( 329 ) 
By putting » = Oorl«, we are led back to the well known 
theory of the ZEEMAN-effect for directions parallel or perpendicular to 
rifnes of force. Indeed, from (13) we deduce for the first case 
.(23) 
§ 5, 
heory 
the lines of 
and for the second 
Further, when 3 
§ = — 5 s or S — T i 
— 0, we have by (14) 
3V T . 
so that in this case, whatever be the value of n, one of the principal 
beams, corresponding to the upper sign, is characterised by a left- 
handed, and the other by a right-handed circular polarisation, lhis 
will require no further explanation. We may, however, say some 
words about the rotation of the plane of polarization that is observed 
along the lines of force, and especially about its amount for 
In this case 
' (S4> 
ill .... (25) 
iv'+g' 
the formulae (23) and (22), the complex index 
5= - 
Hence, according 
of refraction is 
for the left-handed beam, and 
2 P v j 
. , l. . 111 - 1 
0^>=l*.jl- l Or+7 4r»-fy*( 
for the right-handed one. 
Comparing these expressions with (7), we see that the two rays 
are equally absorbed, the index of absorption being for both of them 
n 0 ti 0 Pg 
c (4 v'+in 
but that their real indices of refraction are unequal. Their difference 
is given by . 
f*+ — (i- = 
and, corresponding to it, there is 
zation amounting to 
4ff* t 
4v*+/’ 
i rotation of the plane of polari- 
