What has been said of the motions of the stars applies also, in general,
to the determination of their distances. A vast amount of labor has been
expended on this problem. When at length the distance of a single star
was finally determined, the quantity to be measured was so small as to
be nearly concealed by the unavoidable errors of measurement. The
parallax, or one half of the change in the apparent position of the
stars as the earth moves around the sun, has its largest value for the
nearest stars. No case has yet been found in which this quantity is as
large as a foot rule seen at a distance of fifty miles, and for
comparatively few stars is it certainly appreciable. An extraordinary
degree of precision has been attained in recent measures of this
quantity, but for a really satisfactory solution of this problem, we
must probably devise some new method, like the use of the spectroscope
for determining motions. Two or three illustrations of the kind of
methods which might be used to solve this problem may be of interest.
There are certain indications of the presence of a selective absorbing
medium in space.
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