202 THE RAINBOW 



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 plea- 

 sures of the intellect there is any more pure and con- 

 centrated than that experienced by the scientific man 

 when a difficulty which has challenged the human mind 

 for ages melts before his eyes, and re-crystallises as an 

 illustration of natural law. This pleasure was doubtless 

 experienced by Descartes when he succeeded 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 emer- 

 gence. 



It is said that Descartes behaved ungenerously to 

 Snell that, though acquainted with the unpublished 

 papers of the learned Dutchman, he failed to acknow- 

 ledge 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 emphasise such shortcomings. The 

 temporary weakness 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 colours of the bow to be accounted for ? Here 

 his penetrative mind came to the very verge of the 

 solution, but the limits of knowledge at the time barred 

 his further progress. He connected the colours of the 

 rainbow with those produced by a prism ; but then these 



