i 5 o THE POPULAR SCIENCE MONTHLY. 



sponded to the nitrogen line of the nebular spectrum ; the other, which 

 was still conspicuous, though faint, corresponded to the hydrogen line 



of nebulas. 



That however, was by no means the closing chapter of this sin- 

 gular history. Vogel seems to have ceased from observing the star's 

 spectrum, strangely enough, at the very time when the most remark- 

 able part of the process of change seemed to be approaching. At the 

 Dunecht Observatory also, through pressure of work relating to Mars, 

 no observations were made for nearly half a year. But, on September 

 3d, Lord Lindsay's 15-inch refractor was turned on the star. In the 

 telescope a star was still shining, but with a faint blue color, utterly 

 unlike that of the orb which had shone out so conspicuously last No- 

 vember. Under spectroscopic examination, however, the blue star 

 was found to be no star at all, if we are to regard those orbs only as 

 stars which present a spectrum in some degree analogous to that of 

 our own sun. We regard Sirius as a sun, though in his spectrum the 

 lines of hydrogen are abnormally strong ; and, passing over the class 

 of stars more closely resembling our sun, we regard as a true star the 

 orange orb, Betelgeux, though the lines of hydrogen are wanting in 

 its spectrum ; nor do we reject from among the suns those stars which, 

 like Gamma of Cassiopeia, show the lines of hydrogen bright upon a 

 fainter rainbow-tinted spectrum. There is yet another order of stars 

 those whose spectrum presents bright bands with faintly lustrous 

 internals, which, again, we regard as true suns, though they differ 

 doubtless notably from our own. But we have been in the habit of 

 regarding all the star cloudlets, whether consisting of multitudinous 

 stars, like the clusters, or of luminous star-mist, as differing toto coelo 

 from the sun and from all his fellow-stars. The clusters, indeed, sdve 

 a spectrum resembling the sun's, and we regard them as different only 

 because of their clustering condition. But the nebulas which Sir W. 

 Herschel regarded as consisting entirely of luminous vapor, and which 

 spectroscopic analysis lias proved to be so constituted, we have re- 

 garded not merely as different because of the structure and arrange- 

 ment of their component parts, but as differing altogether in constitu- 

 tion. Now, the object seen as a faint blue star showed the same spec- 

 trum as these gaseous nebulas, or rather as the very faintest of these 

 nebulas. For most of them show three bright lines, and one or two 

 even show four bright lines ; only the faintest shine with absolutely 

 monochromatic or one-tint light. The star in Cygnus now shines like 

 these faintest of the gaseous nebulas that is, with a light which, un- 

 der spectroscopic analysis, presents only one bright line. 



The words in which Lord Lindsay announced this remarkable dis- 

 covery are these : " There is little doubt but that this star has changed 

 into a planetary nebula of small angular diameter,'''' though, he goes 

 on to say, " such a result is in direct opposition to the nebular hy- 

 pothesis." On this last point I venture to express dissent from Lord 



