as possible. It is compared from time to time with a standard curve, in order to insure accuracy. In this second grinding the abrading material is rouge (carefully calcined sulphate of iron). Finally, the lens is polished by being pressed against a piece of cloth powdered with rouge and fastened to the rotating tool. The glass is now loosened from its block, turned over, and the reverse side of the lens ground. When this has been accomplished, the lens must be cut down to the proper shape for mounting in the spectacle-frame. It is placed on a leather cushion and held firmly in position by a rubber-tipped arm, while a diamond glass cutter passing around an oval guide traces a similar oval on the glass below. The superfluous glass outside of the oval is removed by steel pincers, the rough edges are ground smooth on Scotch wheels, and the lens is ready for mounting. The glasses for small telescopes, microscopes, burning-glasses, and the like are ground in the same fashion.
When, however, it comes to grinding the lens for a large astronomical telescope, the process is slightly modified. The work is one requiring considerable skill and patience, though it involves no very great difficulties. It was formerly done entirely by hand and by individual workmen rather than by large firms. It will be remembered that the philosopher Spinoza earned his living by grinding lenses, and since his time less famous workmen have patiently pursued the same trade. At present the grinding of telescope lenses has assumed the proportions of a business, and has nowhere been carried to greater perfection than in America. The firm of Messrs. Alvan Clark and Sons, whose workshops are at Cambridgeport, Mass., have gained a reputation which extends on both sides of the Atlantic, as their lenses exceed in both quality and size even the best products of European skill. A great astronomical telescope is indeed quite cosmopolitan in its genesis. The glass is cast in Paris, the grinding is done in Massachusetts, the mountings are made in London or Berlin, and the telescope itself is pointed toward the heavens from Mount Hamilton or from the Russian Imperial Observatory at Pulkowa.
All the glass ground at Cambridgeport comes from the establishment of M. Mantois in Paris. It is imported in the shape of large disks, which are generally flat on both sides. The first grinding is done by machinery, the abrading material being Tilghmair s chilled iron globules. These are found to be more effective than sand. The finer grinding is accomplished by means of varying grades of emery. It is in the finishing process that the American operations take rank over the foreign. The final touches and the polishing are all done by hand, the rouge being applied on the tip of the finger. It is necessary to employ constant tests during the course of the grinding. At first, these
