THE LIGHT OF THE STARS 291 



spicuously 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 exert- 

 ing 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, 5 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 nebulae 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 pene- 

 trate 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 di- 

 minishes in extra-galactic latitudes. 6 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 ex- 

 planation that the Galaxy is a discoidal aggregation of stars with 

 limits less remote than is sometimes assumed, permits the suppo- 

 sition 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," 7 



5 The central regions of a galactic accumulation of stars may be expected to 

 be relatively free from meteoritic material, for here we have a space swept clean 

 by the stellar attraction which gathers in the material and places it where it can 

 be readily absorbed. In the more distant intergalactie spaces, the meteoritic 

 material is widely dispersed, but upon the borders of the galaxies there are 

 accumulations of finely divided matter, not yet incorporated in the stars. 



6 Contributions from the Mount Wilson Solar Observatory, No. 42, pp. 23-24. 



7 Publications of the Astronomical and Astrophysical Society of America 

 Vol. 1, p. 282; see also Astronomical Journal, No. 558. 



