The facets being ready, the glass discs are cemented to them by centering cement, which may be used quite generally for small lenses. When the cutting of facets has been omitted on a concave surface, the best cement is hard pitch. The grinding tool is generally rather larger than the nest of lenses. Coarse and fine grinding is accomplished wholly on the lathe--the tool being rotated at a fair speed (see infra), and the nest of lenses moved about by its handle so as to grind all parts equally. It must, of course, be held anywhere except "dead on," for then the part round the axis would not get ground; this inoperative portion of the rotating tool must therefore be allowed to distribute its incapable efforts evenly over the nest of lenses. Polishing is accomplished by means of the grinding tool, coated with paper and rouge as before; or the tool may be coated with very thin cloth and used with rouge as before--in this case the polishing goes on fastest when the surface of the cloth is distinctly damp. In working by this method, each grade of emery need only be applied from five to ten minutes. The glass does not appear to get scratched when the emery is changed, provided everything is well washed. A good polish may be got in an hour. The lathe is run as for turning brass of the same diameter as the tool. One side of the lenses being thus prepared, they are reversed, and the process gone through for the other side in a precisely similar manner. [Footnote: Unless the radius of curvature is very short and the lenses also convex, there is no necessity to recess the facets, provided hard pitch is used as the cement. See note on hard pitch.] To save trouble, it is usual, to make such lenses of equal curvature on both faces; but of course this is a matter of taste. Fig. 56. For very common work, bits of good plate glass are employed, and the manufacturer's surface treated as flat (Fig. 56). In this way plano-convex lenses are easily and cheaply made. Finally the lenses have to be centred, an essential operation in this case. This is easily done by the reflection method--the edge being turned off by the file and kerosene and the centering cement being used in making the preliminary adjustment on the chuck. I presume a lens made in this way is worth about a shilling, so that laboratory manufacture is not very remunerative. Fig. 56 shows the method of mounting small lenses for lathe grinding, when only one lens is required. The tool is generally rotated in the lathe and the lens held against it. Sec. 65. Preparing Small Mirrors for Galvanometers.