60 HISTORY OF OPTICS. 



great intensity of the light returned at a certain angle; and the refer- 

 ring the different colors to the different quantity of the refraction ; 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 colors were produced 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 terminated by a shade ; and 

 hence the colors are the same as those produced through a prism." 



The subject was left nearly in the same state, in the work of Gri- 

 maldi, Physico-Mathesis^ de Lumine, Coloribus et Iride, published at 

 Bologna in 1GG5. There is in this work a constant reference' to 



O 



numerous experiments, and a systematic exposition of the science in 

 an improved state. The author's calculations concerning the rainbow 

 are put in the same form as those of Descartes ; but he is further from 

 seizing the true principle on which its coloration depends. He rightly 

 groups together a number of experiments in which colors arise from 

 refraction ; 10 and explains them by saying that the color is brighter 

 where the light is denser : and the light is denser on the side from 

 which the refraction turns the ray, because the increments of refraction 

 are greater in the rays that are more inclined. 11 This way of treating 

 the question might be made to give a sort of explanation of most of 

 the facts, but is much more erroneous than a developement of Descar- 

 tes's view would have been. 



At length, in 1072, Newton gave 12 the true explanation of the facts; 

 namely, that light consists of rays of different colors and different 

 refrangibility. This now appears to us so obvious a mode of interpret- 

 ing the phenomena, that we can hardly understand how they can be 

 conceived in any other manner ; but yet the impression which this 

 discovery made, both upon Newton and upon his contemporaries, shows 

 how remote it was from the then accepted opinions. There appears 

 to have been a general persuasion that the coloration was produced, 

 not by any peculiarity in the law of refraction itself, but by some col- 

 lateral circumstance, some dispersion or variation of density of the 

 light, in addition to the refraction. Newton's discovery consisted in 



9 Sect. ix. p. 193. 10 Prop. 35, p. 254. n Ib. p. 266. 



a Phil. Trans. I, vii. p. 3075. 



