The difficulty is that these stars appear
to us so faint individually, that the investigation of their
spectra is still beyond the powers of our instruments. But the
extraordinary feat performed at the Lick Observatory of measuring
the radial motion of 1830 Groombridge, a star quite invisible to
the naked eye, and showing that it is approaching our system with
a speed of between fifty and sixty miles a second, may lead us to
hope for a speedy solution of this question. But we need not await
this result in order to reach very probable conclusions. The
general outcome of researches on proper motions tends to
strengthen the conclusions that the Keplerian sphere, if I may use
this expression, has no very well marked existence. The laws of
stellar velocity and the statistics of proper motions, while
giving some color to the view that the space in which we are
situated is thinner in stars than elsewhere, yet show that, as a
general rule, there are no great agglomerations of stars elsewhere
than in the region of the Milky Way.
With unity there is always diversity; in fact, the unity of the
universe on which I have been insisting consists in part of
diversity. It is very curious that, among the many thousands of
stars which have been spectroscopically examined, no two are known
to have absolutely the same physical constitution.
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