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DRY PLATES, FILMS AND PAPERS 177
renders possible the action of the light, and the ability which some substances possess of being easily oxidized, and others of being easily reduced, depends on the depolarizer. He considers that sensitizers are either direct or indirect polarizers and are light sensitive, also that they are either reducing agents, or become such when acted upon by light.
Reversal or Solar ization. — Bancroft has also published some lengthy papers dealing with the reversal of the photographic image, or, as it is sometimes called, " solarization " x
Briefly, his views upon this subject may be expressed as follows :
The reversal of the photographic image is due primarily to a change in the silver salt, which is independent of the nature of the medium in which it is imbedded ; the readiness with which the reversal takes place is, however, markedly dependent upon the nature of the gelatine or collodion. It is much easier to obtain a reversal with a gelatine than with a collodion plate. The grain of the emulsion, too, is important in this connection, for reversal takes place much more readily with a coarse-grained emulsion than with a fine-grained one. When pure silver bromide is used reversal is difficult to obtain. A great variety of causes can bring about this action ; thus it can be produced by prolonged exposure of the sensitive plate to light, by the antagonistic action of light of different colours, by exposure to white light followed by immersion in a solution of an oxidizing agent and exposure to the spectrum, by the use of stained plates, by partial development and subsequent treatment with silver nitrate, by the action of white light during development, by the very slow development of an under-exposed plate, by the addition of thio-carbimide, etc., to the developer, by very short and intense exposures,
1 Journ. Phys. Chem., 13, pp. 181-250 and pp. 269-331, 1909 ; 13, pp. 1-90, 1909, pp. 449-468, 1909. M