rofessor Keeler, who considered this the normal type of nebula,
estimated that there were at least 120,000 of such spirals within the
reach of the Crossley reflector of the Lick Observatory. Professor
Perrine has indeed lately raised this estimate to half a million, and
thinks that with more sensitive photographic plates and longer exposures
the number of spirals would exceed a million. The majority of these
objects are very small, and appear to be distributed over the sky in a
fairly uniform manner.

_Planetary_ nebulae are small faint roundish objects which, when seen in
the telescope, recall the appearance of a planet, hence their name. One
of these nebulae, known astronomically as G.C. 4373, has recently been
found to be rushing through space towards the earth at a rate of between
thirty and forty miles per second. It seems strange, indeed, that any
gaseous mass should move at such a speed!

What are known as _ring_ nebulae were until recently believed to form a
special class. These objects have the appearance of mere rings of
nebulous matter. Much doubt has, however, been thrown upon their being
rings at all; and the best authorities regard them merely as spiral
nebulae, of which we happen to get a foreshortened view. Very few
examples are known, the most famous being one in the constellation of
Lyra, usually known as the Annular Nebula in Lyra. This object is so
remote from us as to be entirely invisible to the naked eye. It contains
a star of the fifteenth magnitude near to its centre. From photographs
taken with the Crossley reflector, Professor Schaeberle finds in this
nebula evidences of spiral structure. It may here be mentioned that the
Great Nebula in Andromeda, which has now turned out to be a spiral, had
in earlier photographs the appearance of a ring.

There also exist nebulae of _irregular_ form, the most notable being the
Great Nebula in the constellation of Orion (see Plate XXIV., p. 318). It
is situated in the centre of the "Sword" of Orion (see Plate XX., p.
296). In large telescopes it appears as a magnificent object, and in
actual dimensions it must be much on the same scale as the Andromeda
Nebula. The spectroscope tells us that it is a mass of glowing gas.

The Trifid Nebula, situated in the constellation of Sagittarius, is an
object of very strange shape. Three dark clefts radiate from its centre,
giving it an appearance as if it had been torn into shreds.

The Dumb-bell Nebula, a celebrate