MR. R. T. GLAZEBROOIv ON PLANE WAVES IN A BIAXAL CRYSTAL. 
363 
the theoretical value of /x 2 , we should acid greatly to the error in the case of the inner 
sheet. 
Section VIII .—Effect of Change in Position of Planes of Prisms with reference to the 
Principal Planes of the Crystal. 
We have now to discuss the effect of small variations in the position of the planes 
P It and P Q. 
We take P It first, and we consider its position as determined by C L and the 
angle A L P, while the position of the normals to P and It is fixed by P L (vide 
Section II.). 
Fig. 13. 
Now variations in C L and ALP produce variations in 0 L, O' L, X X', &c., and 
hence in 0 O', while variations in L P only change 0 and O', O L, O' L, X and X' remaining 
constant. 
Let us consider, therefore, the variations of L P first. 
A reference to our tables shows us that through the greater part of the arcs L P, 
P Q', 0—0' is constant, so that SO considered as dependent on L P is equal to SO' 
nearly. 
Again, 
i) cos {OpO') 
and since the change in {0—0') due to a small increment of L P is very small, such an 
increment will not alter the value of g. 2 . 
But if 0—0' is nearly constant for a small motion along L P, 0-\-0' will be nearly so 
for a small motion perpendicular to L P. 
Thus a small motion perpendicular to P L will not alter appreciably the value of y l ; 
3 A 2 
