﻿360 



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



when I is large, but this can be explained. Firstly, the 

 grains used in Slade and Higson's experiments were not all 

 of one size, the variation being about 30 times that in the 

 present case. Secondly, the best curve given in Slade and 

 Higson's paper has actually the same form as that in fig. 4 

 if equal weight is given to each point plotted. Also, in the 

 author's confirmatory experiment, the main point was to show 

 that at low intensities A varied, at any rate approximately, 

 as I 2 ; at high intensities the work was not nearly as accurate 



Fig. 5. 



I— > 



as in the present case. To be certain of the form of the 

 curve in fig. 4 the upper part of it was plotted for another 

 size of grain, and that the same result was obtained is shown 

 by the dotted line. 



The position of a nucleus can be detected by Hodgson's 

 method of partial development of the exposed grains (ibid.). 

 The developer used was made up as follows : — 



200 c.c. saturated Na 2 S0 3 , 

 8 c.c. 10 per cent. KBr, 

 0*3 gm. Amidol. 



This is a weak, slow developer, and is best for this purpose 

 because there is a bigger latitude than if the developer is 

 strong in the time of development necessary to render the 

 position of the nuclei visible and yet distinct from one 

 another. The best development time was found by trial and 

 examination of the grains under the microscope. After 

 exposure the plate was plunged into the developer for a 

 known time, then quickly and thoroughly washed, and dried 

 without fixing. The flat triangular grains used were so thin 

 that the silver deposit was visible without dissolving away 



