_If equidistant Undulations be supposed to pass through a Medium,
of which the Parts are susceptible of permanent Vibrations somewhat
slower than the Undulations, their Velocity will be somewhat lessened
by this vibratory Tendency; and, in the same Medium, the more, as the
Undulations are more frequent._
For, as often as the state of the undulation requires a change in the
actual motion of the particle which transmits it, that change will
be retarded by the propensity of the particle to continue its motion
somewhat longer: and this retardation will be more frequent, and more
considerable, as the difference between the periods of the undulation
and of the natural vibration is greater.
_Corollary._ It was long an established opinion, that heat consists
in vibrations of the particles of bodies, and is capable of being
transmitted by undulations through an apparent vacuum. (NEWT. Opt. Qu.
18.) This opinion has been of late very much abandoned. Count RUMFORD,
Professor PICTET, and Mr. DAVY, are almost the only authors who have
appeared to favour it; but it seems to have been rejected without any
good grounds, and will probably very soon recover its popularity.
Let us suppose that these vibrations are less frequent than those of
light; all bodies therefore are liable to permanent vibrations slower
than those of light; and indeed almost all are liable to luminous
vibrations, either when in a state of ignition, or in the circumstances
of solar phosphori; but much less easily, and in a much less degree,
than to the vibrations of heat. It will follow from these suppositions,
that the more frequent luminous undulations will be more retarded than
the less frequent; and consequently, that blue light will be more
refrangible than red, and radiant heat least of all; a consequence
which coincides exactly with the highly interesting experiments of
Dr. HERSCHEL. (Phil. Trans. for 1800. p. 284.) It may also be easily
conceived, that the actual existence of a state of slower vibration
may tend still more to retard the more frequent undulations, and that
the refractive power of solid bodies may be sensibly increased by an
increase of temperature, as it actually appears to have been in EULER’s
experiments. (Acad. de Berlin. 1762. p. 328.)
_Scholium_. If, notwithstanding, this proposition should appear to be
insufficiently demonstrated, it must be allowed to be at least equally
explanatory of the phenomena with any thing that can be advanced on
the other side, from the doctrine of projectiles; since a supposed
accelerating force must act in some other proportion than that of the
bulk of the particles; and, if we call this an elective attraction,
it is only veiling under a chemical term, our incapacity of assigning
a mechanical cause. Mr. SHORT, when he found by observation the
equality of the velocity of light of all colours, felt the objection
so forcibly, that he immediately drew an inference from it in favour
of the undulatory system. It is assumed in the proposition, that when
light is dispersed by refraction, the corpuscles of the refracting
substance are in a state of actual alternate motion, and contribute to
its transmission; but it must be confessed, that we cannot at present
form a very decided and accurate conception of the forces concerned in
maintaining these corpuscular vibrations.