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

92 THE ADVANCE OF PHOTOGRAPHY Abney's Work. — A very interesting observation was made by Abney, in England, in relation to the foregoing process. He remarked that if an exposed film of gelatine remained a long time in the dark the insolubility increases. Accordingly, a film of this kind which, freshly developed, would only give a faint image, after some hours gives a strongly defined image. This fact allows the time of exposure for pigment pictures to be considerably reduced — that is, a larger number of pictures to be made in the same time. Marion's Work. — Still more interesting is an observasion of Marion, at Paris. He exposed under a negative a sheet of paper sensitized with bichromate of potash, and then pressed it in the dark on a moist film of coloured gelatine also sensitized with bichromate. The gelatine became insoluble wherever the paper had been affected by the light, and on development with hot water a pigment picture was obtained on the paper. We have seen that the parts of a gelatine film containing bichromate of potash, which are exposed to the action of light, become insoluble in water, and do not swell on being moistened ; at the same time they acquire the property of adhering to fatty inks. Thus if an exposed film is moistened with a wet sponge, the unaltered places only absorb the water. If, on the other hand, printer's ink be rubbed over the film, those places only which have been changed by the light retain the ink. This fact was discovered by Poitevin, the author of many valuable discoveries in photographic chemistry. If a piece of paper be pressed on such a film of gelatine, coated with printer's ink, the ink adheres to the paper, and thus a print is obtained of the negative, under which the film had been exposed. This peculiar mode of printing gave at first very imperfect results. The process was rendered difficult from the fragile nature of the gelatine film, the difficulty of