogravure, also certain kinds of 'phantom' photographs, and one method for vitro enamels, depend on the same principle.
The Autotype, Carbon, or Pigmented Gelatine Pro- cess.—Pigmented gelatine paper is an article of commerce, and the Autotype Company supply this 'tissue' sensitised ready for printing. The tissue consists of a thick coat of gelatine, with which has been intimately mixed a certain amount of permanent pigment in very fine powder—if for a black, Indian ink may be employed; other colours are added to modify the tint. The paper so coated is sensitised with ammonium bichromate, and then exposed under a negative till it is supposed to be sufficiently printed. The image is not visible as in a silver or iron print, therefore some indirect plan of telling the proper time of exposure, such as by the use of an actinometer, must be resorted to. The change which takes place in the gelatine film is this: the surface next the negative has been rendered insoluble wherever the light has acted, and that to a depth corresponding to the intensity of the light. It results from this that almost the whole of the surface of the gelatine has been rendered insoluble—to the greatest depth where the light has acted most strongly. Soluble portions, however, remain enclosed between its surface and the paper. No picture is visible till these are removed. To overcome this difficulty—the removal of the soluble portions imprisoned between the insoluble skin and the paper at the back—took many years of experimenting, and all sorts of devices were resorted to. One plan was to expose the back of the tissue to the negative, thus printing through the paper, but the grain of the paper showed offensively. Fargier spread the pigmented gelatine on glass, exposed it thus under a negative, and then coated the film with collodion. On subjecting the whole to the action of warm water, the latter penetrated the collodion and softened the gelatine, which eventually floated off the glass, being held together by the collodion. This was now supported on paper (collodion side down), and washed from the back with warm water, and so the first half-tone photographs in pigmented gelatine were obtained.
Swan experimented on similar lines, and in 1862 he took out a patent for pigmented gelatine films spread upon collodion supported on glass. When dry the whole was stripped from the glass, and thus the first tissue was made. The difficulty of removing the entangled pigment still remained, and this Swan overcame by coating the collodion surface of his tissue with india-rubber solution, and applying it to a piece of paper similarly coated. When both were dry the whole was passed through a press, and then soaked in warm water; thus the soluble portions could be removed, leaving that part of the film acted on by light untouched, and projecting in relief according to the varying action of light as it passed through the gradations of the negative. By this method and a subsequent modification Swan sent out a number of fine pigment prints, but it was so troublesome, expensive, and unsatisfactory in the hands of the average photographic printer that printing by bichromated gelatine never became popular until J. R. Johnson, about the close of the year 1868, discovered that the pigmented gelatine paper ('autotype tissue'), when sensitised by the bichromate and correctly exposed to light under a negative, only required to be soaked in cold water, and then evenly applied to any surface impervious to air, such as glass, zinc, oilcloth, &c., when the gelatine surface would adhere very tenaciously to the support—after the principle of the schoolboy's sucker—by atmospheric pressure alone. Then by soaking in hot water the paper at the back came off, carrying with it much of the unaltered pigment and gelatine, and by laying the image remaining on the support with the hot water the picture in all its delicate gradations appeared clean and perfect. It was this discovery that made what is generally known as autotype or carbon printing a practical working process. The method above described is called the single transfer process, and produces a print reversed as to right and left. The double transfer process merely differs in the adoption of a temporary support of opal glass, zinc, or paper coated with a suitable preparation (the most convenient being Sawyer's flexible support), which holds the print till developed, and from which the print is then transferred to its final support—it is then non-reversed. When practicable it is usual to take a reversed negative, and thus avoid the double transfer.
Powder Process.—By what is called the powder process prints are produced on paper in plumbago, or any other impalpable powder insoluble in water. It has been a good deal used on the Continent. A slightly sticky or 'tacky' preparation of sugar, gum, glycerine, and potassium bichromate, when exposed to light, loses its tackiness in proportion to the intensity of the light acting on it. A glass plate coated with this preparation will therefore, when exposed under a negative, represent the picture, so to speak, by different degrees of tackiness. In this state a fine powder sprinkled over it will adhere in proportion to the stickiness of the surface. When the superfluous powder is removed, and the film coated with tough collodion, it can be detached and, if required, put on any support such as paper, but the soluble portion of the gum, &c. is previously removed by washing.
Photographic Enamels on Glass and Porcelain.—If the image as described in the preceding paragraph be developed by suitable metallic oxides—such as the underglaze colours of the porcelain painter—and the resulting image coated with collodion be washed, the film can be transferred to glass, china, or enamelled metal, and after firing in a suitable kiln it will become vitrified, and will be as permanent as a burnt-in painting on the same substances. Some of the very finest vitreous enamels are, however, produced by a 'substitution' process, in which a collodion transparency is toned to saturation with platinum, or iridium, all traces of silver being carefully eliminated. This modified film is now transferred to an enamel tablet, and burnt in as in the previous method.
