2 9 o THE POPULAR SCIENCE MONTHLY 



The much more searching analysis of Professor J. C. Kapteyn 2 

 favors an actual absorption of light from the more distant stars, but a 

 very much smaller one than that demanded by Comstock's result. 

 Kapteyn's method, however, when applied to bodies more remote than 

 the nearer stars, gives about the same amount of absorption for the 

 easily resolvable clusters, IST.G.C. 7078 and 7089, and for the irre- 

 solvable and very much more distant Andromeda nebula, which indi- 

 cates that his absorbent medium is a local adjunct to these stellar 

 masses, and that it is perhaps a meteoritic envelope of somewhat greater 

 volume than the stellar agglomeration, but not a universal medium 

 filling all space. The circumferential absorption or scattering deple- 

 tion of light by a limited envelope can not be taken as an indication 

 of nebular distance, but will vary with the constitution of the en- 

 shrouding meteoritic swarm. 



To make apparent any general absorption of radiation by the inter- 

 stellar medium, it becomes necessary to investigate the properties of 

 space far beyond the limits of the Galaxy and its outlying shells of 

 sparsely distributed stars, and, crossing the immense voids of surround- 

 ing ether, to inquire whether they contain other galaxies of dimensions 

 comparable with our own, and whether these afford any evidence of a 

 gradual absorption of luminous energy by the intervening medium. 



The first scientific enunciation of the doctrine that there are such 

 external galaxies was given in 1734 by Emanuel Swedenborg in his 

 Principiorum Berum Naturalium, 3 and Herschel's nebular discoveries 

 lent some support to the doctrine; but it was not until after 1864 that 

 further evidence really bearing on the question came. Then, spectro- 

 scopic examination at the hands of Huggins and his successors divided 

 the nebulas into two great classes of the gaseous nebulas with spectra of 

 a few bright lines, and the white nebulas with continuous spectra. This 

 furnished the first real criterion for a fundamental distinction. 



The gaseous nebulas are so closely associated with the Milky Way 

 that they obviously belong to our galactic system; and Eanyard's 

 recognition of wide, dark lanes or spots, often branching or dendritic 

 in form, blotting out extensive regions on Barnard's photographs of 

 the Milky Way, showed that not all of the gaseous bodies in its neigh- 

 borhood are luminous, but that some are to be compared to a dark 

 smoke or mist, obscuring the glories of the brightness which lies back 

 of the widely extended and absorbent cosmic cloud. 4 Among the con- 



2 Contributions from the Mount Wilson Solar Observatory, No. 42. 



8 ' ' Emanuel Swedenborg-Opera quasdam aut inedita aut obsoleta de rebus 

 naturalibus nunc edita sub auspieiis Regias Academise Scientiarum Suecicag. 

 Holmise, 1908." II Cosmologiea-Pars tertia, Paragraphus prima, N. 8 et 11, 

 pp. 271-272. 



4 See A. Cowper Eanyard's completion of Proctor's "Old and New Astron- 

 omy," where the subject is discussed at some length, pp. 739-746. 



