ne, while planning new and still more
important optical achievements.

A heliometer is the most accurate astronomical instrument for relative
measurements of position, as a transit circle is the most accurate for
absolute determinations. It consists of an equatorial telescope with
object-glass cut right across, and each half movable by a sliding
movement one past the other, the amount by which the two halves are
dislocated being read off by a refined method, and the whole instrument
having a multitude of appendages conducive to convenience and accuracy.
Its use is to act as a micrometer or measurer of small distances.[28]
Each half of the object-glass gives a distinct image, which may be
allowed to coincide or may be separated as occasion requires. If it be
the components of a double star that are being examined, each component
will in general be seen double, so that four images will be seen
altogether; but by careful adjustment it will be possible to arrange
that one image of each pair shall be superposed on or coincide with each
other, in which case only three images are visible; the amount of
dislocation of the halves of the object-glass necessary to accomplish
this is what is read off. The adjustment is one that can be performed
with extreme accuracy, and by performing it again and again with all
possible modifications, an extremely accurate determination of the
angular distance between the two components is obtained.

[Illustration: FIG. 92.--Heliometer.]

Bessel determined to apply this beautiful instrument to the problem of
stellar parallax; and he began by considering carefully the kind of star
for which success was most likely. Hitherto the brightest had been most
attended to, but Bessel thought that quickness of proper motion would be
a still better test of nearness. Not that either criterion is conclusive
as to distance, but there was a presumption in favour of either a very
bright or an obviously moving star being nearer than a faint or a
stationary one; and as the "bright" criterion had already been often
applied without result, he decided to try the other. He had already
called attention to a record by Piazzi in 1792 of a double star in
Cygnus whose proper motion was five seconds of arc every year--a motion
which caused this telescopic object, 61 Cygni, to be known as "the
flying star." Its motion is not really very perceptible, for it will
only have traversed one-third of a lunar diameter in the course