LAW OF DISPERSION BY REFRACTION. 385 



The subject was left nearly in the same state, 

 in the work of Grimaldi, Physico-Mathesis, de 

 Lumine, Coloribus et Iride, published at Bologna 

 in 1665. There is in this work a constant refer- 

 ence to 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 colours 

 arise from refraction 10 ; and explains them by say- 

 ing that the colour is brighter where the light is 

 denser: and the light is denser on the side from 

 which the refraction turns the ray, because the in- 

 crements of refraction are greater in the rays that 

 are more inclined 11 . This way of treating the ques- 

 tion might be made to give a sort of explanation of 

 most of the facts, but is much more erroneous than 

 a developement of Descartes's view would have been. 



At length, in 1672, Newton gave 12 the true ex- 

 planation of the facts ; namely, that light consists 

 of rays of different colours and different refrangi- 

 bility. This now appears to us so obvious a mode 

 of interpreting the phenomena, that we can hardly 

 understand how they can be conceived in any other 

 manner; but yet the impression which this dis- 

 covery made, both upon Newton and upon his con- 



10 Prop. 35, p. 254. " Ib. p. 256. 



12 Phil. Trans, t. vii. p. 3075. 



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