The advance of photography : its history and modern applications (1911)

178 THE ADVANCE OF PHOTOGRAPHY with subsequent short exposure to diffused light, or by kathode rays followed by sunlight. Bancroft also finds that the stronger the developer, and the more prolonged the development, the easier it is to obtain a reversed image. On the other hand, the very slow development of an over-exposed plate will produce a normal image. The latent reversed image is more readily destro}^ed by bromine, nitric acid, ammonium persulphate, chromic acid, etc., than the latent normal image. Silver nitrate and potassium nitrate apparently prevent solarization when added to the film before exposure, although it also is probable that the rapid reduction of the silver salts in the presence of these halogen absorbers masks the solarization. According to Eder there are three images on an undeveloped over-exposed plate, viz., a latent normal image, a latent reversed image, and a normal silver image. According to Liippo Cramer the latent reversed image consists of a reduction product of silver bromide, but, according to others, there is no such thing as a latent reversed image, the phenomena being due to the regeneration of silver bromide. The developable image consists of silver bromide and an a-silver sub-bromide which acts as a catalytic agent, causing the developer to reduce the silver bromide ; this a-silver sub-bromide can be reduced by light to a /3-silver sub-bromide which is also reduced by the developer but has no catalytic properties. Solarization occurs when the a sub-bromide has been changed in the high lights very largely to the (3 sub-bromide and this change has not taken place to an}7 great extent in the shadows. The photo-chemistry of the sub-haloids of silver has also been investigated by A. P. H. Trivelli,1 who traces out the suc 1 Trivelli also deals with the problems of solarization in the following papers :—Zcitschr. Wiss. PhoL, 6, pp. 197-216, 237-257, 273-299, 1908;