28 
MR. POWER ON THE ABSORPTION OF THE SOLAR RAYS, ETC. 
consideration, this ratio of distribution should be independent of orientation, and 
consequently the same for both the primary and secondary refracted waves. 
2/. I am not disposed however to leap too suddenly to the conclusion that the 
reflected ray in the action of turning- through the angle w — 26 has no influence in im- 
parting vis viva to the medium*; though it must be conceded that in all cases where 
the brightness of the reflected rays accurately follows the laws of Fresnel as tested 
by Brewster and Arago, the reflected ray has no sensible influence on the medium ; 
for were it otherwise, we ought in an accurate theory of resonance to find a differ- 
ence in the values of s and s', as we do in the intensities of the primary and secondary 
waves, both reflected and refracted, and we have already seen that such difference of 
values will vitiate the law for the secondary reflected ray. It makes all the differ- 
ence in the world, whether the vis viva be supposed to be communicated to the 
medium at the very instant of the shock, or immediately afterwards ; in the first case 
it will be due partly to the reflected and partly to the refracted rays, in the latter 
case it will be due almost entirely to the refracted rays. In all cases, however, it 
is natural to suppose that the refracted rays are chiefly instrumental, and this is 
indicated by the equality of s and s' in isotropical media, without which equality the 
laws of Fresnel, Brewster and Arago, could not, according to the present theory, 
be accurately, though they might very well be approximately true, as in fact they 
would be if s s', though different from each other, were very small compared with 
unity. In clear transparent media, where there is little absorption, s and s' are pro- 
bably very small, and such being the case, the above law of brightness ought to hold, 
independent of the equality s=s'. 
28. I return now to the expressions for the refractive index 
oa 
a=— 
r ?, a i 
1 ga ju. 
^ 1 1 + s gpj 1 + s 
In the first place, it will be remarked that these differ from the value which is 
usually adopted, namely, -• 
a i 
I confess I always considered that the usual mode of deducing this value from the 
spread of the wave, which in fact does not spread, was more elegant than conclusive. 
It is connected, if I mistake not, with the idea that the transverse front of a wave of 
light, as of a wave of sound, consists of particles all of which are in the same phase ; 
* Nevertheless it is worth observing, that at the critical angle (compare No. 39), in the case of s=s', it 
results from the formulae that the absorbing power of the medium has no effect in diminishing the vis viva 
when the ray is turned through an angle tf— 20 in the operation of reflexion, and this is true both for the 
primary and secondary rays. I am therefore inclined to think, and other considerations confirm me in that 
opinion, that the absorbing medium acts something like a file in thinning off the absorbed portions of the ray, and 
requires that the ray should penetrate into its substance before it can exercise any absorbing action upon it. 
