anifested by the denser parts of the
medium, as is the case in our own solar system.

_Ring or Elliptical Nebulae._--These forms of nebulae are so named from
their ring-shaped appearance, sometimes being known as Annular Nebulae.
The elliptical nebulae are usually classed with them, as they are
supposed to be similar kinds of nebulae looked at edgeways. The best
known of this class is that found in the constellation of Lyra, and
known as 57M, which is the number of the star in Messier's catalogue of
stars. It is small but well-defined, so that it looks more like a flat
oval solid ring than a nebula.

The central part is not entirely dark, but is filled up with a hazy
light. Another annular nebula is that situated to the south-west of
Lambda Scorpii. Sir J. Herschel[44] writes of it thus: "It is a delicate
but well-defined annulus. The field is crowded with stars, two of which
are nebulae. A beautiful delicate ring of a faint ghost-like appearance,
about 40" in diameter, in a field of about 150 stars, of 11 and 12
magnitude and under."

Of the elliptical nebulae the best known is the one in the constellation
of Andromeda, which goes by the name of 31M. It is visible on a clear
night, and can be seen by the naked eye as a hazy light. There are
several other elliptical nebulae, lying to the north-west of this great
nebula.

_Planetary Nebulae._--The planetary nebulae represent a number of minute
objects visible in the heavens. They look like globes of a
bluish-coloured gas and are sometimes mistaken for small stars. Sir J.
Herschel writes about them as follows: "Planetary nebulae are very
extraordinary objects. They have, as their name implies, a resemblance
to planets, presenting discs, round or slightly oval, some being quite
sharply defined, terminating in others a little hazy or softened at the
border. They are comparatively rare objects, not more than 25 having
been observed, and of these nearly three-quarters are in the southern
hemisphere. Their disc is circular or slightly elliptic, with sharp,
clear, and well-defined outline, having exactly the appearance of a
planet with the exception only of its colour, which is full blue,
varying somewhat upon green. M. Arago has surmised that they may
possibly be envelopes shining by reflected light from a solar body
placed in their centre, invisible to us because of its excessive
distance."

The suggestion which arises to our mind in view of the atomic and
gravitating