﻿Quantum Theory of Photographic Exposure. 253 



Originally, these experiments were started independently 

 of that theory, onr intention being primarily to study the 

 effect of the clumping, or clustering together in groups, of 

 silver halide grains in a photographic emulsion *. According 

 to Slade and Higson f, " It seems reasonable to assume 

 that each grain acts quite independently and that one grain 

 which has become developable is unable to make a grain, 

 situated in close proximity to it, developable unless the 

 latter grain is developable in itself." From the table 

 (p. 256) it is readily seen that this statement is not true, but 

 the grains when clumped together act as one grain for 

 development to the limit. 



Since each clump acts as one grain a very much broader 

 range of grain sizes, or their equivalent, is obtained extending 

 from the smallest single grains up to the largest clumps 

 (containing 30 or more grains) in a given emulsion. 



It was thought that these results afforded a rigorous test 

 for Silberstehrs theory, and it seemed therefore worth while 

 to compare them with the implications of that theory. 



Silberstein's fundamental formula is essentially, i. e. apart 

 from chromatic complications, and disregarding the lateral 

 dimensions of the light quanta, 



& -j -na 



where N is the original number, per unit of the p'ate, of the 

 class of grains of size (area) a, n the number of incident 

 light-quanta, again per unit area of plate, and k the number 

 of grains affected J. If the finite cross section, a of the 

 light-quantum is taken into account, a is to be replaced by 



'-['Va*- 



At first, the rapid increase of k with the size a as required 

 by that formula seemed (qualitatively speaking) not only 

 attained but even exceeded by the experimental results. 

 This seemed to us to indicate that the sizes (areas) of the 

 clumps of grains were under-estimated by us. In fact, for 



* Extensive experiments are being conducted in this laboratory to 

 study the clumping of grains in different concentrations of the same 

 emulsion. These investigations will be published later. 



t Slade and Higson, "Photochemical Investigations of the Photo- 

 graphic Plate." Proc. Roy. Soc. xcviii. p. 154 (1920). 



% Silberstein, L., "Quantum Theory of Photographic Exposure,'' 

 infra, p. 257. 



