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 Dolionds, 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 (xuinand, 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. 
110 
