This apparatus consists of a cylinder, held within a suitable frame supported on three set-screws, _a_{1}_, _a_{2}_ and _a_{3}_, of hard steel. The lower surface of the frame and consequently the bottom of the cylinder can be adjusted by the wedge _k_ and the set-screws so that it is exactly parallel to the grinding surface of the glass plate. The crystal to be ground down, _p_, is cemented to the bottom of the cylinder, and the whole apparatus rubbed over the grinding surface of the plate. The pressure of the hand on the upper part of the cylinder regulates the pressure of the crystal on the grinding surface. In this way a surface is obtained which is exactly parallel to the surface cemented to the cylinder. [Illustration: FIG. 77. ] Sections of definite thickness can also be obtained by using this apparatus; for the adjustments with the wedge _k_ may be so arranged that, when the stop _t_ in the cylinder has reached the bottom of the slot in the frame, the lower surface of the cylinder will be the required distance from the grinding surface of the glass plate. The wedge is graduated so that the value of one division on the sloping top is known in _mm._ of vertical distance. Saws. Saws,[128] of about six inches in diameter made from ¹⁄₃₆ inch sheet-tin, are convenient for general work, and may be charged with diamond dust by the operator as follows:[129] Crush one or two carats of rough crystals of diamond-bort in a small steel mortar to about the condition of fine sand, care being taken not to reduce the diamond to powder. Transfer the dust to a piece of flat iron and, after collecting it in a heap, moisten with a drop of oil. The cutting edge of the tin disk must now be prepared by making a series of incisions (¹⁄₁₆ inch deep) on the outer margin. This can be done by striking the disk with a sharp, thin knife edge. The larger the number of incisions and the closer they are together the better. Charging the saw is accomplished by gently hammering the edge against the iron plate, upon which is the paste of diamond dust and oil. The disk must be slowly rotated during this charging process, which should be continued until all portions of the edge have been gone over two or three times. Instead of charging with diamond dust the saws may be used with emery. The edges of the saws should first be “upset,” as just described, and then while rotating charged with emery. The emery in the state of mud can be applied with the thumb and fingers, or the saw can be allowed to pass through a tray of emery mud. The finer grades of emery (Nos. 100 to 120) should be used. The method of mounting saws, of course, depends on the way the saws and spindles are constructed. It is well, however, to have the hole in the center of the saw-disk sufficiently large to allow of adjustment for accurate centering. Cutting.