﻿962 Dr. L. Silberstein and Mr. Trivelli on the 



the grains surviving after exposure and development. A 

 glance will show that the majority of clumps, and especially 

 the larger ones, are removed entirely. Of such pairs of 

 samples as figs. 3 a and 3fr, about forty were made, and the 

 behaviour was always typically the same. ' Further and more 

 direct experimental tests of the adopted clump principle are 

 now in progress, notwithstanding that we have but little 

 doubt about its correctness, always, of course, in relation to 

 the material which we are using. And we feel sure that the 

 same principle can be firmly relied upon in what follows. 



3. Let us recall that the theoretical values of the per- 

 centage number y = 100kjN of clumps affected, as given in 

 the fourth column of the table in our first paper, were 

 calculated by means of formula (12), 



% (l-|r) =-««[!- V^P,. • • (12) 

 with the values of the parameters 



?2 = 0572 per y} 

 (7 = 0-097 fx 2 , 



j . . . . (12a) 



the meaning of all the symbols being as before. The agree- 

 ment of these values with the observed ones, ranging over 

 33 classes of grains and clumps, was excellent, thus proving, 

 at any rate, the essential correctness of the formula as far as 

 the dependence on size (area) a goes. 



To test it with regard to the exposure or n, portions of the 

 same plate were subjected to the action of the same light 

 source, cceteris paribus, for one-half, and for one-quarter of . 

 the time of the original exposure. The same method of 

 evaluating N and k being adopted as before, the results 

 tabulated below under y obs . were obtained. Now, without 

 even taking the trouble of retouching the values of the 

 parameters in adaptation to the new observations, a was 

 taken as in (12 a) and n equal to one-half and to one-quarter 

 of its original value, respectively. Since the exposure is, at 

 any rate, proportional to «, our formula with these n- values 

 should represent the two new sets of observations. The 

 following table gives in the first row the number of grains in 

 a clump *, and in the second row the average area a of each 

 class of clumps, in square microns, as before ; the third and 



* Starting from 2, since with these weaker exposures reliable counts 

 of single grains affected could not be secured. 



