REFLEXION AND REFRACTION OF LIGHT. IU 



bably for many alternations. The absolute values, or intensities, 

 of these forces are different in different bodies; but the form of 

 the law, or the function of the distance by which they are ex- 

 pressed, is assumed to be the same for all. * From these postu- 

 lates Newton has rigorously deduced the laws of reflexion and 

 refraction. The problem is the first in which the effects of that 

 important class of forces acting only at insensible distances have 

 been submitted to calculation ; and the solution is regarded by 

 M. Poisson as forming an era in the history of science. 



The reflexion of light at the exterior surface of dense media is 

 ascribed to the repulsive force ; refraction, and internal reflexion, 

 to that inner attractive force which extends up to actual contact. 

 The outermost sphere of action of every body, in this theory, is ne- 

 cessarily attractive, as well as the inmost ; for, were it otherwise, 

 no ray could enter, or emerge from, the medium at an extreme 

 incidence. Sir David Brewster has made an ingenious use of this 

 principle to explain the remarkable fact noticed by Bouguer, that 

 water is more reflective than glass at oblique incidences. 



But though the theory of emission is perfectly successful in 

 explaining the laws of reflexion and refraction, considered as dis- 

 tinct phenomena, yet it is by no means equally so in accounting 

 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 is in general less, the less the difference of the refractive 

 indices of the two media ; and accordingly the reflective and re- 

 fractive forces (if such be the cause of the phenomena) are related 

 to one another in all media, so that one increases or diminishes 

 along with the other.f But how is it that some of the molecules 

 obey the influence of the repulsive force, and are reflected, while 

 others yield to the attractive force, and are refracted? To 

 account 'for this Newton was obliged to have recourse to a new 

 hypothesis. The molecules of light are supposed to pass through 

 certain periodical states, called " fits of easy reflexion and trans- 



* This assumption is tacitly made by Newton, when he takes the function 



as the measure of the refractive power. See Herschel's " Essay on Light,' ' Encyc. Metrop. 



t The reader will find much novel and interesting matter connected with this suh- 



ject in a paper hy Sir David Brewster, " On the Reflexion and Decomposition of 



Light at the separating surface of media of the same and of different refractive powers," 



/'/'/. Trans. 1829. ' 



