﻿366 



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

 Iheoretical. 



Consider what is the effect of a variation in grain size on 

 the nuclei distribution curve shown in fig. 1. 



We will first assume that the sensitivity of a nucleus is 

 quite independent of the size of the grain in which it chances 

 to be, i. e. once a nucleus is formed in a grain, its sensitivity 

 does not change as the grain grows. This is apparently 

 Svedberg's assumption, for he says : " the small and the larger 

 grains in one and the same emulsion are built up of the same 

 kind of light-sensitive material — -just as if they were frag- 

 ments of different size from one homogeneous silver bromide 

 crystal/' If this is the case, then the only result of in- 

 creasing the size of grain is to increase the total number of 

 nuclei, and these will be distributed amongst the different 

 sensitivities in the same proportion as before. This is shown 

 in fig. 11, where the distribution curves for two sizes of 



Kff. 11. 



grain are given. We have made no assumption regarding 

 the relation between total number of nuclei and grain size 

 except that large grains have more than small ones. 



The curves relating I and N (average number of nuclei 

 per grain) which will be obtained from distribution curves 

 such as those in fig. 11 are shown in fig. 12. We have already 

 shown that the N , 1 curve is identical in form with the A, I 

 curve, so that those in figs. 12 and 10 should be of the same 

 form. As a matter of fact, there is a striking difference. 

 The experimental curves in fig. 10 lie practically parallel to 

 one another at the higher intensities, and the point of in- 

 flexion (which corresponds to the maximum ordinate in the 

 nuclei distribution curves in fig. 11) moves towards the origin 

 as the grain size increases. In curves (b), (c), and (d), 

 fig. 10, which are for exceedingly sensitive grains, the point 



