PRINTS 881. Tricolour Transparencies. It is not pos sible in this volume to describe in detail the numerous working methods by which tricolour transparencies can be obtained by superposing in register three images on film. We may men tion, as very suitable for obtaining the three elementary monochromes for this method, the various processes of dye-toning with mordants (§ 602 •to 604), and, among the pigment pro cesses, the process of imbibition of gelatine reliefs, details of which, for multicolour images, have been given (§ 676).
The sole difficulty is to ensure the balance of the three images ; for this the prints, made by contact or by enlargement (when the prints are made on gelatino bromide films), are exposed for the same time and developed together under the same conditions. As a rule, several sets of elementary monochromes are made and dyed to various depths of colour ; by repeated trial, three images are found which give the most faithful reproduction of the subject. Before finally mounting, the three images so chosen are used as patterns, and other prints are made of this depth as far as possible.
When gelatino-bromide films are used for this process they must be chosen with as thin a support as possible, so that the three elemen tary images are in planes as close as possible to each other. Films of the same make should be used, with the object of avoiding failures through differences in the shrinkage of the supports.' 882. Tricolour Prints on Paper. In addition to the process by imbibition of gelatine reliefs, of which we have already described the transfer to paper (§ 676), it is possible to make tricolour prints by the carbon process and its variants (carbro), or by the process of hydrotype printing by discharge of dyes.
Makers of carbon tissue supply on request the special colours required for tricolour work (the blue is usually not green enough, nor is the red purple enough). It is essential to cut the various pieces the same way of the paper so that dis tortion due to wetting and drying may be about the same in the three images. As a temporary support a sheet of glass or celluloid is used, so that the effect may be judged before final transfer. Double-transfer paper is generally used only after several wettings and dryings, with the object of reducing the error from this cause during the final transfer.
In the case of hydrotype prints, the partial discharge of the dye during the trials to test register can be prevented by placing a sheet of very thin celluloid between the relief and the gelatine-coated 883. Kodachrome Film. Kodachrome film, the use of which is at present restricted to sub standard cinematography and to photography in miniature sizes, permits of colour photography with an ordinary camera without the addition of any special and with exposures of roughly the same order as those used for rapid black and white film, but with considerably less latitude in The base receives successively (in addition to the anti-halation backing and the substratum) five superposed layers, which, starting from the base, are an emulsion sensitive to blue and to red, a colourless and very much hardened gelatine isolating layer, an emulsion sensitive to blue and to green, a second isolating layer, and, lastly, an emulsion dyed yellow which is sensitive to blue only. The layers of emulsion,
and especially the isolating layers, are extremely thin, their total thickness not exceeding that of an ordinary reversible emulsion. Owing to the presence in the upper emulsion of the yellow dye, the three emulsions record blue, green, and red respectively (starting from the outer emul sion). After inversion the corresponding posi tive images must therefore be respectively in the same order, yellow, pink, and blue-green.
After the photographs are taken the film is sent to one of the processing works where it is subjected to the following operations.
The film is first developed as a negative, after which the metal silver forming the three selec tion provisional negatives is dissolved. The film then passes in front of a thermo-electric couple excited by a beam of infra-red light which is modulated successively by each of the pictures so as to automatically regulate, by means of a relay, the power of the lamp by which the second uniform exposure is given 044o), thus compensating largely for errors in exposure. Re-development is done in a developer contain ing, besides the developing agent, an adjuvant forming, with the oxidation products of the developer, an insoluble blue-green dye which is deposited on the image in amount proportional to that of the reduced silver (§ 350) forming the provisional positives. The three black positive images are thus each doubled with a blue-green image which ultimately is to remain only in the lowest image. After elimination of the silver bromide not used for the positive images, the film is washed and dried. It is then placed in a bath, the action of which is restricted to the two upper images, which oxidizes the silver to silver chloride and destroys the blue-green dye. After washing, the film is exposed to light and re-developed in a developer similar to the pre ceding one, but giving a pink image, and it is then washed and dried. The film is then placed again in the oxidizing bath, the action of which is now restricted to the superficial layer of emulsion from which the pink dye deposited during the previous operation is thus eliminated. After washing, the film is re-developed in a developer, leaving in the gelatine a secondary yellow image. After washing, the silver of the three images is finally removed, and the film is washed and dried.'