Page:Popular Science Monthly Volume 82.djvu/298

294 the same author explains the dark ring around the nebula near Rho Ophiuchi on the supposition that "the smaller and more slowly moving immigrants. . . are stopped by the particles of the nebulæ," and are detained by the gathering crowd. But even if it could be demonstrated that the stars are arranged in thin sheets, and that celestial bodies exist of sufficient size and momentum to plow their way through great aggregations of stars, demolishing everything in their track, it would still be exceedingly improbable that only one layer of stars should exist in a given direction, or that several rifts should coincide. On the other hand, the presence of widely extended masses of dark absorbent matter in the shape of branching streams, sheets or rings, situated between us and the depths of star-strewn space, is not unlikely.

The gaseous nebulæ which form a part of the galactic structure are often very extensive, and are of a great variety of shapes, being frequently strangely irregular; but the more numerous white nebulas are formed more nearly after a common pattern, although still with infinite variation as to details. In general, what is common to nearly all of the white nebulæ is a tendency to form a two-branched spiral, the branches issuing from opposite sides of a central condensation, and coiling either within the boundaries of a plane circular disk, or forming a helix around a cylindrical directrix. The former figure is the more characteristic, and is well exhibited in the Great Nebula in Andromeda.

Another very remarkable and at present unique type is the transient nebulosity which appeared around Nova Persei, issuing from the star as a center, and expanding into the commencement of a vortical ring. It was an electric phenomenon, an exhibition of canal rays, or positive ions, on a grand scale. Facts from the history of these two bodies will be found useful in preparing one of the necessary means for our quest.

It is obvious that we require for this investigation of external galaxies some scale of distances, and equally obvious that at present such a scale can be only approximate. Indeed, it is probably this uncertainty as to the scale on which the universe is constructed which deters astronomers from attempting to discriminate between different galactic orders. I propose to see if this uncertainty can be, in part, removed.

I propose to take the distance of the Andromeda nebula as our celestial "yardstick," which may be called one andromede, and assuming that when we consider a large number of nebulæ, the average size does not vary with the distance, and that consequently the average distances may be taken inversely proportional to the angular diameters of the objects, I shall classify the nebulæ according to apparent size and brightness. It is essential that the subdivision shall not be too minute.