Rough iron or brass tools do not succeed for fine grinding--i.e. grinding with fine emery, because particles of emery become embedded in the metal so tightly that they cannot be got out by any ordinary cleaning. If we have been using emery passing say a sieve with 60 threads to the inch, and then go on to some passing say 100 threads to the inch, a few of the coarser particles will adhere to the "tool", and go on cutting and scratching all the time grinding by means of the finer emery is in progress. To get over this it is usual to use a rather different kind of grinding tool. A very good kind is made by cementing small squares of glass (say up to half an inch on the side), on to a disc of slate slightly smaller than the lens surface to be formed (Fig. 51). The glass-slate tool is then "roughed" just like the lens surface, but, of course, if the lens has been roughed "convex" the tool must be roughed "concave". The "roughed" tool is then used to gradually improve the fineness of grinding of the glass. For this purpose grinding by hand is resorted to, the tool and lens being supplied continually with finer and finer emery. Fig. 52 gives an idea of the way in which the tool is moved across the glass surface. Very little pressure is required. The tool is carried in small circular sweeps round and round the lens, so that the centre of the tool describes a many-looped curve on the lens surface. The tool must be allowed to rotate about its own axis; and the lens and pedestal must also be rotated from time to time. Every few minutes the circular strokes are interrupted, and simple, straight, transverse strokes taken. In no case (except to correct a, defect, as will be explained) should the tool overhang the lens surface by more than about one quarter the diameter of the latter. After grinding say for an hour with one size of emery fed in by means of a clean stick say every five minutes, the emery is washed off, and everything carefully cleaned. The process is then repeated with finer emery, and so on. The different grades of emery are prepared by taking advantage of the fact that the smaller the particles the longer do they remain suspended in water. Some emery mud from a "roughing" operation is stirred up with plenty of water and left a few seconds to settle, the liquor is then decanted to a second jug and left say for double the time, say ten seconds; it is decanted again, and so on till four or five grades of emery have been accumulated, each jug containing finer emery than its predecessor in the process.