310 
MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
Section VI .—Calculation of the Reciprocal of the Velocity of Wave Propagation on 
Fresnel’s Theory for the First Prism and Comparison with Experiment.—The 
same for Lord Rayleigh’s Theory. 
We are now in a position to calculate the reciprocals of the velocity of wave 
propagation for the first prism on the theory of Fresnel. 
Fig. 7. 
Let 0 O', fig. 7, be the points in which the optic axes cut a unit sphere. 
L M the trace of the principal plane of the prism cutting A C' A' in L, B C' B' 
in M. 
Let N be any point in the plane, in which a wave normal meets the unit sphere. 
Let 
NO = 6> NO 
C'LM=y=l° 12 ' [Section III. (9)]. 
cos 6 = cos LN cos OL+ sin LN sin OL cos y—cos (LN —\) 
if tan \=tan OL cos y. 
Similarly 
if tan X' = tan O'L cos y. 
,,, cos 0 L / T -«r x ,\ 
cos 6 =-— cos (LM—X ) 
cos A 
Now 
AO = 90° —9° 4' 58" [Section V. (6)]. 
= 80° 55' 2" 
AL=2° 31' 58" [Section III. (10)]. 
OL=78° 23' 4" 
whence 
0'L=:96 0 33' 
( 1 ) 
( 2 ) 
