(2) Called Rough Grinding. The surfaces of the glass are to be made roughly convex, plane, or concave, as may be required; the glass is to be equally thick all round the edge. In this process the glass is abraded by the use of sand or emery rubbed over it by properly shaped pieces of iron or lead called "tools." (3) The glass is ground with emery to the correct spherical figure as given by a spherometer. (4) Called Fine Grinding. The state of the surface is gradually improved by grinding with finer and finer grades of emery. (5) The glass is polished by rouge. (6) The glass is "figured." This means that it is gradually altered in form by a polishing tool till it gives the best results as found by trial. In processes 2 to 5 counterpart tool surfaces are required--as a rule two convex and two concave surfaces for each lens surface. These subsidiary surfaces are worked (i.e. ground) on discs of cast iron faced with glass, or on slate discs; and discs thus prepared are called "tools." Taking these processes in the order named, the mode of manufacture is shortly as follows:- (1) The disc of glass, obtained in a roughly circular form, is mounted on an ordinary lathe, being conveniently cemented by Regnault's mastic to a small face plate. The lathe is rotated slowly, and the glass is gradually turned down to a circular figure by means (1) of a tool with a diamond point; or (2) an ordinary hand-file moistened with kerosene, as described in Sec. 42; or (3) a mass of brass or iron served with a mixture of emery--or sand--and water fed on to the disc, so that the disc is gradually ground circular. The operation of making a circular disc of given diameter does not differ in any important particular from the similar operation in the case of brass or iron, and is in fact merely a matter of turning at a slow speed. (2 and 3) Roughing or bringing the surfaces of the glass roughly to the proper convex or concave shape. This is accomplished by grinding, generally with sand in large works, or with emery in the laboratory, where the time saved is of more importance than the value of the emery. Discs of iron or brass are cast and turned so as to have a diameter slightly less than that of the glass to be ground, and are, say, half an inch thick. These discs are turned convex or concave on one face according as they are to be employed in the production of concave or convex glass surfaces. The proper degree of convexity or concavity may be approximated to by turning with ordinary turning tools, using a circular arc cut from zinc or glass (as will be described) as a "template" or pattern. This also is a mere matter of turning.