302 MR. R. T. GLAZEBROOK OR PLANE WAVES IN A BIAXAL CRYSTAL. 
Section IV.— Description of the Second Prism: Position of its Axes.—Observed Values 
of the Deviations and Angles of Incidence, and Calculation of the Reciprocals of 
the Wave Velocity. 
It will be remembered that, in describing the cutting of the crystal, I stated that a 
second prism was formed with its edge nearly parallel to C, and its faces not differing 
much from the m and a faces of the crystal. 
Fig. 4. 
Let the figure, fig. 4, represent a section of the crystal by a plane perpendicular to 
the m faces. 
The traces of the faces of the second prism are A B and C D. 
The greater part of the face A B lies in the twin crystal, so that the results for the 
second prism apply to that crystal. 
On examination, the m face B C was seen to consist of two parts, which are not in 
the same plane. 
Observation showed that the upper half (the prism being placed as in the figure) is 
iit the same zone as the faces D E, E F, while the lower portion is considerably 
removed from this zone. 
I found also that when the faces D E, E F, were levelled, the line of junction of 
the dark and bright portions of the slit, when seen by reflexion from A B, coincided 
with the needle point; so that A B is in the same zone as the m faces; while the 
reflexion from C D, though in the field, was too high. Thus if Q Q /5 fig. 5, be the 
points where the normal to A B, fig. 4, and that to C D, fig. 4, produced backwards 
meet the sphere of reference, A, B, C, fig. 5, being the poles of the principal planes; 
Q lies on the great circle AMB; is slightly below it. 
Let M, M/, fig. 5, be the points where the normal to E F, and that to E D pro¬ 
duced backwards cut the sphere. Then 
AM=AM/ = 58° 28'. 
The mean of several observations gave for MQ=103° 58'. Hence 
AQ = 45° 30' ........... (1) 
The mean of the observations for M Q / and M ' Q /5 gave 
