Manchester Memoirs, Vol. xli. (1897), No. 15. 25 



theory as to the Rontgen rays, is altogether new; so much 

 so that I felt at first rather startled by it ; but I found 

 myself fairly driven to it by the ideas I entertain as to 

 the nature of the Rontgen rays, and I am not aware 

 of any serious objection to it. 



Additional Note. 



The problem of diffraction in the case of a vast system 

 of independent very slender pulses deserves to be treated 

 in somewhat greater detail. It is rather simpler than 

 the problem of diffraction in the case of series of un- 

 dulations such as those which constitute light, because 

 the pulses are to be treated separately and independent!}', 

 like streams of light from different sources ; and as the 

 whole thickness of a pulse in the case of the Rontgen 

 rays may probably be something comparable with the 

 millionth of an inch, we have no need to inquire what 

 will be the disturbance continually passing across a fixed 

 surface in space ; we may treat the shell at any moment 

 as constituting an initial disturbance in the ether, and 

 then examine the efficiency of different parts of the shell 

 in disturbing at a future time the ether at a given point 

 of space in front of the shell. 



The thickness of the shell is not necessarily the same 

 at points situated in widely different directions as regards 

 their bearing from the centre, and the same applies to 

 the direction of disturbance. But in any case for a 

 small portion of the shell the thickness may be deemed 

 uniform, and the direction of disturbance sensibly the 

 same as we pass from point to point in a direction 

 tangential to the shell, while it varies with great rapidity, 



