MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
369 
Thus we have found a plane section of the surface which agrees closely with the 
results of experiment. 
This section passes through L, a point on the principal section A C at an angular 
distance of 1° 21' 42" from C, is inclined to that section at an angle of 60° 20', while 
the normal to the face P lies at an angular distance from L equal to 
35° 0' 19" + 0° 11' 15"=35° 11' 34" 
Under these circumstances it will be necessary to consider the probability of making 
such an error as this result indicates in the position of the normal to P with reference 
to the principal axes. 
But before doing this we must say a few words about the variations in the values 
of fa produced by the correction now considered. 
The final correction will not affect any of them except the first, which will be slightly 
increased by it. 
This follows from the fact that between lines 1 and 2 of Table IX. 6 — 6' varies 
considerably, and increases or decreases with 6-\-0', so that /y, increase or decrease 
together. 
The change considered has increased p, l5 it will therefore increase 
The difference in line 2 is considerable, but it may be noticed that the experimental 
value is clearly too small, falling as it does (Table I., line 18) between two con¬ 
siderably greater values. In the other cases the differences between theory and 
experiment have been diminished but remain of the same sign as before. 
A still further displacement in the same direction would therefore produce still 
further agreement. 
We have now to consider the effect of varying C L. 
Let us trace the effect of increasing C L by 1 O'. 
The value of jx a depends on that of C L being found from it by the formula 
(Section V.) 
H'a 
sin 2 CL 
/W 2 
where 
1-53013 
and r is given from experiment 
v— 1 ’68550 
The new value of a will be found to be 
instead of 
^,,= 1-68571 
1-68560 
This increase in the value of a will increase the angle betwen the optic axes from 
9° 4' 5", the value already found, to 9° 9' 20". 
MDCCCLXXIX. 3 B 
