The advance of photography : its history and modern applications (1911)

244 THE ADVANCE OF PHOTOGRAPHY and blue filter negatives are printed upon a specially prepared celluloid film coated with gelatine containing silver bromide. These are sensitized by immersion in a solution of bichromate of potash for three minutes. They may also be obtained ready for use together with a special case in which they may be stored. The two prints are obtained by making exposures under the two negatives, the celluloid side of the film being placed in contact with the negative and the exposure continued until all the details are apparent as a brownish yellow image. The film is now removed from the frame and immersed in warm water, when the unexposed gelatine will be dissolved and a white image left. The two prints after this treatment are next fixed in clean hypo-sulphite of soda in order that the silver bromide may be removed and a transparent low relief in clear gelatine will remain. Next it is necessary to stain these prints ; that from the negative obtained with a green filter is stained a pink colour in a bath containing a special dye, and that from the blue violet is stained yellow. Suitable dyes for this purpose are on the market. The necessary detailed instructions are supplied with the plates and dyes. The films are dried after being stained, and finally mounted in superposition on the blue transparency. It is, of course, absolutely essential that the three positives should be in exact register. The films are kept in position by means of binding strips, and the mask and ordinary cover-glass are next placed on and the whole bound together in the usual manner. Wood's Process. — In an article to the Philosophical Magazine for April 1899, R. W. Wood describes a process by which diffraction gratings can be used to enable one to obtain a coloured photograph from three negatives. A diffraction grating may consist of a piece of plane glass on which is ruled or photographed a large number of lines