﻿7 racks of the a. Particles hi Sensitive iilms. 423 



Attempts were also made to obtain a halo which is due to 

 the a rays from radium A as well as radium C. For this 

 purpose a metal piece was exposed to radium emanation for 

 a few seconds, and the active deposit quickly detached from 

 it was used as the source of the a rays. Closely examining 

 the haloes thus obtained, it was found that among the trails 

 of grains there are some which have a considerably shorter 

 length than the others and seem to be those produced by 

 the ol rays from radium A. But we have not yet been able 

 to get one clearly shown as a corona, as in the case of the 

 pleochroic halo. 



We have already drawn attention to the fact that the 

 silver grains, which constitute the set of the trails spreading 

 over a wide region, have their seats in the uppermost layer 

 of the film. Fig. 7 is an example, of which the magnification 

 is the same as that of the haloes. This fact suggests the 

 view that these trails of grains are produced by the 

 a particles projected tangentiaily to the surface of the film 

 from the part of the active deposit just above the surface. 

 At first sight, some of the trails seem to be much longer than 

 those constituting the haloes. Carerul inspection, however, 

 shows that this is only apparent and that each of them 

 consists of two or more elementary trails, having the average 

 range of *054, following one another. 



So far, we have presumably treated the problem as the 

 results of the a. ray effect, and no account was taken for 

 the /3 rays which are emitted as well from the active deposit 

 of radium. It has long been known that the ft rays possess 

 the property of acting on a photographic plate ; but owing 

 to difficulties involved in the experiment, very little is known 

 about the effect of an individual ft particle. In the 

 experiment of Kinoshita already referred to, the photo- 

 graphic action of ft rays was found to be about one-third 

 or one-quarter of that of u rays in the case of a thinly 

 coated Wratten's Ordinary Plate. In this calculation it 

 was assumed that active deposit, from radium emanation, in 

 which radium A had already decayed away, emits about the 

 same number of a and ft particles. Since it is now known 

 that the active deposit emits, under this circumstance, about 

 twice as many ft as a particles, the ratio of the photographic 

 action of a /3 particle to that of an a particle reduces to only 

 one-sixth or one-eighth. It therefore seemed likely that 

 a silver halide grain bombarded by a ft particle is not 

 necessarily capable of development. If this be the case, the 

 track of a ft particle would not be so closely filled up with 

 the developed grains as in the case of an a particle. It 



