﻿364 



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



Variation of Grain Size. 



Experimental. 



It will be convenient to deal first with the experimental 

 curves. When the values of x were being found for the 

 curve in fig. 4, the corresponding values for three larger 

 sizes of grain were determined at the same time and in the 

 same way. The characteristic carves for the four sizes are 

 shown in fig. 9. 



Fig. 9. 



Logl-> 



The important points in regard to these curves are that for 

 one and the same emulsion : — (1) a set of large grains is 

 more sensitive than a set of small ones, which confirms 

 Svedberg and Anderson's result (Phot. Jour. 1921, Ixi. 

 p. 325) ; (2) the characteristic curve for small grains has a 

 greater maximum slope than that for large ones, i. e. } the 

 ratio of the intensity which just changes all the grains to 

 that which just causes the smallest possible change is larger 

 the larger the grain size. As will be seen from the figure, 

 the logarithm of this ratio for the smallest size grain is about 

 1'5, which is equal to an intensity ratio of 30 : 1, whilst for 

 the largest size a ratio of 100 : 1 is necessary to give half the 

 curve. 



In Table IV. are given values of x, as read off the experi- 



