MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
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Thus a change in y will not render theory and experiment consistent. 
Thus no change in the position of the plane of the prism will bring Fresnel’s theory 
into agreement with experiment. 
Section VIII.— The Theoretical Investigations for the Second Prism on Fresnel’s 
Theory with Comparison with Experiment. 
Our next step is the theoretical investigation for the second prism. 
Now experiment has shown that this prism lies in the twin crystal ; we must 
therefore find the axes of this crystal. 
Eig. 9. 
Let the figure represent a section of the crystal by a plane perpendicular to the axis 
o a 
The twinning takes place about an axis perpendicular to rn, and consists of a 
rotation through 180° about that axis. 
Its effect, therefore, is to bring O B, fig. 9, into the position 0 B', where 
angle mOB = mOB' 
O 
that is, to turn the axes in the direction B to A, through an angle, 
= 2 (mOB) = 2 ^—AO m)j 
= v —2/x=180°—116° 56' 
= 63° 4' 
and if Q is the point where the perpendicular on the face Q of the second prism meets 
the sphere, we know that 
AQ=45° 30' [Section IV. (1)]. 
Hence since AA'=63° 4' and Q lies in plane A 0 B, 
A / 0Q=17° 34'.(1) 
Now let A' B' C', fig. ID, be the points in which the new axes cut the sphere, and let 
2 t 2 
