518 THE PRINCIPLES OF SCIENCE. [chap. 



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 all bodies are determined 

 in lilce manner by the size of their ultimate particles. 

 A thin plate of a definite thickness will reflect a de- 

 finite 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, 

 according to Newton, from particles being too minute to 

 reflect light ; but if so, every black substance should be 

 transparent. Newton himself so acutely felt this last dif- 

 ficulty as to suggest that true blackness is due to some 

 internal refraction of the rays to and fro, and an ultimate 

 stifling of them, which he did not attempt to explain 

 further. Unless some other process comes into operation, 

 neither refraction nor reflection, however often repeateil, 

 will destroy the energy of light. The, theory therefore 

 gives no account, as Brewster shows, of 24 parts out of 

 25 of the light which falls upon a black coal, and the re- 

 maining part which is reflected from the lustrous surface 

 is equally inconsistent with the theory, because fine coal- 

 dust is almost entirely devoid of reflective power.^ It is 

 now generally believed that the colours of natural bodies 

 are due to the unequal absorption of rays of light of dif- 

 ferent refrangibility. 



Uxperimenhim Crucis. 



As we deduce more and more conclusions from a theory, 

 and find them verified by trial, the probability of the 

 theory increases in a rapid manner ; but we never escape 

 the risk of error altogether. Absolute certainty is be- 

 yond the powers of inductive invest-igation, and the most 

 plausible supposition may ultimately be proved false. 

 Such is the groundwork of similarity in nature, that 

 two very different conditions may often give closely 

 similar results. We sometimes find ourselves therelbre 



' Lrewster's Life of Newton, ist etlit. chan. vii. 



