368 
MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
It will be noticed that tbe differences for this modified theory are much less than 
for the original, but they have nearly all changed sign. In fact, the correction applied, 
calculated so as to produce the closest possible agreement in line 5, has been too great, 
and the differences can be reduced by applying a similar but smaller correction in the 
opposite direction. 
Now this variation in the value of /q has been caused almost entirely by the 
increase of the angle L P ; we must therefore decrease this angle, again, and as the 
numbers in the difference column are each about one-fifth of the corresponding number 
in the column /3, we must decrease L P by one-fiftli of the amount by which it was 
previously increased. 
But this increase was 
S(LP) = 0° 21' 20" 
we must therefore decrease this by 
0° 4' 15' 
leaving the total increase 
0° 17' 5' 
In consequence of this, we must add to the modified theoretical values of /q 
respectively, quantities equal to one-fifth of the corresponding numbers in column j3. 
The result gives us a new set of theoretical values, which agree with experiment 
throughout to a remarkable degree. 
These results are expressed in Table X. 
Column 1 gives the values of /q already modified. 
Column 2 gives one-fifth of quantities in /3, which, when added to the values of 
/q hi Column 1 respectively, give Column 3 the resulting theoretical values. 
Column 4 gives the experimental values, and Column 5 the excess of theory over 
experiment. 
Table X. 
Modified Talue 
of /*!■ 
1/3. 
Einal value of juj. 
Experimental 
value of 
Difference. 
1-68102 
1 
1-68103 
1-68099 
4 
1 
L67707 
7 
1-67714 
1-67721 
— 7 
2 
1-66285 
13 
1-66298 
1-66300 
— 2 
3 
l - 64590 
17 
1-64607 
1-64603 
4- 
4 
1-62806 
18 
1-62824 
1-6280 7 
17 
5 
1-60882 
18 
1-60900 
1-60897 
3 
6 
1-58346 
17 
1-58363 
1 "58365 
—2 
7 
1-55138 
16 
1-55154 
1-58157 . 
-3 
8 
1-53779 
5 
1-53784 
L53774 
10 
9 
These difierences are exceedingly small—sometimes positive, sometimes negative— 
but rarely greater than the possible error of experiment as appeal’s from the tables 
of error in I., II., IIP, and IV. 
