Velocity of ^-particles in passing through Matter. 629 



The apparatus used was the same as that employed by 

 Professor Rutherford and Mr. Robinson* for the analysis 

 of the /3-rays from radium B and radium C, and is shown in 



S represents a source of /3-rays, L is a lead block, and A 

 is a comparatively wide slit. The photographic plate PP is 

 placed on top of the lead block, and is pressed into position 

 by a screw B. The whole is enclosed in a brass box C, 

 rendered air-tight by a ground-glass plate. This box is 

 placed between the pole-pieces of a large electromagnet 

 which is so adjusted as to give a nearly uniform field over 

 the whole area of the pole-faces. The box is evacuated by 

 means of a Fleuss pump and a Gaede mercury-pump. This 

 type of apparatus has the great advantage that the /3-rays 

 of definite velocity comprised in a comparatively wide cone 

 are concentrated so as to form a sharp curved line on the 

 photographic plate. 



In the present experiments one half of the source was 

 covered with a thin layer of the substance whose stopping- 

 power was required, while the other half was left bare. 

 Consequently each line of the magnetic spectrum was replaced 

 by two parallel lines, the displaced line appearing of course 

 nearer the source. It was found to be of extreme im- 

 portance to have the source accurately parallel to the slit 

 in order that the two lines might themselves be parallel. 

 To avoid the scattering by glass which would have occurred 

 with an a-ray tube, and consequent broadening of the lines, 



* Phil. Ma-. Oct. 1913. 



