8cr. XX. ABSORPTION OF LIGHT. 169 



when of glass or platina. When the rays which form 

 the fringes arrive at the screen, they are of different 

 lengths, in consequence of the curved path they follow 

 after passing the edge of the object. The waves are 

 therefore in different phases or states of vibration, and 

 either conspire to form colored fringes or destroy one 

 another in the obscure intervals. The colored fringes 

 bordering the shadows of objects were first described by 

 Grirnaldi in 1665; but besides these he noticed that 

 there are others within the shadows of slender bodies 

 exposed to a small sunbeam, a phenomenon which has 

 already been mentioned to have afforded Dr. Young the 

 means of proving beyond all controversy, that colored 

 rings are produced by the interference of light. 



It may be concluded, that material substances derive 

 their colors from two different causes : some from the 

 law of interference, such as iridescent metals, peacocks' 

 feathers, &c.; 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 

 colored bodies. The latter phenomena have been con- 

 sidered extremely difficult to reconcile with the undula- 

 tory theory of light, and much discussion has arisen as 

 to what becomes of the absorbed rays. But that em- 

 barrassing question has been ably answered by Sir John 

 Herschel in a most profound paper, On the Absorption 

 of Light by colored 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 feet of the ob- 

 struction 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 ap- 

 peals to our ignorance, the latter to our knowledge, for 

 its explanation of the absorptive phenomena. In at- 

 tempting to explain the extinction of light on the corpus- 

 cular doctrine, we have to account for the light so extin- 

 guished 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 



