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XXXI. On the Theory of the Characteristic Curve of a Photo- 

 graphic Emulsion. (Communication No. 22 from the 

 British Photographic Research Association Laboratory.) 

 By F. ( !. Toy, M.Sc, F.Inst.P., F.R.P.S. * 



IN the most recent investigations on the relation between 

 the photographic effect and the light-exposure, special 

 plates containing only a single layer of grains have usually 

 been employed. With such plates the photographic effect 

 is determined by counting the percentage of grains made 

 developable. The curve expressing the relation between this 

 percentage (x) and the logarithm of the exposure may be 

 called the characteristic curve of a single-layer emulsion, 

 corresponding to the ordinary curve of a commercial 

 emulsion, in which, instead of x, values of the density (in the 

 photographic sense) are plotted. 



In a recent paper (Phot. Jour. 1921, lxi. p. 417) the author 

 has shown that such a curve, for a set of grains which are 

 geometrically identical, is of the usual S-shaped tj^pe, i. e. a 

 difference in size or shape does not account for the fact that 

 all the grains do not become developable with the same ex- 

 posure. Now, a set of geometrically identical grains, all in 

 a single layer and similarly orientated to the incident light, 

 represents the simplest possible emulsion which we can in- 

 vestigate experimentally. It also corresponds to the simplest 

 theoretical case, eliminating many complicating factors 

 which, though greatly affecting the form of the characteristic 

 curve, have nothing to do with the primary mechanism of 

 the photograph process. In other words, with this emulsion 

 the curve is reduced to its " purest " form, and is determined 

 almost solely by the photochemical process which takes 

 place. 



It is now generally believed that the primary action of 

 light on the grains is to form in or on the surfaces of them 

 certain " centres " or " points of infection " which act as 

 starting-points for their reduction by the developer. This 

 view has for some time had considerable evidence in its 

 favour. Chapman Jones (Phot. Jour. 1911, li. p. 159) 

 showed that by stopping development at a very early stage 

 it is possible to get particles of silver too small to be visible 

 microscopically, but which can be shown to be present by the 

 colour imparted to the film, and by enlargement to visible 

 dimensions by the deposition on them of mercury. Hodgson 

 (Brit. Jour. Phot. 1917, p. 532) carried development a little 



* Communicated by Prof. A. W. Porter, F.R.S. 



