ADDRESS. 19 



has pointed out too that so far as his observations go the white stars in 

 the constellation of Orion stand alone, with the exception of Algol, in 

 possessing a dark line in the blue which has apparently the same posi- 

 tion as a bright line in the great nebula of the same constellation ; and 

 Pickering finds in his photographs of the spectra of these stars dark lines 

 corresponding to the principal lines of the bright-line stars, and the plane- 

 tary nebulae with the exception of the chief nebular line. The association 

 of white stars with nebular matter in Orion, in the Pleiades, in the region 

 of the Milky Way, and in other parts of the heavens, may be regarded 

 as falling in with the view that I have taken. 



In the stars possibly further removed from the white class than our 

 sun, belonging to the first division of Vogel's third class, which are dis- 

 tinguished by absorption bands with their stronger edge towards the 

 blue, the hydrogen lines are narrower than in the solar spectrum. In 

 these stars the density-gradient is probably still more rapid, the depth of 

 hydrogen may be less, and possibly the hydrogen molecules may be 

 affected by a larger number of encounters with dissimilar molecules. In 

 some red stars with dark hydrocarbon bands the hydrogen lines have not 

 been certainly observed ; if they are really absent, it may be because the 

 temperature has fallen below the point at which hydrogen can exert its 

 characteristic absorption ; besides, some hydrogen will have united with 

 the carbon. The coming in of the hydrocarbon bands may indicate a later 

 evolutional stage, but the temperature may still be high, as acetylene 

 can exist in the electric arc. 



A number of small stars more or leas similar to those which are known 

 by the names of their discoverers. Wolf and Rayet, have been found 

 by Pickering in his photographs. These are remarkable for several 

 brilliant groups of bright lines, including frequently the hydrogen lines 

 and the line D3, upon a continuoiis spectrum strong in blue and violet 

 rays, in which are also dark lines of absorption. As some of the bright 

 groups appear in his photographs to agree in position with corresponding 

 bright lines in the planetary nebulae, Pickering suggests that these stars 

 should be placed in one class with them, although the brightest nebular 

 line is absent from these stars. The simplest conception of their nature 

 would be that each star is surrounded by a nebula, the bright groups beine 

 due to the gaseous matter outside the star. Mr. Roberts, however, has 

 not been able to bring out any indication of nebulosity by prolono-ed 

 exposure. The remarkable star 17 Argus may belong to this class of 

 the heavenly bodies. 



In the nebulae, the elder Herschel saw portions of the fiery mist or 

 ' shining fluid ' out of which the heavens and the earth had been slowly 

 fashioned. For a time this view of the nebulas gave place to that which 

 regarded them as external galaxies, cosmical " sandheaps,' too remote to 

 be resolved into separate stars ; though indeed in 1858 Mr. Herbert 



c 2 



