44(5 
MR. R. T. GLAZEBROOK ON DOUBLE REFRACTION 
While for the violet ray, g, the differences near both axes are negative. To bring 
the two curves into agreement then we should require to decrease both the axes g x , g. z . 
This would produce a corresponding increase in all the positive errors and render the 
variation from Fresnel’s theory near the middle of the arc more marked than in the 
case of the red or green rays. 
In fact, while for the red, supposing the variations in /q, g z contemplated above to 
have been adopted, the greatest difference between theory and experiment would be 
about 
•0001 
for the green ray F it would rise to 
•00015 
and for the violet, g, to 
• 0002 . 
Section IV. 
I. Comparison with previous experiments. 
II. Effect of variation of constants. 
As an additional proof of the accuracy of the experiments it may be worth while 
giving the results of a series of measurements covering the same ground as the second 
part, Table I., made some months previously. Since the prism did not occupy exactly 
the same position relative to the instrument as it did during the experiments in 
Table L, the values of the angle of incidence, and therefore of f, were slightly 
different to those in Table I. 
In making the comparison, therefore, the results of calculation had to be altered by 
interpolation to give the values of g corresponding to the values of f in Table I. 
The result is contained in Table XIY. 
Table XIY. 
y from Table I. 
y from Experiments 
in December, 1878. 
Difference. 
1-64627 
1-64627 
0 
1-64335 
1-64332 
3 
1-64021 
1-64010 
11 
1-63684 
1-63681 
3 
1-63341 
1-63339 
3 
1-62991 
1-62994 
3 
1-62669 
1-62663 
6 
U62356 
1-62365 
9 
1-62095 
1-62097 
2 
1-61862 
1-61859 
3 
1-61680 
1-61680 
0 
