308 
MR. R. T. GLAZEBROOK OR PLANE WAVES IN A BIAXAL CRYSTAL. 
and MP / measures the angle between the wave normal required in the crystal and the 
normal to the face P . 
✓ 
This is the angle we have denoted by (f>. 
From Tables II., III., VIII., and IX. we have corresponding values of 
Other observations gave 
<t>' 
19° 
32' 
21" 
1-68134 
tv 
o 
0 
34' 
27" 
1-68130 
20° 
28' 
9" 
P68119 
19° 
29' 
32" 
1-68123 
f 
P 
19° 
39' 
22" 
1-68126 
20° 
4L 
22" 
1-68129 
Again, none of the experimental values of p in Table III., lines 1 to 11, and 
Table IX., differ greatly from 1‘68125, and this is not far from the mean of the six 
observations recorded above. 
We take, then, this as the radius vector of an ellipse axes p 4 , p c , inclined at 1° 12' 
to p$. 
To determine p c we must have recourse to the second prism. 
Did the plane of the second prism coincide with that of x y, the values of p in 
Table XXI. would all be equal to p c . 
We shall consider later the effect of the inclination of the plane of the prism to that 
of X y, but for the present may remark that the values in Table XXL differ little from 
1‘53013, the value of p c found by Pudberg. 
Let us take therefore 
p,= 1‘53013 .(3) 
We have then in the plane B O C 
1 cos 2 6 sin 2 6 
2 2 *" 2 
H' Pb H'c 
6 being the angle between p and p/ y . 
Now 
p= 1‘68125 
p,= 1-53013 
0 = 1 ° 12 ' 
Whence 
p 4 = 1-68132 .. (4) 
The value found by Pudberg is 1‘68157. 
The difference is considerable, being ‘000-25. 
