ACCORDING TO THE UNDULATORY THEORY. 479 



ference, although I could not at that time explain satisfactorily to myself 

 the manner in which it originated. 



I will now attempt to show that as soon as we assign to matter a 

 very simple property which is no way in opposition to the idea we other- 

 wise conceive of matter, all the phajnomena which we include in the 

 class of phcBnmnena of absorption become mere corollaries of the ge- 

 neral principle of interference. 



Sir John Herschel has lately shown, in a memoir in many respects in- 

 teresting and instructive, On the absorption of light by coloured media 

 vieived in connection with the undulatory theory *, the possibility of con- 

 sidering the phaenomena of absorption as originating in interference. 

 But he has thus traced each individual absorption back to a different 

 cause, by which he is obliged to suppose in the absorbing bodies as many 

 different causes as there are absorptions in the spectrum. If we could 

 conceive of about two thousand separately acting causes in one and the 

 same body, as would be the case for example in nitric acid gas, it would 

 still be difficult to form to ourselves a correct idea of the reason of the 

 great regularity which we must presuppose for the hundred causes in the 

 iodic or bromic gas. Moreover he is of opinion that we must relinquish 

 every notion of a regular functional gradation of this phasnomenon, 

 upon observation of its quantity and apparent irregularity. He states 

 further, that "if the phaenomena were at all reducible to analytical ex- 

 pression, this must be of a singular and complex nature, and must at all 

 events involve a great number of arbitrary constants dependent on the 

 relation of the medium to light, as well £is trascendents of a high and 

 intricate order." 



I will endeavour to prove, on the contrarj'-, that they may be all re- 

 duced to one, or at least to a very limited number of causes, and that 

 they may be all comprehended in one very simple analytical expression, 

 which contains very few constants, and those dependent on the nature 

 of the absorbing medium. The little knowledge we possess of the in- 

 ternal constitution of matter does not permit us to predict what effect it 

 exercises upon a traversing wave of light. If however we may imagine 

 it to be composed of particles which are kept by certain forces at a deter- 

 minate distance from one another, we may also imagine that these par- 

 ticles are capable of offering a resistance to the traversing wave of light, 

 and consequently of partially reflecting it. 



The light thus reflected, which proceeds in a direction contrary to the 

 one it originally had, must be now in like manner reflected in the ori- 

 ginal direction, in order to experience again a partial reflection in the 

 contrary one, and so on ad infinitum. Thus arises an endless series 

 of systems of waves of light, each of which possesses a feebler in- 

 tensity than the one which had immediately jDreceded it, and which has 

 • Phil. Mag. and Annals, vol. iii. p. 401. 



