Page:Popular Science Monthly Volume 82.djvu/296

292 soon cut off by the external galactic limits and have a mean distance one half as great, represented by a double proper motion of 0″.02. This hypothesis fits the observations and reconciles the conflicting results of the two investigators.

Among the many spiral or discoidal nebulæ there are some which have the plane of the disk presented edgewise, and which are foreshortened into long and narrow shapes, sometimes with a central globular condensation. Several of these elongated objects are centrally divided by a dark band. I take it that these dark bands represent the quasi-atmospheric element in question. One of the best examples is the nebula Herschel II 240 Pegasi, which is a fusiform object (as seen in projection) with a strong central condensation, and fading gradually towards the extremities. The bright mass is almost exactly bisected by a longitudinal black band, sharply defined, and about one fourth of the width of the bright part near the ends. It appears to be an equatorial belt of absorbent material, outside of, or an extension of, the margin of a luminous lenticular mass. Other examples are: HV 19 Andromedæ, HV 8 Leonis, HV 41 Canum Venaticorum, HV 24 Comæ Berenices and HI 43 Virginis. It is very probable that our own Galaxy is a similar disk-like aggregation of stars, involving spiral starstreams, and surrounded or interpenetrated by an absorbing medium which is most extensive in the plane of the disk.

In considering the absorption of light in space beyond the farthest reaches of the Galaxy, the investigation is best limited to luminous bodies of the galactic order which are neither themselves involved within the coils of our own starry system, nor situated in an extension of its plane, that is, we must exclude those objects whose galactic latitude is small. The latter, by the hypothesis, will consist of only a few near and relatively brilliant objects whose light has sufficient intensity to penetrate the galactic absorbent medium; but lest the distinction should be considered too fine, or too hypothetical, it may be waived in the present test.

I find only one nebula among those pictured by Mr. Isaac Roberts which is in a conspicuously vacant region. Of this nebula, H IV 74 Cephei = G.C. 4634 = N.G.C. 7023, Roberts says: "The nebula appears in a region almost devoid of stars." It is situated near the border of a branch of the Milky Way. Sir William Herschel has recorded his impression that nebulæ are apt to be found in regions which are poor in stars. This may be so, but an impartial examination of the photographs seems to indicate that the supposed connection between nebulæ and stellar vacuities is mainly a myth. It will require more extensive material than we now have to decide the point. Where such connection does undoubtedly exist, two different causes may be assigned for it: (1) a gaseous nebula between the Milky Way and ourselves may have a wide border of non-luminous absorbent material