318 MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
1 
2-84192 
L 1 
• 50062. 
<r a~ 
From these and the known values of 6, /a or r can be calculated. The table gives 
the result. 
Comparison between Lord Rayleigh’s Theory and Experiment. 
e. 
fx Theory. 
/i Experiment. 
Excess of Theory 
over Experiment. 
1 31 10 
1-68570 
1-68568 
•00002 
8 25 33 
1-68261 
1-68190 
■00071 
11 6 40 
1-68028 
1-67918 
•00110 
14 9 48 
1-67688 
1-67519 
•00169 
15 48 28 
1-67476 
1-67274 
•00202 
The differences between theory and experiment are so marked that it seems un¬ 
necessary to calculate the values of /x for smaller intervals in the values of 6. 
And though some of the apparent difference may be due to the error made in 
assuming the principal plane of the prism to coincide with one of the crystal, that 
cannot account for the whole ; for we have seen that in Fresnel’s surface the error 
made by the same assumption appears only in the fourth place of decimals, in the value 
of the refractive index, while the differences between Lord Rat Leigh’s theory and 
experiment show themselves in the third place, and tend to increase with 6. 
Thus it seems clear that Lord Rayleigh’s theory will not account for the phe¬ 
nomena of double refraction in arragonite. This result agrees with that arrived at by 
Professor Stokes for Iceland spar. 
Section VII .—Effect of Varying Constants of Theory and Position of Plane of Prism 
with reference to Axes of Crystal. 
It remains now to discuss the effects of variations in the values of the axes a, b, c. 
The values of c and b are determined directly from observation, ami in the neigh¬ 
bourhood of these axes the agreement is most close. Any alteration in them would 
affect especially that part of the section which lies between the optic axes, and for 
which the differences between theory and experiment are least; let us consider, there¬ 
fore, a variation in the value of a. 
