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 phenomena which we include in the 
class of phenomena 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 
viewed in connection with the undulatory theory *, the possibility of con- 
sidering the phenomena of absorption as originating in interference. 
But he has thus traced each individual absorption back to a different 
eause, 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 inthe 
iodie or bromic gas. Moreover he is of opinion that we must relinquish 
every notion of a regular functional gradation of this phenomenon, 
upon observation of its quantity and apparent irregularity. He states 
further, that “if the phenomena 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 as trascendents of a high and 
intricate order.” 
I will endeavour to prove, on the contrary, 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 te 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 preceded it, and which has 
* Phil. Mag. and Annals, vol. iii. p. 401. 
