um of the moving
object, in fact, is very slightly _shoved_ hither or thither, according
as it is travelling towards or from the eye; but, for convenience of
measurement, one line is usually picked out from the rest, and attention
concentrated upon it. The application of this method to the stars,
however, is encompassed with difficulties. It needs a powerfully
dispersive spectroscope to show line-displacements of the minute order
in question; and powerful dispersion involves a strictly proportionate
enfeeblement of light. This, where the supply is already to a deplorable
extent niggardly, can ill be afforded; for which reason the operation of
determining a star's approach or recession is, even apart from
atmospheric obstacles, an excessively delicate one.

It was first executed by Sir William Huggins early in 1868.[1436]
Selecting the brightest star in the heavens as the most promising
subject of experiment, he considered the F line in the spectrum of
Sirius to be just so much displaced towards the red as to indicate (the
orbital motion of the earth being deducted) recession at the rate of
twenty-nine miles a second; and the reality and direction of the
movement were ratified by Vogel and Lohse's observation, March 22, 1871,
of a similar, but even more considerable displacement.[1437] The inquiry
was resumed by Huggins with improved apparatus in the following year,
when the velocities of thirty stars were approximately determined.[1438]
The retreat of Sirius, which proved slower than had at first been
supposed, was now announced to be shared, at rates varying from twelve
to twenty-nine miles, by Betelgeux, Rigel, Castor, Regulus, and five of
the principal stars in the Plough. Arcturus, on the contrary, gave signs
of rapid approach, as well as Pollux, Vega, Deneb in the Swan, and the
brightness of the Pointers.

Numerically, indeed, these results were encompassed with uncertainty.
Thus, Arcturus is now fully ascertained to be travelling towards the sun
at the comparatively slow pace of less than five miles a second; and
Sirius moves twice as fast in the same direction. The great difficulty
of measuring so distended a line as the Sirian F might, indeed, well
account for some apparent anomalies. The scope of Sir William Huggins's
achievement was not, however, to provide definitive data, but to
establish as practicable the method of procuring them. In this he was
thoroughly successful, and his success was of incalculable v