66 VOLTAIRE. 



spectral rays by means of a lens, and their forming 

 white in the focus ; but he leaves entirely out the 

 decisive experiment of stopping different portions of 

 the spectrum, and then finding that the focus is no 

 longer white, but of the colour, or mixture of colours, 

 suffered to pass onward. It is perhaps a proof of the 

 same kind, that he states what he certainly never could 

 have learnt in the ' Optics,' the blue colour of the sky 

 as caused by the great attenuation of the vapours aris- 

 ing in the atmosphere (Part ii. chap. 12). Nor could 

 any one who had studied the same admirable work 

 have confined himself almost entirely to one portion 

 of it, and give scarcely any account, except the most 

 general, and indeed meagre, of the colours of thin 

 plates, and none at all of the colours of thick plates. 



With respect to the ' Principia,' he gives with con- 

 siderable fulness the doctrine of equal areas in equal 

 times ; and indeed, from his account, the demonstration 

 as well as the fundamental proposition itself may be 

 gathered. But then comes this very summary state- 

 ment of the planetary law : " Enfin Newton a prouve 

 que si la courbe decrite autour du centre est une ellipse, 

 la force attractive est en raison inverse du carre des 

 distances" (Part iii. chap. 4). He indeed leaves us here 

 to infer, quite contrary to the truth, that the same 

 proportion is peculiar to motion in an ellipse ; and he 

 makes no mention whatever of the inverse problem, 

 the deducing the curve from the force the more im- 

 portant of the two. 



There is a profound view given of the irregularity in 

 the moon's motion caused by disturbance (Part iii. 

 chap. 6), and one or two other parts of the treatise 



