34 COSMOS. 



elucidating the nature of these remarkable planetary vapor 

 ous disks ! Although there is considerable difficulty in ac- 

 quiring a clear conception of the complicated dynamic condi- 

 tions under which, in a globular or spheroidally flattened stel- 

 lar cluster, the rotating crowded suns, whose specific density 

 is greater toward the center, constitute a system of equilibri- 

 um ;* this difficulty increases still more in those circular, 

 well-defined, planetary nebulous disks which exhibit a per- 

 fectly uniform brightness, without any increase of intensity to- 

 ward the center. Such a condition seems to depend less upon 

 sphericity of form (the state of aggregation of many thousand 

 small stars) than on the existence of a gaseous photosphere, 

 which is supposed, as in our Sun, to be covered with a thin, 

 untransparent, or very faintly illuminated stratum of vapor. 

 Does the light in the planetary nebulous disk appear to be 

 thus uniformly diffused simply in consequence of the great 

 distance, which causes the difference between the center and 

 the margins to disappear ? 



The fourth and last order of regular nebulas comprises Sir 

 William Herschel's nebulous stars, i. e., true stars surround- 

 ed by a milky nebula, which is very probably connected with, 

 and dependent upon, the central star. Very different opin- 

 ions exist as to whether the nebula, which, according to Lord 

 Rosse and Mr. Stoney, appears to be perfectly annular in some 

 of these groups (Philos. Transact, for 1850, pi. xxxviii., figs. 

 15 and 16), is self-luminous, forming a photosphere like our 

 Sun, or whether (which, however, is less probable) it is sim- 

 ply illumined by the central Sun. It was the opinion of Der- 

 ham, and to some extent also of Lacaille, who discovered 

 many nebulous stars at the Cape of Good Hope, that the stars 

 were situated far from the nebulas on which they were pro*- 

 jected. Mairan appears (1731) first to have expressed the 

 view that nebulous stars are surrounded by an atmosphere of 

 light appertaining to thern.f We even find that some of the 

 larger stars (of the 7th magnitude, for instance, as No. 675 



* On the development of the dynamic relations manifested in the 

 partial attractions in the interior of a globular cluster of stars, which ap- 

 pears in a telescope of weak power as a round nebula increasing in 

 density toward the center, see Sir John Herschel, in Outlines of As- 

 tronomy, 866 and 872; Observations at the Cape, 44, 111 to 113; 

 Philos. Transact, for 1833, p. 501 ; Address of the President in the 

 Report of the Fifteenth Meeting of the British Association, 1845, p. 

 xxxvii. 



t Mairan, Traite de TAurore Boreale, p. 263 ; Arago, in the Annuaire 

 for 1842, p. 403-413. 



