Constitution of Natural Radiation, 579 



observed, are dispersed into simple wave-trains each travelling 

 in its own appropriate direction. It would seem indeed that 

 this illustration bears more closely on the action of a travelling 

 source impressed on the medium than on the fate of an un- 

 supported pulse travelling across it spontaneously. In default 

 of a constant supply of power to the boat, to be spent, in 

 making new waves, it would soon lose its velocity unless it 

 had a store of kinetic energy great for its size. Thus this 

 close analogy with ordinary dispersion, which is afforded by 

 the dispersed wave-trains excited by a pulse, impressed and 

 maintained from outside, appears to leave where it was the 

 question of the fate of an isolated unsupported pulse, propa- 

 gated into the dispersive medium and then left to itself. The 

 steadiness which in the ordinary dispersion-theory arises from 

 the succession of fresh waves of the train, is obtained in the 

 illustration above by the maintenance of the energy of the 

 pressural source, with results in close analogy in the two cases. 



It thus still seems difficult to evade the force of the 

 argument of Sir George Stokes * : — " When you let a ray of 

 light fall on 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 definite 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 harmoniously together. But in the case of the 

 Rontgen rays, if the nature of them be what I have explained, 

 you have a constant succession of pulses independent of one 

 another. Consequently there is no chance to get up harmony 

 between the vibrations of the ether and the vibrations of the 

 body." 



The distinction may perhaps be put more definitely. White 

 light from an incandescent solid is made up of a vast number 

 of pulses arising from the molecular shocks incessantly 

 occurring in the hampered spaces to which the molecules are 

 confined. On the other hand, the Rontgen rays are made up 



* Wilde Lecture " On the Nature of the Rontgen Rays, ' 1897. Math, 

 and Phys. Papers, v. p. 271. 



2R2 



