lustrates the maxim that
science _differentiates_ as it develops. He was, while much besides, a
specialist in double stars. His earliest recorded use of the telescope
was to verify Herschel's conclusion as to the revolving movement of
Castor, and he never varied from the predilection which this first
observation at once indicated and determined. He was born at Altona, of
a respectable yeoman family, April 15, 1793, and in 1811 took a degree
in philology at the new Russian University of Dorpat. He then turned to
science, was appointed in 1813 to a professorship of astronomy and
mathematics, and began regular work in the Dorpat Observatory just
erected by Parrot for Alexander I. It was not, however, until 1819 that
the acquisition of a 5-foot refractor by Troughton enabled him to take
the position-angles of double stars with regularity and tolerable
precision. The resulting catalogue of 795 stellar systems gave the
signal for a general resumption of the Herschelian labours in this
branch. His success, so far, and the extraordinary facilities for
observation afforded by the Fraunhofer achromatic encouraged him to
undertake, February 11, 1825, a review of the entire heavens down to 15
deg. south of the celestial equator, which occupied more than two years,
and yielded, from an examination of above 120,000 stars, a harvest of
about 2,200 previously unnoticed composite objects. The ensuing ten
years were devoted to delicate and patient measurements, the results of
which were embodied in _Mensurae Micrometricae_, published at St.
Petersburg in 1837. This monumental work gives the places, angles of
position, distances, colours, and relative brightness of 3,112 double
and multiple stars, all determined with the utmost skill and care. The
record is one which gains in value with the process of time, and will
for ages serve as a standard of reference by which to detect change or
confirm discovery.

It appears from Struve's researches that about one in forty of all stars
down to the ninth magnitude is composite, but that the proportion is
doubled in the brighter orders.[102] This he attributed to the
difficulty of detecting the faint companions of very remote orbs. It was
also noticed, both by him and Bessel, that double stars are in general
remarkable for large proper motions. Struve's catalogue included no star
of which the components were more than 32" apart, because beyond that
distance the chances of merely optical juxtapositio