314 M. Arago on Nebulce. 



Nelulce are not uniformly disseminated through all the re- 

 gions of the heavens. — Herschel, on first entering on the study 

 of nebulae, made an important remark; he found that they ge- 

 nerally form strata. One of these strata is very broad, and 

 lying in a direction almost perpendicular to the Milky Way ; it 

 is the stratum in which the Great Bear, Cassiopea, Berenice's 

 Hair, and Virgo, are found. In the middle of one of the stra- 

 ta in question, Herschel perceived no fewer than 31 perfectly 

 distinct nebulae in the short interval of 36 minutes. 



Nebulce considered in their relations to the surrounding 

 spaces. — The spaces which precede and follow simple nebula^ 

 and still more grouped nebulae, generally -contain very few 

 stars. Herschel found this rule invariable. Accordingly, 

 whenever it happened, after the lapse of a short time, that no 

 star was brought by the motion of the heavens within the 

 range of his fixed telescope, he was accustomed to say to the 

 secretary who assisted him, " Make ready to write, nebulae 

 are just approaching." 



The spaces poorest in stars are near the richest Nebulce. — In 

 the body of Scorpio there is a space o^four degrees in breadth 

 in which no stars are to be seen. On the western edge of 

 this vast obscure opening, lies the nebula marked 80 in the 

 catalogue of Connaissance des Te??tps, which Herschel consi- 

 dered the richest and most condensed mass of stars which the 

 firmament can offer to the contemplation of astronomers. 



The same phenomenon recurs near the fourth nebulous 

 group of the Connaissance des Temjjs. This group is likewise 

 situated on the western edge of a space which contains no 

 stars. 



Let us connect these facts with the observation which has 

 shewn that the stars are greatly condensed towards the centre 

 of spherical nebulas, and with that which has afforded the 

 proof that these stars sensibly obey a certain power of conden- 

 sation (or clustering power), and we shall feel disposed to ad- 

 mit with Herschel, that nebulae are sometimes formed by the 

 incessant operation of a great number of ages, at the expense 

 of the scattered stars which originally occupied the surround- 

 ing regions ; and the existence of empty, or ravaged spaces, to 

 use the picturesque expression of the great astronomer, will 



