The Mechanism of Formation of the Latent Photographic Image¹

Abstract

IN a communication to NATURE of Sept. 24, 1927 (vol. 120, p. 441), the preliminary results were described of experiments made in an attempt to correlate the mechanism of the latent image formation with that responsible for producing changes of conductivity on illumination. It was shown that the apparent absence of the photo-conductivity effect in the ultraviolet was due to two things: (1) the small penetration of that light, and (2) the use of thick layers of the silver halide. With thinner layers, of the order of 70µ, the ultra-violet (λ3650) effect in silver bromide was found to be about twice as great as that produced by the blue (λ4358), thus supporting the original prediction that in very thin layers of the order of 1-5µ the effect at λ3650 would rise to nearer ten times that at λ4358, which is the ratio of photographic effects in very thin layers of slow, pure silver bromide emulsions. It was further predicted that in very thin layers the ‘hump’ of maximum sensitivity at λ4600 in the photo-conductivity–wave-length curve would disappear. How completely these conclusions have now been verified can be seen from the accompanying graph (Fig. 1). The inference is that in very thin layers of silver bromide the three curves representing (1) the relative photo-conductivity effects, (2) the relative photographic effects, and (3) the relative light absorptions, each plotted against the wave-length for equal incident intensity, are closely the same, indicating that in all probability the primary stage of the photographic mechanism is intimately connected with that which produces conductivity changes on illumination.