PLATES AND PRISMS OP CRYSTALS OF ARTIFICIAL PREPARATIONS. 891 
strong metal cross-plate, triangular in shape with somewhat coocave edges, repre¬ 
sented at a in the section given in fig. 2. In the centre of this piate, and forming- 
part of the same casting, is carried the outer fixed cone h, in which the vaiious 
movable axes are supported. The vernier plate, of silver, is fixed to a short arm 
springing from between two of the main arms of the cross-plate. Within the outer 
fixed cone h, a second one c is capable of rotation by means of a large ebonite milled 
disc d, which is firmly attached to it immediately below the termination of the fixed 
cone. Above, the upper flange of this movable cone is screwed to the circle e, so that 
rotation of the ebonite disc effects rotation of the circle. 
Fig. 2. 
The circle is fitted with a thich silver tyre, upon which the graduations are 
engraved. These read directly to half-degrees, and with the aid of the vernier to 
single minutes. Immediately below the circle the cone c is loosely encircled by a 
collar f, which can, when desired, be firmly fixed to it by means of a clamping screw 
5x2 