Diazotype.—A new colouring matter or dye of a primrose tint was discovered by Arthur G. Green in 1887, who named it 'primuline.' This dye affixes itself very tenaciously to cotton fibre, so that by merely boiling the fabric in its aqueous solution a permanent yellow colour results; and this yellow basis acts as a mordant, permitting, when acted on by appropriate developing agents, the building up of an immense variety of 'ingrain colours' that admit of wide practical use.
By passing the yellow cloth through a bath of acetic acid and nitrate of soda the material is said to be 'diazotised,' and then is of a brighter yellow colour that is extremely fugitive under the influence of light. This constituted for a time an insuperable objection to its use as a dye, but suggested photographic possibilities. Experiments proved that, if a material containing diazotised primuline be exposed to light under a design, those parts acted on by light speedily decomposed, while the parts protected remained unaltered; the latter on treatment with a phenol or amine produce many permanent compound colours, and the former remain unchanged. Upon this is founded the diazotype process, by means of which every kind of fabric, cotton, muslin, velveteen, wool, linen, silk, &c., as well as colloid films and paper, are dyed by the influence of light. The process is exceedingly simple, very cheap, and gives a positive print, the most opaque objects or parts of a design coming out darkest, the reverse being the case in ordinary silver printing.
Mr Green, in conjunction with his partners Messrs Cross and Bevan, published the process at a meeting of the British Association at Leeds in 1890.
MECHANICAL PRINTING. Woodburytype and Stannotype Processes.—It has long been known that if a leaf, a bit of lace, or any similar object was placed on a sheet of soft metal, and considerable pressure applied, the impression of leaf or lace was sunk into the metal. From this metal plate prints were taken as from an engraved plate, and the process was called Nature-printing (q.v.). If we laid an ordinary autotype print instead of a leaf on a flat piece of iron, covering it at the same time with a smooth piece of sheet-lead, and then put them under sufficient pressure, the result would be an imperfect Woodburytype mould in the soft lead. The metal reverse would be faulty, because in this case the gelatine film is too thin to give enough of relief. In order to obtain a proper mould a layer of sensitised gelatine, considerably thicker than that used for an autotype print, is exposed under a negative. It is developed as in the autotype process, and presents the image in considerable relief. The print is then covered with the lead, and they are pressed together in a hydraulic press, which produces a reverse or mould of the picture in the soft metal without injuring the gelatine relief.
The production of ordinary Woodburytype prints is a purely mechanical operation, the chemical action of light not being called into play; they exhibit true gradation of tint, and in that respect Woodburytype is the only perfect photo-mechanical printing process known. The mould is placed in a printing-press of a peculiar but simple construction, and a warm solution of pigmented gelatine forms, so to speak, the printing-ink. This is poured on the mould, and a thin, hard, strongly-sized paper placed on the top of it. The lid of the press has beneath it a perfectly flat glass plate, which is now brought down on the mould and the lid firmly locked by a catch. The pressure causes all the superfluous gelatine to exude, whilst that in the mould adheres to the paper. In a short time the gelatine sets, when the plate is raised and the print withdrawn. It has now only to be placed in a solution of alum, which renders the gelatine forming the picture insoluble.
The Stannotype (or printing from a surface of tin) has been called a simplified Woodburytype process. Mr Woodbury, to whom it also is due, thus describes it: 'A positive is first made from the negative—preferably by the carbon process. From this carbon or other transparency a negative is made also in carbon; but in this case the tissue possesses much more body and much less colour, so as to obtain a certain amount of relief. This (gelatine) relief negative is then coated with a thin india-rubber varnish. A piece of tinfoil is laid over it, and the whole passed through a pair of india-rubber rollers—a species of mangle, in fact. We have now a printing-mould ready for placing in the press and printing from in gelatinous ink.' This process does not always give the beautiful results obtainable by the original Woodburytype method, but remarkably good results are secured by it.
Heliotype and Phototype Processes.—Both of these are photo-mechanical methods, in which the gelatine relief is itself used to print from in some form of printing-press, instead of being covered with tinfoil as in the stannotype process. Lithographic ink is used. The film or layer of gelatine forming the printing surface requires to be specially and carefully prepared. This process, under the name of Collotype, is much used for book illustration and advertising purposes.
