﻿Theory of Photographic Exposure. 269 



large an extent this is actually the case. But perhaps 

 the most tangible proof of the essential correctness of the 

 assumption of spatially discrete as against continuous action*, 

 seems to be the mere fact, disclosed by microscopic counts, 

 that out of a number of apparently equal grains subjected 

 to a sufficiently weak exposure one or two are affected while 

 the others, nay their next neighbours, remain perfectly 

 intact. It would be in vain to ascribe to these survivors a 

 greater immunity or indifference to light. For it is enough 

 to protract the exposure a little to make them succumb in 

 their turn. Now such a behaviour is most typical of rain 

 as contrasted with flood action, and the discrete light-quanta, 

 hitting now this and now that grain, appear to be a most 

 natural inference, while all attempts to bring into play the 

 individual ''sensitiveness" of the units seem to involve 

 considerable difficulties. 



As to the dependence of: the number of grains affected 

 upon the wave-length, little more of interest in the present 

 connexion is known than the qualitative fact of a shift 

 towards the red of the maximum sensitivity with increasing 

 size of the grain. Moreover, the available curves repre- 

 senting the sensitivity across the spectrum concern the 

 emulsion as a whole and not the separate a- classes of grain 

 with which we are primarily concerned. Spectrographs 

 and spectrophotometry experiments of such a kind, to be 

 aided by direct photo-electric measurements, are now in 

 progress in this laboratory, and all discussions involving 

 wave-length will best be postponed until the results of 

 these experiments and of laborious microscopic counts are 

 forthcoming. 



Before passing to the mentioned quantitative test of the 

 dependence on size, but one more general remark. The 

 reader will have noticed the complete absence of the time- 

 variable in all our formulae, the exposure entering only 

 through the total number n of light-quanta or through the 

 energy E which, in obvious symbols, is ^Idt. The pro- 

 posed theory, therefore, as thus far developed, does not take 

 any account of the little infringements against the reciprocity 

 law t, iu short, of the so-called " failure of the reciprocity 

 law." Now, it is by no means my intention to deny the 



* A rain as against a flood, of light, that is. 



t This early law asserted the dependence of the photographic effect 

 (density) upon I and t only through the total incident energy or ex- 

 posure \ Idt. For constant intensity this is It, whence the name of the 

 law, relating to intensity and exposure-time as factors of a constant 

 product. 



