252 DISCOURSE ON THE STUDY 



colours in the ordinary refraction of light by a prism, 

 of which the theory of Newton gives a complete 

 and elegant explanation ; and the discovery of 

 which by him marks one of the greatest epochs in 

 the annals of experimental science. This, which has 

 been often urged in objection to it, remains still, 

 if not quite unanswered, at least only imperfectly 

 removed. 



(277.) Other phenomena, however, were not 

 wanting to afford a further trial of the explanatory 

 powers of either hypothesis. The diffraction or 

 inflection of light, discovered by Grimaldi, a Jesuit 

 of Bologna, seemed to indicate that the rays of 

 light were turned aside from their straight course 

 by merely passing near bodies of every descrip- 

 tion. These phenomena, which are very curious 

 and beautiful, were minutely examined by Newton, 

 and referred by him to the action of repulsive forces 

 extending to a sensible distance from the surfaces 

 of bodies ; and his explanation, so far as the facts 

 known to him are concerned, appears as satisfactory 

 as could reasonably be then expected ; and much 

 more so than any thing which could at that time be 

 produced on the side of the hypothesis of Huy- 

 ghens, which, in fact, seemed incapable of giving 

 any account whatever of them. 



(278.) Another class of delicate and splendid 

 optical phenomena, which had begun to attract at- 

 tention somewhat previous to Newton's time, seemed 

 to leave both hypotheses equally at a loss. These 

 were the colours exhibited by very thin films, 

 either of a liquid (such as a soap-bubble), or of air, 

 as when two glasses are laid together with only air 



