Cinematographic annual : 1931 (1931)

174 CINEMATOGRAPHIC ANNUAL induction is reduced and the velocity enormously increased by the presence of silver sulphide nuclei. This strongly suggests that the essential for developability is not a nucleus of certain size, but one capable of properly anchoring sufficient silver atoms to start a unit cell of a silver crystal. This might be 16 or less. It has been suggested by E. R. Bullock (22) that "the unit of latent image (or latent fog) in its most effective usual form for chemical or physical development is a single silver atom at rest." Later (23) Bullock indicates Meidinger's results as supporting this conception. It is somewhat difficult to see why sensitivity nuclei, e.g., of silver sulphide, should give enhanced sensitivity on this view, especially if chemical development is essentially a reaction in the solid phase (24) . Hilsch and Pohl's results indicate that exposure of silver halides forms "centres" of photo-silver which are probably not monatomic, but also are not usually of high aggregation — over 100 atoms — since the absorption curves of the photo-silver are very different from those for colloidally dispersed silver. On the other hand, the considerations herein advanced increase the pertinence of Bullock's criticism (25) of the hypothesis that minimum weight of size per se is "the necessary and sufficient condition for developability." The question has been discussed to some extent by A. Steigmann (26) with explicit reference to the question of minimum nucleus size for developability. This author does refer to "the catalytic state of the ripening nuclei," by which apparently he signifies both its presumed hydrogen-activating activity (according to Steigmann and Kogel's theory of sensitizing) but states that it has also the function of a "form catalyst, the task of which is to accelerate a localized crystal growth* [on exposure] , so that on quite short light reduction, that magnitude^ is reached which is required for reduction catalysis in the developer." His discussion therefore still emphasizes the attainment of critical size of the developable nuclei, although a qualifying suggestion in regard to the sensitizing nuclei is made. The suggestion that minimum space lattice requirements were more essential than minimum size per se, seems to have been first clearly stated by H. J. Volger and W. Clark (27). In discussing the latent image and developability, they suggest "the condition be sufficient silver atoms in a group to form a strong space lattice." On following through this hypothesis it is not size of speck as such which is important for development, but aspect, e. g., active edges which can orient and adlineate photo-silver atoms to initiate the silver lattice. From a wavemechanic standpoint, the catalytic action of the specks might be limited to increasing the density of probability of states of equal energy of the systems. + = Ag X >Ag + X and hence localizing or orienting the transition. On this view, size of sensitizing speck still retains an important role in the stage of exposure to light, since the chance of local concentration is increased. * Author's italics. | Writer's italics.