Observations of 500 Nebulae. 



19 



to a total absence of any suspicion of stars indicated by the slightest mott- 

 ling of surface, etc." 



To this class, as is well understood, the planetary and the annular 

 among other nebulse and the globular clusters are consequently to be rec- 

 koned. Concerning Class II and Class III Sir John (1. c, pp. 142, 143) 

 says: "We come now to consider the next great division of our nebulous 

 system II. Irregular nebulce, in which class may be comprehended all which, 

 to a want of complete and in most instances even of partial resolvability 

 by the power of the 20-feet reflector, unite such a deviation from the circular 

 or elliptic form , or such a want of symmetry (with that form) as precludes 

 their beeing placed in Class I, or that of Regular Nebulse. This second 

 class comprises many of the most remarkable and interesting objects in the 

 heavens, as Avell as the most extensive in respect of the area they occupy. 

 Such, for instance, as the great nebulse about e Ononis, v\ Argus, etc.; 

 as well as those smaller but still very extensive and most singular objects 

 IV.41, 8Messier, etc.; the partially symmetrical forms M.51, M.27, etc. ; etc.". 



"The third principal class of subdivision of these objects is that of 

 Irregular Clusters. This class will comprehend all the clusters designated 

 by Sir W. Hebschel as of the VII and VIII classes, and such of those 

 of his VI class as are irregular in their form and can not be referred to 

 the class of Globular Clusters." 



To obtain a convenient form for the descriptions of the nebulse, these 

 classes are further divided into 5 subclasses in respect to magnitude, bright- 

 ness, roundness, condensation and resolubility, which especially among the 

 nebulse of Class I become of greater consequence, as those objects are 

 capable of being systematically described from those several points of view, 

 what is not the case with the nebulse of the two other classes, and in the 

 summary description of which Sir John Herschel consequently proposes 

 only to indicate size and brightness. I have with Herschel admitted 5 de- 

 grees in respect to the 5 principal points in question and thus for the sum- 

 mary descriptions of the nebulse of Class I employed the following scheme. 



