MR. R. T. GLAZEBROOK OR BLARE WAVES IR A BIAXAL CRYSTAL. 
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at that point the theoretical value is greater than the experimental. It continues so 
until we reach line 18, Table V., that is for an arc of about 9°. 
From that point onwards the theoretical value of p. 2 is uniformly less than the 
experimental. 
Thus the result of experiment for the outer sheet is represented by a curve which 
coincides with the theoretical section at the point in which it cuts the plane A O C, 
then lies inside it for an arc of about 9°. 
At this point the two again coincide, and for the rest of the arc experimented on, 
the section given by experiment lies outside that given by theory; the difference 
between the two increasing throughout Tables VI. and VII., and diminishing again in 
Table VIII. 
Section VII.— Effect of possible Errors in the Values of p a , pi, p c discussed. 
The next task will be to determine the effect of any small errors which may have 
occurred : 
(1.) In the determination of p a , p b , p c ] 
(2.) In the angles which fix the position of the planes P It, P Q relatively to the 
axes of the surface. 
(l.) p a . To determine p„ we considered the length of the radius vector common to 
the principal section A O C and the section by the plane P R. This radius vector is 
fixed in length, and is on Fresnel’s theory the radius vector of an ellipse, axes p a and 
p c , inclined at a small angle to p a . 
The increase in the value of p a will therefore increase the value of this angle, that 
is, it will alter the position of the plane P L R' and be more properly considered 
under (2). 
pi is determined by the intersection of the section of the inner sheet by the plane 
P R, with a circle of radius pi, and is therefore fixed at least within the limits of 
experimental error. 
p c is found from an independent and, on the whole, less trustworthy series of 
observations—less trustworthy because the faces were much less plane than P, Q, R. 
Let us consider the effect of decreasing the value of p,.. 
If e be the angle between the optic axes 
tan e= \/ J L 
_1 
Pc 2 Pi 2 
As p c decreases tan e decreases; therefore e decreases. 
Let us consider how this affects 6 and O'. 
