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CHEMICAL RAYS. 241

thing as absolute coldness in our corner of N ature—the
atoms are always in a state of vibration, their vibrations
being executed to and fro across their positions of equilib-
mum.

Into a vapor thus constituted, we have now to pour a
beam of light; which most of you know to be a train of
minute waves, excited in, and propagated through, an al-
most infinitely attenuated and elastic medium, which fills
all Space, and which we name the ether. It is hardly neces-
sary to remind you that these waves of light are not all of
the same size; that. some of them are much longer and
higher than others ; that the short waves and the long ones
move with the same rapidity through space,'just as short
and long waves of sound travel with the same rapidity
through air, and that, therefore, the shorter waves must fol-
low each other in quicker succession than the longer ones;
that the different rapiditieswith which the waves of light
impinge upon the retina, or optic nerve, give rise in con-
sciousness to differences of color ,° that there are, moreover,
numberless waves emitted by the sun and other luminous
bodies which reach the retina, but which are incompetent
to excite the sensation of light; for, if the lengths of the-
waves exceed a certain limit, or if they fall short of a cer-
tain other limit, they cannot generate vision. And it is to
be particularly borne in mind, that the capacity to excite
vision does not depend so much on the strength of the
waves as on their periods of recurrence. I have, as many
of you know, permitted waves to enter my own eye, which,
if their energy were that of light, would have instantly and
utterly ruined the optic nerve, but which failed to produce
any impression whatever upon consciousness, because their
periods were not those competent to excite the retina.

The elements of all the conceptions with which we shall
have subsequently to deal are now in your possession. ,And
you will observe that, thOugh we are speaking of things

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