14 
MR. POWER ON THE ABSORPTION OF THE SOLAR RAYS, ETC. 
the generalisation of ideas, for the expression of which, without such an alternative, 
a new language must necessarily be invented. 
8. The mode of procedure which seemed most likely to lead to a successful result, 
was to assume in the first instance the hypothesis that the vis viva is expended solely 
on the reflected and refracted rays, and afterwards to modify, if possible, the steps of 
the process so as to adapt them to the hypothesis that a portion of it is expended on 
the medium, regarded as distinct from the ether by which it is permeated. 
9. In adopting the more simple hypothesis I was really startled by the formula at 
which I arrived in the course of the investigation, for not only did the general or 
Cartesian law of refraction spring out most unexpectedly, as if by magic, but those 
very same expressions for the intensity of the reflected rays, which were first disco- 
vered by Fresnel, and subsequently verified by the experiments of Brewster and 
Arago, were an immediate consequence of the formulae. 
But while my results are in perfect harmony with experience so far as the latter 
has proceeded, at the same time they differ from those of Fresnel in some particulars. 
In the first place the index of refraction is not the simple quotient of the velocities 
of undulation, but of those velocities each multiplied by the density of the ether in 
the corresponding medium. In the second place, the vibrations of the ethereal par- 
ticles are performed in the plane of polarization (and not perpendicular to that plane, 
as Fresnel supposed), agreeing therein, amongst others, with Maccullagh, Naumann, 
and the earlier researches of Cauchy, but opposed to the more recent investigations 
of the latter and to the experimental determination of Professor Stokes*. Further, 
the expressions for the intensities of the refracted rays differ slightly in other respects 
from those of Fresnel, as given in Airy’s Tracts; I am not aware that these inten- 
sities have been tested by experiment, nor are the refracted rays so readily accessible 
to the experimenter as the reflected rays. I may be permitted however to claim, in 
favour of my own results, that in no one instance do I have recourse to forced analo- 
gies or gratuitous hypotheses, the process I have pursued standing in need of no such 
help. I adopt indeed universally the fundamental hypothesis that the vibrations on 
which light depends, and consequently those of the reflected and refracted as well as 
of the incident rays, are strictly transverse to the directions of the rays. I admit that 
this hypothesis, considered a priori, must be regarded as perfectly arbitrary ; but it 
gains evidence, almost amounting to certainty, a posteriori, when we take into 
account the immense variety of phenomena connected with the polarization and de- 
polarization of light, of which it affords a simple and satisfactory explanation. I am 
aware of the difficulties which have caused other theorists to modify this hypothesis 
in case of the reflected and refracted rays ; but I do not think that those difficulties 
should be objected to me, who approach the problem in an entirely different way, 
and who take into account circumstances which have been neglected by them, 
namely, the vibrations communicated to the medium itself. It is not surprising that 
such difficulties should occur in a dynamical theory which takes no account of such 
* Cambridge Transactions, vol. ix. part 1. 
