Page:Popular Science Monthly Volume 82.djvu/295

Rh vacant spaces in the Milky Way may be noted those running east from Rho Ophiuchi, others east of Theta Ophiuchi, and mingled star clouds and vacancies in Sagittarius near 18 hours right ascension and 11° south declination.

Since there exist these enormously extended masses of gaseous or misty material, capable, whether themselves luminous or dark, of exerting a strong absorption upon the light of any bodies beyond them, and intimately associated with the Milky Way; and since, further, it is inevitable that the broad disk of the galactic accumulation must have gathered into its vicinity great swarms of meteoritic material, acting after the manner of a general, widely distributed mist, forming an envelope analogous to an atmosphere having its greatest depth in the direction of the galactic plane; it follows that this extensive quasi-galactic atmosphere and its associated, but locally limited, gaseous bodies must especially absorb the light from those distant galaxies which lie in or near the plane of the Milky Way. This, it seems to me, is the probable explanation of the extraordinary increase in the numbers of the white nebulæ near the poles of the Galaxy, namely, that the galactic quasi-atmosphere being thinnest along a diameter at right angles to the plane of the swarm, the light of external galaxies is best able to penetrate through the obstructions if coming from this direction.

Kapteyn's recognition of absorption by an interstellar medium also supports the above explanation, since he finds that the absorption diminishes in extra-galactic latitudes. Professor Comstock, it is true, reaches a different result, finding that stars of the 10.5 magnitude have larger proper motions as their galactic latitude increases, whence he concludes that "at right angles to the Galaxy the limits of the stellar system fall within the range of vision," which may be correct, but his explanation that this is so because "the transmission of light through and that this medium offers little obstruction in the direction of the galactic plane does not necessarily follow. The simple explanation that the Galaxy is a discoidal aggregation of stars with limits less remote than is sometimes assumed, permits the supposition that the 10.5-magnitude stars in the galactic plane comprise many relatively bright stars at a double distance and having a mean annual proper motion of 0″.01, whereas the extra-galactic stars are the extra-galactic spaces is impeded by some absorbing medium,"