MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
375 
Thus the new positions of the axes are inclined to the old at angles of 1° 1', 1° 2', 
and 17' respectively for O A, OB, O C. So that the axis of C in its new position 
would be inclined to the line of junction of to m! at an angle of 17', while that of b 
would he inclined to the bisector of these planes at an angle of 1° 2'. 
If variations such as here considered be possible, so that we cannot be certain 
d priori of the position of the axes of elasticity, the only method of testing Fresnel’s 
or any other theory will be by trial. 
Section X .—General Results of the Investigation. 
Combining the results of the two series of experiments, it seems to me most probable 
that Fresnel’s theory is only true as a first approximation. 
Both series of observations have led to the discovery of considerable deviation from 
the theory, unless we assume errors in the experiments, especially in the determination 
of the position of the planes of the prisms witli reference to the axes of the crystals, 
which are greatly in excess of the amounts we can reasonably expect. 
It will be noticed that the values taken for the constants g a and jx h in the two parts 
of the paper differ by ‘0002 and *00017 respectively. This is due to the fact that the 
two crystals used were different. Rttdberg (Pogg. Annalen, vol. xvii., p. 1) found 
differences of as much as *0004 in the values of deduced from two specimens of 
arragonite. 
It may be objected that the variations between theory and experiment are not in 
exactly the same direction in the two Parts ; but we must remember (l) that the 
arcs investigated are taken from entirely different portions of the surface ; (2) in the 
first Part, the approximately elliptic section belongs to the outer sheet; in the second, 
to the inner sheet of the wave surface. 
Fig. 16. 
The accompanying figure will illustrate the results of the investigation. 
