Toi.. 8, 1922 
ASTRONOMY: H. N. RUSSELL 
117 
where the repulsive forces from stars on opposite sides are nearly equal, 
but it can be in no true equihbrium there, and must ultimately escape to 
an indefinitely great distance. 
4. Some force, however, operates to hold these dark clouds together, 
for their outlines are often sharp. This is probably the gravitational 
attraction of the cloud itself. 
Taking a spherical cloud as an example we find that, if its mass is M 
times that of the sun, and its radius R parsecs, the velocit}^ of escape at 
the surface is 0.092M'^i?~-^km./sec. The internal velocity of the neb- 
ular material is known only in the case of the Orion nebula, where the 
luminous gas shows irregular variations in radial velocity from point to 
point, amounting to about 5 km. /sec. ^ on each side of the mean. 
For a nebula 1 parsec in diameter (which may be taken as a rough rep- 
resentation of the small black, almost round spot about 15' in diameter, 
discovered by Barnard^ in Ophiuchus) the mass must be 60 times that 
of the sun, if the escape velocity is to be 1 km. /sec. 
If all this matter were in the form of particles of rock of the optimum 
size, the extinction for light passing centrally through the cloud would 
be 2000 magnitudes. An extinction of 10 magnitudes (quite sufficient for 
opacity) would be produced if the radius of the particles were 72 ji. 
Though these numerical values are largely conjectural, it appears prob- 
able that the aggregate mass contained in one of these great obscuring 
clouds must be very considerable — probably sufficient to form hundreds 
of stars — and that a sensible fraction of the whole mass must be in the form 
of dust less than 0.1 mm. in diameter. 
It can easily be shown that any dust cloud which is impervious to light 
must also be impervious to particles such as those of which it is composed 
(and to free-moving electrons as well) in the sense that such a particle 
could not traverse the cloud without a practical certainty of collision. 
These collisions may account for the existence of dust within the clouds, 
even if it was not a primitive constituent. 
5. The transition from these dark nebulae to luminous nebulae in the 
vicinity of the stars appears to occur in two ways. The first is by simple 
reflection of the light of the stars — which appears to occur in the nebulosity 
surrounding the Pleiades, the star p Ophiuchi, and probably in many other 
cases. The second is by the excitation of gaseous emission, as in the Great 
Nebula of Orion, which is connected with one of the greatest known re- 
gions of obscuration and itself shows signs that obscuring masses lie in 
front of it. 
Both theoretical considerations, as suggested by the writer*^ and the 
facts of observation collected by Hubble,^ indicate that the luminosity 
of gaseous nebulae is probably due to excitation of the individual atoms 
by radiations of some sort (ethereal or corpuscular) emanating from neigh- 
