382 HISTORY OF ASTKONOMY. 



ture much below that of complete fusion ; and it appears 

 to be requisite in this second operation of forming the 

 glass into discs, to stop short of the melting point. If 

 the glass be completely melted, bubbles of air rise 

 through the glass, and are found caught in the glass after 

 it is cooled, diminishing its transparency, and perhaps 

 causing even worse defects. Many discs are spoiled in 

 this manner. The advantage of allowing the glass to 

 cool before it is cast into discs, is, that it affords an op- 

 portunity to inspect the casting, and select such portions 

 as appear less faulty. Each fragment is then put in a 

 separate crucible or mold, having a diameter such as it 

 is proposed to give to the disc, and softened by heat 

 until it accomodates itself perfectly to the mold; and 

 some discs have marks of having been pressed down 

 into the molds by a weight upon the top; It is then 

 annealed by slow cooling in the manner of ordinary glass 

 ware. 



After the death of M. Guinand, his widow and one of 

 his sons set up works in Switzerland, upon the father's 

 principles, and were succeeded by M. Theodore Daguet 

 (of Soleure, near Neuchatel), who sent to the London 

 Exhibition of 1851, several discs of flint-glass, the largest 

 being 15 inches in diameter ; and a disc of crown-glass 

 of 7 inches diameter, which were examined and found to 

 be good. M. Daguet, by a process of his own, gives to 

 flint-glass a degree of hardness not attained by any other 

 manufacturer. His glass, particularly the flint, is distin- 

 guished both by its homogeneousness and its peculiar 



