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CHAPTER IV. 

 DISCOVERY OF ACHROMATISM. 



fllHE discovery that the laws of refractive dis- 

 I persion of different substances were such as to 

 allow of combinations which neutralized the dis- 

 persion without neutralizing the refraction, is one 

 which has hitherto been of more value to art than 

 to science. This property has no definite bearing, 

 which has yet been satisfactorily explained, upon the 

 theory of light ; but it is of the greatest importance 

 in its application to the construction of telescopes ; 

 and it excited the more notice, in consequence 

 of the prejudices and difficulties which for a time 

 retarded the discovery. 



Newton conceived that he had proved by experi- 

 ment V that light is white after refraction, when the 

 emergent rays are parallel to the incident, and in 

 no other case. If this were so, the production of 

 colourless images by refracting media would be 

 impossible; and such, in deference to Newton's 

 great authority, was for some time the general per- 

 suasion. Euler 2 observed, that a combination of 

 lenses which does not colour the image must be 

 possible, since we have an example of such a com- 

 bination in the human eye ; and he investigated 



1 Opticks, B. i. p. ii. Prop. 3. * Ac. Berlin. 1747- 



