MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
293 
the optic axes lie in the plane perpendicular to this axis. A prism formed with its 
refracting edge nearly parallel to the axis of b, would permit of observations being 
made in a plane passing nearly through the optic axes ; and it was found that, by 
inclining the faces so that the axis of a nearly bisected the angle of the prism, and 
making that angle about 42°, light could be passed through the crystal in the direction 
of either optic axis. It was therefore thought right that the prism should be so cut. 
Moreover, by polishing one of the a faces and cutting a face nearly coincident with 
the fourth m face, a second prism was formed with its refracting edge nearly parallel 
to c. 
It was important to get observations in different zones from the same piece of 
arragonite, for its chemical constitution is more or less variable, and the constants— i.e., 
the principal refractive indices—may vary in different specimens. 
The work of cutting was successfully performed by IIilger, and the faces of the 
prisms were very fairly plane. 
The crystal being thus cut, it remained to determine accurately the position of the 
cut faces with reference to the crystallographic axes. 
For this purpose, and throughout the observations, I used a goniometer, by Grubb, 
kindly lent me by Professor Stokes. 
The collimator is fixed to a graduated circle of about 9 inches diameter. The 
graduations, which are in silver, are on the flat face , not the limb of the circle, and the 
circle is divided to arcs of 10'. 
Attached to the reading telescope there are two verniers ; the graduations on the 
verniers being in the same plane as those of the circle; and by means of these the 
circle can be read to angles of 10". 
In the centre of this circle there is a table to hold the prism ; the table being 
attached to a second graduated circle with verniers which can be read to half minutes. 
It was requisite throughout the experiments to have the edge of the prism parallel 
to the axis of revolution of the telescope. 
To attain this the prism was attached to a stand, adjustable with set screws, which 
rested in slots on the table of the goniometer. 
The reading telescope was fitted with a needle point instead of the usual cross wires. 
In order to level the prism, I covered half the slit, so that when viewed directly the 
line of junction of the covered and uncovered portions coincided with the needle 
point. 
I then moved the telescope until the images of the slit, formed by reflexion at the 
faces of the prism, were brought in turn into the field ; and altered the levelling 
screws till the same coincidence was obtained. 
Homogeneous light was secured by using a Bunsen’s burner with a platinum wick , 
one end of which was immersed in a saturated solution of salt-and-water. 
By the advice of Professor Stokes, the observations were made at angles of incidence 
increasing in arithmetic progression. 
