148 THE PRINCIPLES OF SCIENCE. 



according to Newton, from all the particles being too 

 minute to reflect light ; but if so, every transparent sub- 

 stance should appear perfectly black by reflected light, 

 and, vice versa, every black substa,nce should be trans- 

 parent. Newton himself so acutely felt this last difficulty 

 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 further to explain. 

 Unless some other process came into operation, neither 

 refraction nor reflection, however often repeated, would 

 destroy the energy of light. The theory gives no account, 

 therefore, as Brewster shows, of 24 parts out of 25 of the 



light which falls upon a black coal, and the th part 



which is reflected from the lustrous surface is equally in- 

 consistent with the theory, because fine coal-dust is almost 

 entirely devoid of reflective power 1 . It is now generally 

 believed that the colours of natural bodies are due to the 

 unequal absorption of rays of light of different refrangi- 

 bilitv. 



Experimentum 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 most rapid manner ; but we never 

 escape the risk of error altogether. Absolute certainty is 

 beyond the power of inductive investigation, and the 

 most plausible suppositions 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 therefore 

 in possession of two or more hypotheses which both agree 



1 Brewster's 'Life of Newton/ ist edit. chap. vii. 



