ACHROMATIC COMBINATIONS. 
the contrary, the bodies having nearly equal refractive powers, often 
have very unequal dispersive powers, and vice versa . 
66. The high refracting power of the diamond, combined with 
its low dispersive power, were among the circumstances which 
raised the hopes already mentioned, that great improvements in 
microscopic lenses would result from the substitution of that gem 
and others, having like optical properties, for glass. Happily the 
invention of other and better expedients for surmounting the 
imperfections arising from chromatic aberration, have rendered 
unnecessary so expensive a remedy. 
67. The discovery of the fact that the dispersive powers of 
different transparent bodies is not proportional to their refractive 
powers, but on the contrary, that bodies of greater refractive 
powers have sometimes lower dispersive powers, has supplied a 
remedy, which practically speaking, may be said to be completely 
efficacious for the removal of all the injurious effects of chromatic 
aberration. The manner in which this important end has been 
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