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Originally appearing in Volume V21, Page 520 of the 1911 Encyclopedia Britannica.
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FILMS AND SENSITIVE PLATES PAPERS Sensitive Dry Plates.—A special feature of modern photography is the use of trustworthy ready-prepared sensitive dry plates and films in different grades of sensitiveness, so that there is nc necessity for the photographer to prepare his own plates, nor, indeed, could he do so with any advantage. The practice of outdoor and studio photography has thus been very greatly simplified; and although with wet collodion there was the advantage of seeing the results at once and retaking a picture if necessary, the uncertainties connected with the use of the silver bath and collodion, and the amount of cumbrous apparatus necessary for preparing and developing the plates, far outweighed it. There is also an enormous saving of time, in using dry plates as compared with wet, by deferring development. In tropical climates, also, dry plates can be used when work with wet plates would be impossible. On the other hand, the uncertainty of more or less random exposures on ready-prepared Plates must not be overlooked. Besides their use in taking negatives, gelatin dry plates are also largely used for printing transparencies, lantern slides, enlargements, &c. For negative work they are prepared with an emulsion in gelatin of silver bromide, alone or with the addition of silver iodide or chloride, and are to be obtained in five or six degrees of rapidity: " slow," for photo-mechanical or " process " work; " ordinary," for general purposes when quick exposures are not required; " rapid," for landscape and portraits; extra rapid," for instantaneous exposures; and " double extra rapid," for very quick snapshot work in dull weather or for special subjects. These latter kinds are exceedingly sensitive, and require great care in use to avoid fog. In order to prevent halation, or irregular action by reflection from the back surface of the glass, dry plates are coated with a non-actinic " backing," which can easily be removed before development. Self-developing dry plates were introduced in 1906, in which the developing agent is mixed in the film itself, as in the Ilford " Amauto " plate, which only requires immersion in a solution of washing soda for development, or, as in the Wellington " Watalu plates, applied on the back of the plate, plain water only being required for development, this application also preventing halation. The slow plates used for printing lantern slides and transparencies are usually prepared with an emulsion of silver chloride with or without free silver nitrate and other haloids. The rendering of photographic plates isochromatic or sensitive to all colours by dyeing them with eosin, or other suitable dyes, has been greatly improved by the use of new dyes, especially those of the isocyanin group, prepared by Dr E. Konig of the Hoechst factory, and known as " orthochrom T," " dicyanin," " pinaverdol," ' pinachrom " and " pinacyanol," the latter of which can confer on a silver bromide plate as high a degree of sensitiveness for red as erythrosin does for yellow; also F. Bayer's " Homocol," Dr A. Miethe's " ethyl red," and other similar dyes (see E. Jb., 1905, pp. 183, 336). Panchromatic plates are now largely manufactured and used for all photographic work in which a true rendering of the relative colour luminosities is essential, and more particularly for the various methods of colour reproduction in which plates are required to be sensitive to red, green and blue-violet. They are made in different degrees of general and colour sensitiveness, according to the purpose for which they are required, the ordinary " isochromatic " being most sensitive for yellow and green, and the " panchromatic " for red, orange and yellow, as well as for green, blue and violet. To obtain the best results from all these plates it is necessary to screen off the blue and violet rays with yellow or orange transparent screens, or colour filters, made of coloured glass, or glass coated with coloured gelatin, collodion, &c., or with glass cells containing solutions of suitable dyes or salts. For the various processes of three-colour reproduction panchromatic plates and special red, green and blue-violet filters have to be used for taking the three negatives, their intensities and absorptions being carefully adjusted to the particular plates in use; the same applies, but less strictly, to the yellow screens used with ordinary isochromatic plates. Dyes specially suitable for these colour-filters have been prepared by Dr E. Konig. Various kinds of colour screens for ordinary, microscopic and trichromatic work are made commercially, and Messrs Schott of Jena make a special yellow glass in three tints for the purpose. Plates for Colour Photography.—In 1868 Louis Ducos du Hauron, among various trichromatic methods patented for photographically reproducing coloured objects in the colours of nature, described one in which the trichromatic principle, instead of being carried out on three separate plates, was to be combined in one plate by means of a transparent medium covered by a trichromatic screen divided into narrow juxtaposed lines or minute spaces, corresponding to the three primary colours, red, green and blue-violet, the trans-parent colour of each of these lines or spaces acting as a colour filter. A sensitive panchromatic plate was to be exposed in con-tact with this screen to produce a negative with lines or spots corresponding to the relative strength of the three coloured lights passing through it, so that a diapositive print on glass properly registered with the tricolour screen would show the object in its proper colours. This method could not