ASTRONOMY: W. H. WRIGHT 
595 
at 3970A. The component to the right is He, the one to the left is 
of unknown origin. These Hnes are so close that they are only incom- 
pletely resolved on some of the plates, a rough estimate of their relative 
intensities may, however, readily be made, and it will be seen that in 
the upper spectrum the left hand component is much the stronger. 
As we go down the line it loses steadily in strength until in N. G. C. 
6543 and 6826 the two components are equal. In the Orion nebula, 
which has been inserted here, the left hand member is much the weaker 
of the two, while in the last two objects this component is practically 
missing. The same may be said of 3869A; it is strong in the upper 
pictures and finally fades out in the descent. In what appears to be a 
lesser degree there is a corresponding variation of the lines N1-2; that is, 
the so-called chief nebular h*'^es gradually weaken with reference to those 
of hydrogen. The line 3727A, on the other hand, undoubtedly strength- 
ens in the downward progression, though this does not show very clearly 
in these plates by reason of the fact that the 36-inch refractor is an un- 
suitable instrument for studying the ultra-violet, the region of the 
spectrum in which these lines lie. The spectrum of the Orion nebula, 
in which this line shows so prominently, was made with the 12-inch 
telescope; aside from the undue strength of 3727A, which is to be ac- 
counted for by this fact, this spectrum is found to correspond closely with 
those on either side of it, and appears to be intermediate between them. 
There is therefore no essential difference between it and the spectra 
of the planetary nebulae which may be said to belong to the same class. 
In the course of this work the spectra of nine planetary nebular nuclei 
have been examined. In two cases the observations so far made are 
not competent to determine with certainty the nature of the spectra. 
The remaining seven are undoubtedly Class O stars. I think it a fair 
inference that the nebulae and the Class O stars in general stand next 
to each other in the course of stellar evolution. If, as some astronomers 
believe, certain of the red stars stand next to the nebulae, it is remarkable 
that we do not find them associated with these objects. All of the 
nebular nuclei stand high on the temperature scale, if we may judge from 
their great photographic, as compared with their visual, brightness. So 
far as the development of stars from gaseous nebulae may be con- 
cerned the evidence seems to favor the usually accepted theory, rather 
than the red to red progression favored by Lockyer and some others. 
