PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 2 OCTOBER 15. 1916 Number 10 
PRELIMINARY RESULTS ON THE COLOR OF NEBULAE 
By Frederick H. Seares 
MOUNT WILSON SOLAR OBSERVATORY, CARNEGIE INSTITUTION OF WASHINGTON 
Received by the Academy, August 4, 1916 
Important differences in the spectra of gaseous nebulae, even those 
of the same general class, have been known for many years. Although 
the characteristic nebular lines X 5007 and X 4959 seem to maintain a 
constant ratio of brightness, their behavior varies greatly with respect 
to H^, X 4686, X 3727, and other lines. Thus is usually fainter than 
the chief nebular line X 5007, but there is a wide range in the relative 
intensities, and intensity reversals may even occur. Again, lines usually 
present are as yet unobserved in certain objects, and in some we have 
apparently only the monochromatic radiation X 3727.^ 
This diversity is reminiscent of the differences in the spectra of stars. 
These are universally accepted as evidence of evolutionary change, and 
if nebulae have any part in the scheme of development, it is likely that 
their spectral peculiarities will find a similar interpretation. In this 
event spectral correlations must exist, more or less clearly defined 
according to the development of the nebulae compared. The important 
investigations of Wright have already revealed relationships presum- 
ably of this kind. 
Further, experience in other fields suggests the probability of spectrum 
differences within the same nebula. No mixture of luminous vapors 
is homogeneous in its radiation, for the pressure, temperature, electrical 
conditions, etc., are never constant throughout. Thus the spectrum 
changes from point to point in the flame and the electric arc; the radi- 
ation from the attenuated gases of a vacuum tube varies with the region 
examined; and, on a larger scale, the spectrum of the corona, the chro- 
mosphere, and the reversing layer — or, even better, spectroheliograms 
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