much smaller scale. It possesses a nucleus, and on the photographic
plate there can be detected the presence of spiral structure, indicating
the existence of streams of nebulous matter. Adjacent to this nebula is
another of the same class with a double nucleus, and associated with it
is a nebulous star.

SPIRAL NEBULAE.--The great reflector of Earl Rosse at Parsonstown was the
successful means by which nebulae of this form were discovered. This
powerful telescope was capable of defining with greater accuracy the
structural formation of those objects than any other instrument in use.
It was ascertained that spiral coils and convoluted whorls enter into
the structure of most nebulae, indicating a similarity in the process of
change which may be going on in these vast accumulations of cosmical
matter. The most interesting specimen of a spiral nebula is situated in
Canes Venatici. It consists of spiral coils emanating from a centre with
a nucleus and surrounded by a narrow luminous ring. In appearance it
resembles the coiled mainspring of a watch.

PLANETARY NEBULAE.--These have been so named on account of the
resemblance which they bear to the discs of planets. They are of uniform
brightness, circular in shape, with sharply-defined edges, and are
frequently of a bluish colour. They are more numerous than annular
nebulae; three-fourths of their number are in the Southern Hemisphere,
and they are situated in or very near the Milky Way. Those objects were
first described by Sir William Herschel, who was rather perplexed as to
what was their real nature and how he should classify them. He remarked
that they could not be planets belonging to far-off suns, nor distant
comets, nor distended stars. Consequently, he concluded rightly that
they were nebulae. When observed with large telescopes, they lose their
planetary aspect, and their sharpness of outline is less apparent; their
discs become broken up into bright and dark portions, and in some,
numerous minute stars have been observed, whilst others have
well-defined nuclei.

The most prominent nebula of this class is situated in the constellation
Ursa Major, and is called the Owl Nebula, from its fancied resemblance
to the face of that bird. Sir John Herschel describes it as 'a most
extraordinary object, a large, uniform nebulous disc, quite round, very
bright, not sharply defined, but yet very suddenly fading away to
darkness.' When examined in 1848 with Earl Rosse's reflect