768 Prof. A. H. Compton on the 



that the large amount of secondary radiation at these small 

 angles is due in part to the presence of some true scattering. 

 However this may be, the generally satisfactory form of 

 the theoretical curve suggests that we are' working along the 

 right line. 



If this view of the origin of the fluorescent radiation is 

 the correct one, we are supplied with a means of estimating 

 roughly the wave-length of the primary gumma rays. It 

 has been shown above that the wave-length of the softest 

 part of the fluorescent radiation lies between 0*06 and 0'12 

 A.U., and probably nearer the former. But according to 

 the Doppler principle, if the oscillators producing the 

 fluorescent radiation are moving in the direction of the 

 primary rays, the ratio of the wave-length at an angle X to 

 that at an angle 2 is 



( 5 ) 



X 2 1— /3cos# 



Thus, if we take \ 2 to be about 0'08 A.U. at 2 = 135°, 

 and the value of /3 to be 0*5, the wave-length of o the 

 penetrating fluorescent radiation at 45° is about 0*04 A.U. 

 This result does not vary greatly with different values of /9 ; 

 but the extreme hardness of the fluorescent radiation at 45° 

 indicates that it is more nearly similar to the primary rays 

 than to the soft secondary radiation which appears at the 

 larger angles. Thus we shall probably not be far wrong in 

 assigning a value 0*02 to 0'03 A.U. as the wave-length of 

 the most effective part of the hard gamma rays from 

 radium 0. This result is not in disaccord with the calcu- 

 lations of Rutherford * based upon the quantum hypothesis. 



Summary. 



The principal experimental results of this investigation 

 may be summarized as follows : — 



By far the greater part of the secondary gamma radiation 

 from matter traversed by the hard gamma rays from 

 radium C is fluorescent in nature. If any truly scattered 

 radiation is present, at 45° it probably amounts to less than 

 15 per cent., and for angles greater than 90° to less than 

 3 per cent, of the secondary rays. 



* E. Rutherford, Phil. Mag. xxxiv. p. 153 (1917). According to the 

 quantum relation, an electron must have a velocity /3 = 0-8 in order to 

 excite radiation of wave-length O04 A.U. On the present view, there- 

 fore, the radiating beta particle must already have lost a large part of its 

 energy of translation. 



