MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
337 
Section II.-— Determination of the Position of the Planes of the Prism P IP with 
reference to the Axes of Elasticity. 
In order to compare these results with theory we require to know accurately the 
position of the principal planes of the two prisms. We will consider the prism, faces 
P R, first. 
Let O A, O B, O C be the directions of the principal axes of the wave surface, 0 B 
bisects the angle between the faces m ml, O C is parallel to the intersection of these 
faces. 
Let the normals to the faces m m cut a unit sphere, centre O, in M M', the normal 
to the face P in P, and the normal to R in R. 
Let the direction angles of P and R with reference to A, B, C be cq /3 i y 1( a. 2 /3. 2 y 2 
respectively. 
[P lies in quadrant A B C, R in A' B Cf.] 
Fig. 3. 
PM--^, PM'=0/ 
RM=0 3 , RM'=0 3 ' 
Observations on the angles between P and m, P and m give 6 1} 6{ respectively. 
While observations on the angles between R and m, R and m give 0. 2 , 61 respec¬ 
tively. 
Let 
MM'=2/x=2MB. 
Then, as in the first part, from triangles M' B P, M P B we get 
cos 
cos /3, = — 
0f+f 
2 
cos 
COS 
( 1 ) 
with similar formulae for /3 3 , 
MDCCCLXXIX. 
. o x '+o y . e l , -e l 
sin —-— sin —-— 
cos a, — 
COS yU- 
a.i 
X 
(2) 
