avens and the stars in the
chart to correspond; but sometimes it will happen that a point in the
heavens is missing from the chart. His attention is at once arrested; he
follows the object with care, and if it moves it is a planet. Still he
cannot be sure that he has really made a discovery; he has found a
planet, no doubt, but it may be one of the large number already known.
To clear up this point he must undertake a further, and sometimes a very
laborious, enquiry; he must search the Berlin Year-Book and other
ephemerides of such planets and see whether it is possible for one of
them to have been in the position on the night in question. If he can
ascertain that no previously discovered body could have been there, he
is then entitled to announce to his brother astronomers the discovery of
a new member of the solar system. It seems certain that all the more
important of the minor planets have been long since discovered. The
recent additions to the list are generally extremely minute objects,
beyond the powers of small telescopes.

Since 1891 the method of searching for minor planets which we have just
described has been almost abandoned in favour of a process greatly
superior. It has been found feasible to employ photography for making
charts of the heavens. A photographic plate is exposed in the telescope
to a certain region of the sky sufficiently long to enable very faint
telescopic stars to imprint their images. Care has to be taken that the
clock which moves the camera shall keep pace most accurately with the
rotation of the earth, so that fixed stars appear on the plate as sharp
points. If, on developing the plate, a star is found to have left a
trail, it is evident that this star must during the time of exposure
(generally some hours) have had an independent motion of its own; in
other words, it must be a planet. For greater security a second picture
is generally taken of the same region after a short interval. If the
place occupied by the trail on the first plate is now vacant, while on
the second plate a new trail appears in a line with the first one, there
remains no possible doubt that we have genuine indications of a planet,
and that we have not been led astray by some impurity on the plate or by
a few minute stars which happened to lie very closely together. Wolf,
of Heidelberg, and following in his footsteps Charlois, of Nice, have in
this manner discovered a great number of new minor planets, while they