10 BOHLIN, ON THE GALACTIC SYSTEM WITH REGARD TO ITS STRUCTURE. 
form as G.O. 4447. But the actual form of the nebulous body is here even more 
obvious than in the former case. One may construct a model of these nebulx, by 
forming of a slip of paper a cylindrical ring, which, when seen obliquely, almost 
exactly offers their general appearence. 
Fig. 3. Model of Ring Nebula. 
It can hardly be doubted that this also is nearly the actual form of the ring 
nebulz G. C. 4447 and G.C. 2343 and of other Planetary Nebule, which thus chiefly 
consist of a hollow and almost cylindrical shell, the cavity of which contains nebu- 
lous matter of thinner consistency, being moreover condensed at the centre to the 
nucleus there visible.! The fig. 3 shows the projection of a model of Ring Nebula. 
There is, as far as is known, but a single instance of a nebula, fit to explain this 
singular and typical form of ring nebul&e, viz., the Dumbbell-Nebula G. OC. 4532, a re- 
production of which in Plate 4 N:o I shows the features of this body very plainly. 
If it is supposed that this nebula is carried to its actual form from a hollow sphe- 
rical shell, which is broken down at its poles, it becomes at once obvious that the 
nebula, if seen in the direction of its polar axis, would exhibit the general form of 
a ring nebula. As the process once is carried further, the remaining shell will gra- 
dually be reduced to an equatorial zone, more approaching to the cylindrical form 
described above. 
While the nuclei of the ring nebul&e are very small, and the inner parts of these 
nebule are very diluted, as to make the impression of vacuity, the closed planetary 
nebule have generally a circular and comparatively big centre, of great luminosity, 
surrounded by a stratum of atmospheric appearance, which in various instances is 
more or less extended. Such nebule are G. C. 4514, G.C. 4234. In some cases, the 
atmosphere is too insignificant to be marked at all, such nebulx forming a transi- 
tion to nebulous stars,” some of which, on the other hand, present atmospheric appen- 
! J. HerscHeLin his Outlines Ed. 1851, pag. 604, says about the Nebula G. C. 2343 [2 = 115 4m: p = 34? 4]: 
»'The light of this stupendous globe is perfectly equable (except just at the edge, where it is slightly softened) 
and of considerable brightness. Such an appearence would not be presented by a globular space uniformely 
filled with stars or luminous matter, which structure would necessary give rise to an apparent increase of bright- 
ness towards the centre in proportion to the thickness traversed by visual ray. We might, therefore, be induced 
to conclude its real constitution to be either that of a hollow spherical shell or of a flat disc.» 
> A possible connection of planetary nebulxe with nebulous stars is pointed out by W. HERSCHEL in the 
Philosophical Transactions, Vol. 20, 1802 pag. 501, 502. 
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