KEFLEXION AND KEFRACTION OF LIGHT. 43 



mathematician relating to light, the ether is considered as a sys- 

 tem of particles solicited by mutual attractions or repulsions ; and 

 from the partial differential equations which represent their move- 

 ment, he had deduced the laws of propagation in crystallized as 

 well as in homogeneous media. These equations, however, were 

 but approximate, and derived from others of greater generality by 

 the omission of the terms containing the higher powers of the 

 displacements, and of their derivatives with respect to the co- 

 ordinates. Eesuming the problem of the propagation of a plane 

 wave, with the aid of the more general equations, he has finally 

 demonstrated the existence of a relation between the velocity of 

 propagation and the length of the wave.* 



The opacity of bodies is ascribed by Newton to the discon- 

 tinuity of their parts, and to the multitude of internal reflexions 

 which the rays of light undergo within them.f We have many 

 reasons for believing this to be the case ; but as yet we are far from 

 a complete account of the phenomenon. If the reflexions and 

 refractions, which thus arise at each new bounding surface, be 

 similar to those which take place at the outer surfaces of bodies, 

 the molecules of light will indeed be scattered in every direction, 

 but they should undergo no diminution of velocity. How, then, 

 is it that they do not emerge finally from the body as readily as 

 they entered it, and thus render it visible in all directions, not by 

 a superficial reflexion, but by a secondary emission ? To account 

 for the extinction of light, in the theory of emission, we must sup- 

 pose it united to the body which it enters ; and the simplest mode 

 in which we can conceive this union to be brought about is by the 

 direct impact of the molecules of light on those of bodies, whereby 

 they are brought within the sphere of those interior attractive 

 forces to which chemical combinations are referred. This appears 

 to have been the opinion of Newton. " Are not gross bodies and 

 light," says he, "convertible into one another, and may not 

 bodies receive much of their activity from the particles of light 

 which enter their composition ? For all fixed bodies being heated 

 emit light, so long as they continue sufficiently hot, and light 



* Mtmoire sur la Dispersion de la Zumiere.llie attention of the Mathematical 

 Section of the British Association was drawn to this theory by Professor Powell, at 

 the last meeting, chiefly in reference to a limitation which seemed to he required in 

 the physical hypothesis. See Report of Proceedings. 



t Optics, hook 2, part 3. 



