The speed of propagation,
which, as we have seen when discussing the measurement of the velocity
of sound, depends very little on the musical note, is not at all the
same in the case of the various radiations which can be propagated in
the same substance. The index of refraction varies with the duration
of the period, or, if you will, with the length of wave _in vacuo_
which is proportioned to this duration, since _in vacuo_ the speed of
propagation is entirely the same for all vibrations.
Cauchy was the first to propose a theory on which other attempts have
been modelled; for example, the very interesting and simple one of
Briot. This last-named supposed that the luminous vibration could not
perceptibly drag with it the molecular material of the medium across
which it is propagated, but that matter, nevertheless, reacts on the
ether with an intensity proportional to the elongation, in such a
manner as tends to bring it back to its position of equilibrium. With
this simple hypothesis we can fairly well interpret the phenomena of
the dispersion of light in the case of transparent substances; but far
from well, as M.
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