132 
ASTRONOMY: H. SHAPLEY 
urally show a relatively dark central area, but the projection principle 
may not be the only one involved. 
If this nebula is in process of development into a solar system, the 
indications are for a system having certain resemblances to our solar 
system. Our four outer planets have a combined mass 225 times as 
great as that of the four inner planets. Similarly in N. G. C. 7009, there 
is apparently a paucity of material to form planets near the nucleus and 
an abundance of material for planets at greater distances from the 
nucleus. 
In the course of our determinations of nebular velocities in the past 
three years we have noted a relatively large number of ring forms among 
the planetaries, and we have observed no extremely-elongated or highly- 
elliptic gaseous nebulae, and no such forms of gaseous nebulae as would 
result from thin rings seen edge-wise. According to the probabilities 
of the case, if the apparent ring nebulae are really of ring form in space, 
we should have expected to see a number of very elongated elliptic rings 
and a certain number of relatively long and narrow nebulae. We are 
accordingly led to question whether the ring nebulae are true rings or 
whether they are in reality ellipsoidal shells of nebulosity, which appear 
to be rings wholly or in part by virtue of their projection upon the plane 
of sight. 
Students of cometary orbits in the past five years have apparently 
established the Kantian view, that comets are bona fide members of our 
solar system. The observed fact that comets approach the Sun without 
marked preference for any direction of origin has seemed to be difficult 
to account for on the more prominent hypotheses of the evolution of 
the solar system. If the planetary nebula N. G. C. 7009 and other plane- 
taries which involve the ring form have in reality three principal dimen- 
sions and are developing into solar systems, the more or less uniform 
distribution of cometary matter in space of three dimensions does not 
seem to call for surprise. 
A SHORT PERIOD CEPHEID WITH VARIABLE SPECTRUM 
By Harlow Shapley 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Received by the Academy. February 14, 1916 
The most suggestive indication that the stars actually develop from 
one physical condition represented by a definite spectral type into other 
closely alHed conditions, if it is not in fact the only direct observational 
evidence of stellar evolution, is afforded by the spectra of certain short 
