MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BlAXAL CRYSTAL. 
311 
X=78° 22' 55".(3) 
y=96° 33' 5" .(4) 
Also P / being the point in which the normal to the face P, meets the sphere 
LN = LP+PN 
=LP+f 
LP =67° 2 T 25" [Section IIP (8)]. 
LN-X=LP,+f-X 
—(\> —10° 55' 30"..(5) 
LN—A.'=LP / +</) / —V 
— 29° 5' 40".(6) 
Again, if v 1 v z are the velocities of propagation in the direction 9 6', we know that 
Vi 3 4-v 3 3 =a 3 +c 3 --^(a 3 -^<P) cos 9 cos 9' 
v^—v 2 =-(a/ —c 2 ) sin 9' sin 9' 
a c having here their usual meanings of the greatest and least principal velocities. 
Hence 
2v 1 ~=a i + c z — (a 2 —c 2 ) cos ( 9-\-9 ) 
2v 2 2 = cr+c 2 —(a 3 —c 3 ) cos (9—9') 
From these formulae we can find i\ v 2 , and thence the values of jx i jx 2 for 
Taking 
1 
/*i=— 
v v 
1 
ft a = 1-68580 
fx c — 1 '53013 
c 3 =A = -351876 
fL 
cr= 1 0 = *4271 1 7 
/V 
