THE UHE OF HYPOTHESIS. 117 



analogy wliicli doubtless suggested the theory. The 

 failure was in the first and third requisites ; for, as already 

 remarked, the theory did not allow of any precise cal 

 culation of planetary motions, and was so far incapable 

 of rigorous verification. But so far as we can institute a 

 comparison, facts are entirely against the vortices. Newton 

 carefully pointed out that the Cartesian theory was incon 

 sistent with the laws of Kepler, and would represent the 

 planets as moving more rapidly at their aphelia than at 

 their perihelia . Newton did not ridicule the theory as 

 absurd, but showed k that it was pressed with many 

 difficulties. The rotatory motions of the sun and planets 

 on their own axes are in striking conflict with the revo- 



O 



lutions of the satellites carried round them : and comets, 

 the most flimsy of bodies, calmly pursue their courses in 

 elliptic paths, altogether irrespective of the vortices which 

 they intersect. We may now also point to the inter 

 lacing orbits of the minor planets as a new and insuper 

 able difficulty in the way of the Cartesian ideas. 



Newton, though he established the best of theories, was 

 also capable of proposing one of the worst ; and if we 

 want an instance of a theory decisively contradicted by 

 facts, we have only to turn to his views concerning the 

 origin of natural colours. Having analysed, with incom 

 parable skill, the origin of the colours of thin plates, he 

 suggests that the colours of ah 1 bodies and substances are 

 determined in like manner by the size of their ultimate 

 particles. A thin plate of a definite thickness will reflect 

 a definite colour ; hence, if broken up into fragments it 

 will form a powder of the same colour. But, if this be a 

 sufficient explanation of coloured substances, then every 

 coloured fluid ought to reflect the complementary colour of 

 that wdiich it transmits. Colourless transparency arises, 



i Principia, bk. II. Sect. ix. Prop. 53. 

 k Ibid. bk. III. Prop. 43. General Scholium. 

 I, 2 



