REFRACTIVE POWER. 



143 



identity of the refractive powers of wax, when transparent 

 and when opaque, which seemed to be a necessary result, 

 appeared both to the author, to Laplace, and to all the 



The law of refraction may also be more briefly deduced thus : tak- 

 ing the fronts of the incident and refracted rays perpendicular to their 



directions, their inclinations will be determined by the relative veloci- 

 ties with which those fronts advance ; and while the incident front has 

 advanced through a space d, that of the refracted will have advanced 

 through d L proportional to their velocities ; or, 



d v 



d i v i =(J " 



But geometrically for any breadth, 



d = b sin. i d = b sin. r, 

 Hence, 



sin. 



j 



sin. r 

 which is the law of refraction. 



This method, though in a less concise form, is given by Mr. Power 

 ( On Absorption of Rays, $c., Philos. Trans. 1854, part i.,) who never- 

 theless calls in question the principle of the assumption that the front 

 of the rays is strictly perpendicular to their direction, and proposes a 

 more general view : from which, without any assumption as to the 

 nature or law of refraction, he shows that the formula of the sines is 

 directly deducible from his analysis. Objections, however, have been 

 raised against his reasoning. Translator. 



