552 FROFESSOH STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
during which the action is favourable to disturbance is prolonged ; but when the 
action is favourable to quiescence, the effect is just the reverse. Hence, on the 
whole, there is a balance outstanding in favour of disturbance. But if the ether be 
vibrating more slowly than the molecule, it appears from similar reasoning that there 
will be a balance the other way. Hence it is only when the periodic time of the ethereal 
vibrations is less than that of the molecular, that the latter vibrations can be kept 
going by the former. 
232. But it will probably be objected to this explanation, that when a periodic 
disturbing force affects the mean motion of a planet, the mean motion is a maximum, 
not when the force tending to augment it is a maximum, but at a time later by a 
quarter of the period of the force, namely, when the force vanishes in changing sign ; 
and that in a similar manner the change in the periodic time of the vibrations of a 
disturbed molecule will affect equally the duration of the time during which the 
action is favourable to increased disturbance, and that during which it is favourable 
to quiescence, or more exactly will not alter either, since the effects in the first and 
second halves of those times will neutralize each other. The answer to this objection 
is, that we must not treat a molecule as if it were isolated, like a heavenly body, 
since it is continually losing its motion by communication, perhaps to neighbouring- 
molecules, but at any rate to the luminiferous ether ; for without a communication of 
the latter kind there would be no dispersed light. Hence we must consider the imme- 
diate tendency of the disturbing forces rather than their tendency in the long run. 
233. When a molecule itself vibrates in an irregularly periodical manner, the vibra- 
tions which it imparts to the ether are of course of a similar character. The resolu- 
tion of these into vibrations corresponding to different degrees of refrangibility, 
involves some very delicate mathematical considerations, into which I do not propose 
to enter. But without this it is evident that when the ether is agitated by the vibra- 
tions of an immense number of molecules, in all possible states as regards amplitude, 
and consequently periodic time of vibration, the disturbance of the ether must con- 
sist of a mixture of periodic vibrations, having their periods comprised between the 
greatest and least of those belonging to the molecular vibrations ; and corresponding 
to these different periods there will be portions of light of different degrees of refran- 
gibility found in the dispersed beam. These refrangibilities will range between two 
limits, an inferior limit equal to the refrangibility corresponding to the periodic time 
of indefinitely small vibrations, and a superior limit equal to the refrangibility of the 
active light. 
234. This theory seems to accord very well with the general character of dispersed 
beams, as regards the prismatic composition of the light of which they consist. 
When analysed by a prism, these beams are sometimes found to break off abruptly 
at their more refrangible border, but I do not recollect ever to have met with an 
instance in which a beam broke off abruptly at the opposite border, except when the 
whole beam was almost homogeneous. This is just as it ought to be according to 
