XLI. Some experiments on photographic sensitivity

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Abstract
A technique is described for the preparation of optically clear silver bromide in the form of thin plane parallel sheets with flat surfaces. These sheets have been used to investigate the formation of the latent image under illumination. It has been found that the distribution of the latent image between the surface and the interior of the crystals depends on their physical properties, on the nature of the sensitizing layers present on their surfaces, and on the exposure. A surface latent image is formed, together with a weak internal latent image, when well annealed crystals are exposed to light. A surface latent image, on the other hand, is not formed with strained sheets of pure silver bromide, if they have not been annealed at all or only at low temperatures. With such material, an internal latent image is formed by the separation of silver along dislocation lines, which may form the boundaries of an irregular polyhedral structure. The crystals may then be developed after the dissolution of the surface with a solvent for silver bromide. Crystals which give a strong internal image and no surface latent image may be sensitized for the formation of a surface latent image by the condensation on their surfaces of thin films of silver, gold and of certain metallic sulphides from atomic or molecular beams. After a brief exposure to light, a surface latent image is formed on the sensitized crystals which can be developed to a normal negative .image (black in the exposed regions). Longer exposures cause the disappearance of the surface latent image in the exposed areas and the formation of a surface latent image in the unexposed areas. The reversed or positive image (white in exposed regions) can also be developed. This behaviour corresponds to solarization in the photographic process. The internal latent image did not show any trace of reversal or solarization even after very prolonged exposures. The thin films of the sensitizer are shown to provide traps for positive holes but not for electrons. This seems to be the role of the chemical sensitizers. The recognition of this fact necessitates a revision of the ideas on sensitivity specks which have previously been accepted. It is now suggested that sensitivity centres rather than specks are involved and that these arc regions of localized structural imperfection. The traps for the electrons are provided by the silver ions associated with them. All the experimental evidence is consistent with the hypothesis that the latent image consists of a group of silver atoms exceeding a critical size and situated either on the external surface or on an internal surface associated with a mechanical imperfection of the crystal (e.g. a dislocation). The experimental results lead to a new working hypothesis on the formation of the latent image in silver bromide crystals which appears to account satisfactorily for: (1) the distribution of the latent image between the surface and the interior of the crystal under different circumstances; (2) the formation of the latent sub-image and its transformation to a stable latent image; (3) low and high intensity reciprocity failure and the influence of chemical sensitization upon them; and finally, (4) solarization.

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