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

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62 THE ADVANCE OF PHOTOGRAPHY iodide of silver, and not of chloride of silver are exposed in the camera. The image thus produced by ordinary exposure is invisible, but becomes visible through a subsequent process, named the developing process. In daguerreotypes the exposed plate of iodide of silver was further exposed to the vapour of mercury. In this case the vapour was condensed in fine globules on the exposed places, in proportion to the change caused by the light. In the treatment with collodion, the plate was washed over with a solution of green vitriol. This mixed with the adhering solution of silver, and precipitated from it a fine black silver powder, which adhered to the exposed places of the plate. Therefore, in both cases we have a finely pulverized body, which is attracted and retained by the exposed places — a mysterious process, as interesting as it is practically important. From this it appears that it is by no means the colouring of the silver salts which renders the image visible, but the subsequent developing process. If an experiment be made with chloride, bromide, and iodide of silver simultaneously, by exposing and developing them, it is found that chloride of silver gives the feeblest picture under the developer, bromide of silver a stronger one, and iodide of silver the strongest. Therefore, the very body which was most strongly coloured by light is the least coloured under the developer, and the body which is least coloured by the light, viz., iodide of silver, is the most coloured under the developer. The developing process is of immense importance. If it were attempted to produce a picture by exposure in the camera without developing, an exposure of hours would be required before the impression could be seen. The developing process permits, under favourable circumstances, the impression to become visible after an exposure of a very small fraction of a second.