ACHROMATIC COMBINATIONS. 



After Dollond' s discovery, the subject was investigated mathe- 

 matically by Euler, Clairaut, and D'Alembert, but their researches 

 did not lead to any practical improvement, and for a long series of 

 years the lenses produced by the Dollond family enjoyed a mono- 

 poly and a European celebrity. 



The difficulty in constructing achromatic lenses arises from that 

 of obtaining single pieces of flint glass which are pure and uniform 

 throughout their entire dimensions. The slightest impurity, or 

 want of homogeneity in the composition of the glass, produces a 

 streaked and deformed image. 



The method of producing pure flint glass even in pieces of mode- 

 rate magnitude, long remained a secret with the Dollonds, and it 

 formed a very considerable article of exportation. Of late years, 

 however, the art of producing it has undergone immense improve- 

 ment in Switzerland, Bavaria, and other parts of the Continent, 

 by the successful experiments of Gruinand, Frauenhofer, Cauchoix, 

 Korner, D'Artigues, and others. The object-glasses of Dollond, 

 excellent as they were, never could be obtained of greater diameter 

 than about 5 inches. Frauenhofer, however, has succeeded in 

 producing perfect lenses, having diameters measuring from 12 to 

 13 inches. An object-glass, manufactured by Cauchoix, which 

 measures more than 12 inches, is mounted in the great parallactic 

 telescope of Sir James South, at Campden Hill. 



The exact proportion of the ingredients composing these fine 

 specimens is not certainly known, and the excellence of particular 

 pieces depends on accidental circumstances not known or con- 

 trolled by the makers themselves. Korner produced some of his 

 best specimens with the following ingredients : Quartz, previously 

 treated with muriatic acid, 100; litharge, or red lead, 80; and 

 bitartrate of potash, 30. 



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