508 Mr. E. Edser on the Reflexion of Electromagnetic 



light. We have not yet been able to decide if this action 

 can be transmitted from one crystal to another. If it cannot 

 be so transmitted, then it would seem to argue against an 

 electronic transmission such as mentioned above. We pro- 

 pose to test not only the matters of the rapidity of trans- 

 mission along the crystal, and the possibility of transmission 

 from one crystal to another, but also to determine if a pure 

 heat effect can be transmitted by this same mechanism. 



Physical Laboratory, 

 State University of Iowa, 

 May 14, 1914. 



LIV. The Reflexion of Electromagnetic Waves at the Surface 

 of a Moving Mirror. By Edwin Edser, A.R.C.Sc, Head 

 of the Physics Department, Goldsmiths College (^University 

 of London) * m 



1. TT is well known that when waves of any kind are 

 JL reflected from a moving obstacle, the wave-length of 

 the reflected waves generally differs from that of the incident 

 waves. It has also come to be accepted that this alteration 

 of wave-length is the only result produced by the motion of 

 the reflecting obstacle ; in other words, the amplitude of the 

 reflected waves is supposed to be unaffected by the motion 

 of the obstacle, the additional energy conferred on the re- 

 flected waves by this motion being due necessarily to the 

 alteration of wave-length. It is the object of the present 

 communication to show that this supposition is inadmissible 

 with regard to electromagnetic waves ; that, in fact, the 

 amplitude of reflected electromagnetic waves is altered by 

 the motion of the reflector, and that this alteration of ampli- 

 tude is solely responsible for any change in the energy of 

 the reflected wave-train due to the motion of the reflector. 



2. The accepted opinions on this subject have been derived, 

 apparently, from Sir Joseph Larmor's article on Radiation 

 in the revised edition of the Encyclopaedia Brittanica (vol. 

 xxii.). For convenience of reference, an oft-quoted passage 

 (§ 2) in that article will now be transcribed : — 



" Consider a wave-train of any kind in which the displacement 

 is f =a cosm (x + ct), so that it is propagated in the direction 

 in which x decreases ; let it be directly incident on a perfect 

 reflector travelling towards it with a velocity v, whose 

 position is therefore given at time t by x = vt. There will be 

 a reflected train given by ^' = a f cosm' (x — ct), the velocity 

 of propagation c being of course the same for both. The 

 * Communicated by the Author. 



