498 Lord Rayleigh on the 



velocity of aether and matter. A more important difference 

 is the introduction of an additional force of restitution (a 2 «.r), 

 proportional to the absolute displacement of the atoms. His 

 equations are 



"S= a2 f +«*-«. • • • • <»>■• 



»5=^«-^)-^-7 2 J. • • • (10) 



This notation is so different from Maxwell's, that it may be 

 well to exhibit explicitly the correspondence of symbols. 



ielmholtz . 



• 1 $ 



(*• 



[ « 2 



1 v 1 



*.-£ 1 



/3 2 



1 m 



1 a2 



! c 



1 & 



axwell . . . 



•1 V 



1 ? 1 



[ E 



1 x 1 



C \ 



i a P 2 



1 - 



1 o 



1 v 



h// 



When there is no dissipation (R=0, Y 2 = 0), these inter- 

 changes harmonize the two pairs of equations. The terms 

 involving respectively B, and y 2 follow different laws. 

 Similarly Helmholtz's results 



1 Jc 2 fju ft 2 /3 4 run* 



2 m 2 Ji „2„2 «2«2 /„ V1/M 2 „2 /Q2v2i^ # 4^2' 



n 



a 2 )! 2 a 2 7i 2 (inn 2 — a 2 — ffif + yW 



(ii) 



2*_ /3V 1 . . (12) 



c 2 ~ a?~ aV mn 2 ~a 2 -^ • • ' C ,J 



c ?z a 2 7i (mn 2 —a 2 —/3 2 ) 2 -t- <y A n 



identify themselves with Maxwell's, when we omit E and 

 7 2 and make a 2 = 0. 



In order to examine the effect of a 2 , we see that when 

 7 = 0, (11) becomes 



c 2 a 2 a?n 2 m 

 or in terms of v 2 ( = c 2 /c 2 ), 



^l-g. ?-?/"• (14) 



^ mn^ — a^—p 1 



If now in (14) we suppose n=0, or X = co, we find that 

 v = co , unless a 2 = 0. If a 2 = 0, we get, in harmony with (6), 



„ 2=1 ^ , .... (15) 



* What was doubtless meant to be cPg/dy* appears as d 2 $/dz 2 , 

 bringing in x in two senses. 