be carried out successfully for want of efficient panchromatic plates and other difficulties. Between 1892 and 1898 several patents were taken out by J. W. McDonough and J. Joly for various methods of preparing trichromatic ruled screens (Ph. Journ., 1900, p. 191). The Joly method was fairly successful in action, but had several disadvantages owing to the coarseness of the lines, the necessity for having two screens, one for taking and another for viewing, and the cost of making them (B. J. A., 1899, p. 671). The " Florence " chromatic plate (1905), worked out in America by J. H. Powrie and Florence M. Warner, was an improvement on the Joly method, the colour screen being photographically printed on a glass plate, coated with pan-chromatic emulsion and exposed to the coloured object through the screen (Penrose Pictorial Annual, 1905-1906, p. III). Some good results were produced, but it has not come into use. After several years of laborious research, Messrs Lumiere, of Lyons, adopting Ducos du Hauron's coloured grain method, succeeded where he had failed, and in 1907 brought out their " Auto-chrome " plates, in a very complete and practical form, making it possible to produce photographs in the colour of natural objects by one exposure instead of three, as in the ordinary three-colour processes. Glass plates are coated with an adhesive medium over which is spread a mixture of potato starch grains, of microscopic fineness, stained violet, green and orange, the interstices being filled in with fine carbon powder to form a tricolour screen, dark by reflected and of a pinkish, pearly appearance by transmitted light. This is varnished and coated with a thin sensitive panchromatic emulsion of gelatino-silver bromide. The plates are exposed in the camera from the back, through the tricolour films, using also a special compensating orange-yellow screen, before or behind the lens, then developed as usual, producing a negative coloured image in the complementary colours, which is then treated and re-versed so as to produce a positive coloured image by transmission, showing the picture in its proper colours. The results thus obtained are remarkably good and practically solve the problem of direct colour photography in a simple and fairly inexpensive manner (see Agenda Lumiere, 1909). In C. L. Finlay's " Thames " colour plate (1908) the tricolour screen is formed by rows of circular dots coloured alternately orange-red and green and the intermediate spaces blue. It is used alone, the coated surface being placed in contact with a panchromatic plate, the uncoated side towards the lens. It carries register marks for adjusting it to the finished picture after development and reversal of the image. These screens, being more transparent than the " Autochrome," require less exposure, but the colour rendering is not so perfect. In the Jougla " Omnicolore " plate (1909) the tricolour screen and sensitive surface are combined on one plate as in the " Autochrome," but the screen is made up of a series of blue-violet parallel lines, with intermediate alternate broken lines of orange-red and yellowish-green at right angles to them, the red narrower than the green. The relative sizes of the coloured dots in the three plates are approximately: " Autochrome " starch grains . -Ei to ssa in. " Thames " plate, dots, diameter . . sia „ " Omnicolore " plates, blue line red square E. Fenske's " Aurora " plate (1909) is a tricolour screen formed by coating a glass plate with a mixture of finely divided particles of gelatin, dyed orange-red, green and blue-violet, without any intervening spaces. The grain generally is coarser and more irregular than in the " Autochrome " plates, but optically corresponds more closely to them than the " Thames " or " Omnicolore screens do. These plates are issued uncoated for use with any suitable panchromatic plate. A later process is due to Dufay. With the exception of the " Autochrome,” these processes are still more or less in the experimental stage. Celluloid Films.—In order to avoid the weight of glass plates, which may become burdensome on a tour, and also the risk of breakage of valuable records, thin films or sheets of celluloid coated with sensitive emulsions can be used, with great saving of bulk and weight and no loss of efficiency, though such films are some-times liable to deterioration by long keeping before or after exposure. They are made in two thicknesses, stiff or flexible, the stiff being used exactly as plates, but held in a carrier or simply backed with a card or glass plate, while the flexible are made up in separate sheaths with cardboard backing, as in the " Kodoid films, or in convenient packages of twelve or more in " film packs " of various patterns. Flexible films of this kind on celluloid have for many years past also been prepared in long strips of different widths suitable for use in hand cameras of the Kodak types and in roll-holders. In the early forms of roll-holders the films were used alone, and being unprotected had to be changed in the dark room, but, as already stated, they are now supplied on spools in cartridges which can be changed in daylight. C. Silvy seems to have been the first to employ this method in 187o. In these cartridges the film is attached to a much longer strip of black paper, and rolled up with it, so that several turns of the paper have to be unrolled before the film is ready for exposure, this point being marked on the outside paper for the successive exposures, with numbers visible through a red screen at the back of the holder. When all have been exposed, the black paper is rolled on for several turns, and when taken out of the holder the loose end is fastened up till the film is developed. As these films are principally used for landscape work, it is now usual to make them