[Fig. 624.] shows piece-meal and in perspective, a part of the lapidary’s wheel-mill. There we see the table c c, the grind-plate I, whose axis is kept in a vertical position by the two square plugs a a, fixed into the two summers by the wedges b b. On the two sides of the wheel-plate we perceive an important instrument called a dial, which serves to hold the stone during the cutting and polishing. This instrument has received lately important ameliorations, to be described in [fig. 625.] The lapidary holds this instrument in his hand, he rests it upon the iron pins u u fixed in the table, lest he should be affected by the velocity of the revolving wheel-plate. He loads it sometimes with weights e, e, to make it take better hold of the grinding plate.
One of the most expert lapidaries of Geneva works by means of the following improved mechanism, of his own invention, whereby he cuts and polishes the facets with extreme regularity, converting it into a true dial.
[Fig. 625.] shows this improvement. Each of the two jaws bears a large conchoidal cavity, into which is fitted a brass ball, which carries on its upper part a tube e, to whose extremity is fixed a dial-plate f f, engraved with several concentric circles, divided into equal parts, like the toothed-wheel cutting engine-plate, according to the number of facets to be placed in each cutting range. The tube receives with moderate friction the handle of the cement rod, which is fixed at the proper point by a thumb-screw, not shown in the figure, being concealed by the vertical limb d, about to be described.
A needle or index g, placed with a square fit on the tail of the cement rod, marks by its point the divisions on the dial plate f f. On the side m n of the jaw A, there is fixed by two screws, a limb d, forming a quadrant whose centre is supposed to be at the centre of the ball. This quadrant is divided as usual into 90 degrees, whose highest point is marked 0, and the lowest would mark about 70; for the remainder of the arc down to 90 is concealed by the jaw. The two graduated plates are used as follows:—
When the cement rod conceals zero or 0 of the limb, it is then vertical, and serves to cut the table of the brilliant; or the point opposite to it, and parallel to the table. On making it slope a little, 5 degrees for example, all the facets will now lie in the same zone, provided that the inclination be not allowed to vary. On turning round the cement rod the index g marks the divisions, so that by operating on the circle with 16 divisions, stopping for some time at each, 16 facets will have been formed, of perfect equality, and at equal distances, as soon as the revolution is completed.
Diamonds are cut at the present day in only two modes; into a rose diamond, and a brilliant. We shall therefore confine our attention to these two forms.