﻿Theory of Photographic Exposure. 2G7 



Another way is to take account of the possibly /im'^ trans- 

 versal dimensions of: the light parcels, which may perhaps be 

 comparable with those of the lesser silver halide grains. 



Let us assume, therefore, that a light-quantum, of suffi- 

 ciently high frequency, becomes efficient in affecting a grain 

 only if it strikes it fully, or almost so. To fix the ideas, let 

 r be the equivalent radius of a grain, i. e., such that 



and similarly let p be the average radius of the transversal 

 section of a light parcel (so to call the space occupied by a 

 quantum of energy). Then the efficient area of a grain to be 

 substituted instead of a, will be 



a' = 7r(r — p, 2 ~a 1— ^ , 



and we shall have for the exponent s, instead of (10), 



s = na'=/3Fa [l-^Tx, for p<r< \< : \„. (10a) 



and 5 = for p > r or X > X c . 



It remains to assume in a general way that p, which may 

 be the average of section-radii different even for parcels of 

 the same wave-length, is itself a function of the wave-length 

 increasing with X, without prejudicing, however, the parti- 

 cular form of this function. Certain easily ascertainable broad 

 features of such a function and thence also of the resulting 

 factor in s, 



£(\)=\[ 



(ii) 



will suffice to ensure a maximum of the sensitivity exponent 

 between X = and X = X e . The value of X c itself may still 

 turn out to depend on the size a of the grain and on its 

 physical conditions as well. Every process which will make 

 the liberation of a photoelectron from the grain (crystal) 

 easier will lengthen X c . Part of the effect of sensitizing 

 may arise in this manner. But questions of this kind must 

 necessarily be postponed until some further experimental 

 data are gathered. Of such a kind is also the question 

 whether p (which, for a given X, may also extend over a 

 whole range of values) attains at all the semi-diameter r of 

 even the smallest of the actual grains, and whether the 

 corresponding wave-length A entailing the vanishing of s, 

 exceeds or is smaller than X c as derived from direct photo- 

 electric experiments. In absence of all knowledge concerning 



