DIFFUSE MATTER IN SPACE 
387 
Returning to the question whether the usual assumption of perfect 
transparency of space (apart from specially obscured regions) is justified, 
we must answer that we think it is; the only serious risk in this conclusion 
is that it neglects the possibility of absorption by meteor dust. 
Dark Nebulae and Diffuse Nebulae. 
263. We now consider diffuse nebulae such as the Orion Nebula which 
show bright line emission spectra, and dark nebulae which appear as 
obscuring patches in the sky hiding the light of the stars behind. Whilst 
it may not always be possible to discriminate correctly between an 
obscuring patch and an actual lacuna in the distribution of the stars, 
there are many cases where the existence of dark nebulosity is undoubted. 
Dark and diffuse nebulae are closely connected and grade into one another 
insensibly; sometimes a diffuse nebula is continued as an obscuring patch; 
sometimes part of a dark nebula is faintly luminous where the proximity 
of bright stars gives the necessary stimulation. 
It is now generally agreed that the luminosity of a diffuse nebula is 
stimulated by the radiation of the stars contained in it; it is often described 
as a fluorescence. The dependence of the nebular light on the'stellar radia 
tion is well shown by Hubble’s Variable Nebula, where the stimulating 
star is a variable and accordingly the nebula itself is variable. The idea is 
that the atoms in the nebula are excited by absorbing the radiation coming 
from the stimulating star or stars, and emit their characteristic bright line 
spectrum as they relapse. 
Consider for example the lines of the Balmer series ( H a , H etc.) 
which appear in the diffuse nebulae. The wave-length of the radiation 
required to raise the normal hydrogen atom to the required state is 
1025-5 A for H a ranging up to 911-5 A for the highest members of the 
series. As this is far in the ultra-violet the hot stars will be much more rich 
in the required radiation than cool stars. Accordingly luminous nebulae 
showing the hydrogen lines are generally found surrounding groups of 
B type stars. The dark nebulae may be considered to be precisely similar 
intrinsically, but lacking stars hot enough to stimulate them. It may be 
remarked, however, that it is possible for opacity to be stimulated by 
radiation—the radiation, for example, liberating more free electrons and 
thus providing absorbing mechanism. I do not suppose that the opacity 
of ordinary dark nebulae is a stimulated opacity; but an examination of 
the photographs of Hubble’s Variable Nebula strongly suggests that the 
dark patches of obscuration which appear and disappear in certain regions 
of it are governed by a varying stimulus from the associated star. 
Since the state of the nebula is far removed from thermodynamic 
equilibrium, the emission is not generally in the same wave-lengths as