The Theory of Photographic Sensitivity

Abstract

SINCE 1920 our knowledge of the formation of the latent image has been greatly increased by investigations which divide themselves roughly into three chapters. The first notable advance was made by Svedberg during the years 1920–1922, and consisted in the proof that, the latent image consists of small centres distributed on the silver halide grains of an exposed photographic plate, the distribution of the centres being entirely according to the law of chance. A grain is not developable unless it contains at least one such centre. The second step was to decide upon the origin of these development centres. Two theories were advanced. In one of these it was assumed that the light itself formed the centres, the grains themselves being supposed to be homogeneous. In the other, it was assumed that there exist in the grains, prior to exposure, nuclei which are, not silver halide and which are formed during the manufacture of the emulsion, and that the action of light is to change their condition in such a way that they become centres at which development can start. The first theory was extensively investigated and modified by Silberstein and others in the Research Laboratories of the Eastman Kodak Company, whilst the second was developed by Toy in the laboratories of the British Photographic Research Association. Direct evidence in favour of the second theory was at length obtained by Clark in the latter-named laboratories.