MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 377 
Let us suppose that an equation of this form holds in crystalline media also, only 
that a, b, c, &c., instead of being constants are functions of the directions of propagation 
and vibration; and, further, let us suppose that Fresnel’s construction is true for 
waves of infinite length, so that the equation -=a gives us a Fresnel’s wave surface. 
From the known values of the constants of the wave surface for different rays of the 
spectrum the constants of the surface for infinite -wave lengths can be found, and hence 
the value of a calculated in a,ny given direction. 
1 
If from experiment we find the value of - or p for any wave length, the difference 
p —a ought on this theory to be equal to —. 
And if we find the values of p for different wave lengths (p l5 p 2 , p 3 say) in the same 
direction, we have 
b 
^~ a= \~z 
b 
p 3 a — 
b 
P- 3 a — 
whence 
CO Xj 
fjio—Ct _ \ 3 2 
yL6 3 CC A/£ 
To verify this I observed the values of p in two directions for the rays C, D, and F, 
with the following results :— 
Q- = 1-2403 
Pd a 
Pc 
—a 
=: 1 *2875 (first experiment) 
1-2770 (second experiment) 
—) =1-46978 
& 
/^D a 
:1'47208 (fh’st experiment) 
= 1-47348 (second experiment). 
The numbers, especially in the last case, are sufficiently close to make it seem worth 
while continuing the investigations. 
3 c 
MDCCCLXXIX. 
