TfiANSACTlONS OF THE SECTIONS. 373 



law connecting the planes of polarization was the same as that 

 already found in the case of external conical refraction*. 



On the Absorption of Light by coloured Media, viewed in con- 

 nexion with the undulatory Theory. By Sir John F. \V. 

 Herschel, F.R.S, 



The absorption of light by coloured media having been of 

 late regarded as offering peculiar difficulties when attempted 

 to be reconciled with the undulatory theory, the object of this 

 paper is to point out considerations which appear capable of 

 reconciling most of the phajnomena with that theory, at least 

 in so far as to show that they involve nothing contrary to sound 

 dynamical principles and obvious analogies when regarded as 

 particular cases of the general doctrine of undulatory motion. 



The extmction of light in general is first considered. In the 

 corpuscular hypothesis, hght,to be extinguished, must be anni- 

 hilated, or transformed either into caloric or some other form 

 of matter. On the undulatory hypothesis, the extinction of 

 light resolves itself into the more general and highly interest- 

 ing dynamical question, " What becomes of Motion?" This 

 question is therefore considered, and by tracing the progress 

 of an undulation through a body imperfectly elastic or a system 

 of bodies placed in communication, it is shown that a continual 

 subdivision of the original undulation, and a perpetual internal 

 reflection of the subdivided portions, will speedily have the 

 effect of agitating every molecule of the system at one and the 

 same instant, with vibratory motions, in every possible phase 

 and direction. The dynamical state of a molecule so agitated 

 is identical with a state of perfect rest, so that the general fact 

 of the extinction or absorption of light within a medium imper- 

 fectly transparent is so far from being repugnant to dynamical 

 principles, that it is one of their immediate and most natural 

 consequences ; imperfect transparency, in this view of the sub- 

 ject, consisting in the juxtaposition of parts unequally elastic, 

 or of portions of the etherial medium unequally loaded, or re- 

 strained from their natural aptitude to motion by the gross par- 

 ticles of material bodies with which they are connected. 



This connexion of the etherial particles with the grosser 

 molecules which constitute our solids, liquids, and gases, may 

 be considered as giving rise to compound vibrating systems, 

 each of which, it is easy to conceive, may have a greater apti- 



• A full account of these experiments is given in a memoir read before tlie 

 Royal Irish Academy, on the 28th January 1833, and ordered to be pub- 

 lished in the 17th volume of its Transactions. 



