NEBULA. 33 



greater number are round, or somewhat oval, and either 

 sharply defined, or indistinct and vaporous at the margins. 

 The disks of many of these nebulae present a very uniform 

 light, while others appear mottled, or of a peculiar texture 

 as if curdled. No trace of condensation round a central point 

 has ever been observed. Lord Rosse has recognized five plan- 

 etary nebulous spots to be annular nebulae, having one or two 

 central stars. The largest of these planetary nebulae is sit- 

 uated in the Great Bear (near /3 Ursae Maj.), and was discov- 

 ered by Mechain in 1781. The diameter of the disk* is 2' 

 40". The planetary nebula in the Southern Cross (No. 3365, 

 Observations at the Cape, p. 100), with a disk having a di- 

 ameter scarcely equal to 12", exhibits the brightness of a star 

 of the 6 -7th magnitude. Its light is indigo-blue, and the 

 same color, which is very remarkable in nebulae, is observed 

 in three other objects of the same form, although in the lat- 

 ter the blue is less intense.! The blue color of some plan- 

 etary nebulae does not militate against the possibility of their 

 being composed of small stars ; for we find blue stars not only 

 as the individual members of a pair of double stars, but even 

 stellar clusters composed entirely of blue stars, or of the lat- 

 ter interspersed with small red and yellow stars. $ 



The question whether planetary nebulae are very remote 

 nebulous stars, in which our telescopic vision is unable to rec- 

 ognize the difference between a luminous central star and the 

 vaporous envelope surrounding it, has already been considered 

 in the beginning of my Delineation of Nature. k Would that 

 Lord Rosse's colossal telescope might finally be the means of 



* If we consider the planetary nebula in the Great Bear to be a 

 sphere having an apparent diameter of 2' 40", " and assume its distance 

 to be equal to the known distance of 61 Cygni, we shall obtain an act- 

 ual diameter for the sphere, which is seven times greater than the orbit 

 described by Neptune." — Outlines, § 876. 



t Outlines, p. 603; Observations at the Cape, § 47. There is an or- 

 ange-red star of the eighth magnitude in the vicinity of No. 3365 ; but 

 the planetary nebula retains its deep indigo-blue color when the red 

 star is not in the field of the telescope. The color is, therefore, not the 

 effect of contrast. 



X Cosmos, vol. hi., p. 136, 208, and note. The companion and the 

 main star are blue, or bluish, in more than 63 double stars. Indigo- 

 blue stars are intermixed in the splendid, many-colored clusters of stars, 

 No. 3435 of the Cape Catalogue (Dunlop's Catalogue, No. 301). An en- 

 tirely uniform blue cluster of stars is observed in the southern heavens 

 (No. 573 of Dunlop ; No. 3770 of Sir John Herschel). This cluster has 

 a diameter of 3£', with prolongations measuring 8' in length; the stars 

 are of the 14th and 16th magnitude. (Observations at the Cape, p. 119.) 



§ Cosmos, vol. i., p. 85, and note. Compare Outlines, § 877. 



B 2 



