304 MR, R. T. GLAZEBROOK ON PLANE WAVES IN A 
CRYSTAL. 
The position of the plane Q Q / is completely determined by the arc A Q and the 
angle A Q Q y ; we have already found, 
AQ = 45° 30' 
For the angle A Q the triangle Q A Q / gives 
sin AQQ = m ^ sin QAQ 
sin QQ, ^ 
whence 
AQQ / = 4° 37' 20" .(4) 
Thus we see that the principal section of the second prism is inclined to the plane 
c at an angle of 4° 37' 20". 
I mentioned above that the second prism lay in the twin crystal. This was proved 
by passing light through the crystal, in a plane nearly parallel to the principal plane 
B C, when four images of the slit appeared. On cutting off the end containing the 
second prism, two images vanished ; on cutting off the other end, only the second pair 
vanished, and the first pair reappeared. The work for the first prism was done with 
the second pair, the first pair being rarely sufficiently distinct for measurement. 
Throughout the work, cj) <\> denote the angles which the normals to the incident or 
emergent wave, and the wave in the crystal make with the normal to the face Q. 
The same remarks as to the reversal of the crystal hold as in the case of the first 
prism. 
