or by masses of
this fluid lying beyond the regions of the seventh-magnitude stars. This
fluid might exist independently of stars. If it is self-luminous, it
seems more fit to produce a star by its condensation, than to depend
upon the star for its own existence. Such were a few of the theorems to
which his discovery of this nebula led him. The hypothesis of an elastic
_shining fluid_ existing in space, sometimes in connection with stars,
sometimes distinct from them, was adopted and never abandoned. How well
the spectroscope has confirmed this idea it is not necessary to say.
We know the shining fluid does exist, and in late years we have seen the
reverse of the process imagined by HERSCHEL. A star has actually, under
our eyes, become a planetary nebula, and the cycle of which he gave the
first terms is complete.

In five separate memoirs (1802, 1811, 1814, 1817, and 1818) HERSCHEL
elaborated his views of the sidereal system. The whole extent of his
views must be gained from the extended memoirs themselves. Here only the
merest outline can be given.

In 1802 there is a marshaling of the various objects beyond our solar
system. The stars themselves may be _insulated_, or may belong to
_binary_ or _multiple_ systems, to _clusters_ and groups, or to grand
groups like the Milky Way. Nebulae may have any of the forms which have
been described; and, in 1811, he gives examples of immense spaces in the
sky covered with diffused and very faint nebulosity. "Its abundance
exceeds all imagination."[38] These masses of nebular matter are the
seats of attracting forces, and these forces must produce condensation.
When a nebula has more than one preponderating seat of attracting
matter, it may in time be divided, and the double nebulae have had such
an origin. When nebulae appear to us as round masses, they are in reality
globular in form, and this form is at once the effect and the proof of a
gravitating cause.

The central brightness of nebulae points out the seat of the attraction;
and the completeness of the approximation to a spherical form points out
the length of time that the gravitating forces have been at work. Those
nebulae (and clusters) which are most perfect in the globular form, have
been longest exposed to central forces. The planetary nebulae are the
oldest in our system. They must have a rotatory motion on their axes.

By progressive condensation planetary nebulae may be successively
converted into bright stella