384 HISTORY OF OPTICS. 



colour, those which endeavour to move only a little 

 more strongly produce yellow." Here we have a 

 clear perception that colours and unequal refrac- 

 tion are connected, though the cause of refraction 

 is expressed by a gratuitous hypothesis. And we 

 may add, that he applies this notion rightly, so far 

 as he explains himself 8 , to account for the colours 

 of the rainbow. 



It appears to me that Newton and others have 

 done Descartes injustice, in ascribing to De Dominis 

 the true theory of the rainbow. There are two 

 main points of this theory, namely, the showing 

 that a bright circular band, of a certain definite 

 diameter, arises from the great intensity of the light 

 returned at a certain angle ; and the referring the 

 different colours to the different quantity of the re- 

 fraction ; and both these steps appear indubitably 

 to be the discoveries of Descartes. And he informs 

 us that these discoveries were not made without 

 some exertion of thought. "At first," he says 9 , 

 "I doubted whether the iridal colours were pro- 

 duced in the same way as those in the prism ; but, 

 at last, taking my pen, and carefully calculating 

 the course of the rays which fall on each part of 

 the drop, I found that many more come at an angle 

 of forty-one degrees, than either at a greater or a 

 less angle. So that there is a bright bow termi- 

 nated by a shade ; and hence the colours are the 

 same as those produced through a prism." 



8 Meteor. Sect. ix. 9 Sect. ix. p. 193. 



