Sec. 55. The next step is to rough out the lens, and this may easily be done by rotating it more slowly, i.e. with a surface speed of ten feet per minute, and turning the glass with a hard file, as explained in Sec. 42. If it is desired to employ the slide rest, it is quicker and better to use a diamond tool--an instrument quite readily made, and of great service for turning emery wheels and the like,--a thing, in fact, which no workshop should be without. A bit of diamond bort, or even a clear though off-colour stone, may be employed. An ordinary lathe tool is prepared by drawing down the tool steel to a long cone, resembling the ordinary practice in preparing a boring tool. The apex of the cone must be cut off till it is only slightly larger than the greatest transverse diameter of the diamond splinter. The latter may have almost any shape--a triangular point, one side of a three-sided prism is very convenient. A hole is drilled in the steel (which must have been well softened), only just large enough to allow the diamond to enter--if the splinter is thicker in the middle than at either end, so much the better--the diamond is fastened in position by squeezing the soft steel walls tightly down upon it. Personally I prefer to use a tool holder, and in this case generally mount the diamond in a bit of brass rod of the proper diameter; and instead of pinching in the sides of the cavity, I tin them, and set the diamond in position with a drop of soft solder. Fig. 47. In purchasing diamond bort, a good plan is to buy fragments that have been employed in diamond drilling, and have become too small to reset; in this case some idea as to the hardness of the bits may be obtained. Full details as to diamond tool-making are given in books on watch-making, and in Holtzapffell's great work on Mechanical Manipulation; but the above notes are all that are really necessary--it is, in fact, a very simple matter. The only advantage of using a diamond tool for glass turning is that one does not need to be always taking it out of the rest to sharpen it, which generally happens with hard steel, especially if the work is turned a little too fast. I recommend, therefore, that the student should boldly go to work "free hand" with a hard file; but if he prefer the more formal method, or distrust his skill (which he should not do), then let him use a diamond point, even if he has the trouble of making it. When using a diamond it is not necessary to employ a lubricant, but there is some advantage in doing so.