isochromatic, and they may be used with or without a yellow screen. They are also made " non-curling " by being coated with gelatin on both sides. Negatives taken on these thin films have the advantage that they can be printed from either side without perceptible loss of definition, which is useful in printing by the single transfer carbon process, and in some of the photo-mechanical printing methods. Flexible transparent films in sheets and rolls have also been prepared upon hardened gelatin, but it is difficult to retain the original dimensions of the film owing to expansion of the gelatin. Paper coated with sensitive emulsions has been successfully used for making negatives in the same way as the celluloid films, and is cheaper, but much more liable to deterioration from atmospheric action before and after exposure, and unless developed soon after exposure the impressed images may fade and become undevelopable. Such papers are, however, still used in meteorological and other self-recording instruments. Stripping films of thin celluloid upon a paper support were introduced by Messrs Wellington and Ward, and had advantages for printing from either side, but are not now made. Photographic Printing Papers.—Pari passu with the supply of ready-prepared plates, all kinds of photographic printing papers can now be obtained ready for use, so that the photographer has nothing to do with the preparation of his sensitive plates or papers. The old albuminized papers have been generally superseded by ready-prepared sensitive papers coated by machinery with emulsions of silver haloids in gelatin, with or without citrate or other organic silver salts, the chloride being used for most of the " P.O.P." or " printing out papers," which contain more or less free silver nitrate, and in the " self-toning " papers some salt of gold. Some of these printing out papers are also made with emulsions of silverchloride in collodion, and known as " C.C." or " collodiochloride." The basis of most of the developable bromide papers used for enlargements and direct copying, containing no free silver nitrate, and with which an invisible image is brought out by development, much in the same way as with dry plates, is silver bromide. These papers are made in great variety of tints and surfaces, " smooth " and " rough," " glossy " and " matt," for producing different effects. They are largely used for direct printiqg by artificial light or daylight, for enlargements, and for printing photographic post-cards, &c., in large numbers by machinery, the prints being made on a long band with an almost instantaneous exposure, and developed and fixed by being passed through the proper solutions on large rollers or otherwise. Papers for the platinotype processes, sensitized with salts of platinum and iron, are also manufactured for printing out entirely or for development with potassic oxalate. Prints on these papers have the advantage of being permanent. Messrs York Schwartz and J. Mallabar's process of developing and toning prints made on a special sensitive paper prepared with an emulsion of silver phosphate was introduced by Messrs Houghton in 1908 under the name of " Ensyna." Very short exposures to day or artificial light are required, and with a special developer (" Ensynoid ") permanent prints are obtained with a varied scale of tones similar to those given by toning with gold, the colour of the print being determined by the exposure, short exposures giving purple and long exposures brown or reddish tones. The process is a rapid one, the operations of printing, developing, fixing and washing being completed within about ten minutes or even less. For the various methods of printing in permanent pigments (" Autotype," &c.) tissues are prepared coated with pigmented gelatin in various colours, and very successful results in colour photography have been obtained by printing from suitable negatives in three colours with specially prepared yellow, blue and pink tissues. Similar papers, prepared with pigmented gum instead of gelatin, are used in the " gum bichromate process, and " single transfer " papers, coated with plain gelatin, are used in the pigment printing processes to receive the developed print, and are also useful for photo-lithography, the new " oil-printing " methods, and in trichromatic printing on paper by the Sanger-Shepherd method and Dr Konig's " Pinatype." For Manly's " Ozotype " and " Ozobrome " processes special gelatinized and pigmented papers are made. " Cyanotype " and " Ferrogallic " papers are prepared for the use of architects, engineers, &c., in rolls of consider-able width, for the direct reproduction of tracings and drawings as blue or black prints by these and similar methods. Apparatus for Development.—The recognition of the fact that the two principal factors in the development of modern photo-graphic dry plates with a suitable developer are time and temperature, and also that a prolonged immersion in dilute solutions is in many cases a more convenient and equally efficient method of development, has led to the construction of apparatus for enabling the operation to be carried out almost automatically and for timing its duration. In 1894 A. Watkins brought out his factorial system of development based on the principle " that with a correct exposure on a given plate with a given developing agent, the time of development required for a given printing opacity has a fixed arithmetical ratio to the time of appearance of the high lights of the image, provided the developing power of the solution remains constant during development; and this rule holds good for all variations of strength, amount of alkali or bromide, and temperature within those limits which have been found safe in practice " (Photo. News, 1894, 38, pp. 115, 729; and further, Ph. Journ., 