REFLEXION AND REFRACTION. 27 



a distance a little greater, the attractive force is changed into 

 a repulsive one ; and that these attractive and repulsive 

 forces succeed one another for many alternations. Nothing 

 can be more reasonable than this hypothesis, granting that 

 light is material ; for the succession of attractive and repul- 

 sive forces, here assumed, is altogether similar to tha. 

 to which the known phenomena of molecular action are 

 ascribed. 



On these suppositions Newton has rigorously deduced the 

 laws of reflexion and refraction. In the case of reflexion, it is 

 shown that the whole perpendicular velocity of the molecule 

 is restored to it in an opposite direction, by the operation of 

 the supposed repulsive force ; and, therefore, that the angles 

 which its path makes with the perpendicular to the surface, 

 before and after reflexion, are equal. In the case of refrac- 

 tion, it is proved that the effect of the attractive force is to 

 increase the square of the perpendicular velocity of the mole- 

 cule, by an amount which is constant for the same medium ; 

 from which it follows, that the sines of the angles which its 

 course makes with the perpendicular to the surface, before 

 and after refraction, are in the inverse ratio of the velocities 

 in the two media. This problem was the first in which the 

 effects of molecular forces were submitted to calculation ; and 

 its solution is justly regarded as forming an era in the history 

 of science. 



(34) But although the theory of emission is successful 

 in explaining the laws of reflexion and refraction, considered 

 as distinct phenomena, it is by no means equally so in ac- 

 counting for their connexion and mutual dependence. When 

 a beam of light is incident on the surface of any transparent 

 medium, part is in all cases transmitted, and part reflected ; 

 the intensity of the reflexion being less, the less the differ- 

 ence of the refractive indices of the two media, and the re- 



