554 
ASTRONOMY: F. H. SHARES 
for hydrogen and calcium — reveal the selective distribution of vapors 
within the solar atmosphere. 
Lack of homogeneity has already been observed in numerous nebulae ; 
but probably the phenomenon is more nearly universal than has hither- 
to been supposed, and one naturally inquires whether internal differences 
may not provide useful criteria of development supplementing those 
based on spectral variations from nebula to nebula. In fact Wright 
has already called attention, in the case of planetary nebulae, to the 
gradually increasing concentration of X 4686 within the nucleus as we 
approach the Wolf-Rayet stars in his graded series of spectra. ^ He 
has made this circumstance a basis for classification, and it seems likely 
that such criteria will be even more used in the future. 
Various methods for investigating the constitution of nebulae are 
available. Measures with a spectral photometer should reveal differ- 
Seed 27 10™ 
FIG. 1. MESSIER 51. 
Iso 
ences in the distribution of the gases; but few objects are bright enough 
for an application of the method, which at best is tedious and uncertain. 
Monochromatic photographs for each spectrum line can be obtained 
with a slitless spectrograph or an objective prism; but if the nebula 
is large, or the dispersion insufficient, the images overlap. The method 
is useful for planetary nebulae, which are small and relatively bright. 
For these the ordinary slit spectrograph gives valuable results, the dis- 
tribution of luminosity being indicated by the length of the lines. That 
localized in the nucleus produces short lines, while longer lines originate 
in gases more widely distributed. The method has been used effectively 
by Campbell, Wright, and Wolf. 
For a general survey photographs on ordinary and photovisuaP 
plates should yield useful information. This method was first used by 
