SECT. XXXVI. NUMBER OF NEBULA. 407 



appear to us to be 10,057 days instead of 10,000. Were the star 

 to approach to the sun by the same quantity instead of receding, 

 the apparent periodic time would be diminished by 57 days. 



As the sun is only a unit in the stellar system, so the Milky 

 Way, and all the stars that adorn the firmament of both hemi- 

 spheres, constitute a group which is but a unit among the infinite 

 numbers of starry clusters and nebulas that are profusely scat- 

 tered throughout the universe. 



By the aid of a good telescope there may be seen on the clear 

 vault of heaven, between the stars of our own stellar system, and 

 far in the depths of space, an immense multitude of objects like 

 comets or clouds of white vapour of all forms and sizes. Some 

 are mixed with stars, others are entirely formed of them. Many 

 appear as if they were stellar, but required a telescope of higher 

 power to resolve them, and vast numbers appear to be matter 

 rarefied in the highest possible degree, giving no indication of a 

 stellar nature; and these are in every state of condensation, from 

 a vague film hardly to be discerned to such as have actually 

 arrived at a solid nucleus of stars. The cloudy appearance is 

 merely the blending of the rays of innumerable stars which are 

 themselves invisible from their extreme distance, like parts of the 

 Milky Way. Sir William Herschel was at first of that opinion, 

 and the nebulas that have been resolved by Lord Rosse's telescope 

 have led astronomers to believe that such is the case. Yet the 

 tails of comets, the zodiacal light, and the extensive luminous 

 atmospheres which encompass many of the stars, show that, in 

 all probability, a self-luminous phosphorescent material substance 

 in a highly diluted or gaseous form exists in vast abundance. 



The number of the nebulas, like that of the stars, is only limited 

 by the imperfection of our instruments, for each improvement in 

 the telescope only enables us to penetrate a little farther into the 

 infinity of space to see a few more of these shadowy existences 

 in the far distance, and to resolve a few more of those that are 

 comparatively near. Sir William Herschel examined the nature 

 and determined the position of 2500 nebulas in the northern 

 hemisphere whose places were computed from his observations, 

 reduced to a common epoch, and arranged into a catalogue, in 

 order of right ascension, by his sister, Miss Caroline Herschel, 

 who added lustre to the name she bore by her eminence in astro- 

 nomical knowledge and discovery. Sir John Herschel revised 



