20 



Stokes, on the Nature of the Rontgen Rays. 



the ethereal vibrations start and reach a set of molecules, 

 they are somewhat impeded by the molecules, and they 

 tend also to move the molecules. But as the molecules 

 are relatively very heavy, it may be that it takes some 

 considerable time for the molecules to be set sensibly in 

 motion. Now, if the system of molecules is exceedingly 

 complex, a mode of motion of the molecules, or it may 

 be of the constituent parts of the molecules, may be 

 found such that the system tends to vibrate in practically 

 any periodic time that you may choose ; only as you 

 choose one time or another the mode of vibration will 

 be different ; and, again, according to the direction in 

 which the molecules are successively made to vibrate 

 the actual mode of vibration will be dfferent. Well, I 

 conceive that the difference between the propagation 

 of the Rontgen rays and rays of ordinary light with 



where the cathodic rays fall on the wall of the Crookes' tube. This 

 place is indicated to the eye by the fluorescence of the glass. But we 

 are not on that account to regard the fluorescence as the cause of the 

 Rontgen rays, or even to regard the Rontgen emission as a sort of 

 fluorescence. I have seen it remarked, as indicating no very close 

 connection between the two, that with a metallic target we have a 

 copious emission of Rontgen rays though there is no fluorescence, and 

 that when a spot on the glass wall of a Crookes' tube has for some 

 time been exposed to a rather concentrated cathodic discharge, though 

 the fluorescence which it exhibits under the action of the cathodic 

 discharge becomes comparatively dull, as if the glass were in some way 

 fatigued for fluorescence, it emits the Rontgen rays as well as before. 



Fluorescence is undoubtedly indicative of a molecular disturbance ; 

 but in what precise way this disturbance is brought about by the 

 cathodic discharge, is a matter on which I refrain from speculating. 

 But whatever be the precise nature of the process, it seems pretty 

 evident that it can only be by repeated impacts of molecules from the 

 cathode that a sufficient molecular disturbance can be got up to show 

 itself as a visible fluorescence. 



Suppose a shower of molecules from the cathode to be allowed 

 suddenly to fall on the anti-cathode, and after raining on it for a little 

 to be as suddenly cut off. According to the views I entertain as to 

 the nature of the Rontgen rays, the moment the shower is let on the 



