THE USE OF HYPOTHESIS. 147 



analogy which 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 1 . 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- 

 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 decisr\ 1 v 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 which it transmits. Colourless transparency arises, 



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

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

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