and Refraction for the Principal Section of Media in motion. 413 

 normal of the effective surface, we can write :— * 



C E sin a E ^E + C R sin a R ^ R =C D sin a D /M D ** J 



Therefore not merely will the vis viva of the reflected and the 

 refracted wave result from that of the incident, but the algebraic 

 sum of the just-noted components of the quantities of the motion 

 in the incident and the reflected wave will also be equal to the 

 corresponding components in the refracted wave. 



I have here to elucidate the two new notions of the effective 

 dividing plane and the reduced aether-masses. 



In regard to the former, the particles of the incident wave do 

 not all reach the moving partition in one and the same position. 

 The effect of this is as if the partition were turned through a 

 certain angle 



S=— sin a cos ylr. 

 co T 



where yjr denotes the angle between the perpendicular and the 



direction of the motion, and — the ratio of the velocities of trans- 



co 



lation and light. The actual partition is thus, so to say, indif- 

 ferent toward the incident wave ; and the Active rotating plane 

 is at the same time also the effective one. 



If, further, both media consisted of homogeneous aether of 

 different densities, then the reduced masses would be equal to 

 the actual ones. But now a ponderable transparent medium is 

 an aggregate of particles of aether and particles of ponderable 

 matter. Nearly the same constitution may be attributed to the 

 aether in the interior of the medium as to the aether in cosmical 

 space; and the material particles oscillating with it, which are 

 certainly not hurried along entirely without resistance, but per- 

 form oscillations according to laws of their own (in the state of 

 rest isochronously, and in general in smaller amplitudes than 

 the aether particles), will act almost entirely as ballast, and 

 usually contribute little to the development of force. Hence, if 

 by m be understood the mass of aether, and by m' that of the 

 body of the same volume, and if C, C be the mean velocities of 

 vibration corresponding to any wave of the individual aether- and 

 body-particles, the velocity of propagation of this wave is given 



* I prefer this form, first advanced by Cornu (Ann. de Chim. et de Phys. 

 vol. xi. p. 283) for media at rest, to Fresnel and Neumann's continuity- 

 equation, inasmuch as it brings out the distinction between velocity of 

 oscillation and amplitude, and is applicable to crystals without bringing in 

 other principles. 



