SECT, xxxvii.] TWO CLASSES OF NEBULAE. 435 



its being an unfathomable congeries of stars, which there is 

 reason to believe has changed its form in some parts within 

 the last fifty years. Vast multitudes of nebulae of this kind 

 are so faint as to be with difficulty discerned at all till they 

 have been for some time in the field of the telescope, or are 

 just about to quit it. Occasionally they are so vague, that 

 the eye is conscious of something being present, without 

 being able to define what it is ; but the unchangeableness of 

 its position convinces the mind that it is a real object "an 

 image was before mine eyes, but I could not discover the 

 form thereof." 



The other class of nebulae, vastly inferior in size, of 

 definite forms and great variety of character, are scattered 

 through the remote heavens, or congregated in a great 

 nebulous district far from the Milky Way. Many cling to 

 stars like wisps of clouds, others are exactly like comets 

 with comae and tails, some are annular, either circular or 

 like an enormous flat ring seen very obliquely, with a len- 

 ticular vacancy in the centre (N. 226). A very remarkable 

 example of an annular nebula is to be seen mid-way be- 

 tween /3 and y Lyrge. It is elliptical in the ratio of 4 to 5, 

 and is sharply defined, the internal opening occupying about 

 half the diameter. This opening is not entirely dark, but 

 filled with a faint hazy light, aptly compared by Sir John 

 Herschel to fine gauze stretched over a hoop (N. 227). Its 

 diameter must be 1300 times greater than that of the earth's 

 orbit if as far from us as 61 Cygni dimensions that are most 

 astounding. These objects are like hollow shells, whose 

 borders seem brighter because the substance, whatever it 

 be, is more condensed to appearance than the central part. 

 They are exceedingly rare, there are only seven known, three 

 of which have stellar or nebulous nuclei in the centre ; two 

 of the latter are in the northern hemisphere, and one in the 

 southern. Two nebulae in the northern hemisphere are 

 described as most amazing objects, one like a dumb-bell, or 

 hour-glass of bright matter, surrounded by a thin, hazy 



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