Manchester Memoirs, Vol. xli. (1897), No. 15. 19 



the other are freely refracted ? This difficulty led me 

 to conceive of a theory, which I believe to be new, as 

 to the nature of refraction itself — as to the nature of 

 what takes place, for example, when light is refracted 

 through a prism. Suppose we have light of a definite 

 refrangibility, and a prism on which it may be made 

 to fall. When the light is admitted we commonly 

 imagine — at least, I believe so — that the light is imme- 

 diately refracted, and with proper appliances you get the 

 spectrum. Immediately ? I do not think so. How is it 

 that light travels more slowly through refracting medium 

 than through vacuum ? There are different conjectures 

 which have been advanced. One is that the ether within 

 refracting media is more dense than the ether in free 

 space. Another is that while the density is the same 

 the elasticity is less. Then, there have been speculations 

 as to the ether being loaded with particles of matter. 



Take a piano. If you strike a note a string is set 

 in vibration. You would hardly hear any sound at all 

 if it were rigidly supported. But it rests on a bridge 

 communicating with a sounding-board, and the sounding- 

 board presents a broad surface to the air, and is set 

 in motion by the string. The sounding-board and the 

 string form a compound vibrating system. In the same 

 way it may be that the molecules of the glass, or other 

 refracting medium, and the ether form between them a 

 compound vibrating system, and, when the motion is fully 

 established, the two vibrate harmoniously together. But 

 how does it get to be established ? We can hardly 

 imagine otherwise than that the ether is excessively rare 

 compared with ponderable matter.* Well, supposing 



* The views as to the nature of refraction, which I have endeavoured 

 to explain, lead me incidentally to make a remark on another subject 

 not, indeed, very closely connected with it. From the first, Rontgen 

 recognised as the seat of the X rays which he had discovered the place 



