Professor Campbell's spectroscopic measurements.

Herschel's conclusion as to the advance of the sun among the stars was
not admitted as valid by the most eminent of his successors. Bessel
maintained that there was absolutely no preponderating evidence in
favour of its supposed direction towards a point in the constellation
Hercules.[78] Biot, Burckhardt, even Herschel's own son, shared his
incredulity. But the appearance of Argelander's prize-essay in 1837[79]
changed the aspect of the question. Herschel's first memorable solution
in 1783 was based upon the motions of thirteen stars, imperfectly known;
his second, in 1805, upon those of no more than six. Argelander now
obtained an entirely concordant result from the large number of 390,
determined with the scrupulous accuracy characteristic of Bessel's work
and his own. The reality of the fact thus persistently disclosed could
no longer be doubted; it was confirmed five years later by the younger
Struve, and still more strikingly in 1847[80] by Galloway's
investigations, founded exclusively on the apparent displacements of
southern stars. In 1859 and 1863, Sir George Airy and Mr. Dunkin
(1821-1898),[81] employing all the resources of modern science, and
commanding the wealth of material furnished by 1,167 proper motions
carefully determined by Mr. Main, reached conclusions closely similar to
that indicated nearly eighty years previously by the first great
sidereal astronomer; which Mr. Plummer's reinvestigation of the subject
in 1883[82] served but slightly to modify. Yet astronomers were not
satisfied. Dr. Auwers of Berlin completed in 1866 a splendid piece of
work, for which he received in 1888 the Gold Medal of the Royal
Astronomical Society. It consisted in reducing afresh, with the aid of
the most refined modern data, Bradley's original stars, and comparing
their places thus obtained for the year 1755 with those assigned to them
from observations made at Greenwich after the lapse of ninety years. In
the interval, as was to be anticipated, most of them were found to have
travelled over some small span of the heavens, and there resulted a
stock of nearly three thousand highly authentic proper motions. These
ample materials were turned to account by M. Ludwig Struve[83] for a
discussion of the sun's motion, of which the upshot was to shift its
point of aim to the bordering region of the constellations Hercules and
Lyra. And the more easterly position of the solar apex