Manchester Memoirs, Vol. xli. (1897), No. 15. 21 



reference to passing through a prism depends upon that. 

 When you let a ray of light fall upon a refracting 

 medium such as glass, motions begin to take place in 

 the molecules forming the medium. The motion is at 

 first more or less irregular ; but the vibrations ultimately 

 settle down into a system of such a kind that the regular 

 joint vibrations of the molecules and of the ether are 

 such as correspond to a given periodic time, namely, 

 that of the light before incidence on the medium. That 

 particular kind of vibration among the molecules is kept 

 up, while the others die away, so that after a prolonged 

 time — the time occupied by, we will say, ten thousand 

 vibrations, which is only about the forty thousand 

 millionth part of a second — the motion of the molecules 

 of the glass has gradually got up until you have the 

 molecules of the glass and the ether vibrating har- 



emission of Rontgen rays begins, it lasts as long as the shower, and 

 ceases the moment the shower is cut off. But the fluorescence only 

 gradually, quickly though it may be, comes on when the shower is 

 allowed to fall, and gradually fades away when the shower is cut 

 off. So far from the fluorescence being in any way the cause of 

 the Rontgen emission, there seems reason to think that if it exercises 

 any effect upon it at all, it is rather adverse than favourable. For it 

 has been found that when the target is metallic, and gets heated, the 

 Rontgen discharge falls off; and fluorescence, like a rise of temperature, 

 involves a molecular disturbance, though the kind of disturbance, is 

 different in the two cases. 



As the fluorescence of the glass wall and the emission of X rays 

 are two totally different effects of the same cause, namely, the molecular 

 bombardment from the cathode, the intensity of the one must by no 

 means be taken as a measure of the intensity of the other, even with 

 the same tube. The former effect would appear to be the more easily 

 produced. This consideration removes a difficulty mentioned at p. 10 

 of the paper by Prince Galitzin and M. v. Karnojitzky, as attending 

 the supposition that the X rays originate in the points in which the 

 cathodic rays fall on the wall of the tube or other target. Nor need it 

 surprise us that in some cases the shadows seem to indicate more than 

 one source of action, when we remember that from a given point more 

 than one normal can be drawn to a given closed surface. 



