306 FOURTH REPORT 1834. 



but the form of the law, or the function of the distance by which 

 they are expressed, is assumed to be the same for all*. From 

 these postulates Newton has rigorously deduced the laws of re- 

 flexion and refraction. The problem is the first in which the 

 efl'ects 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 re- 

 flexion, to that inner attractive force which extends up to actual 

 contact. The outermost sphere of action of every body, in this 

 theory, is necessarily 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 inge- 

 nious 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 

 distinct phenomena, yet it is by no means equally so in account- 

 ing for their connexion and mvitual 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 in- 

 tensity of the reflexion is in general less, the less the difterence 

 of the refractive indices of the two media ; and accordingly the 

 reflective and refractive forces (if such be the cause of the phe- 

 nomena,) are related to one another in all media, so that one 

 increases or diminishes along with the otherf. 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 transmission," which modify the effects of 

 the attractive and repulsive forces, and in which they are dis- 



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



u'' — 1 



— as th^ measure of the i-efractive power. See Herschel's " Essay on 



Light," Encyc. Met. 



^ The reader will find much novel and interesting matter connected with 

 this subject in a paper by Sir David Brewster, " On the Reflexion and Decom- 

 position of Light at the separating surface of media of the same and of differ- 

 ent refractive powers," Phil. Trans. 1829. 



