ASTRONOMY: H. N. RUSSELL 
409 
One of the most attractive of unexplored fields is the investigation of 
the Magellanic Clouds. The small amount of work which has been done, 
mainly on the Smaller Cloud, has led to the discovery of a remarkable 
relation between the periods and absolute magnitudes of the variables 
in the Cloud,^^ to the estimate that its distance is 20,000 parsecs,^^ and 
to the discovery that the nebulae within it, and probably the Cloud as a 
whole, have a very high radial velocity.^^ The great instruments which 
are now being erected in the southern hemisphere may well be actively 
directed toward this region. 
13. (a) Foremost among the many problems presented by the 
nebulae is the cause of their luminosity. In spite of our ignorance of the 
origin of the characteristic nebular lines, the appearance of such lines as 
X 4686 in the spectra of nebulae, and of the Wolf-Rayet spectrum in their 
nuclei, suggests that in them 'Sve are presented" (in Fowler's words)''* 
''with phenomena which result either from the effects of powerful elec- 
trical actions or of very elevated temperatures." Though such condi- 
tions may easily enough exist in the nuclei, it is very hard to see how 
high temperatures can prevail throughout the whole volume of a 
nebula.* There are several possible ways out, however. 
The electrical action may be a bombardment of the outer region by cor- 
puscles emitted from the nucleus. Or perhaps the luminosity of the gases 
is fluorescent, like that of the sodium or bromine vapors studied by 
Wood.^^ Or, as Fabry has recently suggested," we may have to do with 
a body which absorbs and emits radiation only in narrow regions of 
short wave-length, and may therefore attain a very high temperature in 
thermal equilibrium with the radiations from a distant, but still hotter, 
source. To determine the true explanation among these and many 
other possibilities may tax the resources of both experimental and 
theoretical spectroscopy. 
The association of gaseous nebulae with stars of ' early' spectral type 
might be anticipated on any of these theories. For such stars are very 
hot bodies, and would be the most powerful sources both of corpuscular 
and ultra-violet radiation. Hence the association of these stars with 
nebulae does not prove that the stars originate from the nebulae. It is 
entirely conceivable that, on the contrary, the nebulae, as visible ob- 
jects, owe their existence to the radiation of the stars, and are their 
offspring, and not their parents. Some gaseous nebulae, however, are 
not near bright stars, and the nuclei of planetary nebulae appear to be 
* Fabry's calculated temperature of 15,000° for the Orion Nebula,^^ as he points out, 
is liable to be diminished by an unknown amount on account of the widening of the lines 
of the spectrum by turbulent motion of the nebulaj matter in the line of sight. 
