of Qoo new Nebula, and Clusters of Stars . 501 
XI. Planetary Nebula. 
This seems to be a species of bodies that demands a particu- 
lar attention. To investigate the planetary nature of these 
nebulas, is not an easy undertaking. If we admit them to con- 
tain a great mass of matter, such as that of which our sun is 
composed, and that they are, like the sun, surrounded by dense 
luminous clouds, it appears evidently that the intrinsic bright- 
ness of these clouds must be far inferior to those of the sun. A 
part of the sun’s disk, equal to a circle of 15" in diameter, would 
far exceed the greatest lustre of the full moon ; whereas, the 
light of a planetary nebula, of an equal size, is hardly equal to 
that of a star of the 8th or 9th magnitude. If, on the other 
hand, we should suppose them to be groups, or clusters of stars, 
at a distance sufficiently great to reduce them to so small an 
apparent diameter, we shall be at a loss to account for their 
uniform light, if clusters; or for their circular forms, if mere 
groups of stars. 
Perhaps they may be rather allied to nebulous star?. For, 
should the planetary nebulae with lucid centres, of which the 
next article will give an account, be an intermediate step be- 
tween planetary nebulae and nebulous stars, the appearances of 
these different species, when all the individuals of them are fully 
examined, might throw a considerable light upon the subject. 
XII. Of planetary Nebula with Centres. 
In my second catalogue of nebulae, a single instance of a 
planetary nebula with a bright central point was mentioned; 
and, in the annexed one, No. 73 of the 4th class, is another of 
very nearly the same diameter, which has also a lucid, though 
JMDCCCII, 3 T 
