Just as the stars about us are all in motion, so the starry systems
beyond our sight may be in motion, and our system as a whole may be
moving in concert with them. If this be so, then after interminable
ages the aspect of the entire system of systems must change, its
various members assuming new positions with respect to one another. In
the course of time we may even suppose that our universe will approach
relatively close to one of the others; and then, if men are yet living
on the earth, they may glimpse through the openings which reveal
nothing to us now, the lights of another nearing star system, like the
signals of a strange squadron, bringing them the assurance (which can
be but an inference at present) that the ocean of space has other
argosies venturing on its limitless expanse.
There remains the question of the luminiferous ether by whose agency
the waves of light are borne through space. The ether is as mysterious
as gravitation. With regard to ether we only infer its existence from
the effects which we ascribe to it. Evidently the ether must extend as
far as the most distant visible stars. But does it continue on
indefinitely in outer space? If it does, then the invisibility of the
other systems must be due to their distance diminishing the quantity
of light that comes from them below the limit of perceptibility, or to
the interposition of absorbing media; if it does not, then the reason
why we cannot see them is owing to the absence of a means of
conveyance for the light waves, as the lack of an interplanetary
atmosphere prevents us from hearing the thunder of sun-spots.
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