414 Prof. Ketteler on the Boundary-Conditions of Reflection 

 by the expression (Astr. Und. p. 195) : — 



* 



1 + 



CW 



in which, evidently, instead of the oscillation-velocities C, C, 

 may be inserted the components taken along any direction, e. g. 

 C sin Civ, C sin Ad* The compound medium in question there- 

 fore behaves for every direction as a homogeneous sether would 

 whose reduced mass was 





C A'T 



n' being the absolute index of refraction. If we write ■*? = T" tp 



A' 

 the ratio of amplitudes — appears as a constant characterizing 



the medium and remaining unaltered during the motion. 



The further investigation is attended by a certain difficulty in 

 ascertaining the three masses or volumes m E ,m R ,m D set in motion 

 under the influence of the translation. The solution of this 

 problem, however, is facilitated by the fact that, referred to the 

 moving isotropic medium, the continuity-conditions of Case I. 

 permit the form of the equation of the vires vivce belonging to 

 them to be deduced ; and this gives a hint for the behaviour of 

 anisotropic media generally. 



Lastly, with respect to the intensity-formulae themselves 

 which are thus to be obtained, we get, e. g. for the external re- 

 flection from moving isotropic media (rc' E = l, G>E = fc>R=v) and 

 for the first principal case, 



sin (a — fl D ) 

 sin (a -f a D ) 

 and for the second, 



tan (a — a D ) 

 tan (a + a D ) 



A moving mirror consisting of any isotropic medium behaves, 

 therefore, as one of the same substance at rest, to which instead 

 of the previous face a new one, turned the angle ft, is given, and 

 for which the former ratio of refraction n is in any way reduced 

 to 



I sin a __ v 



" sin dp co + g/c cos (OLn — TJ/) 



in which ^ is referred to the component in the plane of inci- 

 dence of the motion of translation, and k signifies FresneFs co- 



p sni^a — u D j r 





