1887.] NEW YORK ACADEMY OF SCIEKCES. 101 



materials of the pot and stirring-rod, which become mixed in 

 with the glass in consequence of the intense heat to which all 

 are subjected. These veins must, so far as possible, be ground 

 or chipped out with the greatest care. The glass is then melted 

 again, pressed into a fiat disc, and once more put into the an- 

 nealing oven. In fact, the operation of annealing must be re- 

 peated every time the glass is melted." Annealing consumes 

 two months each time for the large discs. "When cooled it 

 is again examined for veins, of which great numbers are sure 

 to be found. The problem now is to remove these by cutting 

 and grinding without either breaking the glass in two or cut- 

 ting a hole through it. If the parts of the glass are once sep- 

 arated they can never be joined without producing a bad scar 

 at the point of junction. So long, however, as the surface is 

 unbroken, the interior parts of the glass can be changed in 

 form to any extent. Having ground out the veins as far as 

 possible, the glass is to be again melted and moulded into 

 proper shape. In this mould great care must be taken to have 

 no folding of the surface. Imagining the latter to be a sort 

 of skin inclosing the melted glass inside, it must be raised up 

 wherever the glass is thinnest, and the latter allowed to slowly 

 run together beneath it. 



"If the disc is of flint, all the veins must be ground out on 

 the first or second trial, because after two or three mouldings the 

 glass will lose its transparency. A crown disc may, however, 

 be melted a number of times without serious injury. In many 

 cases — perhaps the majority — the artisan finds that after all his 

 months of labor he cannot perfectly clear his glass of the 

 noxious veins, and he has to break it up into smaller pieces. 

 When he finally succeeds, the disc has the form of a thin grind- 

 stone two feet or upward in diameter, according to the size of 

 the telescope to be made, and from two to three inches in thick- 

 ness. The glass is then ready for the optician. 



" The first process to be performed by the optician is to grind 

 the glass into the shape of a lens with perfectly spherical sur- 

 faces. The convex surface must be ground in a saucer-shaped 

 tool of corresponding form. It is impossible to make a tool per- 

 fectly spherical in the first place, but success may be secured oa 



