RECENT ADVANCES IN SCIENCE n 



it difficult to form a rational theory of their structure and life- 

 history. When to this difficulty is added the necessity of 

 accounting at the same time for the complicated spectrographic 

 information, the problern would seem at first sight to be im- 

 possible to solve. Curtis classifies the forms shown by these 

 objects into seven general types, and proves that some of the 

 simpler types can be reasonably explained on the hypothesis 

 of oblate spheroidal shells of matter, which in some cases may 

 be approximately homogeneous, but which in others must be 

 thinner at the equatorial zone than at the poles. A falling in 

 of the matter from the polar zones would then give a possible 

 explanation of the peculiar S-shaped spectral lines given by 

 some of the nebulae. The planetaries are difficult objects to 

 fit into the scheme of stellar evolution, and this matter is not 

 solved in the present volume. The only statement which can 

 definitely be made is that the planetary stage of existence 

 must be relatively very brief — otherwise the number of plane- 

 taries would be much larger. In this they resemble the Wolf- 

 Rayet stars, with which their nuclei show an essential identity. 

 It is very improbable that the rarity of the planetary nebulae 

 is due to large numbers remaining to be discovered ; the 

 Harvard spectrographic survey of the sky has increased the 

 number of classified stellar spectra from about 9,000 to over 

 200,000, but this increase of over 2,000 per cent, has only 

 added one new object with a spectrum of planetary type. 



Part IV, the spectrographic velocities of 125 bright-line 

 nebulae, by W. W. Campbell and J. H. Moore, is an important 

 contribution to our knowledge of the velocities of nebulae 

 and of the complicated structure of various nebular lines. 

 Full details are given of the observations of each nebula. 

 Several results of interest are obtained. The average velocity 

 of the planetary nebulae, after separating the component of 

 the solar motion, is found to be about 30 km. per sec. Since 

 this is many times larger than the average velocity of the B- 

 type stars, it does not seem probable that the latter have evolved 

 from planetary nebulae. Seventeen nebulae have been ob- 

 served in the Greater Magellanic Cloud, and one in the Lesser 

 Cloud. The former give a mean velocity of 363 km. per sec. 

 recession, and the latter gives a velocity of 307 km. per sec. 

 recession. The velocity of the Greater Magellanic Cloud is 

 considered in more detail by R. E. Wilson in the next part. 

 The probable masses of three planetary nebulae whose paral- 

 laxes have been determined are estimated on reasonable 

 assumptions, and found to be much greater than the mass of 

 our solar system. The behaviour of the spectral lines taken 

 in conjunction with the photographic evidence leads Campbell 

 and Moore to conclude that many of the planetaries may be 



