or, two bright
stars were discovered in its interior; each was in the centre of a
circular dark space surrounded by whorls of nebulous matter--hence the
origin of its name. This nebula gives a bright line spectrum indicative
of gaseous composition. It is believed to consist chiefly of hydrogen
and other gases which form a globe of such stupendous magnitude that, if
we surmise its distance from the earth to be sixty-five light years--an
estimate much too low--'its diameter would exceed that of the orbit of
Neptune upwards of 100 times.'[10] Within its compass the orbs of
hundreds of solar systems as large as that of ours would be able to
perform their revolutions, having spacious intervals existing between
each system. Another interesting planetary nebula is in the
constellation of the Dragon, near to the pole of the ecliptic; it is
slightly oval, of a pale blue colour, and contains a star of the
eleventh magnitude in its centre. It gives a gaseous spectrum. Attempts
have been made to determine its parallax, but without success, and
during the eighty years it has been under observation it has remained
apparently motionless. Its light period, if estimated at 140 years,
would indicate the existence of a globe with a diameter equal to
forty-four diameters of the orbit of the planet Neptune.[11] A nebula of
this class was discovered by Sir John Herschel in the Centaur. He
described it as resembling Uranus, but larger; its colour was of a
beautiful rich blue, and its light equalled that of a star of the
seventh magnitude.

NEBULOUS STARS.--These stars are each surrounded by a luminous haze
several minutes of arc in diameter and of a circular form. Sir William
Herschel, by his observation of those objects, arrived at the conclusion
'that there exists in space a shining fluid of a nature totally unknown
to us, and that the nebulosity about those stars was not of a starry
nature.' Thirteen stars of this type have been enumerated by him and
many others have since been discovered. The 'glow' which surrounds them
has been observed in a few instances to have vanished without leaving
any trace of nebulosity behind, but the causes which have brought about
such a result are entirely unknown. The nature of those stars is
involved in considerable obscurity, and one class of nebula would seem
to merge into the other; nebulous stars with faint aureolae do not differ
much from small nebulae interspersed with stellar points.

LARGE IRREGUL