On the Photographic Action of a, ß and y Rays emittod from Radioactive Substances. W 



may i)e cunipared with- that deduced from its photometric density 

 D, viz. 



M=kD, (8) 



where k is the photometric constant, being for green light equal to 

 rOo.lO* gr. per sq. cm. of the plate. 



Giving tlio following values experimentally found: 

 .s = l-21). lOMor an Instantaneous Plate, for which i) = '412, and 

 s= 1 -06. l(y for an Ordinary Plate, for which 7>=-210, we get 

 iH=4-52. 10"' gr. per sq. cm. by (7) and 



M=4'24A()'^ gr. per sq. cm. by (8) for the Instantaneous Plate, 

 and 



i¥=2"55. ]()"■' gr. per sq. cm. by (7) and 

 -^i" = 2'16. 10'* gr. per sq. cm. by (8) for the Ordinary Plate. 

 Bearing in mind the fact that the conversion of silver 

 bromide to metallic silver is not carried out to completion, the 

 agreement betw^een the values obtained l)y the two methods is 

 seen to be quite satisfactory. 



The average radius of silver grains calculated by equation 

 (6) corresponds, for the reason just stated, to the superior limit. 

 It is 



r=*43 j" for the Instantaneous Plate, and 

 r='38 fJ- for the Ordinary Plate. 

 The size of the silver grains was actually measured under a 

 microscope, by means of an ocular micrometer for a number of 

 grains. It was found that the above calculated values of r are 

 within the range over which the size of the observed grains varies. 

 (1)). Ikeuti showed that the average radius /'o of halide 

 grains can als(j be deduced from the average number Si of silver 

 grains per unit length along an a ray track, when the thickness 

 of the emulsion film and the mass of silver halide contained per 

 unit area of it are known from other determinations. 



Rememljering that every halide grain becomes subsequently 

 developable whenever struck by an a particle, it can easily be 

 >^eeYi that silver grains presenting themselves as an « ray track 

 on a developed plate must have their centres within a circular 



