On the Photographic Action of a. ß and y Eays emitted from Radioactive Substances. 7 



particles traversed, taken per unit volume, is least in that region, 

 and this fact can be explained as the result of an intense ionisation 

 near the extreme end of the range of the « particles^^ 



A theory put forward by Kinoshita to explain the singularity 

 ol the photographic action was that, if some of the halide 

 molecules witliin a grain are initially ionised by one or more a 

 particles, the whole grain l)ecomes subsequently capable of 

 development, l)ut the reduction cannot extend to other grains 

 which have not been initially ionised. When the film is 

 completely developed, all the grains struck by the « particles are 

 reduced to a constant limit, which depends on the size of hahde 

 grains in the emulsion film but not on the degree of the primary 

 ionisation in them, and we thus obtain silver grains as a secondary 

 consequence of the ionisation, so that the photographic action is 

 constant throughout the whole range of the a particles in the 

 substance. A further consideration will be presented later^\ 



In variance with the above theory, MichP^ states that only a 

 part of the halide grains encountered l)y an a particle become 

 subsequently developable. This conclusion is based on a micro- 

 scopic comparison between the compactness of silver grains on a 

 photographic plate which was obtained by exposing it to light, 

 and the difïuse arrangement of the grains on an « ray track 

 obtained on the same plate. 



In order to show that the above reasoning is by no means 

 adequate, the original paper of Kinoshita must be again referred 

 to. It was shown that the photographic density D of a photo- 

 graphic plate produced by normally falling a particles varies with 

 their numljer n per unit area of the plate as 



D=D,(l-e-), (1) 



where D^ is the maximum value of D attainable by an indefinitely 

 large number of the a particles, and c- is a constant depending on 

 the quality and the thickness of the emulsion film on the plnte. 



As is generally known, the photometric density of a plate is 



1) J. Jo]y, /. c.\ E. Rutherford, 'Radioactive Substances and their Radiations'(1913),p.310. 



2) cf. p. 15. 



3) I. c. 



