PLATES AND PRISMS OE CRYSTALS OP ARTIFICIAL PREPARATIONS. 903 
hand, if necessary, by resting the little finger upon the top of the left column. One 
can detect so accm’ately by the delicate sense of touch how the grinding is proceeding, 
whether the crystal is bearing it easily, or whether there is too much strain, and can 
either reduce the pressure by gently adding to the weight of the lever by slight 
downward pressure of the finger and thumb upon the counterpoise, or can increase it 
by exerting a slight upward pressure, and thus diminishing the counterpoising effect. 
Moreover, as the crystal is ground away, one is able to preserve contact with the 
grinding surface by the same slight upward pressure upon the lever, which, even 
when very friable crystals are under operation, may be safely exerted at frequent 
intervals. With fairly hard crystals (potassium sulphate, for instance), the effect of 
the counterpoise may be entirely removed every few seconds by lifting the lever out 
of action, without fracturing a good specimen, provided the rotation of the grinding 
surface is steady and its rate does not exceed two revolutions per second. For still 
harder crystals, those that ar’e only just softer than glass, the cup at the top of the 
axis may be weighted with more or less small shot or other convenient weighting 
material, but the grinding must be slow, and carefully watched. If, on the contrary, 
the crystal is soft or brittle, both levers must be brought into action, the horizontal 
one by lowering its screw support, and the pressure regulated as before by manipula¬ 
tion of the elbow lever. If cleavage is largely developed there is less chance of 
splitting if the grinding is made to occur in the direction of the trace of the cleavage 
plane, and not at right angles to it. 
It is best in all cases to finish grinding with both levers in action, as the relative 
coarseness of the ground surface is rendered considerably smoother thereby, and the after 
polishing is much more rapidly achieved. The crystal holder may at any time Ijc 
removed in order to inspect the ground surface, and to see whether grinding has 
proceeded sufficiently far, without any danger of disturbing the adjustment, the groove 
in the rod of the holder running tightly along its guiding rib. When this is the 
case, and the final gentle grinding has been done, it is advisable, before removing the 
grinding plate, to again test the correctness of adjustment of the crystal in order to 
be certain that no movement has occurred during the grinding. The crystal is well 
cleansed from oil with a silk handkei’chief, the goniometer lamp, whose small by-pass 
has been left burning, is re-lit, and the images from the various faces of the adjusted 
zone are reviewed. If they are still, as is usually the case, symmetrical to the 
horizontal cross-wire, polishing can be proceeded with ; if there is any slight .evidence 
of movement, due, perhaps, to softening of the wax by the heat caused by too rapid 
grinding, the crystal must be re-adjusted, again ground for a minute or so, and the 
images again reviewed, when, they should be perfectly satisfactory. The grinding 
plate is then removed, and the polishing performed upon the lower permanent 
polishing disc, likewise lubricated with oil, the same method of manipulating the 
elbow lever according to the “ feel ” of the polishing being followed. As a rule, the 
grinding need not occupy more than fifteen minutes, and the polishing five ; polishing 
