NEBULA. 321 



samp colour, which is very remarkable in nebulae, is observed 

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

 latter the blue is less intense. 10 The blue colour of some 

 planetary nebula3 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 latter interspersed with small red and yellow stars." 



The question whether planetary nebula? are very remote 

 nebulous stars, in which our telescopic vision is unable to 

 recognize the difference between a luminous central star, and 

 the vaporous envelope surrounding it, has already been consi- 

 dered in the beginning of my Delineation of Nature* Would 

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

 of elucidating the nature of these remarkable planetary va- 

 porous discs! Although there is considerable difficulty in 

 acquiring a clear conception of the complicated dynamic 



10 Outlines, p. 603; Observations at the Cape, 47. 

 There is an orange-red star of the eighth magnitude in the 

 vicinity of No. 3365; but the planetary nebula retains its 

 deep indigo-blue colour when the red star is not in the field 

 of the telescope. The colour is, therefore, not the effect of 

 contrast. 



61 Cosmos, vol. iii. pp. 184, 283, 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-coloured clusters of stars, No. 3435 of the Cape Cata- 

 logue (Dunlop's Catalogue, No. 301). An entirely 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.) 



53 Cosmos, vol. i. p. 68 and note. Compare Outlines- 

 $877. 