Photo-lithography and Zincography.—The only difference between these is that a lithographic stone is used in the one case and a plate of zinc in the other for the mechanical printing. It is necessary that the original drawings should be done in lines and not in half-tint. At least, it is doubtful whether much success has as yet attended the production of half-tint photo-lithographs. A negative is taken from the drawing by the camera, and from it a print is made on paper coated with bichromatised gelatine much in the same way as in the antotype process. But before the print is developed it receives a coating of lithographic transfer ink specially prepared for the purpose. It is next floated in warm water till the lines are seen as depressions. With the aid of a sponge and water at a temperature of about 150° F. the soluble portion is removed, leaving the picture in insoluble gelatine with its coating of transfer ink. It now only requires to be washed, dried, and transferred to the stone or zinc plate (see LITHOGRAPHY). These processes have been greatly superseded by the various photo-engraving and zinc processes, fully described in the article ILLUSTRATION, by which printing blocks suitable for a typographic press can be produced in a few hours.
Photography is generally employed as a means of reproducing drawings on wood blocks for the engraver. This process is of much importance, as the original drawing is preserved, not only for comparison with the finished engraving, but it may be for its artistic value. The original drawing also may be made of any convenient size, and reduced on the wood—a great consideration when minute objects are to be represented. The necessities of wood-engraving require that the drawing-on-wood should be reversed; hence the necessity of a reversed negative in any direct printing process. The negative may either be printed direct on to the wood, or a modification of the carbon process employed. In the first case, one process is first to render the surface of the block waterproof, and then it is whitened with Chinese white. The block is then sensitised with chloride of silver, and printed under a reversed negative. It is then toned with gold and fixed with hyposulphite of soda, washed and dried, and is then ready for the engraver.
In the carbon process, a carbon tissue is made with very little gelatine and a large amount of carbon or other pigment. The block is rendered waterproof and whitened with baryta; the carbon print is developed on the wood with warm water, and, when dry, is ready for the engraver.
Photo-micrography consists in photographing microscopic objects by causing a microscope to take the place of the ordinary photographic lens in the camera, so that the enlarged image is cast upon the sensitive surface of the collodionised or gelatine plate. Such photographs, again enlarged by the optical lantern, are much used for class instruction.
By reversing the arrangement necessary for the enlargement of microscopic objects it will be seen that minute photographs of engravings, or other objects, may be produced which would require a microscope for their inspection. In this way communication was maintained during the investment of Paris, when copies of letters and newspapers were inserted in quills, and fastened to carrier pigeons.
ASTRONOMICAL PHOTOGRAPHY.—The application of photography to astronomy has within the past few years assumed immense importance, and great results have been achieved through the wedging of the camera with the telescope. Until recent times the only remarkable photographs of the celestial bodies were those of the moon, which were executed by Warren de la Rue, Rutherford, and others. The moon being from its size and brightness a comparatively easy body to photograph, the old processes were sufficient for the purpose, and most perfect results were obtained. Jansen and others have secured photographs of the sun which exhibit markings upon its surface with great distinctness, and many photographs of the corona when the orb has been under eclipse have been taken when the somewhat rare opportunity has occurred. But it is in picturing the distant stars and nebulae that the greatest work has been achieved by photography, and results obtained which would have been impossible without the aid of the highly sensitive dry plates now at the disposal of the astronomer. Among the triumphs already obtained in this direction may be mentioned Roberts' photograph of the 'Andromeda Nebula,' Common's photograph of the Nebula in Orion, and several similar negatives obtained by the brothers Henry of Paris. One by these last workers, a photograph of the Pleiades, should receive special mention. A certain star in this well-known group appeared in the photograph in question with a nebulous haze attached to it. This star was not known to be associated with a nebula, and the astronomers of Paris in vain endeavoured to detect it by aid of the most powerful telescope at their disposal. The nebulous mass was therefore discredited, until another photograph of the Pleiades arrived shortly afterwards from America which exhibited exactly the same peculiarity. Once more the nebula was searched for, and at length was declared to be faintly discernible. From this it would seem that the photographic film is more sensitive to faint impressions than is the retina of the eye, and in a certain sense this is true. These star photographs are often exposed to the action of the light from those distant bodies for three or four hours, during which time the clock-work train attached to the telescope keeps the images of the tiny points of light stationary on the plate, in opposition to the rotation of the earth. Each image, however faint, has therefore a comparatively long time to make an impression on the sensitive chemical surface, and exerts a cumulative action, with the result that the images of stars are registered which no human eye has ever beheld. To put the matter more plainly, it may be said that a certain section of the sky covered by the field of a powerful telescope is seen to contain a definite number of stars. When this same space is photographed their number is often doubled. At a convention of astronomers held in Paris in 1887 it was decided to take steps for photographing the whole of the heavens. For this purpose the sky has been chartered out into squares, and each observatory helping in the work will photograph a certain number of these spaces. The work is estimated to entail ten years' labour, this long time being partly accounted for by the circumstance that, owing to the occurrence of unsuitable weather and the interruptions caused by moonlight, there are only about fifty nights in the year when sidereal photography is possible. There are many difficulties in photographing the stars, some of which have led to discussion, and have caused doubts to be raised as to the accuracy of the results attained. The chief of these is represented by the circumstance that the photographs exhibit discs of light, varying in size according to the brightness of the stars, instead of mere points of light, which the extreme distance of the bodies should secure. This expansion of size is believed to be due to irradiation, want of complete achromatism in the lenses, and reflection from the back of the photographic plate. These difficulties will no doubt be surmounted in the future, and it may be mentioned that they are only observable when a refracting telescope is employed. Hence the use of reflecting telescopes for this work has been suggested; but, although by this means some of the faults mentioned are banished, other inconveniences arise which form obstacles to good work. Dr Huggins has done much valuable work in photographing the spectra of the heavenly bodies.
