here fortunately comes into play. "Proper motions" are only angular
velocities. They tell nothing as to the value of the perspective
element they may be supposed to include, or as to the real rate of
going of the bodies they are attributed to, until the size of the
sphere upon which they are measured has been otherwise ascertained.
But the displacement of lines in stellar spectra give directly the
actual velocities relative to the earth of the observed stars. The
question of their distances is, therefore, at once eliminated. Now the
radial component of stellar motion is mixed up, precisely in the same
way as the tangential component, with the solar movement; and since
complete knowledge of it, in a sufficient number of cases, is rapidly
becoming accessible, while knowledge of tangential velocity must for a
long time remain partial or uncertain, the advantage of replacing the
discussion of proper motions by that of motions in line of sight is
obvious and immediate. And the admirable work carried on at Potsdam
during the last three years will soon afford the means of doing so in
the first, if only a preliminary investigation of the solar
translation based upon measurements of photographed stellar spectra.

The difficulties, then, caused either by inaccuracies in star
catalogues or by ignorance of star distances may be overcome; but
there is a third, impossible at present to be surmounted, and not
without misgiving to be passed by. All inquiries upon the subject of
the advance of our system through space start with an hypothesis most
unlikely to be true. The method uniformly adopted in them--and no
other is available--is to treat the _inherent_ motions of the stars
(their so-called _motus peculiares_) as pursued indifferently in all
directions. The steady drift extricable from them by rules founded
upon the science of probabilities is presumed to be solar motion
visually transferred to them in proportions varying with their
remoteness in space, and their situations on the sphere. If this
presumption be in any degree baseless, the result of the inquiry is
_pro tanto_ falsified. Unless the deviations from the parallactic line
of the stellar motions balance one another on the whole, their
discussion may easily be as fruitless as that of observations tainted
with systematic errors. It is scarcely, however, doubtful that law,
and not chance, governs the sidereal revolutions. The point open to
question is whether the workings of