n become
considerable; but the immense preponderance of extremely close over (as
it were) loosely yoked bodies is such as to demonstrate their physical
connection, even if no other proof were forthcoming. Many stars
previously believed to be single divided under the scrutiny of the
Dorpat refractor; while in some cases, one member of a supposed binary
system revealed itself as double, thus placing the surprised observer in
the unexpected presence of a triple group of suns. Five instances were
noted of two pairs lying so close together as to induce a conviction of
their mutual dependence;[103] besides which, 124 examples occurred of
triple, quadruple, and multiple combinations, the reality of which was
open to no reasonable doubt.[104]

It was first pointed out by Bessel that the fact of stars exhibiting a
common proper motion might serve as an unfailing test of their real
association into systems. This was, accordingly, one of the chief
criteria employed by Struve to distinguish true binaries from merely
optical couples. On this ground alone, 61 Cygni was admitted to be a
genuine double star; and it was shown that, although its components
appeared to follow almost strictly rectilinear paths, yet the
probability of their forming a connected pair is actually greater than
that of the sun rising to-morrow morning.[105] Moreover, this tie of an
identical movement was discovered to unite bodies[106] far beyond the
range of distance ordinarily separating the members of binary systems,
and to prevail so extensively as to lead to the conclusion that single
do not outnumber conjoined stars more than twice or thrice.[107]

In 1835 Struve was summoned by the Emperor Nicholas to superintend the
erection of a new observatory at Pulkowa, near St. Petersburg, destined
for the special cultivation of sidereal astronomy. Boundless resources
were placed at his disposal, and the institution created by him was
acknowledged to surpass all others of its kind in splendour, efficiency,
and completeness. Its chief instrumental glory was a refractor of
fifteen inches aperture by Merz and Mahler (Fraunhofer's successors),
which left the famous Dorpat telescope far behind, and remained long
without a rival. On the completion of this model establishment, August
19, 1839, Struve was installed as its director, and continued to fulfil
the important duties of the post with his accustomed vigour until 1858,
when illness compelled his virtual resignation