REFLEXION AND REFRACTION OF LIGHT. 45 



coloured media, in the theory of emission, it seems necessary to 

 suppose that an attractive force is exerted at a distance between 

 the molecules of the tody and those of light, and that the absolute 

 value of this force varies with the colour. It does not seem easy 

 to reconcile these suppositions to the Newtonian account of refrac- 

 tion ; and the difficulty is still further increased when we proceed 

 to apply the same considerations to the absorption of definite 

 rays, and introduce the hypothesis of specific actions, varying in 

 the most abrupt and irregular manner with the refrangibility of 

 the ray.* 



The absorption of light, and the opacity of bodies, were long 

 since urged by Halley as difficulties in the wave-theory. The ether 

 is supposed to penetrate all bodies freely, and why not also the 

 undulatory motion in which light consists ? To this difficulty we 

 find a full and complete solution in the principle of interference. 

 When a wave enters a discontinuous substance, it will be broken 

 up, and its parts undergo continued 'subdivision by internal re- 

 flexions ; so that when these parts reach the second surface of the 

 body, they are found in every possible phase, and must destroy one 

 another by interference. The phenomenon, as has been observed 

 by Sir John Herschel, is analogous to the impeded propagation of 

 sound in a mixture of gases differing much in elasticity as com- 

 pared with their density. 



The same writer has given an ingenious and natural account 

 of the absorption of specific rays on the principles of the wave- 

 theory, in a paper read before the Association last year, f He con- 

 siders the molecules of the body and those of the ether as forming, 

 conjointly, compound vibrating systems, which are more disposed 

 to transmit vibrations of some determinate period than others. 

 Other vibrations, however, not in unison with these systems, may 

 be propagated through them. These forced vibrations, as he calls 

 them, will be obstructed in their progress, and their amplitudes 

 diminished by the mutual influence of the motions of the parts 

 of the systems ; and he shows that it is possible to conceive sys- 

 tems, which will be wholly impervious to a vibration of a par- 

 ticular period, while they freely transmit others not differing from 



* See Sir David Brewster's Report on Optics. 



t "On the Absorption of Light by coloured Media, viewed in connexion with tho 

 Undulatory Theory." Phil. Mag., Third Series, vol. iii. 



