﻿354 Mr. F. C. Toy on the Theory of the 



These facts are at any rate sufficient to justify an attempt 

 to explain the relation of the number of grains changed to 

 the light-intensity on the basis of the existence in the 

 grains of: actual particles which are not silver halide. These 

 are not necessarily all changed to reduction centres from in- 

 active particles by the same light-energy as they would be 

 if they were single molecules of the halide. We shall make 

 no assumptions as to the composition of these centres, and 

 the theory does not depend on their being composed of 

 colloidal silver. We shall use the term " nucleus " rather 

 than centre to indicate the presence in the grains of actual 

 particles before exposure. 



Ihe Characteristic Curve of a Set of Geometrically 

 Identical Grains. 



llieoretical. 



Our first object is to consider the case of a set of grains of 

 identical size and shape, and to determine the relation we 

 should expect to find between the percentage of these which 

 are made developable and the light-intensity. The time of 

 exposure is kept constant throughout. 



If we consider a volume V of the silver halide which is 

 very large compared with that of a single grain, we may 

 assume that the total number of nuclei in any such volume 

 of the emulsion is the same, though the number contained in 

 individual grains in this volume may vary. We will define 

 the sensitivity of a single nucleus as the minimum intensity 

 which must be incident upon it in order to make it " active " 

 in the presence of the developer. For a given intensity of 

 the incident light there will be a definite number of such 

 active nuclei in every volume V, and they will be distributed 

 amongst the grains entirely haphazard, according to the laws 

 of chance. Every grain which happens to have at least one 

 active nucleus will be developable. 



When the intensity of the light increases, more grains are 

 changed. On any " nucleus " theory this happens because. 

 more nuclei are present, so that a single grain has a greater 

 chance of having at least one of them. This may be ex- 

 plained in one of two ways. Firstly, all nuclei may have the 

 same sensitivity, say I, but owing to the rapid absorption of 

 light, those nuclei which are situated in the grain at some 

 distance from the surface on which the light is incident, do 

 not receive an intensity of I when the incident intensity is 

 small. As the latter becomes greater, the volume of silver 

 halide, throughout which the intensity is at least I, increases, 



