234 SPECIAL RESULTS IN THE URANOLOGICAL 



are the aqueous nebulae of our atmosphere, the clouds. The 

 normal form ( 402 ) of the celestial nebulae is considered to be 

 elliptical or spheroidal. With equal telescopic power, such 

 nebulae are most easily resolvable into star-clusters when 

 they are most globular; and on the other hand when the 

 compression in one direction and elongation in the other 

 is greatest they are the most difficult of resolution ( 403 ). 

 We find in the heavens gradually varying forms from 

 round to elliptic more or less elongated. (Phil. Trans., 

 1833, p. 494, PI. ix. figs. 1921.) The condensa- 

 tion of the milky nebulosity is always progressive towards 

 a centre, or as in some cases even towards several central 

 points or nuclei. It is only in the class of round or oval 

 nebulae that double-nebulae are known ; and in these, as 

 there is no perceptible relative motion of the individuals 

 in respect to each other, (either because no such motion 

 exists, or that it is exceedingly slow), we are without the 

 criterion which would enable us to demonstrate the reality 

 of a mutual relation, and which in the case of double stars 

 we possess for distinguishing those which are physically from 

 those which are merely optically double. (Drawings of 

 double-nebulae are to be found in the Philosophical Trans- 

 actions for 1833, figs. 68 71. Compare also Herschel, 

 Outlines of Astronomy, 878, and Observations at the 

 Cape of Good Hope, 120.) 



Annular nebulae are among the rarest phenomena with 

 which we are acquainted. In the northern hemisphere, 

 according to Lord Rosse, seven are known to us. The 

 most celebrated annular nebula is the one situated between 

 J3 and y Lyrae (No. 57, Messier; No. 3023 of Sir John 

 HerscheFs Catalogue) ; it was first observed by Darquier at 



