SECT. XXXVI. NEBULOUS STARS. 411 



have perforations as if the black sky was seen through openings 

 in the interior haze, for in no instance is the central opening 

 quite dark. 



Some nebulas are like very elliptical annular systems seen 

 obliquely. If they be elliptical flat rings, the dark centre may be 

 a real opening ; but should the systems be a series of very long 

 elliptical concentric shells surrounding a hollow, the dark axis 

 may be merely a line of comparative darkness. 



The connection of the elliptical nebulas with double stars is 

 mentioned as very remarkable. In one elliptical nebula whose 

 longer axis is 50" there are two individuals of a double star each 

 of the 10th magnitude symmetrically placed rather nearer the 

 vertex of the ellipse than the foci ; in another the stars are un- 

 equal, but placed exactly at the extremities of the major axis, 

 as in plate 8 : besides these there are several other instances. 



Double nebulas are not unfrequent in both hemispheres, ex- 

 hibiting all the varieties of distance, position, and relative bright- 

 ness, with their counterparts the double stars. The rarity of 

 single nebulas as large, faint, and as little condensed in the centre 

 as these, makes it extremely improbable that two such bodies 

 should be accidentally so near as to touch, and often in part to over- 

 lap each other, as these do. It is much more likely that they 

 constitute systems ; and, if so, it will form an interesting object 

 of future inquiry to discover whether they possess orbital motion. 

 Nebulous stars are beautiful objects, quite different from all 

 the preceding. They are round or oval, increasing in density 

 towards the centre. Sometimes the central matter is so vividly 

 and sharply condensed and denned that the nebula might be 

 taken for a bright star surrounded by a thin atmosphere. One 

 is a star of the 8th magnitude exactly in the centre of a round 

 bright atmosphere 25" in diameter ; the star is quite stellar, and 

 not a nucleus : it has not the smallest appearance of being resolv- 

 able. Another nebulous star is / Qrionis, which has a broad at- 

 mosphere in which is a dark cavity not symmetrical with the star, 

 and a small double star with a similar opening on the edge of the 

 atmosphere. Lord Eosse observes that these openings appear to 

 .be of the same nature with that within the bright stars in the 

 trapezium of Orion, the stars being at its edge ; and he is con- 

 vinced that the stars are not only connected with the nebula, but 

 that they are equidistant with it ; hence, if their parallax can be 



T 2 



