PORTION OP THE COSMOS. NEBULAE. 237 



still greater in those circular, well-defined, planetary nebulous 

 disks which show an entirely uniform brightness not in- 

 creasing towards the centre. Such a state of things seems 

 less compatible with the form of a globe (or with thousands 

 of small stars in a state of aggregation) than with the idea 

 of a gaseous photosphere, which in our sun is supposed to 

 be covered with a thin, untransparent, or at least very faintly 

 illuminated vaporous stratum. May it be that in the 

 planetary nebula the light appears so uniformly distributed 

 only because the difference between the margin and the 

 centre disappears by reason of the great distance ? 



Among the nebulse of regular forms, the fourtli and last 

 class consists of Sir William HerschePs "nebulous stars/' 

 *'. e. actual stars surrounded by a milky nebulosity, which is 

 very probably in relation with the central star and dependent 

 on it. Whether the nebulosity which, according to Lord 

 Eosse and Mr. Stoney, appears in some cases quite annular, 

 (Phil. Trans, for 1850, PL xxxviii. figs. 15 and 16), 

 should be regarded as self-luminous, and as forming a pho- 

 tosphere as in our sun, or whether, (as seems less probable), 

 it be merely illuminated by the central sun, are points on 

 which very different opinions prevail. Derham, and to a 

 certain degree Lacaille, who at the Cape of Good Hope dis- 

 covered several nebulous stars, believed the stars to be 

 distant from and unconnected with the nebulae on which 

 they appeared projected. Mairan (1731) appears to have 

 been the first who put forward the opinion of nebulous stars 

 being surrounded with a luminous atmosphere of their own 

 (4ii j. There are even larger nebulous stars (for example of 

 the 7th magnitude, as No. 675 of the Catalogue of 1833), 



