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THE POPULAR SCIENCE MONTHLY. 



of a circle, a ring, an ellipse, or even a straight line. Nebulas of this 

 latter kind are represented in Fig. 9 and in Fig. 10. When an ellip- 

 tical ring is extremely elongated, and the minor axis is much smaller 

 than the major one, the densitj 7 and brightness of the ring diminish a& 

 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. 



Pig. 12. 



Double Nebula. 



Annular Nebula with Centre. 



Those nebulas, 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 neb- 

 ulae ; in form, however, they vary considerably, some of them being 



Fig. 13. 



Planetart Nebula with Two Stars. 



spiral and some annular. Some of these planetary nebulas are repre- 

 sented 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 



