sponded to the nitrogen line of the nebular spectrum; the other, which was still conspicuous, though faint, corresponded to the hydrogen line of nebulæ.
That however, was by no means the closing chapter of this singular history. Vogel seems to have ceased from observing the star's spectrum, strangely enough, at the very time when the most remarkable 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 November. 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, save a spectrum resembling the sun's, and we regard them as different only because of their clustering condition. But the nebulæ which Sir W. Herschel regarded as consisting entirely of luminous vapor, and which spectroscopic analysis has proved to be so constituted, we have regarded not merely as different because of the structure and arrangement of their component parts, but as differing altogether in constitution. Now, the object seen as a faint blue star showed the same spectrum as these gaseous nebulæ, or rather as the very faintest of these nebulæ. 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 nebulæ—that is, with a light which, under spectroscopic analysis, presents only one bright line.
The words in which Lord Lindsay announced this remarkable discovery 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 hypothesis." On this last point I venture to express dissent from Lord
