32 
MR. POWER ON THE ABSORPTION OF THE SOLAR RAYS, ETC. 
ray, and if sin 6 exceed the critical value there will be no refracted ray, but the 
incident light will be totally reflected without any diminution of intensity, but with 
a change of phase (compare No. 39, and the Note to No. 42 at the end of this paper). 
Hence we see the possibility of particular rays, on which the medium exerts a 
powerful absorbing action at small angles of incidence, being totally reflected at 
larger incidences, whilst the remainder of the incident beam is partly refracted, the 
refracted and reflected light thus being of completely different colours. Specimens 
of coloured glass partaking of this property are not uncommon, and I have recently 
been shown a glass which is deep blue seen by reflected light, and reddish brown by 
refracted light, an effect which Professor Stokes, who showed me the specimen, 
assures me is not of the nature of fluorescence, the name he has finally chosen for the 
phenomena discovered by himself. 
At the other limit, for which 5=0, sin^ attains its minimum value - sin0; the 
rays, of which no portion has been absorbed, therefore emerge on the most refracted 
n, 
side of the spectrum; the same thing appears from the expression jo< y =yq^, which 
shows that the refractive index is then a maximum. 
32. Although it is possible in this manner to account for a considerable range of 
spectrum extending, with rapidly decreasing intensity, from the most refracted end, 
where the rays have suffered least absorption and where the intensity is the greatest, 
towards the least refracted end, where the intensity decreases without limit, never- 
theless there is nothing in this theory which necessarily connects the degree of 
refraction with the colour, or more generally speaking, the period of the ray. 
Experience shows that the most refracted rays have the smallest period ; if then we 
would account for the chromatic dispersion in this way, we must admit that media 
are more acted upon by the rays of longer period, than by rays of shorter period, by 
the red than by the violet ; but this is contrary to experience. The effect of absorp- 
tion upon the index of refraction must therefore be regarded as antagonistic to the 
chromatic dispersion. 
33. M. Cauchy has, I consider, given a satisfactory theory of chromatic dispersion, 
which is perfectly consistent with every thing which has been advanced in the pre- 
sent theory. But I claim for the latter that it gives a satisfactory account of the 
phenomena of absorption and the spectral spaces discovered by Fraunhofer, and com- 
monly known by the name of Fraunhofer’s lines, especially when taken in connexion 
with the consideration of luminous resonance, to which subject, I think, the attention 
of scientific men is here directed for the first time. 
In fact, according to the laws of resonance, those rays will act most forcibly on 
the medium, which find amongst the particles of the latter some capable of vibrating 
in unison, or in harmonic consonance less perfect than unison, with themselves, the 
unison of course giving rise to by far the most energetic action, but the other conso- 
nances producing effects more decided as the coincidences of phase are more frequent ; 
