"This may be justly asserted to be one of the most sublime truths
which astronomical science has hitherto disclosed to the researches of
the human mind."

Latterly the best work on double stars has been done by
S. W. Burnham,[14] at the Lick Observatory. The shortest period he
found was eleven years (kappa Pegasi). In the case of some of
these binaries the parallax has been measured, from which it appears
that in four of the surest cases the orbits are about the size of the
orbit of Uranus, these being probably among the smallest stellar
orbits.

The law of gravitation having been proved to extend to the stars, a
discovery (like that of Neptune in its origin, though unlike it in the
labour and originality involved in the calculation) that entrances the
imagination became possible, and was realised by Bessel--the discovery
of an unknown body by its gravitational disturbance on one that was
visible. In 1834 and 1840 he began to suspect a want of uniformity in
the proper motion of Sirius and Procyon respectively. In 1844, in a
letter to Sir John Herschel,[15] he attributed these irregularities in
each case to the attraction of an invisible companion, the period of
revolution of Sirius being about half a century. Later he said: "I
adhere to the conviction that Procyon and Sirius form real binary
systems, consisting of a visible and an invisible star. There is no
reason to suppose luminosity an essential quality of cosmical
bodies. The visibility of countless stars is no argument against the
invisibility of countless others." This grand conception led Peters to
compute more accurately the orbit, and to assign the place of the
invisible companion of Sirius. In 1862 Alvan G. Clark was testing a
new 18-inch object-glass (now at Chicago) upon Sirius, and, knowing