216 SPECIAL RESULTS IN THE URANOLOGICAL 



so the experience of the resolvability of a considerable 

 number of nebulae has led in the present and most recent 

 times to inferences as to the non-existence of any true 

 nebula;, and even of any cosmical or celestial nebulosity 

 whatsoever. Whether, however, the well-defined nebulae of 

 which I have spoken be indeed composed of* self-luminous 

 nebulous matter, or whether they are merely remote, closely 

 crowded, and rounded clusters of stars, they must ever con- 

 tinue to be regarded as highly important features in our 

 knowledge of the arrangement of the structure of the Uni- 

 verse and of the contents of celestial space. 



The number of nebulae whose places in Eight Ascension 

 and Declination have been determined already exceeds 3600, 

 and some of those of irregular form and indefinite outline 

 extend over a breadth equal to eight diameters of the moon. 

 According to a former estimate of William Herschel (made 

 in 1811), at least ^f- n - of the entire surface of the heavens is 

 occupied by nebulae. Seen through colossal telescopes their 

 contemplation leads us into regions from whence, according 

 to no improbable assumptions, a ray of light requires millions 

 of years ere it can reach our eyes, to distances for which 

 the dimensions of the nearest sidereal stratum (distances of 

 Sirius, or the calculated distances of the double stars in 

 Cygnus and Centaurus), scarcely afford an adequate unit of 

 measure. Supposing the well-defined nebulae to be ellip- 

 tical or spherical clusters of stars, then their " conglome- 

 ration^ itself indicates the existence of some mysterious 

 mode of action in the gravitating forces whose influence 

 they obey. If, on the other hand, they are vaporous masses 

 having one or more nebulous nuclei, then the difference of 

 the degree of condensation exhibited tells us of the possi- 



