The much more searching analysis of Professor J. C. Kapteyn 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, N.G.C. 7078 and 7089, and for the irresolvable and very much more distant Andromeda nebula, which indicates 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 depletion of light by a limited envelope can not be taken as an indication of nebular distance, but will vary with the constitution of the enshrouding meteoritic swarm.
To make apparent any general absorption of radiation by the interstellar 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 surrounding 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 Rerum Naturalium, 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, spectroscopic examination at the hands of Huggins and his successors divided the nebulæ into two great classes of the gaseous nebulæ with spectra of a few bright lines, and the white nebulæ with continuous spectra. This furnished the first real criterion for a fundamental distinction.
The gaseous nebulæ are so closely associated with the Milky Way that they obviously belong to our galactic system; and Ranyard'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 neighborhood 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. Among the con-
- Contributions from the Mount Wilson Solar Observatory, No. 42.
- "Emanuel Swedenborg-Opera quædam aut inedita aut obsoleta de rebus naturalibus nunc edita sub auspieiis Regiæ Academiæ Scientiarum Suecicæ. Holmiæ, 1908." II Cosmologica-Pars tertia, Paragraphus prima, N. 8 et 11, pp. 271–272.
- See A. Cowper Ranyard's completion of Proctor's "Old and New Astronomy," where the subject is discussed at some length, pp. 739–746.