328 M. A. J. Angstrom's Optical Researches. 



consequence of absorption, make no impression on our organs 

 of sight, but generally fall in the domain of feeling. 



If therefore the colours of bodies are to be explained by reso- 

 nance, this property must also be ascribed to the aether, and thus 

 we arrive with ease at the third of the principles referred to, 

 namely to that of interference. 



2. It is necessary to distinguish strictly the absorption of 

 light from its diffusion. Botb, indeed, produce the common 

 result of weakening the passing rays ; but they differ from each 

 other essentially in the circumstance, that the diffused light pos- 

 sesses the same properties as the incident light ; the absorbed 

 portion, on the contrary, exhibits itself as heat, or as a chemical 

 agent ; in other words, in the former case the time of oscillation 

 is unchanged, but in the latter this is not the case. This is an 

 essential distinction, which has not always been borne in mind 

 in optical investigations, although Melloni proved it long ago in 

 the case of heat. That the diffused light possesses the same 

 properties as the incident light is most evident from the fact 

 that the lines of Fraunhofer, in the solar spectrum, are also ex- 

 hibited by the light of the planets, and by all bodies illuminated 

 by the sun. This is not disproved by the newest investigations 

 of Stokes, in which he seeks to show, that, in the case of absorbed 

 light, the time of vibration is altered, because the facts there 

 brought forward belong to the latter class. 



Diffused light must therefore, according to the foregoing, be 

 produced by modifications of the undulatory motion of thesether 

 itself, while absorbed light is transferred to the molecules of the 

 body. These are thereby moved from their positions of equili- 

 brium and thrown into vibrations, which vary with the peculiar 

 characteristics of the body. It is, however, to be well borne in 

 mind, that a medium absorbs, not only those vibrations which it 

 most readily assumes, but also those which occupy a simple rela- 

 tion to its own time of vibration, such as the octave, third, &c. 

 As bodies in general are not luminous, it follows that the ab- 

 sorbed light must escape the cognizance of the eye. 



That this, however, is not always the case, is proved by Brew- 

 ster's remarkable discovery in the case of the alcoholic solution of 

 the green colouring matter of leaves. When a beam of light is 

 transmitted through such a medium, the liquid in the path of 

 the beam is coloured blood-red by unpolarized light. This phas- 

 nomenon, to which Brewster has applied the name of inner 

 dispersion, furnishes a manifest proof that the absorbed light does 

 not always pass to oscillating series of a lower order. The so- 

 called epipolarized rays of Hersehel belong to the ordinary case, 

 in which the absorbed light reappears as heat, to the extent 

 that here it is chemical rays which are absorbed, and which, 



