und by an ex-postmaster of the Prussian town of
Driessen, by name Hencke, who, in spite of the general disbelief in the
existence of any more planets, set himself diligently to search for them,
and toiled for fifteen long years before at length reaping his reward.
Others then resumed the search; Hind, the observer of an English amateur
astronomer near London, found Iris a few weeks after Hencke had been
rewarded by a second discovery in 1847, and in the following year Mr.
Graham at Markree in Ireland (who is still living, and has only just
retired from active work at the Cambridge Observatory) found Metis; and
from that time new discoveries have been added year by year, until the
number of planets now known exceeds 500, and is steadily increasing.

[Illustration:

_By permission of Messrs. Macmillan & Co._
I.--J. C. ADAMS.]

[Illustration:

II.--A. GRAHAM.
DISCOVERER OF THE NINTH MINOR PLANET (METIS).]

[Sidenote: The photographic method.]

You will see the great variety characterising these discoveries; some of
them are the result of deliberate search, others have come accidentally,
and some even contrary to expectation. Of the great majority of the
earlier ones it may be said that enormous diligence was required for each
discovery; to identify a planet it is necessary to have either a good map
of the stars or to know them thoroughly, so that the map practically
exists in the brain. We need only remember Hencke's fifteen years of
search before success to recognise what vast stores of patience and
diligence were required in carrying out the search. But of late years
photography has effected a great revolution in this respect. It is no
longer necessary to do more than set what Sir Robert Ball has called a
"star-trap," or rather planet-trap. If a photograph be taken of a region
of the heavens, by the methods familiar to astronomers, so that each star
makes a round dot on the photographic plate, any sufficiently bright
object moving relatively to the stars will make a small line or trail, and
thus betray its planetary character. In this way most of the recent
discoveries have been made, and although diligence is still required in
taking the photographs, and again in identifying the objects thus found
(which are now very often the images of already known members of the
system), the tedious scrutiny with the eye has become a thing of the past.

TABLE SHOWING THE NUMBER OF MINOR PLANETS DISCOVERED IN EACH DE