REFLEXION AND REFRACTION OF LIGHT. 33 



in general, sufficient to overcome altogether the effect of the 

 attractive force, and subject the molecule to the repulsive, it is 

 obvious that the phase of the fit must modify the effects of these 

 forces in every intermediate degree ; and that the molecules which 

 do obey the attractive force must have their velocities augmented 

 in different degrees, depending on the phase. Consequently, as 

 the direction of the refracted ray depends on its velocity, the 

 transmitted beam will consist of rays refracted in widely different 

 angles, and will be scattered and irregular. 



In some of his writings, Newton attributes the reflexion and 

 refraction of light to a difference in the density of the ether within 

 and without bodies ; or rather he refers the attractive and repul- 

 sive forces to this, as to a more general principle. The ether is 

 supposed to be rarer within dense bodies than without ; and the 

 rays of light, in crossing the bounding surface, are pushed from 

 the side of the denser ether, so that their motion is accelerated if 

 they pass from the rarer to the denser body, and retarded in the 

 opposite case. Reflexion at the surface of the rarer medium is ex- 

 plained on the same suppositions ; but to account for the ordinary 

 reflexion by a denser medium, Newton was obliged to introduce 

 new and gratuitous hypotheses respecting the constitution of the 

 ether at the confines of two media in which its density is different.* 



The velocity of propagation, in the wave-theory of light, de- 

 pends solely on the elasticity of the vibrating medium as compared 

 with its density. If, then, a plane wave be incident obliquely on 

 the bounding surface of two media, it is obvious that its several 

 portions will reach that surface at different moments of time ; 

 and each of these portions will become the centre of two spherical 

 waves, one of which will be propagated in the first medium with 

 the original velocity, while the other will be propagated in the 

 new medium, and with the velocity which belongs to it. But, by 

 the principle of the coexistence of small motions, the agitation of 

 any particle of either medium is the sum of the agitations sent 

 there at the same instant from these several centres of disturbance ; 

 and the surfaces on which they are accumulated at any instant 

 will be the reflected and refracted waves. These surfaces are 

 those which touch all the small spherical waves at any instant. 

 It is easy to see that they are both plane ; and that the reflected 



* Birch's History of the Royal Society, vol. iii. p. Ml. Optics, Query 19. 

 D 



