13t) 
or, in terms of wave-lengths, 
nn <*_ 1 V yHNXon* 
K > c ‘"~ 2n (À* — /l 0 2 ) 2 - Vi 2 
where & = y%/2 tcc. 
§ 5. In the paper alluded to we considered the behaviour of 
a «mono-electronic» body, supposed to contain only one class of 
movable electrons. As an example of the application of the fore¬ 
going general formulae let us suppose we are dealing with such 
a substance. With this assumption it follows from equations (III), 
(IV) and (V) of § 4., that if we take 
(1) n 0 2 — n 2 + y%n — (by ceN= 4nyceNcP 
(2) 2kn = 4nyceN(ä 
then the following relations hold — 
(3a, b) 
Thus 
x 2 — 1 = 
& 2 -\-@ 2 1 
2vx — 
<8 
cP 2 —j— (§L 2 ' 
(4) 2*• — \ (l + j ,~+ (^ + et ) — «*) ' 
This is the exact expression which must be used for the calcula¬ 
tion of x. In some cases however, we shall find it more convenient 
to have recourse to an approximate expression. Let us suppose (8 
great in comparison to unity and éP small in comparison to (8. 
Then, to a first approximation, 
(5) 
2x 
<ä 
0 ?2_|_T(p- 
Writing now x 1 for the right-handed circularly polarized wave 
and x 2 for the left-handed circularly polarized wave (see § 4.) we 
obtain 
from (1), (2) and (5) 
(6) 
16n%y 2 
Xl Xl ~ + 
ceNkn 2 4 
\{G % * + 4k t n*y 
where 
(7) 
II 
O 
to 
to 
1 
wyceN 
(8) 
G 1 = F -[- y % n 
(9) 
G. j = F — y % n . 
