iscoveries became a stream of
increasing volume, until in 1897 the total number reached about 430.
Their distribution in the solar system is somewhat as represented in
Fig. 55. By the improvement of astronomical telescopes, and by the
devotion with which certain astronomers have applied themselves to this
interesting research, a special method of observing has been created for
the distinct purpose of searching out these little objects.

It is known that the paths in which all the great planets move through
the heavens coincide very nearly with the path which the sun appears to
follow among the stars, and which is known as the ecliptic. It is
natural to assume that the small planets also move in the same great
highway, which leads them through all the signs of the zodiac in
succession. Some of the small planets, no doubt, deviate rather widely
from the track of the sun, but the great majority are approximately near
it. This consideration at once simplifies the search for new planets. A
certain zone extending around the heavens is to be examined, but there
is in general little advantage in pushing the research into other parts
of the sky.

The next step is to construct a map containing all the stars in this
region. This is a task of very great labour; the stars visible in the
large telescopes are so numerous that many tens of thousands, perhaps we
should say hundreds of thousands, are included in the region so narrowly
limited. The fact is that many of the minor planets now known are
objects of extreme minuteness; they can only be seen with very powerful
telescopes, and for their detection it is necessary to use charts on
which even the faintest stars have been depicted. Many astronomers have
concurred in the labour of producing these charts; among them may be
mentioned Palisa, of Vienna, who by means of his charts has found
eighty-three minor planets, and the late Professor Peters, of Clinton,
New York, who in a similar way found forty-nine of these bodies.

[Illustration: Fig. 55.--The Zone of Minor Planets between Mars and
Jupiter.]

The astronomer about to seek for a new planet directs his telescope
towards that part of the sun's path which is on the meridian at
midnight; there, if anywhere, lies the chance of success, because that
is the region in which such a body is nearer to the earth than at any
other part of its course. He steadfastly compares his chart with the
heavens, and usually finds the stars in the he