CHAPTEE IV. 



ABSORPTION AND EMISSION. 



(67) ALL bodies absorb a portion of the light which enters 

 their substance ; and this absorption is different for rays of 

 different refrangibilities, so that white light always becomes 

 coloured after passage through a medium of sufficient thick- 

 ness. There is, accordingly, no body perfectly transparent. 

 On the other hand, no known body is perfectly opaque. Gold 

 itself, which is the densest of the metals, transmits a faint 

 greenish light, when in the state of gold leaf. 



The phenomena of absorption are all explained on the 

 supposition that light, which has traversed a given thickness 

 of any medium, loses by absorption the same fractional part 

 of its intensity, in passing through any given thickness of 

 the medium. According to this hypothesis, the intensity of 

 the light must diminish in geometrical progression, as the 

 thickness increases in arithmetical. For if i denote the in- 

 tensity of the light incident on the medium, and ia the 

 intensity after it has traversed the unit of thickness, the 

 intensity, after passing through two, three, &c., units, will be 

 * a 2 , i a 3 , &o. And generally, the intensity of the light, I, 

 after traversing the thickness 0, will be 



The quantity a is the coefficient of transmission ; and it is 

 different for the light of different colours. Hence, when the 

 incident light is white, the elements of which it consists will 

 be absorbed in different proportions, and the emergent light 





