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LXXXV. Quantum Theory of Photographic Exposure. 

 (Second Paper.) By L. Silberstein and A. P. H. 

 Triyelli *. 



IN the present paper an account will be given of some 

 additional experimental tests of the light-quantum 

 theory o£ photographic exposure proposed in the first paper 

 of the same title f, and some further theoretical formulae will 

 be deduced from the fundamental one given in that paper. 

 First of all, however, due mention must be made of certain 

 very valuable experimental investigations, since published by 

 Svedberg, which seem again to corroborate the theory, also 

 of a paper by Svedberg and Andersson published somewhat 

 earlier, but not brought to our notice until the first paper had 

 been dispatched for publication. 



1. Concerning " The Effect of Light/ 5 Svedberg and 

 Andersson's paper (Phot. Journal, August 1921, p. 325),. 

 dealing under that head with only a very few size-classes of 

 grains (each class, moreover, of a very considerable breadth),, 

 contains only the qualitative though definite conclusion that 

 ' for equal exposure the percentage of developable grains is 

 always greater in the class of larger grains." The quanti- 

 tative, viz. exponential dependence of this percentage upon 

 the size (area) of the grains, is obtained and well verified 

 experimentally in the case of bombardment by u-rays, 

 Kinoshita's experiments of 1910 having made it very 

 probable that each silver halide grain hit by an a-particle 

 is made developable. The latter being granted and the 

 discrete nature of a-rays being a palpably established fact,, 

 the validity of the exponential formula, in our symbols 

 k = JSi (1 — e~ na ), had to follow as a necessary consequence. 

 Its verification is properly a verification of Kinoshita's 

 statement, and by having thus tested it experimentally 

 Svedberg and Andersson have certainly done an important 

 piece of work, especially as Kinoshita's result was contested 

 by St. Meyer and v. Schweidler. In the next section 

 analogous experiments with /3-rays are described, but the 

 results thus far obtained are not conclusive apart from 

 enabling the authors to state that one or two /3-particles 

 striking a grain do not as a rule make it developable. 

 Finally, returning once more to the effect of light (p. 332), 



* Communication No. 149 from the Research Laboratory of the^ 

 Eastman Kodak Company. Communicated by the Authors. 

 t L. Silberstein, Phil. Mag. July 1922, p. 257. 



