324 MR. R. T. GLAZEBROOK OR PLANE WAVES IN A BIAXAL CRYSTAL. 
P Q M be the plane of the prism; P being any point in the plane ; Q the normal to 
the face, and M the point in which the plane cuts B' C', 0, O' the optic axes. 
Then if P is the pole of any wave in the crystal, PQ = </>'. 
Fig. 10. 
Let VO=9, PO'= &, we shall require the values of 6, 6'. 
Now in triangle A' M Q 
angle Q=4° 37' 20” [Section IV. (4)]. 
A'Q = 17° 34' 
cot MQ= cos Q cot A'Q 
whence 
MQ=17° 37' 14”.(2) 
Let A' M Q=x- 
Then from triangle A' M Q. 
cos M= cos QA' sin Q. 
whence 
Now, 
put 
x =85°35'40". 
cos 0'= cos PM cos MO'+ sin PM sin MO' cos X 
= cos MO'(cos PM + sin PM tan MO' cos X ) 
tan MO' cos x = tan X' 
n , cos MO'cos (PM—A,') 
cos 0 — 
(3) 
cos A.' 
(4) 
