PLATES AND PRISMS 01^ CRYSTALS OE ARTIFICIAL PREPARATIONS. 911 
Th e Grinding of Prisms. 
Prisms can be prepared by means of the instrument as readily as section-plates. 
The mode of setting- any desired imaginary plane in the crystal parallel to the 
grinding disc, in order to grind a surface in that direction, will be clear from the fore¬ 
going. In the case of prisms two such surfaces are required, inclined at about 60° to 
each other. It is especially convenient that the two surfaces shall be equally inclined 
to one of the principal planes of optical elasticity, and this can be achieved in a very 
simple manner by use of the instrument now described. The most convenient mode 
of proceeding is to adjust this plane of optical elasticity parallel to the grinding 
plane, and the direction of the edge of the desired prism parallel to the lower tangent 
screw. Then, by movement of the upper tangent screw at right angles to the first, 
the corresponding segment may be rotated for an angle of about 30°, the exact 
amount of which should be noted. A surface is theii ground and polished in this 
direction. As a movement of 120° would be required in order to bring the plane of 
the other desired surface parallel to the grinding plane if the same setting were 
retained, the crystal is unmounted. The hard black optician’s wax used lends itself 
particularly well to unmounting, for, after detaching the wax surrounding the crystal 
at the side with the point of a penknife, the crystal may usually be detached, intact 
and unsoiled by wax, by a gentle pressure. If this is not the case, the crystal is 
loosened by the application of benzene or other solvent incapable of attacking the 
crystal. It is then cleansed from oil by a silk handkerchief, and re-set upon the 
crystal holder, after turning over, so that the second surface may be conveniently 
adjusted. After the re-adjustment of the same plane of optical elasticity parallel to 
the grinding plane, and the direction of the edge parallel to the lower tangent screw, 
the upper tangent screw is rotated for exactly the same number of degrees in the 
neighbourhood of 30° as in adjusting the first surface. The second surface is then 
ground and polished precisely similarly to tire first. The two surfaces will then be 
inclined at about 60°, exactly 60° if desired; they will also be symmetrical to the 
plane of optical elasticity in question, and the refracting edge will be parallel to the 
desired axis of optical elasticity. It will frequently happen that a zone of faces will 
be developed perpendicular to the principal optical plane in question, so that its 
adjustment can be immediately effected, and the adjustment of the direction of the 
desired edge will usually be achieved in a simple manner with reference to existing 
faces. Even in more complicated cases a little consideration will enable the move¬ 
ments provided with the instrument to be utilised so as to achieve the desired result 
with accuracy. If the crystal is not very small a pair of surfaces may be ground 
while adjusted in each position, one 30° on each side of the plane of optical elasticity, 
so that a pair of prisms may be obtained, and the refractive indices thus determined 
in duplicate upon the same crystal. It is quite easy, moreover, to grind another pair 
of prisms symmetrical to another plane of optical elasticity, so that all three refractive 
