frightful inaccuracy of the scientific people's terms, which
is the consequence of their always trying to write mixed Latin and
English, so losing the grace of the one and the sense of the other.
And, in this point of the diffraction of light I am stopped dead by
their confusion of idea also, in using the words undulation and
vibration as synonyms. "When," says Professor Tyndall, "you are
told that the atoms of the sun _vibrate_ at different rates, and
produce _waves_ of different sizes,--your experience of water-waves
will enable you to form a tolerably clear notion of what is meant."

'Tolerably clear'!--your toleration must be considerable, then. Do
you suppose a water-wave is like a harp-string? Vibration is the
movement of a body in a state of tension,--undulation, that of a
body absolutely lax. In vibration, not an atom of the body changes
its place in relation to another,--in undulation, not an atom of
the body remains in the same place with regard to another. In
vibration, every particle of the body ignores gravitation, or
defies it,--in undulation, every particle of the body is slavishly
submitted to it. In undulation, not one wave is like another; in
vibration, every pulse is alike. And of undulation itself, there
are all manner of visible conditions, which are not true
conditions. A flag ripples in the wind, but it does not undulate as
the sea does,--for in the sea, the water is taken from the trough
to put on to the ridge, but in the flag, though the motion is
progressive, the bits of bunting keep their place. You see a field
of corn undulating as if it was water,--it is different from the
flag, for the ears of corn bow out of their places and return to
them,--and yet, it is no more like the undulation of the sea, than
the shaking of an aspen leaf in a storm, or the lowering of the
lances in a battle.

And the best of the jest is, that after mixing up these two notions
in their heads inextricably, the scientific people apply both when
neither will fit; and when all undulation known to us presumes
weight, and all vibration, impact,--the undulating theory of light
is proposed to you concerning a medium which you can neither weigh
nor touch!

All _communicable_ vibration--of course I mean--and in dead matter:
_You_ may fall a shivering on your own account, if you like, but
you can't get a billiard-ball to fall a shivering on _its_ own
account.[18]

Yet observe that in thus signalizing the inaccuracy of t