the double stars are explicable by
the ordinary laws of gravitation, thus confirming the assumption that
Newton's laws apply to these sidereal bodies. Henceforth there could be
no reason to doubt that the same force which holds terrestrial objects
on our globe pulls at each and every particle of matter throughout the
visible universe.

The pioneer explorers of the double stars early found that the systems
into which the stars are linked are by no means confined to single
pairs. Often three or four stars are found thus closely connected into
gravitation systems; indeed, there are all gradations between binary
systems and great clusters containing hundreds or even thousands of
members. It is known, for example, that the familiar cluster of the
Pleiades is not merely an optical grouping, as was formerly supposed,
but an actual federation of associated stars, some two thousand five
hundred in number, only a few of which are visible to the unaided eve.
And the more carefully the motions of the stars are studied, the more
evident it becomes that widely separated stars are linked together into
infinitely complex systems, as yet but little understood. At the same
time, all instrumental advances tend to resolve more and more seemingly
single stars into close pairs and minor clusters. The two Herschels
between them discovered some thousands of these close multiple systems;
Struve and others increased the list to above ten thousand; and Mr.
S. W. Burnham, of late years the most enthusiastic and successful of
double-star pursuers, added a thousand new discoveries while he was
still an amateur in astronomy, and by profession the stenographer of a
Chicago court. Clearly the actual number of multiple stars is beyond all
present estimate.

The elder Herschel's early studies of double stars were undertaken in
the hope that these objects might aid him in ascertaining the actual
distance of a star, through measurement of its annual parallax--that
is to say, of the angle which the diameter of the earth's orbit would
subtend as seen from the star. The expectation was not fulfilled. The
apparent shift of position of a star as viewed from opposite sides of
the earth's orbit, from which the parallax might be estimated, is so
extremely minute that it proved utterly inappreciable, even to the
almost preternaturally acute vision of Herschel, with the aid of any
instrumental means then at command. So the problem of star distance
allured and elu