ion,
owing to the increasing absorption of its remaining light in the
atmosphere surrounding it.

The method employed for the spectroscopic determination of the motion of
a star in the line of sight is the same as the method we have described
in the chapter on the sun. The position of a certain line in the
spectrum of a star is compared with the position of the corresponding
bright line of an element in an artificially produced spectrum, and in
this manner a displacement of the stellar line either towards the violet
(indicating that the star is approaching us) or towards the red
(indicating that it is receding) may be detected. The earliest attempt
of this sort was made in 1867 by Sir William Huggins, who compared the F
line in the spectrum of Sirius with the same line of the spectrum of
hydrogen contained in a vacuum tube reflected into the field of his
astronomical spectroscope, so that the two spectra appeared side by
side. The work thus commenced and continued by him was afterwards taken
up at the Greenwich Observatory; but the results obtained by these
direct observations were never satisfactory, as remarkable discrepancies
appeared between the values obtained by different observers, and even by
the same observer on different nights. This is not to be wondered at
when we bear in mind that the velocity of light is so enormous compared
with any velocity with which a heavenly body may travel, that the change
of wave length resulting from the latter motion can only be a very
minute one, difficult to perceive, and still more difficult to measure.
But since photography was first made use of for these investigations by
Dr. Vogel, of Potsdam, much more accordant and reliable results have
been obtained, though even now extreme care is required to avoid
systematic errors. To give some idea of the results obtainable, we
present in the following table the values of the velocity per second of
a number of stars observed in 1896 and 1897 by Mr. H.F. Newall with the
Bruce spectrograph attached to the great 25-inch Newall refractor of the
Cambridge Observatory, and we have added the values found at Potsdam by
Vogel and Scheiner. The results are expressed in kilometres (1 km. =
0.62 English mile). The sign + means that the star is receding from
us,-that it is approaching.

Newall. Vogel. Scheiner.
Aldebaran + 49.2 + 47.6 + 49.4
Betelgeuze + 10.6 + 15.6 + 18.8
Procyon