PLATES AND PRISMS OF CRYSTALS OF ARTIFICIAL PREPARATIONS. 905 
outer cylinder is held in position round the inner one by small milled-headed screws 
passing through the slots and screwing into the solid cylinder at opposite sides, the 
dnection of the screws being approximately parallel to the upper tangent screw 
when the holder is fixed in its socket. After adjustment of the crystal these screws 
can be used as clamping screw^s to fix the outer cylinder rigidly to the inner core. 
In order to carry out the four operations above specified, the crystal is cemented in 
the usual manner to the end of the hollow cylinder of the special holder, with the 
zone of faces to be adjusted placed approximately parallel to the axis of the cylinder, 
and with the clinopinacoid, the symmetry plane, arranged not very far from parallel to 
the direction of the clamping screws arranged at the centres of the slots. The wax 
employed sets so rapidly that there is only time to make the. roughest approximation 
to this position, which, however, is all that is necessary. The operation may be con¬ 
veniently carried out with the inner cylinder inserted. For the purpose of adjusting 
a face exactly parallel to either tangent screw, a small plate of microscope cover-glass 
is cemented to the face of the lowest portion of the low'er segment, immediately 
above the position to be occupied by the crystal holder, and parallel to the plane of 
movement of the segment. Before attaching the cylindrical crystal holder the axis is 
lowered until the glass plate is about the height of the axes of the telescope and 
collimator, the image of the slit reflected from the surface of the plate is adjusted to 
both cross-wires, and the reading of the circle for this position recorded. The cylin¬ 
drical holder is now attached, the circle is set to the recorded reading if the face is 
to be adjusted parallel to the lower tangent screw, or at 90° from that position if 
the face is to be made parallel to the upper tangent screw, the outer cylinder of the 
crystal holder is rotated until the image of the slit reflected from the face is bisected 
by the vertical cross wire, and the tangent screw at right angles to the face is 
manipulated, if necessary, so that the image is also bisected by the horizontal cross 
wire. The cylinder is then fixed to its core by means of the small clamping screws. 
Having in this manner adjusted the clinopinacoid parallel to the upper tangent 
screw, the third operation of adjusting the other faces of the zone perpendicular to 
the grinding plane is then carried out by use of this upper tangent screw. Lastly, 
the whole zone now being exactly perpendicular to the grinding plane, the reading of 
the scale of the upper segment is noted, and the tangent screw is worked until the 
segment has moved over the required arc (the angle of extinction Avith respect to the 
axis of this zone) correctly set to within ten minutes, when the direction of the median 
line will likewise be perpendicular to the grinding plane. Grinding and polishing is 
then carried out precisely as in the first case. 
3. The case may next be considered of a rhombic crystal which only exhibits one of 
the three principal planes parallel to two of the crystallographic axes, the remaining 
planes being of prismatic, domal, or pyramidal character. Suppose, for instance, the 
only faces exhibited are those of the basal plane and four prism faces belonging to the 
same form, and that it is desired to grind a plane parallel to one of the undeveloped 
MDCCCXCIV.—A. 5 Z 
