"ROUGHING " LENSES.
There are two methods of shaping glass, one being to heat it till it becomes plastic, and the other to break off or grind away the superfluous parts of a rough piece un til it is suitably formed. Cheap lenses are sometimes made by the former method, hut it is quite unsuitable for the manu facture of high-class objectives, which can only be properly obtained by the latter method, as now to be described. Unless intended for a large lens, one plate of glass will probably sullice for several lenses, and will have to be cut in slices to make it thinner. This is done by sawing These variations are of immense value to the optician, rendering it possible to produce lenses of almost perfect correction, suited to widely different pur poses and requirements. It is evident, however, that the de signing of modern objectives is no longer the simple matter it was at one time, when there were practically but two kinds of glass obtainable. With the many varieties of glass now obtainable the work has be come far more complicated and diffn-nit_ rerinirina hio-hlv specialised knowledge and skill, although, at the same time, the inherent optical defects of a lens can now be more satisfactorily corrected.
it with a rotating disc, with tiny diamond points set in its rim. This slowly and
steadily makes its way through the glass, dividing it into slices of suitable thick ness. From these pieces circular discs are made, which are then roughly shaped to the spherical curvature required by a Emery is used at first, carefully sorted into different grades by placing in water and allowing to settle ; the operation be ing repeated many times, so that a finer sediment is secured at each. Very ac curate grinding tools are necessary ; these are made in pairs, one convex and the revolving spindle having attached to it a metal tool shaped to the counterpart of the desired carve (see Fig. 531). This tool is supplied with a coarsely powdered other concave, the two being ground to gether until not only of the correct curva ture, but so perfectly spherical that they meet perfectly in any position of contact (Fig. 532). Large numbers of these accurate tools are required, varying in size and curvature. Racks containing these tools, with their profile gauges, are shown in Fig. 533. With these appliances and z.brasive, such as emery or carborundum, and with water to secure cool w orking.