390 
DIFFUSE MATTER IN SPACE 
54 volts, we consider that the elements in general are ionised down to a 
level of 40-50 volts. Bearing in mind the high temperature of the central 
star, this is about the value predicted by the method of § 260. 
The inferences in the last paragraph are based on the argument (§ 258) 
that in highly diffuse material not only will the spectrum of the ion 
ordinarily present appear but also that of the next lower ionisation. For 
example the ion He + will after excitation emit its proper spectrum. But 
it is also able to capture an electron; the captured electron does not 
necessarily occupy the lowest orbit in the first instance but it will soon 
come down to the lowest orbit in one or more steps and emit the spectrum 
of neutral He in so doing. 
The fact that so large a proportion of the nebular lines are unidentified 
is evidence that the conditions of ionisation are more extreme than those 
generally attained in a vacuum tube. 
It is a natural suggestion that the nebula should be regarded as an 
extended chromosphere. There is, however, little real resemblance. 
Whether radiation pressure is concerned in the phenomenon or not, it 
can scarcely be the main support of the nebula as it is of the chromosphere. 
The hydrogen is mainly ionised in the nebula and the solitary hydrogen 
nuclei are unabsorbent, so that they could not be supported. It seems 
evident that the chief support of the nebula must be rotation. 
265. Two possible lines of explanation of the annular form of many of 
these nebulae are open. It may be that the limits of the annulus mark 
the places where the conditions for emission of the light concerned cease 
to exist; or it may be that the annulus represents an actual condensation 
of the element concerned. In either case an observational result of great 
interest is the variation of size of the annulus as studied in different kinds 
of monochromatic light. When viewed through a prism a number of 
monochromatic images of the nebula are seen—one for each of the chief 
emission lines in the spectrum. These images are rings of different sizes, 
the nebulium rings being noticeably large and He + noticeably small. 
It may well happen that as we go away from the star the ionisation 
rises to a maximum and then declines. At small distances the density 
is too great, and at large distances the high frequency radiation is reduced 
by absorption in the nebula. It is, however, difficult to explain the hydrogen 
annulus in this way since ionisation of the hydrogen would seem to be 
wholly disadvantageous. 
With regard to the other explanation Milne’s investigation showed that 
a chromosphere must necessarily stand on a base and cannot float above 
the star detached from it (§ 254). But the nebula differs from the chromo 
sphere in two essential respects: firstly, it has the additional support of 
rotation; secondly, it is transparent to much of its own radiation.