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

262 THE ADVANCE OF PHOTOGRAPHY The dyed print plates can be used for obtaining coloured pictures on paper over and over again. After the print is finished it is advisable to immerse it in a fixing bath, since this hardens the gelatine and makes the colours more permanent. Transparencies in colour can be made if desired. In this case a glass is coated with a bichromated gelatine, and this when dry is exposed under one of the positives, washed, and stained up in the corresponding dye ; the plate again coated and exposed under a second transparency, washed and stained ; and similarly for the third, unless it is desired to make the yellow image on a second prepared plate, and to use this as the cover glass. Imbibition Process. — Another process somewhat similar in principle is known as Sanger Shepherd's Imbibition Process. The positives for this are printed upon a celluloid film coated with gelatine containing silver bromide, the gelatine being sensitized with potassium bichromate. Thus, in this process, use is made of the relief surface of hardened gelatine obtained by developing such a film after it has been exposed under one of the three negatives. When the positive film has been exposed and developed in warm water, the white bromide of silver which remains is removed by hyposulphite of soda. Three positives are made, one from the negative obtained with the red colour-filter, one with the green, and one with the blue. These are next dyed up ; the print from the green negative being placed in a pink dye, the one from the red in a blue dye, and that from the blue in a yellow dye. Now it is known that if a gelatine relief stained with a suitable dye-stuff is squeegeed in contact with a damp sheet of gelatinized paper, the dye will leave the harder gelatine surface and adhere to the softer gelatine coating on the paper. Hence after this operation the positives are squeegeed in the following order on to the surface