ASTRONOMY: W. H. WRIGHT 
167 
In another publication reference has been made to the fact that the 
nuclei of the planetary nebulae are in general Class 0, or to use an older 
term, Wolf-Rayet stars. ^ The inquiry into the relationship between 
the spectra of the nebulae and their nuclei has necessitated a study of 
the distribution of light in the nebulae, line by line. This has been 
pursued by various means, only one of which need be referred to here. 
If the slit of the spectroscope be placed across the image of one of these 
usually symmetrical nebulae, those lines which are emitted throughout 
its full extent will be longer than those given out say only close to the 
center. In this way the length of a line may be taken as the measure 
of the extent of its occurrence in the source, just as is the case in labora- 
tory studies of the arc, spark and other terrestial sources. 
It has been found that in the majority of cases the nebulae are not 
homogeneous,^ that is, lines observed in the above described manner 
have different lengths, and in general there is a certain order of length 
for the different lines. Selecting a few from the sixty or more nebular 
lines we may say that 4686 is always among the shortest, followed by 
the Hues of helium, 'nebulium' and hydrogen in the order of . length. 
An interpretation of this phenomenon on the assumption that the nebu- 
lae are in the process of condensation into stars is that the lines shrink 
into the nucleus in this order. We have examples of nebulae in many 
stages of the process. The present purpose however is not to enter into 
a speculative discussion of the development of stars, but to sketch the 
behavior of the line 4686 and the members of the related series. 
The line 4686 is found in two forms : (a) as a narrow line in the nebu- 
losity of some of these objects, and {h) as a broad band confined to the 
nuclei of certain others. Its occurrence in both forms in the same 
nebula is rare, one case only of this sort having been observed. 
{a) Occurring in the first form. The line presents the appearance of 
a monochromatic radiation, under the strongest dispersion used, that 
of three dense prisms. It is generally (perhaps always) accompanied 
by very faint ^ Puppis lines (5411, 4541, 4200) of a similar character. 
It is to be looked for in those nebulae in which the lines 5007 and 4959 
have their greatest strength relatively to the hydrogen lines. A case 
in point is the nebula N. G. C. 7027, the brightest known planetary. 
In this object the position of the line, derived from four photographs 
secured with a three prism spectrograph, is 4685.93, (R), a quantity 
in satisfactory agreement with Fowler's laboratory determination of 
4685.98 (Rowland). The wave-lengths of the three related hues were 
measured with a dispersion of one prism. They are 5411.4, 4541.7, and 
4202.2. The last value is not very trustworthy and all three are sub- 
