REPORT ON PHYSICAL OPTICS. 317 



the constancy of the velocity of wave- propagation is regarded 

 but as the approximate result of an incomplete analysis. The 

 problem presented itself to M. Cauchy in a similar point of 

 view. In the profound researches of this matheinatician relating 

 to light, the ether is considered as a system of particles solicited 

 by mutual attractions or repulsions ; and from the partial diffe- 

 rential equations which represent their movement, he had de- 

 duced the laws of propagation in crystallized as well as homo- 

 geneous 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 coordinates. Re- 

 suming the problem of the propagation of a plane wave, with 

 the aid of the more general equations, he has finally demon- 

 strated the existence of a relation between the velocity of pro- 

 pagation 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 undei-go within themf . 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 di- 

 rections, — not by a superficial reflexion, but by a secondary 

 emission ? To account for the extinction of light, in the theory 

 of emission, we must suppose 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 mole- 

 cules 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, " con- 

 vertible into one another, and may not bodies receive much of 

 their activity from the particles of light which enter their com- 

 position ? For all fixed bodies being heated emit light, so long 

 as they continue sufficiently hot, and light mutually stops in 

 bodies as often as its rays strike upon their partsj ." 



* Memoire siir la Dispersion de la Lumiere. — The attention of the Mathe- 

 matical 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 be required in the physical hypothesis. — See Report of Prnceedinf/!i. 



t Oplic.1, book 2, part 3. I Optics, Query 30. 



