﻿Photographic Action of u, ft, and y Bays. 837 



the photographic plate. Reinganum * was the first to 

 obtain tracks of single « particles in sensitive films ; Michl f 

 and, subsequently, Baisch % and Mayer § also studied, in 

 some detail, the photographic action of <x particles. The 

 tracks obtained by Reinganum and Michl were mostly 

 curved, but, as Michl points out, this was probably due to 

 the particular way in which the gelatine film contracts 

 on drying. Walmsley and Makower || were the first to 

 publish a microphotograph of a-ray tracks. Some inter- 

 esting micropbotographs of a-ray tracks have been published 

 in a recent paper by Kinoshita and Ikeuti ^f. 



In the course of an investigation by the present writer, 

 into certain properties of a and /3 rays, by the photographic 

 method, it was found that most photographic plates, on 

 development, show, under the microscope, a large number 

 of blackened grains, even when they have not been previ- 

 ously exposed to light or some other stimulus, .such as 

 radiations from an active substance. It is clear that with 

 a plate free from this defect, the examination of photo- 

 graphic films under the microscope should afford a very 

 sensitive way of studying the properties oE a rays, and the 

 method would seem to be applicable to all the phenomena 

 which have hitherto been investigated by the method of 

 scintillations. The method has, in fact, been used by 

 Mayer to study the scattering of a particles by metallic 

 films. Both Reinganum and Mayer recommend the photo- 

 mechanical and the Sigurd dia-positive plates, made by 

 R. Jahr (Dresden), as being better than any other plates 

 used by them ; but, as Mayer points out and as his results 

 also show, even with these plates the number of grains 

 visible under the microscope on an exposed plate was too 

 large to permit the counting of <x particles with accuracy. 



In this short paper it is intended to describe a plate 

 suitable for various kinds of investigations with the photo- 

 graphic method, and briefly to refer to some of the experi- 

 ments which have been undertaken with it. 



Selection of plate. — After considerable search with 

 different kinds of plates, sensitive films, and papers with 

 suitable developing solutions, it was found that Wratten 

 and Wainvvright's lantern-plate presented an absolutely 

 clear surface. For a temperature o£ 18° C, which was 



* Keinganum, Phys. Zeit. xii. pp. 1076-77 (1911). 



t Michl Akad. Wis*. Per. Wien, pp. 1431-1447 (1912). 



% Baisch, Ann. der Phys. xxxv. p. 565 (1911). 



§ Mayer, Ann. der Phys. xli. (1913). 



|| Walmsley & Makower, Proc. Phvs. Soc. xxvi. (1914). 



11 Kinoshita & Ikeuti, Phil. Mag-. March 1915. 



