(1584 
W = DE sin (DE), 
An 
or, by (73) 
WU. iy) a ACR 
a result which was to be expected. 
13. Let us now examine the wave-surface somewhat closer. The 
section with a plane of coordinates degenerates into 2 ellipses, which 
Benen tons 0: 
v2 
Co ne en? LE ne we 
and 
u? 
5 Ds Ze el 
Gm” Nm gen Ge Hm w° 
The first ellipse is similar to the section of the electric ellipsoid 
9 dus 50 
rar =1 with the plane «=0, and the second ellipse to 
sas Wet ans rlr Ae 
the section of the magnetic ellipsoid mate EE PT 1 with the 
plane x=0. I shall call the first ellipse an electric ellipse and 
the second a magnetic one. The same applies to the sections with 
the planes y=0O and «<= 0. It is easy to find that the electric 
ellipse in one plane of coordinates intersects the magnetic ellipse situated 
in another plane of coordinates (of course in a point of a coordinate axis). 
If bj > bp > 3, the electric ellipse in the Y Z-plane lies quite outside, 
and that in the X Y-plane quite inside the magnetic ellipse, while in the 
X Z-plane the two ellipses intersect in 4 points. These four points 
are conic points of the wave-surface. (It is easy to find analytically 
that the wave-surface can only have conic points in the three planes 
of coordinates and in the plane at infinity, which projectively may 
also be considered as plane of coordinates. The section of the wave- 
surface with each of these four planes degenerates into 2 conic sections ; 
so that every plane furnishes four conic points; in all, 16 conic points, of 
which however only the four lying in the XZ-plane are real. The 
waye-surface intersects the plane at infinity along the sections 
10% 
