(58 ) 
VU 
Go 
—_— 
Ter Tm sin Us 11)’ 
if the electric induction falls along I, and: 
Vv 
en ° ’ 
Perm sin (I, II) 
5 = 5 5 N Sj Terr Tr 
if the magnetic induction falls along I, so that —=————. 
83" Nel 7am 
The two values s, ‘and s, are the same if rey : tmy =Teqy : “mr 
i.e. if the wave-front is a section of similitude, and ®B and D are 
therefore indeterminate in it. 
8. We shall now derive an equation which we shall use in order 
to determine the wave-surface. We apply the before-mentioned 
transformation, which if we take the axes of the magnetic ellipsoid 
as axes of the coordinates, is represented by (37). We distinguish 
by accents the transformed quantities, so that we get: 
f=SV tr I =IV ky Wh un 
d=eVun Valur dze ur 
The equation: 
le + my + nz=s 
represents a plane parallel to the wave-front at a distance s from 
0. This plane is changed by the transformation into: 
al ym z'n 
Veco Villy Vie 
= <8; 
or 
al tym +2n=s, 
where 7’ m' and «#' are the direction-cosines of the normal, and 
s the distance from O to the transformed plane. In this: 
l m P n 
Js m' = - n = ——, 
PY Hy PV Mz Pp 
DV Mex 
