18 Stokes, on the Nature of the Rontgen Rays. 



with a single pulse, whereas in the case of light we are 

 dealing with an indefinite succession of disturbances. 

 In order to understand the sharpness of the shadows 

 produced by the Rontgen rays, we are not obliged to 

 suppose that the disturbance is periodic at all. It must 

 be partly negative and partly positive, and that being 

 the case, if the thickness of the shell is very small, the 

 amount of diffraction will be very small, too. Those who 

 have attempted to obtain evidence of the diffraction of 

 the Rontgen rays have been led to the conclusion that if 

 the rays are periodic at all the period is something enor- 

 mously small — perhaps thirty times, perhaps a hundred 

 times, as small as the wave-length of green light. It 

 seems difficult to imagine by what process you could 

 get such very small vibrations, if vibrations there be. 

 It is easier to understand how the arrival of charged 

 molecules at the cathode might produce disturbances 

 which are almost abrupt. 



Well, then, this is what I conceive to constitute the 

 Rontgen rays. You have a rain of molecules coming 

 from the electrically-charged cathode, which you may 

 think of as the rain-drops in a shower. They strike 

 successively on the target, each molecule on striking the 

 target producing a pulse, as I have called it, in the ether, 

 which is essentially partly positive and parti) 7 negative ; 

 and you have a vast succession of these pulses coming 

 from the various points of the target which are not 

 protected by some screen interposed for the purpose of 

 experiment. 



This explains the absence, or almost complete absence, 

 of diffraction. But that is not all we have to explain ; 

 we have still a very serious thing behind. What is it that 

 constitutes the difference between the Rontgen rays and 

 rays of ordinary light in consequence of which the one 

 are not refracted, or only in an infinitesimal degree, while 



