THE VARIATION OF ANGLES OBSERVED IN CRYSTALS. 
493 
in contact with the crystal, it may he possible to measure either its specitic gravity or 
its refractive index, and from these to detei:mine the degree of concentration. 
Now the goniometer used in tlie earlier part of this researcli lends itself admirahly 
to the measurement of the retractive indices of solutions by the method of total 
reflection. If a glass prism he adjusted on the crystal holder, its angles and 
refractive index can be measured ; it may then he immersed m the solution, and if 
we substitute for the square trough previously used a cylindrical trough (fig. I G, 
Plate 13)’'' with a plane glass front set perpendicular to the telescope, it will not lie 
difficult to measure the angle of total internal reflection in tlie prism, and so the 
index of refraction of the liquid. 
Further, by using a sufficiently small prism, the index of tlie liquid may be 
ascertained at any desired point, and by suspending a crystal in the solution and 
moving it gradually towards the siu'face of the prism, it may be possible to explore 
the liquid and to trace the change in refractive index as we approach the growing 
crystal. 
A few observations were made in this way witli a small glass prism of about 45°, 
and a crystal of alum immersed in concentrated solution in the square trougli of 
fig. 1. A face of the crystal and the reflecting surface of the prism were accurately 
adjusted parallel to each other in the following way. The crystal was fixed for the 
purpose on a wire rod curved over the edge of the trough and mounted in a crystal 
holder provided with the ordinary tangent and traversing screws for centring and 
adjusting ; the holder was clamped to a heavy stand placed beside the goniometer. 
The crystal was first adjusted until it reflected the image of the collimator slit upon 
the cross-wires of the telescope; it was then parallel to the axis ot the goniometer. 
The crystal having been withdrawn to a short distance by the traversing screw, the 
prism was next adjusted so that its hypothenuse or totally reflecting surface also 
reflected the collimator signal on to the cross-wires. The prism being then turned 
through 180° had its reflecting surflice parallel to and facing the crystal plane, and 
the crystal could be brought as close to it as is desired by means of the traversing 
screw. The telescope was next set at right angles to the second prism face, and then 
moved from this position through an angle equal to the angle of emergence for the 
saturated solution ; the front of the trough being also moved so as to remain 
perpendicular to the telescope. Finally, therefore, the prism and telescope are in the 
approximate position for measuring the index of the liquid, and the crystal, having a 
face parallel to the reflecting- surface of the prism, can he brought into absolute plane 
contact with it. 
* The trough is a cylindrical vessel with truncated front; it is held in position liy three spring clips, 
two of which press the plate of glass which constitutes the front against the ground edges of the vessel, 
and prevent leakage. The trough is adjusted upon its table by three levelling screws, and this table is 
fixed in any desired position upon the circular table of fig. 1 by means of two catches at the back and a 
clamping screw in front. 
