MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
317 
axes, there is considerable difference between Fresnel’s theory and experiment: that 
the differences between the two are most marked in the neighbourhood of the optic 
axes, and amount there to ’0005 about. For the outer sheet of the surface (except 
just in the neighbourhood of the principal axes), the theoretical values of the radius 
vector are uniformly greater than the experimental, while for the inner sheet the 
reverse is the case. 
In fact, the curves of section of the surface, as given by experiment, approach more 
nearly to one another than those of the surface of wave slowness on Fresnel’s theory. 
Fig. 8. 
In the above figure, fig. 8, the dotted line gives the result of experiments, while the 
strong line gives the form of the section on Fresnel’s theory. 
To compare the results with those given by Lord Rayleigh’s theory, we have the 
equation to the surface, a, b, c being the principal velocities, 
V2 -i 
vr , ri" 
+ ^ W “ ° 
—-1 —-1 
& 2 c 3 
Put Y = r is a radius vector of the surface of wave slowness 
r 
0 70 7 0 0 
err . b i m i 
nr 
1— a?! 3, 1 —6 V 3 1—e 3 r 3 
Let us suppose as a first approximation that m=0, i.e., that the plane of the prism 
is coincident with a principal plane of the crystal, 
crP(l — cV 3 ) + c 2 ft 2 ( 1 — ctV 2 ) = 0 
rW=« 2 /- + km 3 
Also l— cos 6, n~ sin 6 
0 being the angle between r and the maximum radius vector 
