﻿Quantum Theory of Photographic Exposure. 959 



The existence of the aforesaid " centres '" as seats of 

 incipient development, around which the developer's action 

 gradually spreads, has been known for some years *, and has 

 been observed, among others, by Trivelli. But the important 

 discovery that these centres are haphazardly distributed is 

 entirely due to Professor Svedberg. If his results are 

 ultimately confirmed by further experiments, especially for a 

 series of different exposures, it will be possible to consider 

 these centres as an intermediate link in the theory proposed 

 in our first paper (the centres marking the spots where the 

 grain's were hit by the light-darts). In the meantime, 

 however, our further tests have to be conducted by con- 

 sidering the last link of the chain, i. e., by counting the 

 grains of each size-class affected and ultimately developed. 



2. Before passing on to the description of our further 

 experimental results, a few words must be said in defence of 

 the property attributed in our first paper to clumps (aggre- 

 gates) ol* grains which apart from some single grains con- 

 stituted our chief material. An explanation seems the more 

 necessary, as another recent paper by Svedberg f contains 

 results apparently clashing with what we believe to be the 

 behaviour of clumps with respect to light. The property 

 assumed by us, as the expression of experimental facts, was 

 that a clump, i. e., an aggregate of grains in contact with one 

 another, behaves as a photographic unit, by which is meant 

 that if any one of its component grains is made developable, 

 the whole clump will be reduced by a sufficiently long develop- 

 ment. AVe have since been able to test this behaviour in a 

 variety of ways. 



On the other hand, Svedberg concludes from his experi- 

 ments that there is no transference of reducibility (develop- 

 ability) from one grain to another ' f in direct contact'' with 

 it. (See especially p. 184, loc. cit.) 



This apparent discrepancy seem to be due to the circum- 

 stance that Professor Svedberg worked with an emulsion (a 

 single kind only) consisting of rather small and almost 

 spherical grains, whereas our material, and especially the so- 

 called AV-12-C experimental emulsion, with which all the 

 work in question is being done, consists predominantly of 

 large and very thin, flit polygonal plates or tablets which are 

 in mutual contact either along- a v\hoie edoe or, still more 

 intimately, are partly piled upon or overlapping each other. 



* Cf. M. B. Hodgson, Journ. Franklin Inst., November 1917. 

 f On "The Reducibility of the Individual Plalide Grains," Phot. 

 Journal, 1922, pp. 183-186. 



