﻿Theory of Photographic Exposure. 271 



of the larger, flat grains piled upon each other in part, 

 although the smaller, spherical grains, in less intimate 

 contact, may perhaps behave differently.) Such being the 

 case, their experimental results should be covered by our 

 formula with a written for the area of the whole clump, no 

 matter how large and how numerous its components. This 

 has seemed a rather severe test but the more so tempting and 

 instructive. Since all the classes of clumps were given, in 

 each trial, a unique exposure (through a blue filter specified 

 toe. cit.) and there was no question of varying X, it will be 

 most convenient to retain in the corresponding formula the 

 original light- quantum number n as the parameter common 

 to all clumps. Thus the formula to be tested becomes 



— =1 — e , s=na =na 



(»-»)'■ 



or somewhat more conveniently for computations, if o~ = 7rp 2 

 be the (average) area of the transversal section of a light 

 parcel, 



lo g (l-4) = -™[l-^]\ . . (12) 



In the following table the first column gives the number 

 of grains in a clump, the second the average area a of a 

 clump in square microns, and the third column the per- 

 centage of clumps affected out of all (i\ 7 ) clumps of each 

 kind originally present, i.e. 



100 k 



as deduced by Trivelli and Righter from their observations. 



Clumps of ain^. y oba> y c ^ Ay. 



1 grain O-75-i 16-5 162 +0-3 



2 grains 1-925 44-9 48'4 -3"5 



3 , 3-03 76-6 68-9 +8-3 



4 4-88 87-i 87-3 -0-2 



5 „ Crls 96-" 933 2-7 



6 ., 7-4-2 98-2 964 1-s 



7 „ (8-6) 100 98-0 2-o 



8 , (9-8) 100 99-6 0-4 



9 , (11- ) 100 99-8 0-2 



10 „ (12- ) 100 100 0-0 



etc., etc. etc., etc. idem. idem. idem. 



32 grains >24 100 100 0-0 



33 „ >25 100 100 00 



The most reliable a-values are those for the clumps of one 

 and of two grains, being averages of the largest numbers 



