GL A SS-MAKING. 6 1 9 



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 re- 

 verse 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 re- 

 moved 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 as- 

 tronomical telescope, the process is slightly modified. The work 

 is one requiring considerable skill and patience, though it in- 

 volves 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 qual- 

 ity 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 es- 

 tablishment 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 



