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176 THE ADVANCE OF PHOTOGRAPHY
soluble in the fixing bath ; (2) a partial breaking up of the haloid forming a rapidly developable product which is prevented from development by oxidizers and is insoluble in the fixing bath until oxidized ; and (3) a complete breaking up of the haloid with the liberation of the halogen, forming an undevelopable product which is not acted upon by oxidizers and is insoluble in the fixing bath. The last of these products appears at the latest about the beginning of the reversal period.
Sheppard and Mees,1 who have investigated this question from the standpoint of the electron theory, consider that the photographic process consists in the passage of ionized silver to the metallic state, with a suboxidation stage probably intermediate. " Ripening " seems due (a) to the formation of resonating systems or clumps of halides, and (6) to the formation of an intermediate reduction product, and the function of the gelatine is to assist in both of these. On exposure, light is absorbed and electrons are set free which ionize the haloid and the surrounding air ; the gelatine having a high dielectric constant conserves the electrons ; and the gas, according as it removes the electrons, or not, may reduce the sensitiveness ; electrons may be emitted either from the halide or from the sensitizers used. The gelatine combines with the free halogen, and the ionization of the halide leads to chemical reduction, probably a half halide of the form Ag2X is produced in solid solution with the halide present, this being the latent image.
According to W. D. Bancroft 2 only those rays of light which are absorbed by the film have the power to produce any chemical action upon the contained silver salts ; further, all the absorbed rays have not equal powers in this direction.
The presence of a depolarizer increases, or, it may be,
1 Roy. Soc. Proc, Ser. A, 78, pp. 461-472, 1907.
« Journ. Phys. Chem., 12, pp. 209-278 and pp. 318-376, 1908.