﻿Quantum Theory of Photographic Exposure. 957 



the authors remark only briefly that an analogous concep- 

 tion might also assist in the interpretation of the mode of its 

 action ; but add that if the quantum hypothesis be assumed, 

 " the difficulty arises that the real blackening curve has not 

 the exponential form prescribed by this hypothesis if we 

 suppose each halide grain to be made developable when 

 struck by a single light quantum/' They seem to forget 

 that the simple exponential formula yielded by a quantum 

 theory relates to the case of equal grains, which is not that 

 of real emulsions, and that in order to obtain the blackening 

 curve (say density 1) plotted against the logarithm of 

 exposure) that elementary formula has to be integrated over 

 the range of sizes, which presupposes the knowledge of the 

 frequency curve of the emulsion, and the somewhat intricate 

 question of the relation between the photographic " density " 

 and the total of blackened areas has to the treated *. The 

 latter question, simple though it be for one-layered coatings, 

 becomes particularly intricate in the usual case of many 

 layers of grains. It is for this reason that the best way of 

 testing a similar theory consists in microphotographic counts 

 and planimetric measurements of the individual grains. At 

 any rate, Svedberg and Andersson propose to turn to another 

 more complicated assumption f which, they expect, " will 

 actually predict a blackening curve of S-shape."" They 

 propose to discuss this possibility on another occasion. 



The second of tho papers alluded to, due to Professor 

 Svedberg himself (Phot. Journal, April 1922, p. 186), has a 

 more direct bearing upon our subject, and may turn out to 

 supplement our own tests by offering, as it were, an inter- 

 mediate link in the conjectured mechanism of the action of 

 impinging quanta or light darts. In this paper Svedberg 

 proposes to explain the behaviour of the grains noted in his 

 preceding paper by a single hypothesis, and to test the latter 

 directly. His hypothesis is that the product of the light 

 action on the halide grain consists of " small centres distri- 

 buted through the grain or through the light-affected part of 

 the grain according to the laws of chance/' and that a grain 

 will become developed if it contains one or more such centres. 

 If v be the average number of centres per grain, the per- 

 centage probability that a grain will contain at least one 

 centre (and will therefore be developable) is P = 100 (1 — e~ v ~). 



* Concrete examples of such a kind will be treated in the third paper 

 on our subject. 



t Namely, that a certain minimum number of quanta must strike the 

 grain within a certain maximum part of its area in order to " build up a 

 silver nucleus large enough to act as a reduction centre." 



