of a circle, a ring, an ellipse, or even a straight line. Nebulæ of this latter kind are represented in Fig. 9 and in Fig. 10. When an elliptical ring is extremely elongated, and the minor axis is much smaller than the major one, the density and brightness of the ring diminish as its distance from the central nucleus increases; and this takes place to such a degree sometimes, that at the farthest points of the ring, the ends of the major axis, it ceases to be visible, and the continuity seems to be broken. The nebula has then the appearance of a double nebula, with a central spot as represented in Figs. 11, 12.
| Fig. 11. | Fig. 12. |
 | |
| Double Nebula. | Annular Nebula with Centre. |
Those nebulæ, which appear with tolerably sharply-defined edges in the form of a circle or slight ellipse, seem to belong to a much higher stage of development. From their resemblance to those planets which shine with a pale or bluish light, they have been called planetary nebulæ; in form, however, they vary considerably, some of them being
Fig. 13.
Planetary Nebula with Two Stars.
spiral and some annular. Some of these planetary nebulæ are represented in Figs. 12, 14, 15. The first has two central stars or nuclei, each surrounded by a dark space, beyond which the spiral streaks are disposed; the second has also two nuclei, but without clearly separable
