Manchester Memoirs, Vol. xli. (1897), No. 15. 17 



ment or velocity with respect to a radius vector drawn 

 from O. Consequently the positive and negative parts 

 of the disturbance will reach P in succession. But if 

 instead of the small portion A B of the shell we take 

 C D, lying in a direction from P not very near the 

 normal, it is easy to see that the positive and negative 

 parts of the disturbance expressed by our double integral, 

 reaching as they do P simultaneously, almost completely 

 cancel each other. And this cancelling is so much more 

 nearly complete as the obliquity is greater, and likewise 

 as the thickness of the shell is smaller. If, then, 'the 

 disturbance in the ether consequent on the arrival of 

 any projected molecule at the target is very prompt, 

 lasting it may be only a very small fraction of the period 

 of a single vibration of the ether in the case of light, 

 our shell will be so thin that a small isolated portion 

 of the Rontgen discharge is propagated so nearly wholly 

 in the direction of a normal to the wave that the almost 

 complete absence of diffraction is thus accounted for.* 



The explanation which has just been given of the 

 apparent absence of diffraction in the case of the 

 Rontgen rays is closely analogous to the ordinary 

 explanation of the existence of rays and shadows. It 

 differs, however, in this respect, that here we are dealing 



* It is known that there is a difference of quality in Rontgen rays, 

 and that the Rontgen discharge may be filtered by absorption. It is 

 known also that the increased exhaustion in a Crookes' tube, which is 

 accompanied by increasing difficulty in sending a discharge through it, 

 has the effect of giving rise to increasing penetrative power in the 

 Rontgen rays which it gives out. It seems to me probable that this 

 difference of quality corresponds to a more or less close approach to 

 perfect abruptness in the production of disturbance in the ether when a 

 molecule propelled from the cathode reaches the target, and accordingly 

 to a less or a greater thickness in the outward-travelling shell of 

 disturbance in the ether ; and that at relatively high exhaustions the 

 molecules are propelled with a higher velocity, and so give rise to a 

 more prompt disturbance when they reach the target. 



