PLATES AND PRISMS OF CRYSTALS OF ARTIFICIAL PREPARATIONS. 897 
after adjustment to the focus of the Jens and to the vertical position. Both telescope 
and collimator may be fixed at any angle to each other, in any position upon the 
circular guide, by means of clamping screws passing througlr the rabbeted base of 
each support and actuated in each case by a short lever. 
The grinding plane consists of a circular disc of ground plate-glass, mounted in a 
strong supporting disc of brass with raised edges, bevelled upon the outside. The 
finely ground surface of the glass is made as truly plane as possible. The brass 
supporting disc is screwed beneath concentrically by three screws to a pulley firmly 
fixed to the stout axis of rotation; tlie latter projects a little above the upper plane 
surface of the pulley, so as to fit tightly into a central hole bored in the under side 
of the brass supporting disc, wliich ensures the attainment of concentricity, and 
passes downwards into a rigid cylindrical bearing. The axis of rotation is carefully 
fixed truly perpendicular to the pulley and the grinding plane. The adjustment of 
the latter exactly perpendicular to the axis of rotation of the circle is provided for in 
the mode of supporting the cylindrical bearing. From the upper part of the bearing- 
radiate three legs, terminating in strong levelling screws, which rest directly upon 
the metal base. The bearing itself passes down through a fairly wide hole in the 
base, which is raised somewhat from tire supporting wooden base (intended for the 
reception of a protective glass shade when the instrument is not in use) and which is 
also somewhat hollowed underneath to afford room for the purpose ; some little 
distance below the metal base the bearing cylinder terminates in a iDi-oad head, 
between which and the under surface of the base a strong spiral spring is confined, so 
that the ends of the three levelling screws are pulled tightly down upon the base. 
The axis of the grinding disc is prevented from rising in the cylindrical bore of the 
bearing by means of a, suitable flange, and l^otli the broad upper end of the bearing 
and the boss of the pulley which bears upon it are worked quite plane. A small 
quantity of lubricating oil can be introduced into the bore of the bearing by means 
of a small bent side tube which rises from it at a convenient angle. The grinding- 
plane thus rotates without a trace of wobbling, and with a minimum of friction. 
The rotation is effected by means of the pulley seen to the right in fig. 1, whose 
diameter is about the same as that of the grinding disc. This pulley is mounted in a 
true bearing upon a stable fixed support, which raises it to the same height as the 
smaller pulley fixed to the brass disc which supports the grinding plate, and is 
provided with an ivory handle, fitted loosely upon a vertical rod furnished with suit¬ 
able head. In order to equalise the pressure on both sides of the axis of the grinding 
disc a third pulley, of the same size as that beneath the latter, is introduced to the 
left, similarly fitted to the large driving pulley, but without handle. The band of 
strong cat-gut crosses on each side of the central pulley, and j^rovision is made for 
tightening it whenever necessary by making the support of the third pulley capable 
of a certain amount of sliding in a short slot in the metal base, rigid fixation in the 
desired position being effected by means of a strong broad-headed screw manipulated 
MDCCCXCIV.—A. 5 Y 
