ACHROMATIC COMBINATIONS. 



the contrary, the bodies having nearly equal refractive powers, often 

 have very unequal dispersive powers, and vice versa-. 



Fig. 33. 



Fig. 34. 



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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