Durability of Photographic Prints.—Experience has proved that silver prints, however carefully prepared, cannot be depended upon for permanency. Much vexation has frequently arisen from the fading of these, and on this account they are no longer used for book illustration. They will keep better unmounted than mounted, and they should be kept in a dry situation, as damp increases their tendency to fade. Platinotype prints are believed to be permanent by those best able to judge. Autotype, Woodburytype, and other prints in pigmented gelatine are permanent if stable colours are employed, and of course those obtained by any of the photo-mechanical processes are certainly so when printed, as they usually are, in lithographic or printers' ink.
Miscellaneous Applications and Improvements.—The report that the art of photographing in the colours of nature has been discovered crops up year after year with curious persistency, and may be generally traced to the work of unscrupulous persons who seek to deceive the public for their own advantage. It is difficult to see how the much-talked-of photography in colours as popularly understood can ever be achieved. By the introduction of specially prepared gelatine dry plates—known as orthochromatic ('right colour') or isochromatic ('equal colour'), both very vague terms—it is possible to reproduce colours in their true shade relation to one another. For instance, suppose that one seeks to photograph by ordinary plates a blue vase containing yellow flowers. In the resulting picture the vase will be white and the flowers will be black. But if we use isochromatic plates the vase will be rendered as a gray and the flowers will appear almost white, which is obviously more true to the way in which the eye observes the original. This change in the behaviour of the sensitive surface is brought about by adding to the gelatine emulsion of which it is composed a minute quantity of certain dyes. Vogel in 1873 discovered that certain coal-tar dyes produce a change of sensitiveness in silver compounds; and in the same year Tailfer and Clayton secured a patent for the preparation of colour-sensitive plates prepared by the same agency. They use an ammoniacal solution of eosine; and plates made under the patent are now supplied commercially. They are much used in copying all coloured objects, such as oil-paintings; and there is little doubt that they will play an important part in sidereal photography, in the registration of coloured stars. Meteorologists are now depending upon photography to furnish them with cloud studies, and with pictures of Lightning (q.v.). A study of the latter is likely to extend our knowledge concerning the phenomena connected with thunderstorms, and has already elucidated a few problems. Murybridge, in the United States, introduced the system of analysing by means of photography the motions of a trotting horse, running dog, &c. By means of special apparatus he found it possible to take a dozen consecutive pictures of a single movement. Marey of Paris further developed this phase. In the cinematograph a series of photographs taken in rapid succession from a moving scene is thrown on a screen by the aid of a magic-lantern in the same rapid manner in which they were taken, the result being a life-like reproduction of the original scene.
Photography through an opaque object by means of the Röntgen rays (see RÖNTGEN) has already been of immense use in surgery in fixing the exact position of bullets, &c., in the human body as a preliminary to their extraction.
Although photography in colours has long occupied the attention of scientific photographers, no direct method of doing so has yet been described. One, and perhaps the most important of the indirect methods, is as follows: Three separate (colourless) negatives of a coloured object are taken through coloured screens. From these positives are taken, and colour is supplied by means of inks or dyes. When these three-coloured monochromatic positives are superimposed, and seen by projection on a screen or through a photo-chromoscope, a coloured image of the original is the result.
BIBLIOGRAPHY.—Chapman Jones, Science and Practice of Photography; Abney, Instruction in Photography, and Photography with Emulsions; Eder, Modern Dry Plates; Robinson and Abney, Art and Practice of Silver Printing; Robinson, Picture-making by Photography; J. R. Sawyer, The A.B.C. Guide to Autotype; Liesegang, Manual of Carbon Process; Meldola, Photographic Chemistry; Chadwick, The Magic Lantern Manual; Hepworth, Photography for Amateurs and Book of the Lantern; Wilkinson, Photo-engraving and Lithography; Monckhoven, Photographic Optics; Burton, Optics for Photographers; Dallmeyer, Choice and Use of Photographic Lenses; Harrison, History of Photography; Schnauss, Collotype and Photo-lithography. See also periodical literature, and especially the annual publications, Year-book of Photography, Almanac, &c.