Planetary nebulæ have much more in common than irregular nebulæ, and their minor varieties might, with some plausibility, be associated with differences in relative age. They are marked chiefly by the character of the nuclear star which, in nearly all such objects, appears to act as the pivot of the surrounding vaporous structure. The supposition lies close at hand that it is designed as a provision for the nourishment of the star--that the star gains in mass and light at the expense of the nebula, which it is eventually destined wholly to absorb and supersede. On this view, planetaries like the green glow-lamp at the pole of the ecliptic (N.G.C. 6,543) should be regarded as the most advanced, while Webb's planetary in Cygnus (N.G.C. 7,027) would exemplify an inchoate condition. In the former the central star is of 9·6 magnitude, and sharply stellar; in the latter it is double and diffuse,[85] perhaps a wide binary system in embryo. The question is, however, still open as to the real nature of the connection between planetaries and their central stars. The pabulum theory is a promising conjecture; but no facts with which we are acquainted stringently enforce it. Ideas on the subject will need complete revision if the traces of spirality noted from time to time in some of these peculiar objects prove to be of radical significance. The _oculi_, distinctive of the 'Owl nebula' (N.G.C. 3,587), as originally shown by the Parsonstown reflector, consisted of luminous traceries coiled round _two_ interior stars,[86] but the appearance was either due to illusion, or became effaced by change, since the camera has refused to endorse it as genuine. The 'helical' planetary in Draco[87] is doubtless essentially a spiral conformation;[88] and Professor Schaeberle, by means of exposures with a 13-inch reflector of 20 inches focus, has compelled, not only the Ring nebula in Lyra,[89] but the Dumb-bell in Vulpecula, to betray the surprising secret of their whorled structure. Both these nebulæ give a spectrum of bright lines, and inventiveness is at a loss to devise means for building up gaseous materials into edifices of strongly characterized architectural forms. The materials, indeed, may not be wholly gaseous;[90] or we possibly see (as Professor Darwin long ago suggested) merely illuminated stream-lines of motion furrowing an obscure mass. But if this be indeed so, there is the further question to be asked: What direction does the motion take? Do the tides set inward or outward?