ASTRONOMY: W. H. WRIGHT 
269 
The interesting theory of radiation proposed by Bohr, to which ref- 
erence has already been made, predicts lines separated by about two 
angstroms from the members of the B aimer series. Their verification 
in the spectra of the nebulae would be a matter of great importance, 
but the problem presents unusual difficulties which I have not yet been 
able to overcome. It is possible that future efforts will meet with 
greater success. 
An attempt has recently been made, by an excellent observer, to clas- 
sify the nebulae on the basis of the intensity of 4686. The fine is com- 
mon to both the nebulae and the Class O stars, and in the proposed sys- 
tem of classification it is assumed that the brighter a line in a nebula 
the closer the latter approaches to the stellar type. While I believe 
that the fine in question, with others, will eventually serve as the basis 
of such a classification, I am strongly of the opinion that the matter of 
the intensity of the line alone affords no clue to the position of the spec- 
trum in the scale of development unless indeed it be in a sense entirely 
opposite to the one suggested. It is not until the line settles into the 
nucleus, and is practically eliminated from the nebula proper, that the 
Class O stage is approached. In such objects as N. G. C. 2d Index 
418, N. G. C. 6826 and N. G. C. 6572, which have Class O nuclei, the 
line is confined to the nucleus, and is a very inconspicuous affair. In 
general the more prominent this Kne in the spectrum of a nebula the 
further removed is the object from the stellar stage. The nebula N. 
G. C. 7027 apparently has no Class O characteristics, but 4686 is among 
its strongest lines. 
1 London, Mon. Not. R. AsL Soc, 73, 62 (1913); E. J. Evans, Nature, London, 92, 5 (1913). 
2N. Bohr, Phil. Mag., London, ser. 6, 26, 1 and 476 (1913); A. Fowler, London, Phil. 
Trans. R. Soc, 214, 225 (1914). 
^ Astrophys. J., December, 1914. 
* A number of cases of non-homogeneity of nebulae have been reported by previous ob- 
servers. Campbell found the hydrogen lines relatively strong in the outer parts of the Orion 
and Trifid nebulae and also of N. G. C. 2d Index 418 = S. D. M. - 12° 1172. See also Hart- 
mann, Astrophys. J., 21, 389 (1905). See also Burns's observations of the Ring nebula, 
Lick Obs. Bui., 6, 92 (1910). Wolf has observed the Ring nebula in Lyra, by a method iden- 
tical with that described in this paper, and finds 4686 to be a short line. Sitz. Heid. Akad. 
Wiss., 27, 1 (1911). 
Miss A. J. Cannon, Ann. Obs. Harvard Coll., 56, 66 (1912). 
