﻿368 



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



plotted from these distribution curves are shown in fig. 14, 

 and it will be seen that they are similar to the experimental 

 curves in fig. 10. The reason why, for very sensitive grains, 

 the lower half of the 8-shaped curve appears to vanish (b), 

 (c), and (d), fig. 10, is that the value of Ij is very nearly 

 zero, but it would be shown if the points were plotted on a 

 bigger scale. 



Fig. 14. 



The evidence thus points to there being two reasons why 

 large grains are more sensitive than small ones. Firstly, 

 there are more nuclei present in the larger grains, so that a 

 single grain has a greater chance of having at least one ; and 

 secondly, the average sensitivity of the nuclei increases with 

 the size of grain. 



Svedberg in his most recent paper (ibid.) discusses the 

 relation between the average number of nuclei per grain and 

 the grain size. He says: — " The rapidity of the increase of 

 the average number of nuclei per grain N with size of grain 

 would depend on two factors : the ability of the developer 

 to penetrate into the grain, and the homogeneity of the field 

 of light in the grain. If the developer is not able to get 

 into the interior of the grain, but only attacks the surface 

 layer, then N would mean the number of centres in that 

 surface layer, and therefore would increase in approximate 

 proportion to the grain surface even in cases where the field 

 of light in the grain was not homogeneous (because of strong- 

 light absorption). On the other hand, if the developer is to 

 penetrate the grain, N~ would depend upon the field of light 

 in the grain. If the absorption of light were feeble, N would 

 increase in proportion to the volume of the grain ; if the 

 absorption were very strong, N would increase approximately 

 proportionally to the cross-section of the grain/' Later in 



