Spectrum of the ft Rays excited by y Rays. 283 



•circular orbits, and after passing through a wide slit fall on 

 the photographic plate PP. The great advantage of this 

 method is that the ft rays in a comparatively wide cone are 

 concentrated into a narrow band on the photographic plate, 

 thus making it possible to detect the presence of groups of 

 ft rays of feeble intensity and to deduce their velocities with 

 accuracy. The source of y radiation S was a fine-bore glass 

 tube containing about 50 millicuries of radium emanation. 

 The glass walls were sufficiently thick to absorb all the a rays 

 and the low velocity primary ft rays. This source was 

 surrounded b] r a cylinder of the absorbing substance which 

 in different experiments varied between 0*1 and 1 millimetre 

 in thickness. 



From the work of Baeyer and Danysz it is known that 

 the velocities of the groups of ft rays are reduced by their 

 passage through a thin sheet of matter, and that the ft radia- 

 tion at the same time ceases to be homogeneous. After 

 passing through a small thickness of matter, the sharp bands 

 initially observed on the photographic plate become broader 

 and more diffuse, and with increasing thickness become 

 undetectable against the general background of continuous 

 radiation. Even the swiftest groups from radium C are 

 undetectable after passing through the glass tube and an 

 additional thickness of 014 mm. of lead or gold. All the 

 low velocity ft rays escaping under these conditions arc 

 those liberated by the passage of the y rays through the 

 absorbing screen. The magnetic spectrum of these excited 

 ft rays was examined in exactly the same manner as that of 

 the primary ft rays. 



A number of fairly broad bands were clearly visible on 

 the photographic plates, above the general fog due to the 

 penetrating y radiation. The outside edges of the bands, 

 corresponding to the swiftest rays of the groups, were 

 sharply defined, but the bands gradually decreased in inten- 

 sity on the low velocity side. The appearance of the bands 

 is easily explained on general considerations. If the passage 

 through matter of y rays of definite frequency gives rise to 

 a ft particle of definite speed, then the ft rays excited at the 

 outer surface of the absorbing screen should escape with no 

 loss of velocity, but those generated some distance below 

 decrease in velocity before they are able to escape. Conse- 

 quently we should expect that the bands would show a 

 well-marked outer edge corresponding to the swiftest rays, 

 and a gradual fading of intensity on the inner side due to 

 the effect of particles with lower velocities. In this respect 

 •the spectrum of the ft rays excited by y rays differs from 



