378 HISTOEY OF ASTRONOMY. 



foreign glass ; but it has been found impossible to obtain 

 large discs possessing that entire homogeneity and free- 

 dom from veins which are demanded in a lens in order 

 that it may produce a perfect image. 



In the years 1846 and '48, Mr. Alvan Clark, of Boston, 

 made two telescopes of East Cambridge flint-glass, having 

 an aperture of five inches, which will show the division 

 of the close pair in Zeta Cancri, and- Zeta Bootis, whose 

 distance is about one second. He has, however, ex- 

 pressed his determination to make no more telescopes of 

 American glass, until he can find specimens of a better 

 quality. It may be safely asserted, notwithstanding some 

 pretensions to the contrary, that no good telescope of 

 large dimensions has yet been manufactured of American 

 glass. 



Ever since the invention of the achromatic telescope by 

 Dollond, about a century ago, one of the greatest obstacles 

 to the construction of large telescopes, has been the diffi- 

 culty of obtaining large discs of glass of perfectly uniform 

 density and free from veins. The chief difficulty seems 

 to arise from the difference in the specific gravity of the 

 constituents of glass ; some melt at a lower temperature, 

 and sinking through the mixture, leave a streak in de- 

 scending; some decompose in a heat required for the 

 fusion of others. It has been said that the glass em- 

 ployed by Dollond in the manufacture of his best tele- 

 scopes was all made 'at the same time ; and the largest 

 achromatic object glasses constructed in England, until 

 recently, did not exceed five inches in diameter. More 



