MR. POWER ON THE ABSORPTION OF THE SOLAR RAYS, ETC. 
13 
as a point, and X the length of the undulation. Then, if -us be the area of the pupil, 
properly directed towards the luminous poiut at the successive distances r and r', 
^2 and will be the quantities of vis viva which pass through the pupil in equal 
times, and being condensed on the retina produce the sensation of light with bright- 
nesses proportional to their magnitudes. This accords with the views of all writers 
on the subject of physical optics. On the other hand, we know by experience, that 
these brightnesses are as 4* and consequently p=p ; the ether intermediate to the 
two spheres in transmitting the luminous wave has therefore delivered over the via 
viva unchanged in quantity. 
5. Let us now consider what will become of the vis viva when the luminous wave 
is incident upon the plane surface of a refracting medium. I shall confine my atten- 
tion in the present communication to a singly refracting isotropical medium, amongst 
the comparatively grosser particles of which the incomparably more subtle and more 
numerous particles of the ether are supposed to be diffused in a different state of den- 
sity to that which prevails in the surrounding spaces, such altered density being due 
to the attractions or repulsions which the particles of the medium exercise on those 
of the ether. 
This being premised, we may regard the expenditure of the vis viva as of two kinds, 
according as it is distributed to the particles of the ether, giving rise to the reflected 
and refracted rays, or to the particles of the refracting medium. If it be expended 
solely on the ether, the sum of the vires vivce of the reflected and refracted waves 
ought to be exactly equal to the vis viva of the incident wave ; but if a portion of the 
vis viva be communicated to the particles of the medium, the vis viva of the incident 
wave ought to surpass the sum of the vires vivce of the reflected and refracted waves 
by a certain excess. 
6. The object with which the present inquiry commenced was to take into account 
the effect of such supposed excess, in the hope of arriving at some explanation of the 
Stokesian phenomena. The remarkable result I have obtained, that every loss of vis 
viva will be accompanied by a diminution of the refractive index , is quite in the direc- 
tion of the author’s own idea of “ a change of refrangibility but I confess it throws 
no light on the change of period, which it is also necessary to account for. The latter, 
1 am inclined to think, is due to an action of the nature of harmonic resonance , and from 
some calculations which I have made, I think it probable that the light produced in 
the Stokesian experiments may be due to resonant vibrations excited in the medium, 
which are about a major or minor third lower in pitch than those of the invisible 
rays producing them, the medium afterwards communicating those vibrations to the 
ether as a new source of light. 
7- Some apology maybe required for borrowing from the language of music, terms 
explanatory of phenomena which cannot be heard, and in some cases neither heard 
nor seen ; but critical taste must be prepared to yield a general licence to physical 
inquirers to indulge in such catachreses of language, whenever they are called for by 
