196 DIFFRACTION OF LIGHT. [SECT. xx. 



colours from two different causes : some from the law of inter- 

 ference, such as iridescent metals, peacocks' feathers, <fec. ; 

 others from the unequal absorption of the rays of white light, 

 such as vermilion, ultramarine, blue, or green cloth, flowers, 

 and the greater number of coloured bodies. The latter phe- 

 nomena have been considered extremely difficult to reconcile 

 with the undulatory theory of light, and much discussion has 

 arisen as to what becomes of the absorbed rays. But that 

 embarrassing question has been ably answered by Sir John 

 Herschel in a most profound paper, On the Absorption of 

 Light by coloured Media, and cannot be better given than 

 in his own words. It must, however, be premised, that, as all 

 transparent bodies are traversed by light, they are presumed 

 to be permeable to the ether. He says, "Now, as regards 

 only the general fact of the obstruction and ultimate extinction 

 of light in its passage through gross media, if we compare the 

 corpuscular and undulatory theories, we shall find that the 

 former appeals to our ignorance, the latter to our knowledge, 

 for its explanation of the absorptive phenomena. In attempt- 

 ing to explain the extinction of light on 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 comparatively stagnant. 

 The heating power of the solar rays gives a prima facie plau- 

 sibility to the idea of the 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 luminous rays are not 

 the most calorific ; but that, on the contrary, the calorific energy 

 accompanies, in its greatest intensity, rays which possess com- 

 paratively feeble illuminating powers? These and other 

 questions of a similar nature may perhaps admit of answer in 

 a more advanced state of our knowledge ; but at present there 

 is none obvious. It is not without reason, therefore, that the 



