The nature and formation of the photographic latent image

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Abstract
A review is given of a theory of the nature and mode of formation of the photographic latent image and of the experimental evidence on which the theory is based. Earlier hypotheses due to Eggert and to Gurney and Mott are used, according to which the primary action of the light is to free photoelectrons and positive holes and the formation of silver specks involves the movement of silver ions. The stable latent image is seen as a positively charged silver speck on the surface of a halide grain, or in the case of internal image at a dislocation. The minimum stable image is a positively charged group of four silver atoms (Ag4 +), the formation of which requires three electrons. The sub-image is a neutral pair of silver atoms and requires two electrons for its formation. A main role of silver sulphide is to provide traps for positive holes; after doing 80 the sulphide molecule dissociates into AgS and an interstitial cation. In terms of this model a discussion is given of reciprocity failure, development, the role of iodide and of other phenomena.

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