us aroused that some planet might, after all, be moving in this
seemingly empty space, gave rise to the gradual discovery of a great
number of small bodies; the largest of which, Ceres, is less than 500
miles in diameter. Up to the present day some 600 of these bodies have
been discovered; the four leading ones, in order of size, being named
Ceres, Pallas, Juno, and Vesta. All the asteroids are invisible to the
naked eye, with the exception of Vesta, which, though by no means the
largest, happens to be the brightest. It is, however, only just visible
to the eye under favourable conditions. No trace of an atmosphere has
been noted upon any of the asteroids, but such a state of things is only
to be expected from the kinetic theory.

For a good many years the discoveries of asteroids were made by means of
the telescope. When, in the course of searching the heavens, an object
was noticed which did not appear upon any of the recognised star charts,
it was kept under observation for several nights to see whether it
changed its place in the sky. Since asteroids move around the sun in
orbits, just as planets do, they, of course, quickly reveal themselves
by their change of position against the starry background.

The year 1891 started a new era in the discovery of asteroids. It
occurred to the Heidelberg observer, Dr. Max Wolf, one of the most
famous of the hunters of these tiny planets, that photography might be
employed in the quest with success. This photographic method, to which
allusion has already been made in dealing with Eros, is an extremely
simple one. If a photograph of a portion of the heavens be taken through
an "equatorial"--that is, a telescope, moving by machinery, so as to
keep the stars, at which it is pointed, always exactly in the field of
view during their apparent movement across the sky--the images of these
stars will naturally come out in the photograph as sharply defined
points. If, however, there happens to be an asteroid, or other planetary
body, in the same field of view, its image will come out as a short
white streak; because the body has a comparatively rapid motion of its
own, and will, during the period of exposure, have moved sufficiently
against the background of the stars to leave a short trail, instead of a
dot, upon the photographic plate. By this method Wolf himself has
succeeded in discovering more than a hundred asteroids (see Plate XIII.,
p. 226). It was, indeed, a little streak of this