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ASTRONOMY: W. H. WRIGHT 
ject to revision with further observation. The agreement with the 
values calculated by theory: 5411.74, 4541.79, and 4200.02, is good. 
N. G. C. 7027 appears to be an unusually homogeneous nebula, 4686 
being probably coextensive with the helium and hydrogen Hnes. 
In addition to the ^ Puppis series of lines, another series lying in the 
ultraviolet was found by Fowler to be associated- with 4686. The first 
member of this series, 3203, has been searched for with a quartz spectro- 
graph attached to the Crossley reflector. It could not be photographed 
with a prolonged exposure, but this does not prove its nonexistence, as 
the reflectivity of silver is very low at 3200. It should be looked for 
with more suitable apparatus. 
{h) Occurrence as a nucleus line. As has been remarked, the line when 
confined to the nucleus is broadened out into a band, which is frequently 
bordered on the edge of shorter wave-length by a broad dark Hne. The 
maximum of this band is commonly displaced toward the red end of 
the spectrum, the amount of the shift being of the order of an angstrom. 
The lines of the ^ Puppis series are present, in planetary nuclei, in some 
instances as bright and in others as dark bands. In this respect they 
parallel their behavior in Class O stars in general, as described by the 
observers of the Harvard College Observatory.^ As an example we 
may cite the nebula B. D. -f 30^3639. The nucleus of this object 
is a well known Class O star, and contains numerous bright bands in 
addition to that at 4686. Nearly all of these are of the same width, 
about 10 A. A very few bands have widths of about a quarter of this 
amount, and among them are the members of the ^ Puppis series. 
These, with 4686, are bordered on the violet edge by broad dark lines. 
In only one instance has 4686 been found to exist in both nebula and 
nucleus. In N. G. C. 2392 the narrow nebular Kne contains a knot 
where the continuous spectrum of the nucleus crosses it. From the 
apparent rarity of this condition it seems probable that the transition 
from one form of radiation to the other, in any progressive change 
which may be going on in these objects, takes place with comparative 
rapidity, once it has begun. 
It is an unsettled question among, physicists whether the lines here 
discussed are due to hydrogen or helium. On this phase of the matter 
the nebulae appear to shed no light, as they afford no indication that 
the radiations belong to either of these elements. Fowler however 
noticed certain peculiarities of their locahzation in vacuum tubes, and 
it is possible that their behavior in this respect in the planetary nebulae 
may lead to some understanding of the conditions obtaining in these 
objects. 
