322 
MR. R, T. GLAZEBROOK OR PLARE WAVES IR A BIAXAL CRYSTAL. 
For about 10° on either side of the major axis (the passage across which is indicated 
by the dark line in the table) tire results of theory and experiment agree fairly well. 
The average difference irrespective of sign is (if we neglect the values for 6= 2° 30' 43" 
and for 9= 9° 14' 14", which gives a point on the circle of radius 1‘68125) ‘000047, 
and only in two cases does the difference amount to '0001. 
But when we come to values for 6 greater than 11° we see that the experimental 
value of i-i is, as before, always greater than the theoretical, and that the difference 
increases with 6, and at last becomes nearly the same as in Table XXV. 
Thus this change does not produce agreement between the two. 
We may, however, consider shortly what alteration in jx c would in this case bring 
theory and experiment more closely into agreement. 
We have 
_1_ 
Q 
w 
cos 2 9 
H'a 
sin 2 6 
o 
l^c“ 
. '.Six = — sm~9Sa c 
Taking: the last values on Table XXVIII. 
O 
Sp= -00024 
/9=15° 48' 
l±— 1'67274 
T53013 
Whence 
Sp,,— ‘00248 
so that the minor axis of the ellipse, which having the same major axis as the experi¬ 
mental curve would pass through the extreme experimental point, is 
p ( — 1'53261 
instead of 1' 5 3 013 
And on referring to Table XXI., which gives the values of p c very approximately, we 
see that this value is quite out of the question. 
Thus a decrease in the angle y will not produce the required effect. 
Neither will an increase. For the first part of the inner sheet in Table III., and 
throughout Table IX., we must have the theoretical value of /p and therefore of 6-\-9' 
nearly constant; and this is clearly impossible along a plane section inclined at a finite 
angle to the plane of the optic axes. 
