378 FORMS OF THE NEBULAE. SECT. XXXVII. 



telescope, or are just about to quit it. Occasionally 

 they are so vague that the eye is conscious of some- 

 thing, without being able to define what it is : but the 

 unchangeableness of its position proves that it is a real 

 object. Many present a large ill-defined surface, in 

 which it is difficult to say where the center of the 

 greatest brightness is. Some cling to stars like wisps of 

 cloud ; others exhibit the wonderful appearance of an 

 enormous flat ring seen very obliquely, with a lenticular 

 vacancy in the center (N. 226). A very remarkable in- 

 stance of an annular nebula is to be seen exactly half- 

 way between /9 and y Lyrae. It is elliptical in the ratio 

 of 4 to 5, and is sharply defined, the internal opening oc- 

 cupying about half the diameter. This opening is not 

 entirely dark, but filled up with a faint hazy light, aptly 

 compared by Sir John Herschel to fine gauze stretched 

 over a hoop (N. 227). There is a very remarkable 

 nebula in Orion, in which there is some reason to believe 

 that a new star has recently appeared. Two nebulae 

 are described as most amazing objects : One like a 

 dumb-bell or hour-glass of bright matter, surrounded by 

 a thin hazy 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 

 (N. 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 sep- 

 arated at an angle of 45 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" (N. 229). 

 It appears that double nebulae are not unfrequent, ex- 

 hibiting 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 center as these, makes it very improbable 

 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 systems ; 

 and if so, it will form an interesting subject of future in- 

 quiry to discover whether they possess orbitual motion. 



