of Light by Coloured Media, 403 



the corpuscular doctrine, we have to account for the light so 

 extinguished as a material body, which we must not suppose 

 annihilated. It may, however, be transformed ; and among 

 the imponderable agents, heat, electricity, &c, it may be that 

 we are to search for the light which has become thus compa- 

 ratively stagnant. The heating power of the solar rays gives 

 a prima facie plausibility to the idea of a transformation of 

 light into heat by absorption. But when we come to examine 

 the matter more nearly, we find it encumbered on all sides 

 with difficulties. How is it, for instance, that the most lumi- 

 nous rays are not the most calorific, but that, on the contrary, 

 the calorific energy accompanies, in its greatest intensity, rays 

 which possess comparatively feeble illuminating powers? These 

 and other questions of similar nature may perhaps admit of 

 answer in a more advanced stage of our knowledge ; but at 

 present there is none obvious. It is not without reason, there- 

 fore, that the question " What becomes of light?" which ap- 

 pears 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 W What becomes of light?" merges in the more ge- 

 neral one, " What becomes of motion?" And the answer, 

 on dynamical principles, 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 

 each other. 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 continues 

 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 which they successively impinge. 

 The superposition of such waves will, it is easily seen, at length 

 operate their mutual destruction, which will be the more com- 

 plete, the more irregular the figure of the body and the greater 

 the number of internal reflections. 



In the case of a body perfectly elastic and of a perfectly 

 regular figure, the internal reflection of a wave once propa- 

 gated within it in some particular direction might go on for 

 ever without producing mutual destruction ; and in sonorous 

 bodies of a highly elastic nature we do in fact perceive it to 

 continue for a very long time. But the least deviation from 



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