140 MALUS. 



than the grounds for a decision between the claims of the 

 two rival theories of light. 



The celebrated physicist Wollaston, some years before, 

 had proposed a method by means of which to deduce the 

 refractive power of all substances whether transparent or 

 opaque. This method rests on the determination of the 

 angle under which these substances applied immediately 

 in contact with one of the surfaces of a prism of glass, 

 through which we look at them, begin to cease to be 

 visible. 



Now according to the theory of reflexion* expounded 



* To render what follows intelligible, many readers may find it per- 

 haps desirable if we here explain, very briefly, the view of ordinary 

 reflexion and refraction of light as explained respectively by the emis- 

 sion and the loave theories. 



On the former a molecule of light resembles an elastic body, which 

 if projected obliquely against a hard plane surface, by the principles 

 of mechanics rebounds at an angle equal to that at which it im- 

 pinged. 



In refraction the investigation is more difficult: a molecule of light 

 is here supposed to enter, projected with great velocity, among the 

 molecules of the refracting transparent medium which are at such 

 relative distances as to allow it freely to pass among them; but at its 

 first entry among them it is of course attracted by them; it then be- 

 comes a problem of dynamics, requiring the aid of the higher mathe- 

 matics, to determine what will be the path which it will pursue under 

 their influence. In general it is clear, that under these united attrac- 

 tions urging it on, its velocity will be accelerated: but to go into the 

 complete solution, would be beyond the limits of a note. It was fully 

 investigated by Newton (Principia, lib. i. sect. xiv. prop. 94), where 

 he demonstrates that on these principles the deviation of the refracted 

 ray will follow the law that the sines of the angles of incidence and re- 

 fraction are in a constant ratio. 



Similar investigations have been pursued by Ltiplace, more espec- 

 ially with regard to atmospherical refraction, the atmosphere being 

 supposed to consist of strata of different densities. (Mec. Celeste, vol. 

 iv. liv. x. ch. i. 2, 3.) 



On the wave hypothesis, the explanation admits of a very simple 

 kind of illustration. 