1900, 24, p. 221). By a series of observations he ascertained the multiplying factors of most of the developers in ordinary use, and in 1905 brought out his " factorial calculator " and a " dark-room clock " for facilitating the working of the method. The former is made of aluminium, and consists of two circular disks, the upper smaller one rotating and carrying a pointer. The outer disk is marked with a scale of Watkins' factors for the different developers, as given in the " instructions " accompanying the instrument, and is used to denote the " time of development " in minutes. The scale on the inner Calculator. room Clock. disk shows the " time of appearance " in seconds or minutes. In use the pointer is set to the factor for the developer in use, and against the " time of appearance " on the inner scale will be found the total number of minutes required for complete development (fig. 72). The " calculator " can be used with any ordinary clock or watch, but the " dark-room clock " (fig. 73) has been specially constructed for the factorial system. It is an improvement on the earlier forms of Watkins' Eikronometer," and has a 4 in. dial with so minute and 100 seconds divisions, very plain for dark rooms, centre seconds hand, stop action and outside indicator to mark the completed time. The seconds hand completes the revolution in Too seconds, while the minute hand does so in to minutes, or sufficient for the longest ordinary development, though it runs on, if necessary, very much longer, both hands starting together always at O. In 1908 Watkins brought out another system of " thermodevelopment " by time dependent on the use of a standard " time developer," the duration of the development, at a given temperature, being modified according to the make and speed of the particular plate in use. The temperature variations are indicated by a movable scale, or " thermo-calculator," on the bottle of developer, the variations for development speed of various plates being given approximately on the ' Watkins' Plate Speed List," which thus shows the " speed of plate " and " speed of development " with the standard developer at 6o°. This method is well adapted for plates, films and stand development in tanks or machines, no observation of the plate being required, and the times are most conveniently observed with the " dark-room clock." Full details of these two distinct methods of development will be found in the 4th edition of the Watkins' Manual of Exposure and Development. C. W. Piper's " photographer's stop clock " (1906) is a more elaborate clock, intended for use not only in " time development " but for all photographic operations in which accurate control in regard to time is of importance. It is fitted with a gong and arranged to work by " time " or " bulb." Once started, by pressure on a lever or on the bulb, it will continue to go until stopped, striking the gong at the completion of every minute, when the seconds hand reaches the zero point. A second pressure on the bulb stops the clock, so long as the pressure is continued, while pressure on a lever stops it permanently. It is thus useful for timing any intermittent operations, whilst the clock adds up the separate times and prevents the occurrence of errors difficult to avoid when timing with an ordinary watch. By an additional attachment a prolonged time exposure with the camera may be terminated, or an " instantaneous " or short " time " exposure given at any prearranged time. Messrs Houghton's " Ensign " clock for time development has a dial with 6o divisions, a single hand, and is fitted with a gong. It can be set to ring an alarm bell at the expiration of any period from one minute to one hour, can be started or stopped immediately and is easily read in the dark-room. It requires no winding up, the action of setting providing the tension for the recording movements. It can be stopped and started at will and the bell arranged to give a short or prolonged ring. S. Stanley's is another convenient form, with a 41 in. dial, divided into 6o seconds and 6o minutes, the thick hand recording the seconds and the thin hand the minutes. Several forms of developing tanks and machines have been constructed for developing a number of exposed plates, together with ordinary or dilute developers, with the aid of the factorial system or independently of it. The Kodak " Automatic Developing Tank " (1905) is a useful arrangement by which bands of ex-posed roll films can be developed in daylight, without any need of a dark-room (fig. 74). The exposed film is wound from the spool into a red celluloid apron contained in a box A, then placed in the tank B, where it is left in a dilute developer for about twenty minutes, and requires no attention. It gives very good results. For the " Brownie " films a special daylight developing box is made. With the Kodak " Eastman Plate-developing Tank " (1908) the exposed plates are removed, in the dark-room, from the plate holders and placed, in pairs back to back, in a special framework holding six pairs, which is lowered into a metal tank containing the developer, and is fitted with a watertight lid so that it can be inverted during development. A clock face, with pointer, by which the period of development may be noted is fitted outside the tank. Another apparatus of the kind is made for developing celluloid films exposed in the " Premo Film Packs " (fig. 75). Other forms are made, and in some the fixing and washing can also be effected. These tanks undoubtedly save much time arid trouble in developing a large number of exposed plates or films, and have been found to work with efficiency and regularity. Eastman Kodak Co. brought out in 1907 a machine for developing paper prints on bromide or gaslight papers.
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