702 Sir J. Larmor on the Reflexion of Electromagnetic 



of the electrons corresponding to the room temperature, 

 V = l*l . 10 7 cm. per second. We thus find 



Metal. 



T. 



! 

 L. 



Copper 



2-8 10-14 

 8-1 10" 15 

 4-2 10-i 2 

 2-1 IO-1 5 

 2-2 10-14 

 5-0 10-14 



31 10~ 7 



8-9 10~ s 

 4-6 10- 5 

 2-3 10~ 8 

 2-4 10" 7 

 5-5 10~ 7 



Aluminium 



Bismuth (H = 7200) 



Platinum 



Gold 



Silver 





We wish to thank Mr. W. C. Duryea, mechanician of 

 the laboratory, for his skill in constructing the apparatus 

 employed. 



Palmer Physical Laboratory, 

 Princeton, N.J. 



LXXIII. On the Reflexion of Electromagnetic Waves by a 

 moving perfect Reflector, and their Mechanical Reaction. 

 By Sir Joseph Larmor, F.R.S* 



I FIND that I cannot accept the criticism of my views on 

 this subject which is contained in Mr. Edser's paper in 

 the present volume of the Phil. Mag. pp. 508-527. 



In a train of radiation, the magnetic and the electric force- 

 intensities must be of the types of velocity and strain in the 

 transmitting medium, if we are to liken the radiation to a 

 train of dynamical waves at all. The amplitude with which 

 we are naturally concerned is the amplitude of the swaying 

 displacement or shift of that medium, a shift of which the 

 magnetic force-intensity represents in some way the time- 

 gradient or velocity. In reflexion of a wave-train directly 

 incident, the amplitude of this displacement, which is the 

 vector referred to in the last sentence o£ the quotation made 

 by Mr.Edser, undergoes no change ; but when the reflecting 

 surface is receding, the amplitude of its velocity, here the 

 magnetic force, is no longer thereby preserved. 



If the reflector is receding with velocity v, equal to c, that 

 of the radiation, it just keeps ahead and there is no reflexion, 

 and so there ought to be no pressure. The argument in the 

 quotation made by Mr. Edser (which will be found in Brit. 

 Assoc. Report. 1900, expressed more concretely in terms of 



* Communicated by the Author. 



