AR NEBULAE.--These are found in both hemispheres, and are
remarkable on account of the varied appearances which they present, and
the large extent of space which many of them occupy. In some, the
nebulous matter of which they are composed can be seen like masses of
tufted flocculi, sometimes piled up, and at other times promiscuously
scattered, resembling in appearance the foam on the crested billows of a
surging ocean rendered suddenly motionless, or cirro-cumuli floating in
a tranquil sky. Islands of light with intervening dark channels,
promontories projecting into gulfs of deep shade, sprays of luminous
matter, convoluted filaments, whorls, wreaths, and spiral streams all
enter into the structural formation of a great nebula.

The Great Nebula in Argo, in the Southern Hemisphere, is one of the most
remarkable objects of this class. It consists of bright irregular masses
of luminous matter, streaks and branches, and occupies an area about
equal to one square degree. At its eastern border is situated the
variable star Eta Argus, which fluctuates between the first and seventh
magnitudes in a period of about seventy years.

A rich portion of the Galaxy lies in front of the nebula, which creates
an effect as if it were studded over with stars. Sir John Herschel, in
describing this nebula, writes as follows:--'The whole is situated in a
very rich and brilliant part of the Milky Way, so thickly strewed with
stars that, in the area occupied by the nebula, not less than 1,200 have
been actually counted. Yet it is obvious that these have no connection
whatever with the nebula, being, in fact, only a simple continuation
over it of the general ground of the Galaxy. The conclusion can hardly
be avoided that, in looking at it, we see through and beyond the Milky
Way, far out into space, through a starless region, disconnecting it
altogether from our system. It is not easy for language to convey a full
impression of the beauty and sublimity of the spectacle which this
nebula offers as it enters the field of view of a telescope, fixed in
right ascension, by the diurnal motion, ushered in as it is by so
glorious and innumerable a procession of stars, to which it forms a sort
of climax, and in a part of the heavens otherwise full of interest.'
Another large bright nebula (called 30 Doradus), also in the Southern
Hemisphere, is composed of a series of loops with intricate windings
forming a kind of open network against the background