nalogy
to form a fair estimate of its force. You believe that in society you
are surrounded by reasonable beings like yourself. You are, perhaps,
as firmly convinced of this as of anything. What is your warrant for
this conviction? Simply and solely this: your fellow-creatures behave
as if they were reasonable; the hypothesis, for it is nothing more,
accounts for the facts. To take an eminent example: you believe that
our President is a reasonable being. Why? There is no known method
of superposition by which any one of us can apply himself
intellectually to any other, so as to demonstrate coincidence as
regards the possession of reason. If, therefore, you hold our
President to be reasonable, it is because he behaves _as if_ he were
reasonable. As in the case of the aether, beyond the _'as if'_ you
cannot go. Nay, I should not wonder if a close comparison of the data
on which both inferences rest, caused many respectable persons to
conclude that the aether had the best of it.

This universal medium, this light-aether as it is called, is the
vehicle, not the origin, of wave-motion. It receives and transmits,
but it does not create. Whence does it derive the motions it conveys?
For the most part from luminous bodies. By the motion of a luminous
body I do not mean its sensible motion, such as the flicker of a
candle, or the shooting out of red prominences from the limb of the
sun. I mean an intestine motion of the atoms or molecules of the
luminous body. But here a certain reserve is necessary. Many
chemists of the present day refuse to speak of atoms and molecules as
real things. Their caution leads them to stop short of the clear,
sharp, mechanically intelligible atomic theory enunciated by Dalton,
or any form of that theory, and to make the doctrine of 'multiple
proportions' their intellectual bourne. I respect the caution, though
I think it is here misplaced. The chemists who recoil from these
notions of atoms and molecules accept, without hesitation, the
Undulatory Theory of Light. Like you and me they one and all believe
in an aether and its light-producing waves. Let us consider what this
belief involves. Bring your imaginations once more into play, and
figure a series of sound-waves passing through air. Follow them up to
their origin, and what do you there find? A definite, tangible,
vibrating body. It may be the vocal chords of a human being, it may
be an organ-pipe, or it may be a stretc