LAW OF DISPERSION BY REFRACTION. 389 



tion of different colours, as to some expressions 

 used by Newton, which, they conceived, conveyed 

 false notions respecting the composition and nature 

 of light. Newton had asserted that all the different 

 colours are of distinct kinds, and that, by their 

 composition, they form white light. This is true of 

 colours as far as their analysis and composition by 

 refraction are concerned ; but Hooke maintained 

 that all natural colours are produced by various 

 combinations of two primary ones, red and violet 14 ; 

 and Huyghens held a similar doctrine, taking, how- 

 ever, yellow and blue for his basis. Newton an- 

 swers, that such compositions as they speak of, are 

 not compositions of simple colours in his sense of 

 the expressions. These writers also had both of 

 them adopted an opinion that light consisted in 

 vibrations; and objected to Newton that his lan- 

 guage was erroneous, as involving the hypothesis 

 that light was a body. Newton appears to have 

 had a horrour of the word hypothesis, and protests 

 against its being supposed that his "theory" rests 

 on such a foundation. 



The doctrine of the unequal refrangibility of 

 different rays is clearly exemplified in the effects of 

 lenses, which produce images more or less bordered 

 with colour, in consequence of this property. The 

 improvement of telescopes was, in Newton's time, 

 the great practical motive for aiming at the improve- 

 ment of theoretical optics. Newton's theory showed 



14 Brewster's Newton, p. 54. Phil. Trans, viii. 5084, (5086. 



