﻿258 Dr. L. Silberstein on a Quantum 



" exposure " of wave-length X, the number of light-quanta 

 contained in it will be 



n= Tc X (1) 



From a recent conversation with Einstein, there are 

 weighty reasons for making a=l. But since we do not 

 prejudice its value, there is no harm in retaining this 

 coefficient in the formula. 



Now, let us assume that the necessary and sufficient con- 

 dition for a silver-halide grain to be affected, i. e., to be made 

 developable (entirely or in part) is that it should absorb one 

 light-quantum. 



Moreover, let us assume that a grain * does absorb a light 

 quantum whenever it is fully hit by one, of a sufficiently 

 high frequency v c or of a wave-length not exceeding a 

 certain value X c . 



There are perhaps some experimental hints or more or less good 

 reasons for making these two assumptions, but we need not stop to 

 consider them here. It will be time to reject or to modify them when 

 they are contradicted by photographic experiments. Nor is it necessary 

 to enter here upon the mechanism by means of which a silver-halide 

 grain is affected by a light-quantum, whether it be the knocking out of 

 an electron, as suggested by Joly, or something entirely different. For 

 none of such details will influence our main argument, to be treated in 

 the next section. Only when we come to consider the dependence of the 

 photographic effect upon the wave-length will it be interesting to con- 

 sider the photoelectric hypothesis and necessary to take account of the 

 fact that a photo-electron is not liberated unless the frequency exceeds 

 a certain, the so-called critical value. Under these circumstances 

 A e appearing in our second assumption will stand for the critical 

 wave-length as known from Photo-Electricity. 



Again, whether a grain being " affected " is made developable in part 

 only or throughout its whole area (no matter how large) is, in view of 

 the* kind of the contemplated experimental tests, of no great importance. 

 As a matter of fact, however, there is good evidence that a grain is 

 always made developable as a whole, no matter what its size, and this 

 seems even to hold for " clumps " or aggregates of several smaller 

 grains, as will be explained hereafter. If so, then our formulae, to be 

 "developed presently, will give not merely the number of affected grains 

 but also, by integration, the total " mass " made developable and hence 

 also the photographic density. But we may as well remain content with 

 the formulae for the number (~k) of grains affected, and count these in all 

 experimental tests. This, far from being a disadvantage, will enable us 

 to subject the proposed theory to more precise, though at the same time 

 more severe tests. 



One more remark. It will be understood that when we come to adopt 

 the photo-electric hypothesis, a grain " affected " will stand for a grain 



* Or perhaps, more generally, one of every p grains hit, where p 

 is a number to be determined by experiment, but presumably equal 

 to unity. 



