Oa the Photographic Action of a, ß and y Eays emitted from Radioactive Substances. 9 



strikes the grain. For, if, in the case of tlie Instantaneous Plate, 

 the numl)er of the overlapping he taken as 37 per cent, of the 

 total, eacli « particle would in the average change To? grains on 

 its passage through the film; in other words, the numher of a 



particles required to change one halide grain would l)e ^r^^ or 



•73, which is the value of ~^ calculated from the values of c and 



.?o for that plate. Similarly, in the case of the C)rdinar3' Plate, if 

 the area uncovered with the grains be taken as 10 per cent, of the 

 totals, PI a particles on average would be required to change one 

 halide grain. We are thus lead to the conclusion that one a 

 particle is sufficient to change a halide grain into the developable 

 state, whenever it encounters the grain and whatever may l)e the 

 quality of the plate. 



In dealing with an « ray track, it must be rememlK^red that 

 we are observing those grains only, the centres of which lie, as 

 w^ill be directly shown, within a very nari'ow cylinder of a cross 

 section equal to that of the halide grains, l)ut not those having 

 their centres outside the cylinder. Since grains at various depths, 

 the difïerence of which is much greater than their linear dimen- 

 sions, can be seen simultaneously in a microscopic vision even of 

 fairly high magnification, mere comparison of the compactness 

 of visi1)le grains on the two difïerent plates, one acted on by fight 

 and the other by individual a particles, would never lead to the 

 conclusion already stated. 



8. The size of (/rains. The size of the silver grains has been 

 determined, as far as we know, only by microscopic methods, 

 visual, projecting or photographing. These methods, bowever, do 

 not seem to give very reliable results, unless care is taken with re- 

 gard to the diffraction phenomenon, because the grains to be mea- 

 sured are so small that their linear dimensions are of the same order 

 of magnitude as the wave-lengths of the visible light. In the fol- 

 lowing, we shall descril)e two methods of deducing the size of halide 

 grains from the data obtained in the investigation on the « i-ay 

 photographs. Although the methods are rather indirect, tlR\v may 

 still be of use in practical application. 



