otion
obtained visually. That the similar work on the stars begun at Greenwich
in 1874, and carried on for thirteen years, remained comparatively
unfruitful, was only what might have been expected, the instruments
available there being altogether inadequate for the attainment of a high
degree of accuracy.

The various obstacles in the way of securing it were overcome by the
substitution of the sensitive plate for the eye. Air-tremors are thus
rendered comparatively innocuous; and measurements of stellar lines
displaced by motion with reference to fiducial lines from terrestrial
sources, photographed on the same plates, can be depended upon within
vastly reduced limits of error. Studies for the realisation of the
"spectrographic" method were begun by Dr. Vogel and his able assistant,
Dr. Scheiner, at Potsdam in 1887. Their preliminary results,
communicated to the Berlin Academy of Sciences, March 15, 1888, already
showed that the requirements for effective research in this important
branch were at last about to be complied with. An improved instrument
was erected in the autumn of the same year, and the fifty-one stars,
bright enough for determination with a refractor of 11 inches aperture,
were promptly taken in hand. A list of their motions in the line of
sight, published in 1892,[1534] was of high value, both in itself and
for what it promised. One noteworthy inference from the data it
collected was that the eye tends, under unfavourable circumstances, to
exaggerate the line-displacements it attempts to estimate. The
velocities photographically arrived at were of much smaller amounts than
those visually assigned. The average speed of the Potsdam stars came out
only 10.4 miles a second, the quickest among them being Aldebaran, with
a recession of thirty miles a second. More lately, however, Deslandres
and Campbell have determined for Zeta Herculis and Eta Cephei
respectively approaching rates of forty-four and fifty-four miles
a second.

The installation, in 1900, of a photographic refractor 31-1/2 inches in
aperture, coupled with a 20-inch guiding telescope, will enable Dr.
Vogel to investigate spectrographically some hundreds of stars fainter
than the second magnitude; and the materials thus accumulated should
largely help to provide means for a definite and complete solution of
the more than secular problem of the sun's advance through space. The
solution should be complete, because including a genuine determinatio