PROFESSOR STOKES ON THE CHANGE OF REFRANGI BILITY OF LIGHT. OoJi 
the above theory, because the amplitude of vibration decreases indefinitely in ap- 
proaching- the less refrangible limit. In the case of a solution of chlorophyll, we 
may suppose that the part of the molecular forces of restitution depending on first 
powers of the displacements is considerable, on which supposition, the effect ought 
to approach to what would take place were there no other part. But were the forces 
of restitution strictly proportional to the displacements, the vibrations would be 
isochronous, and could only be excited by ethereal vibrations having almost exactly 
the same period, but would be powerfully excited by such. Accordingly, in a solu- 
tion of chlorophyll the dispersion comes on very suddenly; a large part of it is pro- 
duced by active light of nearly the same refrangibility as the dispersed light; and 
the latter, by whatever active light produced, has nearly the same refrangibility that 
it had at first. This supposition, combined with the preceding theory, accounts also 
for the transparency of the fluid with respect to rays of less refrangibility than the 
first absorption band, for the great intensity of that band, for the rapidity with which 
opacity comes on at its less refrangible border, and the comparatively slow resump- 
tion of transparency on the other side. A difference of the same nature on opposite 
sides of a maximum of opacity seems to be a very common phenomenon in absorption. 
On the other hand, in those numerous cases in which the dispersion comes on gra- 
dually, in the manner described in Art. 44, we may suppose the part of the forces of 
restitution depending on first powers of the displacements to be but small. 
235. It may appear at first sight to be a formidable objection to the theory here 
brought forward, that in the experiment mentioned in Art. 216, the intensity of the 
dispersed light did not appear to be more than doubled when the intensity of the 
incident disturbance was doubled ; and that in the experiment described in Art. 215, 
the rays of low refrangibility did not appear to exercise any protecting influence. But 
the difficulty may, I think, be got over by a very reasonable supposition. It seems 
very natural to suppose that a given molecule remains for the greater part of the 
time at rest, or nearly so, and only now and then gets involved in vibrations. On 
this supposition, it is only a very small per-centage of the molecules that at a given 
instant are vibrating to an extent worth considering. Conceive now a stream of light 
consisting of the highly refrangible rays to be incident on a sensitive medium, and to 
cause 1 per cent, of the sensitive molecules to vibrate considerably, the rest 
vibrating so little that they may be regarded as at rest. Now imagine a second 
stream, similar in all respects to the first, to influence the medium which is already 
under the influence of the first stream. Of the 1 per cent, of the molecules already 
vibrating, many are vibrating, we may suppose, nearly with their maximum ampli- 
tude, and consequently are not much affected. Besides, it is a great chance if the 
epoch of the ethereal vibrations belonging to the second stream is such as to pro- 
duce any great tendency either towards quiescence or towards disturbance in a mole- 
cule just for the short time that it is vibrating strongly under the influence of the 
first stream. But of the 99 per cent, of quiescent molecules 1 per cent, are made to 
4 B 2 
