SECT. XX. ABSORPTION OF LIGHT. 175 



slender beam of light. If the edge of a knife or hair, for example, 

 be held in it, the rays, instead of proceeding in straight lines 

 past its edge, are bent when quite close to it, and proceed from 

 thence to the screen in curved lines called hyperbolas ; so that 

 the shadow of the object is enlarged, and, instead of being at 

 once bounded by light, is surrounded or edged with coloured 

 fringes alternating with black bands, which are more distinct 

 the smaller the pin-hole (N. 202). The fringes are altogether 

 independent of the form or density of the object, being the same 

 when it is round or pointed, when of glass or platinum. 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 coloured fringes or destroy one another in the obscure in- 

 tervals. The coloured fringes bordering the shadows of objects 

 were first described by Grimaldi 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 coloured rings are pro- 

 duced by the interference of light. 



It may be concluded that material substances derive their 

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

 ference, 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 coloured bodies. The latter phenomena have 

 been considered extremely difficult to reconcile with the undu- 

 latory 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 can- 

 not 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 



