MR. R. T. GLAZEBROOK QN PLANE WAVES IN A BIAXAL CRYSTAL. 
295 
Section III.— Determination of the Position of the Principal Plane of the First Prism 
with reference to the Crystallographic Axes .— Values of the Quantities observed in 
the Experiments, and Calculation of the Reciprocal of the Wave Velocity for the 
First Prism. 
We proceed now to the measurements made to determine the position of the 
principal plane of the first prism, of which the faces are P IV 
Fig. 2. 
Let A B C be the points in which the axes of the crystal cut a sphere, with its 
centre at the origin. 
Let M M' be the poles of the m faces of the crystal, M M" lie in the great circle 
A B, and the angle M M' is known; also B M = B M'; let P P, be the poles of the 
cut faces of the prism. 
Let 
PM=d, PM'=A 
p,m=6>, vw=e; 
Let a, /3, y be the angles which O P makes with the axes. 
Let MA=/x. 
From triangle PAM, 
cos PM — cos AP cos AM+ sin AP sin AM cos PAM 
or 
cos 6= cos a cos /x-f-sin & sin y cos A 
From PAM', 
cos 0 '= — cos a cos yf sin a sin y cos A 
cos 6 cos a cos y— cos O'f cos « cos y 
cos 0— cos 6’ 
cos a=—-- 
2 cos y 
. 9+e f . e>-B 
sin —~— sin 
2 2 
COS 
(i) 
