436 STELLAK NEBULE. [SKCT. xxxvn. 



atmosphere, so as to give the whole an oval form, or the 

 appearance of an oblate spheroid. This phenomenon bears 

 no resemblance to any known object (K 228). The other 

 consists of a bright round nucleus, surrounded at a distance 

 by a nebulous ring split through half its circumference, and 

 having the split portions separated at an angle of 45 degrees 

 each to the plane of the other. This nebula bears a strong 

 similitude to the Milky Way, and suggested to Sir John 

 Herschel the idea of a " brother system bearing a real 

 physical resemblance and strong analogy of structure to our 

 own " (K 229). 



Double nebulae are not unfrequent in both hemispheres, 

 exhibiting all the varieties of distance, position, and relative 

 brightness, with their counterparts the double stars. The 

 rarity of single nebulae as large, faint, and as little con- 

 densed in the centre as these, makes it extremely im- 

 probable that two such bodies should be accidentally so 

 near as to touch, and often in part to overlap each other, as 

 these do. It is much more likely that they constitute sys- 

 tems ; and, if so, it will form an interesting object of future 

 inquiry to discover whether they possess orbital motion. 



Stellar nebulas are beautiful objects, differing from all of 

 these, and occur rarely. They are round or oval, increasing 

 in density towards the centre. Sometimes the central 

 matter is so highly and rapidly condensed as to give the 

 whole the appearance of a star with a blur, or like a candle 

 shining through horn, and occasionally the central matter 

 is so vivid and so sharply defined, that the nebula might be 

 taken for a bright star surrounded by a thin atmosphere. 

 There is an elliptical one of great beauty, faintly visible to 

 the naked eye in the girdle of Andromeda. The extensive 

 and delicate atmosphere of these nebulous stars assumes all 

 degrees of ellipticity from the circular to the spindle-shaped 

 ray, or almost the straight line (N. 230). 



The zodiacal light or lenticular-shaped atmosphere of the 

 sun, which may be seen extending beyond the orbits of 



