388 HISTORY OF OPTICS. 



coigne, a friend of Linus, still maintained that he 

 and others had seen what the Dutch physician had 

 described; and Newton, who was pleased with the 

 candour of Gascoigne's letter, suggested that the 

 Dutch experimenters might have taken one of the 

 images reflected from the surfaces of the prism, of 

 which there are several, instead of the proper re- 

 fracted one. By the aid of this hint, Lucas of Liege 

 repeated Newton's experiments, and obtained New- 

 ton's result, except that he never could obtain a 

 spectrum whose length was more than three and a 

 half times its breadth. Newton, on his side, per- 

 sisted in asserting that the image would be five 

 times as long as broad, if the experiment were 

 properly made. It is curious that he should have 

 been so confident of this, as to conceive himself cer- 

 tain 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 dis- 

 covery of which we have next to speak. 



Newton was attacked by persons of more impor- 

 tance than those we have yet mentioned; namely, 

 Hooke and Huyghens. These philosophers, how- 

 ever, did not object so much to the laws of refrac- 



13 Brewster's Newton, p. 50. 



