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

90 THE ADVANCE OF PHOTOGRAPHY apparatus, which would not pay in the limited sphere of portrait photography. By this relief process a good deal of work was at one time done by printing on glass ; thus Goupil prepared copies of oil paintings by this method which were much in request. The Woodbury process was also used for preparing slides for projection by lantern. Poitevin's Discoveries. — We have seen above that gelatine mixed with bichromate of potash becomes insoluble in the light. This fact was made by its discoverer, Talbot, the basis of heliography — that is, of photographic steel engraving. Poitevin, a Frenchman who did much to promote photograprry, founded on the same method another process : he produced pictures in various colours. He first used carbon as a pigment, thus obtaining carbon pictures. The process is simple : Poitevin coated paper with prepared gelatine coloured with lampblack, and exposed this under a negative ; he then washed the film in hot water, which dissolved those parts of the gelatine unaffected by the light, whilst the insoluble parts retained their colouring matter and thus formed a picture. Practical difficulties occur even in this simple process. It has been already mentioned that the action of the light does not always penetrate the whole thickness of the film. The half-tones have, therefore, no support, and are torn off in washing (see fig. 22 d, p. 87). Before treating the films with hot water, it is therefore necessary to transfer them, as described in the part dealing with photoreliefs. An albumenised sheet is pressed in the dark on the coloured film of gelatine, and then the whole is plunged into hot water ; the half-tones adhere to the paper pressed upon them, and the image appears uninjured on it, as in fig. 22 e. The picture is, of course, reversed — that is, what was