PLATES AND PRISMS OF CRYSTALS OF ARTIFICIAL PREPARATIONS. 909 
in diameter, resembling the one employed for re-grinding the surface of the 
grinding plate; one side of this disc is made a true plane, and to the centre of 
the other side the steel grooved attaching rod is fixed as in the other holders, 
special care, however, being taken to attain 90° exactly. Upon the side to which 
the rod is attached a shallow white metal cap is fitted and rigidly fixed by 
means of three small screws ; it envelopes the thick disc down to half its depth and 
extends outwards for a quarter of an inch as a flange parallel to the plane surface. 
The flange is bored with three small holes at symmetrical points. The lower portion, 
constructed entirely of very hard white metal, resembles the cap in shape, and the 
uncovered lower half of the thick disc fits neatly in it; the outer flange is of like 
diameter and width to the one carried b37- the upper part of the arrangement, and 
carries three fixed projecting screws, which pass through the holes in the latter. In 
the centre of this lower cap a circular depression has first been braced out of such 
diameter and depth that any of the glass discs used for mounting the crystal will 
nicely fit in it, but cannot sink quite flush ; a concentric hole of slightly smaller 
diameter has then been cut quite througb. The thickness of the cap is such that 
the little annidus thus left to support the disc is only about the thickness of ordinary 
note paper. 
When it is desired to use the arrangement, the upper portion is placed in position 
beneath the ordinary adjusting motions at the lower end of the axis of the instru¬ 
ment, and the rod firmly fixed in its socket by means of the milled-headed screw. 
The axis is then lowered by means of the large milled head at its summit until the truly 
plane surface of the thick disc is within one-eighth of an inch of the grinding plane. 
It is then gently lowered by manipulating the near counterpoised lever until it all but 
touches the plane. By placing a white screen in the background the relatively large 
l-inch disc can be adjusted by means of the tangent screws, so that its truly plane 
surface is exactly parallel to the grinding plane, as evidenced by the equal thickness 
of the fine line of white background seen between the two planes upon sighting with 
the eye at the same level. This should also be the case when the axis and holder 
are rotated 90°, and, of course, likewise for all positions of the circle. The holder 
may then be removed from its socket ; as its attaching rod is grooved and the groove 
is guided by a closely-fitting rib in the socket, the same position will be taken up 
when it is again placed in position. The disc carrying the cemented crystal is now 
placed in the circular depression of the lower part of the arrangement, crystal down¬ 
wards, so as to pass through the hole, care being taken that there is no cement left on 
the margin of the disc, where it is supported by the thin annulus. The upper part 
is then inserted and the two parts are screwed together by means of three small 
milled-headed nuts, seen in fig. 1 in the centre of the front of the baseboard, which 
engage with the screws projecting through the upper flange. As the upper surface 
of the glass disc is not quite flush with the inner surface of the lower cap, it is firmly 
pressed against the truly plane surface of the thick disc when the nuts are screwed 
