a. Particles with Light Atoms. 



543 



vessel was filled with the gas to be examined either by 

 exhaustion or displacement. It is a great advantage to have 

 the zinc sulphide screen outside the apparatus, in order to 

 avoid contamination due to volatilized active matter, and 

 for the easv introduction of absorbing material between the 

 end plate and the screen. 



Fiff. 1. 





r~ 



<s 



\Y 





- 



% 







■ 



\ 



X 



In practice, the source was introduced info the brass 

 vessel at a convenient distance from the screen, and the air 

 exhausted. The a. rays after traversing the end plate fell on 

 the screen, and the marked luminosity due to them was a 

 guide in fixing the microscope M in the centre of the opening. 

 The diameter of the field of view (2 mm.) was less than the 

 width of the opening (3 mm.). 



Since the number of H atoms observed under ordinary 

 conditions is less than one in a hundred thousand of the 

 number of a particles, H atoms, projected in the direction 

 of the a particles, can only be detected when the a, rays are 

 stopped by the absorbing screens. It was not found possible 

 to bring an intense source closer tiian 3 cm. from the screen 

 on account of the luminosity excited in it by the <y rays and 

 swift /3 rays, which prevented counting of weak scintillations. 

 A strong magnetic field was necessary to bend away the 

 /3 rays which caused a very marked luminosity on the screen. 

 A field of 6000 gauss was generally employed for this 

 purpose. 



