( 150 ) 
These equations are the same as (27) to (32), with the exception 
that ® and € and in the same way Sand $ have been interchanged, 
ae Q EM: 
and that — is replaced by —. By changing in the same way 
§ U 
(39), (40), (41) and (42), we find: 
Na a Be 5 ih: 
v An U 
nr ss 
ya Oe RR 
tnU~1 = sin (DE) re 
from which last equation we further deduce: 
HE? sin? (D €) € 5 
tv Nn (DE) 
(42 U)? D cos (D E) B cos (BH) ’ 
=v 
and so, according to (9) and (18), 
OS OTe NRE en en CED 
€ "5 
This equation expresses that g is equal to v times the area of 
the parallelogram, described on the radii vectores of the reciprocal 
electric and the reciprocal magnetic ellipsoid, resp. in the directions 
of € and S. 
The two values of g for the same ray are equal only when the 
ray-plane is a section of similitude of the two reciprocal ellipsoids, 
and the electric and magnetic force are accordingly indeterminate 
ant. 
12. By Poynrine’s theorem the flow of energy is greatest 
through a plane through § and €, i.e. through the ray-plane. 
The amount of energy W, which according to that theorem flows 
through the ray-plane per unit of time and per unit of area is: 
