ON RAINBOWS. 661 



from its back, and again refracted on their emergence. Descartes 

 was acquainted with the law of Snell, and, taking up his pen, he cal- 

 culated, by means of that law, the whole course of the rays. He 

 proved that the vast majority of them escaped from the drop as diver- 

 gent rays, and, on this account, soon became so enfeebled as to produce 

 no sensible effect upon the eye of an observer. At one particular 

 angle, however namely, the angle 41 aforesaid they emerged in a 

 practically parallel sheaf. In their union was strength, for it was this 

 particular sheaf which carried the light of the " primary " rainbow to 

 the eye. 



There is a certain form of emotion called intellectual pleasure, 

 which may be excited by poetry, literature, nature, or art. But I 

 doubt whether among the pleasures of the intellect there is any more 

 pure and concentrated than that experienced by the scientific man 

 when a difficulty which has challenged the human mind for ages melts 

 before his eyes, and recrystallizes as an illustration of natural law. 

 This pleasure was doubtless experienced by Descartes when he suc- 

 ceeded in placing upon its true physical basis the most splendid meteor 

 of our atmosphere. Descartes showed, moreover, that the " secondary 

 bow " was produced when the rays of light underwent two reflections 

 within the drop, and two refractions at the points of incidence and 

 emergence. 



It is said that Descartes behaved ungenerously to Snell that, 

 though acquainted with the unpublished papers of the learned Dutch- 

 man, he failed to acknowledge his indebtedness. On this I will not 

 dwell, for I notice on the part of the public a tendency, at all events 

 in some cases, to emphasize such short-comings. The temporary weak- 

 ness of a great man is often taken as a sample of his whole character. 

 The spot upon the sun usurps the place of his " surpassing glory." 

 This is not unfrequent, but it is nevertheless unfair. 



Descartes proved that, according to the principles of refraction, 

 a circular band of light must appear in the heavens exactly where the 

 rainbow is seen. But how are the colors of the bow to be accounted 

 for ? Here his penetrative mind came to the very verge of the solu- 

 tion, but the limits of knowledge at the time barred his further prog- 

 ress. He connected the colors of the rainbow with those produced 

 by a prism ; but then these latter needed explanation just as much as 

 the colors of the bow itself. The solution, indeed, was not possible 

 until the composite nature of white light had been demonstrated by 

 Newton. Applying the law of Snell to the different colors of the 

 spectrum, Newton proved that tbe primary bow must consist of a se- 

 ries of concentric circular bands, the largest of which is red, and the 

 smallest violet ; while in the secondary bow these colors must be re- 

 versed. The main secret of the rainbow, if I may use such language, 

 was thus revealed. 



I have said that each color of the rainbow is carried to the eye by 



