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ASTRONOMY: H. N. RUSSELL 
comparable with some of the faintest stars in luminosity. Clearly, noth- 
ing final can be said on this subject until we know what it is that shines 
in the gaseous nebulae, and why. It may be remarked, however, that 
the wide-spread assumption that the origin of the stars is to be sought 
in the visible nebulae appears to have had very little solid basis. All 
classes of nebulae except the extended gaseous nebulae have already 
been excluded from consideration as observational knowledge increased. 
(b) A few nebulae, like those in the Pleiades,^^ appear to shine by light 
reflected from neighboring stars, and Slipher's spectroscopic work is 
steadily adding to the list of his discoveries in this field. Hertzsprung^^ 
has shown photometrically that the brightness of the nebulosity in the 
Pleiades is entirely consistent with the reflection hypothesis. Similar 
studies of other nebulae, and especially of the remarkable variable 
nebulae recently observed by Slipher,^° would be of value. 
Barnard's long continued researches'^ have made it highly probable 
that there exist many dark nebulae, revealed only by the effects of their 
opacity in concealing whatever lies beyond them. It is highly signifi- 
cant that the most remarkable of these dark regions is obviously directly 
connected with one of the nebulae which shines by reflected light, — 
that surrounding Rho Ophiuchi^^ — ^nd that the whole mass is com- 
paratively near us in space, at a distance of 100 to 150 parsecs. If 
such masses of practically opaque material are scattered through the 
galactic and extra-galactic regions at distances comparable with this, 
the resulting absorption of light must play a very important role in 
limiting the apparent extent of the universe. If this absorption is of the 
type which is produced by dust, or even by particles of the size of the 
drops of water in ordinary clouds, it will affect all wave-lengths to 
substantially the same extent, and be much more difficult to detect 
than the gaseous scattering, increasing for the shorter wave-lengths, 
which several investigators have sought for, but whose existence Shapley 
has apparently disproved.^^ It seems appropriate to remark in this 
connection that absorption independent of the wave length seems 
a priori much more likely to occur than the other, since the same quan- 
tity of matter in the form of a fog is incomparably more effective than 
in gaseous form, (compare the opacity of a few meters of cloud with 
that of all the rest of the atmosphere) and also since most forms of 
matter are likely to be in the solid or liquid state at the temperatures 
prevailing in interstellar space. 
(c) The forms of nebulae — especially of planetary and ring nebulae — 
deserve careful study. As Campbell suggests,^^ it is difficult to account 
for them without assuming the existence of some repulsive force which 
