SECT, xx.] ABSORPTION OF LIGHT. 197 



question, 'What becomes of light?' which appears to have 

 been agitated among the photologists of the last century, has 

 been regarded as one of considerable importance as well as 

 obscurity by the corpuscular philosophers. On the other 

 hand, the answer to this question, afforded by the undulatory 

 theory of light, is simple and distinct. The question, ' What 

 becomes of light,' merges in the more general one, 'What 

 becomes of motion?' And the answer, on dynamical prin- 

 ciples, is, that it continues for ever. No motion is, strictly 

 speaking, annihilated; but it may be divided, and the 

 divided parts made to oppose and, in effect, destroy one 

 another. A body struck, however perfectly elastic, vibrates 

 for a time, and then appears to sink into its original repose. 

 But this apparent rest (even abstracting from the inquiry 

 that part of the motion which may be conveyed away by the 

 ambient air) is nothing else than a state of subdivided and 

 mutually destroying motion, in which every molecule con- 

 tinues to be agitated by an indefinite multitude of internally 

 reflected waves, propagated through it in every possible 

 direction, from every point in its surface on whiclji they suc- 

 cessively impinge. The superposition of such waves will, it 

 is easily seen, at length operate their mutual destruction, 

 which will be the more complete the more irregular the 

 figure of the body, and the greater the number of internal 

 reflections." Thus Sir John Herschel, by referring the 

 absorption of light to the subdivision and mutual destruction 

 of the vibrations of ether in the interior of bodies, brings 

 another class of phenomena under the laws of the undulatory 

 theory. 



The ethereal medium pervading space is supposed to 

 penetrate all material substances, occupying the interstices 

 between their molecules; but in the interior of refracting 

 media it exists in a state of less elasticity compared with its 

 density in vacuo; and, the more refractive the medium, the 

 less the elasticity of the ether within it. Hence the waves 

 of light are transmitted with less velocity in such media as 



