Besohmble Nebiilce, 311 



tensive as the first. To that succeeded, in 1802, a third cata- 

 logue of five hundred new nebula?. Tivo thousand five hundred 

 nebulae — such, then, was Herschel's contribution to a branch 

 of astronomy scarcely entered upon before his time. The ex- 

 tent of it, at the same time, is the least merit of this great work, 

 as we shall see. 



RESOLVABLE NEBULJE. 



Their form. — Nebulae — even those to which that name is 

 improperly given, or which can be resolved into stars by means 

 of powerful telescopes, — present themselves under a great va- 

 riety of forms. There are some of them which, being greatly 

 elongated and very narrow, may almost be taken for simple 

 luminous lines, straight or serpentine ; others, opening in the 

 shape of a fan, resemble an aigrette diverging from a strongly 

 electrified point. In some cases, the contours have no regu- 

 larity ; in others, one would suppose they had the head and 

 nucleus of a comet. Let us attend to more detailed defini-. 

 tions. 



Circular Nebulce. — The circular form is that which resolva- 

 ble nebulae appear most commonly to assume. Herschel de-. 

 voted himself to the examination of circular nebulae in a most 

 particular manner. He has deduced from his observations im- 

 portant results, of which I shall endeavour to give an exact 

 idea. 



The circular form is only apparent ; the real form must be 

 globular or spherical. An observation which I shall imme- 

 diately refer to will render this evident. 



In general, the stars of which these nebulae are composed 

 appear to be very nearly of the same size.* They are distri- 



* Although the rule I have prescribed for myself prevents me entering 

 upon memoirs posterior to those of William Herschel, I cannot re- 

 sist the temptation of bringing forward in this place two curious observnt- 

 tions by James Dunlop. This astronomer, during his residence at Para- 

 matta, New Holland, remarked, at 1 Ih 29^ 20s of right ascension, and 

 29"*1C' of southern polar distance, a resolvable nebula of 10' diameter, in 

 which shone three red stars and a yellow one, displaying these peculiar kinds 

 of light in the midst of a multitude of white stars. On another occasiox:^ 



