PLATES AND PRISMS OF CRYSTALS Ob' ARTIFICIAL PREPARATIONS. 8'J3 
The gun-mefcal axis i is internally bored in the manner shown in fig. 2, the bore 
being fairly wide for the upper two-thirds of its length, but more constricted in its 
lower portion, in order to permit a central axis of steel r to slide in it freely but without 
lateral play, independent rotation being likewise prevented by means of a groove and 
rib as in the case of the axis i. This inner steel axis carries at its lower extremity 
the crystal holder and the movements necessary for adjusting the crystal, and 
terminates at the upper end in a rapid-threaded screw. By means of a milled head s 
and attached nut t, which latter passes through the cap u closing the bore of the gun- 
metal axis i, the steel axis and the crystal may be raised or lowered so as to remove 
or approach the latter from or to the grinding plane. The emergent upper portion of 
the screw thread is protected by means of a tubular cap v, which screws down upon 
the milled head s. Over this cylindrical cap may be placed a short tube carrying 
above a brass cup, shown in fig. 1, which is intended for the reception of small shot or 
weights, whenever it is considered desirable to increase the pressure between the 
crystal and the grinding disc over and above that which can be effected by manipulation 
of the levers. 
The centering and adjusting apparatus carried at the lower end of the inner steel 
axis r consists of two centering motions, acting in directions at right angles to each 
other and actuated by milled-headed traversing screws, and two circular adjusting 
motions of the type first employed by von Lang, actuated by tangent screws also 
arranged at right angles to each other. These movements are constructed rather 
more strongly than for ordinary goniometrical purposes. For centering, an arrangement 
has been adopted which has been employed for some time by the firm of Troughton and 
Simms for centering purposes, and which was used by them in the vertical circle 
goniometer described by Miees.* This arrangement affords greater strength and is 
less liable to develop looseness than the usual rectangular form. The centering is 
attained by the relative movement of two circular discs id, x about each other, and of 
these two about a third y. The third disc y is rigidly fixed to the lower end of the 
steel axis r by means of the bridge 2 . The second disc x is pivoted to y at a point 
near the circumference, and the movement of x about y is limited by means of a pin 
screwed into x and passing through a curved slot, concentric to the pivot, cut out of 
y, close contact of the two discs being maintained by means of a spring washer, 
pressed between the broad head of the pin and the disc y. The rotation of x about 
the pivot is effected by means of the upper milled-headed traversing screw, which is 
arranged along the diameter at right angles to that joining the pivot and the pin and 
passes under the bridge 2 . The end of the screw presses against a short upright 
fixed to the disc x and passing through a central hole in y, and the upright is main¬ 
tained pressed against the end of the screw during retrocession by a piston and spring 
arrangement iv’ similar to that employed in the fine adjustment of the circle. The 
lower disc iv is made capable of rotation about the central disc x in a precisely similar 
* ‘ Miiieralogical Magazine,’ March, 1891, p. 214. 
