n of double stars (in which Struve
chiefly employed it), and for such subtle measurements as might serve to
reveal or disprove the existence of a sensible stellar parallax.
Fraunhofer, moreover, constructed for the observatory at Koenigsberg the
first really available heliometer. The principle of this instrument
(termed with more propriety a "divided object-glass micrometer") is the
separation, by a strictly measurable amount, of two distinct images of
the same object. If a double star, for instance, be under examination,
the two half-lenses into which the object-glass is divided are shifted
until the upper star (say) in one image is brought into coincidence with
the lower star in the other, when their distance apart becomes known by
the amount of motion employed.[68]

This virtually new engine of research was delivered and mounted in 1829,
three years after the termination of the life of its deviser. The Dorpat
lens had brought to Fraunhofer a title of nobility and the sole
management of the Munich Optical Institute (completely separated since
1814 from the mechanical department). What he had achieved, however, was
but a small part of what he meant to achieve. He saw before him the
possibility of nearly quadrupling the light-gathering capacity of the
great achromatic acquired by Struve; he meditated improvements in
reflectors as important as those he had already effected in refractors;
and was besides eagerly occupied with investigations into the nature of
light, the momentous character of which we shall by-and-by have an
opportunity of estimating. But his health was impaired, it is said, from
the weakening effects of his early accident, combined with excessive and
unwholesome toil, and, still hoping for its restoration from a projected
journey to Italy, he died of consumption, June 7, 1826, aged thirty-nine
years. His tomb in Munich bears the concise eulogy, _Approximavit
sidera_.

Bessel had no sooner made himself acquainted with the exquisite defining
powers of the Koenigsberg heliometer, than he resolved to employ them in
an attack upon the now secular problem of star-distances. But it was not
until 1837 that he found leisure to pursue the inquiry. In choosing his
test-star he adopted a new principle. It had hitherto been assumed that
our nearest neighbours in space must be found among the brightest
ornaments of our skies. The knowledge of stellar proper motions afforded
by the critical comparison of recent with e