2 9 2 THE POPULAR SCIENCE MONTHLY 



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 nebulae there are some which 

 have the plane of the disk presented edgewise, and which are fore- 

 shortened 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 Andromeda, HV 8 Leonis, HV 41 Canum Yenaticorum, HV 24 

 Comce Berenices and HI 43 Virginis. It is very probable that our own 

 Galaxy is a similar disk-like aggregation of stars, involving spiral star- 

 streams, 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 in- 

 tensity to penetrate the galactic absorbent medium; but lest the dis- 

 tinction 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 Eoberts 

 which is in a conspicuously vacant region. Of this nebula, H IV 74 

 Cephei = G.C. 4634 — N.G.C. 7023, Eoberts 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 nebula? 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 nebulae 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 our- 

 selves may have a wide border of non-luminous absorbent material 



