236 SPECIAL RESULTS IN THE URANOLOGICAL 



texture as if curdled/* They never show traces of conden- 

 sation towards the centre. Five planetary nebulae have been 

 recognized by Lord Rosse as annular nebulae with one or 

 two central stars. The largest planetary nebula is situated 

 in the Great Bear, (not far from ft Ursae raaj.) and was 

 discovered by Mechain in 1781. The diameter of the 

 disk ( 406 ) is ' 40". The planetary nebula in the Southern 

 Cross, (No. 3365, Cape Observations, p. 100,) with a disk 

 of scarcely 12" diameter, has the brightness of a star of 

 between the 6th and 7th magnitudes. The colour of its 

 light is an indigo-blue, and (among nebulae) this remark- 

 able hue is found also in three objects of a similar form, in 

 which, however, the blue is less intense ( 407 ). The blue tint 

 of some planetary nebulae by no means contradicts the possi- 

 bility of their being composed of small stars, for we are 

 acquainted with blue stars, not only as forming both members 

 of a pair or double-star, but also in clusters consisting 

 either entirely of blue, or of blue mixed with red and yellow 

 small stars ( 408 ). 



The question whether the planetary nebulae are very 

 distant nebulous stars in which the difference between an 

 illuminating central star and a surrounding vaporous en- 

 velope escapes our telescopic vision, has been alluded to in 

 an earlier portion of my work ( 409 ) . May Lord Rosse's giant 

 telescope at length afford the means of investigating the nature 

 of these wondrous planetary vaporous disks ! Difficult as 

 it is to form a clear conception of the complicated dynamic 

 conditions under which, in a spherical or spheroidically 

 elliptical cluster of stars, the rotating, congregated suns, 

 becoming specifically denser as the centre is approached, 

 form a system in equilibrium ( 41 ), this difficulty becomes 



