100 . TKANSACTIONS OF THE [fEB. 21, 



by Feil, a grandson of P. L. Guinand. Fell made the discs for 

 the great Austrian refractor, 27 inches in diameter. He also 

 made the discs for the Princeton telescope, and furnished 

 the discs for the great Russian telescope of 30 inches diam- 

 eter, and those of 36 inches for the Lick Observatory of Cali- 

 fornia. 



The process of making these large discs seems to be well un- 

 derstood by Messrs. Chance and Peil, so that the only difficulty 

 in getting the large discs is the long delay. The Russian discs 

 were received by the opticians (the Clarks), who do the polish- 

 ing, in about two years after being ordered. 



The flint disc for the Lick telescope was ready in about 

 one year, and the crown was nearly ready in nine months 

 after, but was broken in the handling. Newcomb thinks 

 that the secret of the manufacture consists principally in the 

 constant stirring of the molten glass during the process of 

 making. 



The reason why the glass-makers require so long a time to 

 make the large discs may be understood from the following ac- 

 count: 



''As optical glass is now made, the material is constantly 

 stirred with an iron rod during all the time it is melting in the 

 furnace, and after it has begun to cool, until it becomes so stifE 

 that the stirring has to cease. It is then placed, pot and all, in 

 the annealing furnace, where it is kept nearly at a melting heat 

 for three weeks or more, according to the size of the pot. 

 When the furnace has cooled off the glass is taken out, and the 

 pot is broken from around it, leaving only the central mass of 

 glass. Having such a mass, there is no trouble in breaking it 

 up into pieces of all desirable purity, and sufficiently large for 

 moderate-sized telescopes. But when a great telescope of two- 

 feet aperture or upward is to be constructed, very delicate and 

 laborious operations have to be undertaken. The outside of the 

 glass has first to be chipped off because it is filled with im- 

 purities from the material of the pot itself. But this is not 

 all. Veins of unequal density are always found extending 

 through the interior of the mass, no way of avoiding them hav- 

 ing yet been discovered. They are supposed to arise from the 



