764 Prof. A. H. Compton on the 



correction must be applied to this value to make allowance 

 for the fact that while the greater part of the secondary 

 radiation which enters the ionization is absorbed, in the 

 present experiment only about half of the primary beam was 

 thus absorbed. Taking this correction factor to be about 

 0*7, we find that approximately 50 per cent, of the absorbed 

 primary rays is trail stormed into radiation of sufficiently 

 high frequency to penetrate 0*15 cm. of lead *. The 

 efficiency oi: transformation of the energy is therefore of a 

 much higher order than that observed in an X-ray tube 

 • operating at usual potentials, in which case not as much as 

 1 per cont. of the energy of the cathode rays appears as 

 ~X-rays. It is possible that this difference is to be accounted 

 lor by an excitation of gamma rays when the secondary beta 

 particles are liberated in addition to that produced when 

 they collide with other electrons. 



A question of great theoretical importance is — What kind 

 of oscillator can give rise to radiation which not only is 

 more intense in one direction than in another, but also differs 

 in wave-length in different directions ? Since the secondary 

 'radiation differs in frequency from the primary rays, it 

 would seem impossible to invoke any interference between 

 the radiation from the different oscillators to account for 

 this phenomenon. Such an explanation is rendered the 

 more difficult by the fact that to explain the different hard- 

 ness of the rays in different directions, oscillators of different 

 frequencies would have to be present, between which there 

 could be no fixed phase relations. An obvious means of 

 accounting for the observed phenomenon is to suppose that 

 the radiator which gives rise to the secondary rays is moving 

 at high speed in the direction of the primary beam. In 

 this case, both the intensity and the frequency ■ of the 

 fluorescent radiation will be greater in the forward than 

 in the reverse direction, as is demanded by the experiments. 



A rigid calculation of the relative intensity of the fluores- 

 cent radiation at different angles, according to this hypothesis, 

 is not nt present possible, because the scattering of the beta 

 particles results in an irregular distribution of their velocities. 

 It will nevertheless be instructive to consider the relative 

 energy radiated in different directions by an oscillator 

 moving at a speed comparable with that of light. It can be 



* The estimate here made of the efficiency of transformation is, of 

 course, based upon the assumption that unit energy of one frequency 

 produces the same total number of ions as unit energy of another 

 frequency. Though this assumption has not been tested over the range 

 of frequencies here considered, it does not appear probable that any 

 error thus introduced can change the order of magnitude of the result. 



