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APPARATUS] are remarkably good and practically solve the problem of direct colour photography in a simple and fairly inexpensive manner (see Agenda Lurmère, 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:— 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” lates, 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 sometimes 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 1870. 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 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 silver

chloride in collodion, and known as “C.C.” or “collodiochloride.” The basis of most of the develop able 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 printing 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 König’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 considerable 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 photographic 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

. 72.—Watkins’s Factorial Calculator.

. 73.—Watkins’s Dark-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 4 the “time of appearance” on the inner scale will be found the total number of minutes required for complete development (fig. 72). 