132 THE ROYAL SOCIETY OF CANADA 



the thickness of the plate P, an indication that practically the whole 

 of the secondary rays are softened. If there had been no change in 

 frequency all the figures in column 3 of the table would have been 

 practically equal to 1 .00. 



Similar results to those outlined above were obtained with 

 aluminium and water radiators so that, for radiators of low atomic 

 weight, the softening of the secondary rays was independent of the 

 nature of the radiator. Results recently obtained indicate that, with 

 soft X-rays, the softening is somewhat greater with radiators of high 

 atomic weight. 



That the effect found above was not a spurious one was decisively 

 proved in the following manner. The average frequency in the 

 primary beam was a little greater than the characteristic absorption 

 frequency of silver. Consequently, if the secondary rays had a 

 lower average frequency, they would possibly have a lower absorption 

 coefiticient in silver than the primary rays. This would, of course, 

 depend on the magnitude of the change. To test these points, the 

 same angle of 110° was used, the primary rays were hardened some- 

 what by a plate of aluminium 1 . 63 mm. thick, and the plate P was of 

 silver of mass 0. 103 gramme per cm.^ The ratio A/B turned out to 

 be 0.57, the mass absorption coefficient of the primary rays in silver 

 being 17.6, that of the secondary rays 12.8. Further experiments 

 showed that the softening of the secondary rays was not so marked 

 as the angle they made with the primary rays became smaller. For 

 small angles the effect is negligible. 



In these experiments the primary beam was not strictly homo- 

 geneous "but A. H. Compton,*' using as a primary beam one reflected 

 from a crystal, has shown that there is a change in the secondary 

 rays similar to that found when ordinary X-rays of corresponding 

 penetrating power are used. He has found that, particularly with 

 small angles between the primary and secondary rays, part of the 

 secondary radiation is of the same frequency as that of the primary. 



In these papers Compton suggests that the softened radiation is 

 produced by the secondary /3-rays which are always ejected from the 

 radiator when it is struck by the primary beam. On the other hand, 

 the writer has always been of the opinion that the effect could not be 

 explained in this way. The following considerations show why this 

 point of view has been taken. Secondary /3-rays may excite X-rays 

 (1) in collision with atoms of the radiator: (2) during the course of 

 their expulsion from the parent atom. At first, Compton thought 



«A. H. Compton, Phys. Rev., vol. 18, p. 96, 1921; Nature, Nov. 17, p. 366, 

 1921; Phys. Rev., vol. 19, p. 267, 1922. 



