Vol. 8, 1922 
ASTRONOMY: H. N. RUSSELL 
115 
In the calculations involving the atmosphere and hydrosphere the thick- 
ness of the crust is assumed to be ten (or twenty) miles, 
* Published with the permission of the Director of the U. S. Geological Survey. 
^ H. S. Washington, "Chemical Analyses of Igneous Rocks," U. S. Geol. Survey, 
Prof. Paper 99 (1917). 
2 F. W. Clarke, "The Data of Geochemistry," U. S. Geol. Survey, Bull. 695, 1920 (22 
and 33). 
DARK NEBULAE 
By He:nry Norris Russell 
Research Associate, Mount Wilson Observatory, Carnegie Institution of 
Washington 
Communicated March 14, 1922 
1. It is now generally believed that many of the dark markings in the 
Milky Way, and dark starless regions in the sky, are produced by the in- 
terposition of huge obscuring clouds between us and the remoter stars. 
A long list of such dark markings has been given by Barnard,^ who has 
done more than any one else to point out their importance and probable 
nature. In some cases, as in the Pleiades, Orion and Ophiuchus, these 
"regions of obscuration" merge into faintly luminous nebulosity in the 
vicinity of certain stars, in such a way that there can be no doubt that 
they lie near these stars in space. 
It thus appears that the obscuring masses or dark nebulae in Ophiuchus 
and Scorpius are at a distance of from 100 to 150 parsecs, those in Taurus 
at probably about the same distance, and those in Orion some 200 parsecs 
from us, while the dimensions of the individual clouds are themselves 
measured in parsecs. 
The occurrence of these three great regions of obscuration within a 
distance which is so small compared with that of the galactic clouds in- 
dicates that such objects are probably of great cosmical temperature. 
2. These dark nebulae usually appear to be quite opaque. In some 
cases the stars can be seen faintly through them, apparently without much 
change in color; but in certain instances^ stars imbedded in dense luminous 
nebulosity are abnormally red. 
Of the various forms in which matter may be distributed in space, by 
far the most efficient in producing obscuration is fine dust, since this has 
the greatest superficial area per unit of mass. In a cloud composed of 
spherical particles of radius r and density p, distributed at random so that 
the average quantity of matter per unit volume is the extinction of a beam 
of light in passing through this cloud will be e stellar magnitudes per unit 
of distance, where e = 0.814 qd/pr. 
