62 HISTORY OF OPTICS. 



coigne's letter, suggested that the Dutch experimenters might havt 

 taken one of the images reflected from the surfaces of the prism, of 

 which there are several, instead of the proper refracted one. By the 

 aid of this hint, Lucas of Liege repeated Newton's experiments, and 

 obtained Newton's result, except that he never could obtain a spectrum 

 whose length was more than three and a half times its breadth. 



o 



Newton, on his side, persisted in asserting that the image would be 

 five times as long as it was broad, if the experiment were properly 

 made. It is curious that he should have been so confident of this, as 

 to conceive himself certain that such would be the result in all cases. 

 We now know that the dispersion, and consequently the length, of the 

 spectrum, is very different for different kinds of glass, and it is very 

 probable that the Dutch prism was really less dispersive than the 

 English one. 13 The erroneous assumption which Newton made in 

 this instance, he held by to the last ; and was thus prevented from 

 making the discovery of which we have next to speak. 



Newton was attacked by persons of more importance than those we 

 have yet mentioned ; namely, Hooke and Huyghens. These philoso- 

 phers, however, did not object so much to the laws of refraction of 

 different colors, 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 colors 

 are of distinct kinds, and that, by their composition, they form white 

 light. This is true of colors as far as their analysis and composition 

 by refraction are concerned ; but Hooke maintained that all natural 

 colors are produced by various combinations of two primary ones, red 

 and violet ; u and Huyghens held a similar doctrine, taking, however, 

 yellow and blue for his basis. Newton answers, that such composi- 

 tions as they speak of, are not compositions of simple colors 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 language was erroneous, as involving the hypothesis 

 that light was a body. Newton appears to have had a horror 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 mere or 



13 Brewster's Newton, p. 50. 



14 Brewster's Newton, p. 54. Phil. Trans, viii. 5084, 6086. 



