THE MANUFACTUKE OF TELESCOPES. 381 



Guinand made was not equal in quality to that which 

 Fraunhofer made at a later period. There can, however, 

 be little doubt that much of the reputation of the Munich 

 telescopes has resulted from Guinand's experiments in 

 the manufacture of glass. The art of making this glass 

 is kept a secret. Many particulars of this manufacture 

 therefore can only be conjectured. Faraday found the 

 specific gravity of Guinand's flint-glass to be about 

 3*616, and that its composition was silica 44*3, oxyd of 

 lead 43-05, and potash 11-75. It is said that Guinand's 

 original practice was to saw the blocks of glass which he 

 obtained at one casting, into horizontal sections, supposing 

 that every part of the same horizontal section would have 

 the same density. A fortunate accident conducted him 

 to a better process. While his men were one day carry- 

 ing a block of this glass on a hand-barrow to a saw fl\ 

 the mass slipped from its bearers, and rolling down a 

 declivity was broken to pieces. Guinand selected those 

 fragments which appeared perfectly homogeneous, and 

 softened them in circular molds in such a manner that 

 on cooling, he obtained discs that were afterward fit fc 

 working. To this method he adhered, and contrived a 

 way of cleaving his glass while cooling, so that the frac- 

 tures should follow the most faulty parts. When flaws 

 occur in the large masses, they are removed by cleaving 

 the pieces with wedges, and then softening them again in 

 molds which give them the form of discs. 



It will be remembered that glass softens so as to be 

 readily molded into any required shape, at a tempera- 



