﻿958 Dr. L. Silberstein and Mr. Trivelli on the 



Now, it would be enough to assume that these centres are 

 produced by' discrete light-quanta impinging upon the grain, 

 and the formula P = 100 (1 — e' na ) would follow at once. 

 (For, if n be the number of light-quanta per unit area, and a 

 the area of a grain, v = na.) But Svedberg does not make this 

 assumption *, and devotes instead the remainder of his paper 

 to testing directly the above formula for the occurrence of at 

 least one centre and the corresponding chance formula for 

 the percentage number of grains having no centres, of those 

 having one or two or three centres, etc., having succeeded in 

 making these centres or, in Svedberg's own words, "the 

 nuclei corresponding to the developable centres," visible and 

 accessible to measurement. For details of these elegant 

 experiments the reader must be referred to the original 

 paper. Here it will be enough to say that the recorded 

 " dots " or visible traces of those centres were found distri- 

 buted very much in accordance with the probability formulae, 

 namely, in one experiment with light and one with X-rays. 

 Only two si,r<?-classes of grains were treated in each of these 

 experiments, and with regard to the dependence upon 

 exposure Professor Svedberg (p. 192) has thus f;tr only 

 roughly stated that the percentage number of developable 

 grains " increases approximately exponentially as function 

 of exposure," at least for normal and for over-exposures in 

 the case of light (and probably for all exposures in the case 

 of X-rays) though not for under-exposure to light. The 

 paper is concluded by the remark that to account for the 

 deviation from the exponential formula in the case of low 

 light-exposure, we should probably have to adopt the 

 quantum point of view, and that in the case of light (a 

 quantum of visible light containing 5000 times less energy 

 than an X-ray quantum) " several quanta would have to be 

 absorbed very near one another to forma developable centre }> 

 Such a view, however, can easily be shown to be untenable. 

 At any rate, Professor Svedberg proposes to test it by experi- 

 mental investigations which are planned in this direction. 



* In the discussion which followed upon the reading- of Svedberg's 

 paper, Prof. T. M. Lowry mentioned such an assumption of a " bombard- 

 ment by light corpuscles " as the simplest interpretation of Svedberg's 

 photographs (of the " centres "). Other speakers, however, were rather 

 hostile to such a view, and Mr. B. V. Storr considered it even equally 

 conceivable that the " centres" distributed haphazardly might be present 

 before the light action, but such a state of things would have hardly 

 escaped Svedberg's notice. At any rate, Professor Svedberg will no 

 doubt meet Mr. Storr's objection by appropriate control experiments. 

 Control experiments of such a kind, viz. desensitizing experiments, are 

 now being made by Sheppard and Wightman. 



