WRIGHT— RECENT SPECTROSCOPIC OBSERVATIONS. 523 



second group, the extended white nebulae.^ The gaseous nebulae, 

 from their peculiar distribution along the Milky Way, are undoubt- 

 edly to be regarded as forming part of our system of stars, and 

 in that sense furnish available material for our speculations. 

 Furthermore they are the only ones whose spectra we are able to 

 study with any degree of completeness, so that from the point of 

 view of spectroscopy, our hope, for the present, must lie very 

 largely with them. For these and other reasons it is between the 

 stars and the gaseous nebulae that a relationship has generally been 

 sought. The connection has been claimed between the nebulae and 

 the Class O, or Wolf-Rayet stars, by those favoring one hypothesis 

 on the basis of certain spectral similarities, and on the rather pecu- 

 liar distribution of both groups of objects along the path of the 

 Milky Way. The opposition, on the other hand, points to the 

 occurrence of the bright lines of hydrogen in both the gaseous 

 nebulae and some of the red stars; and quite recently a number of 

 the nebulium lines have been observed by Merrill at Mount Wilson 

 to occur temporarily in the spectra of stars of that class. It will 

 be seen that as between these two hypotheses, the matter of this 

 connection with the nebulae is one of vital concern. If the gaseous 

 nebulae can be shown to be related to the Wolf-Rayet stars the 

 first hypothesis is strengthened, if, on the other hand, the con- 

 nection is with the red stars the favor goes to the other theory. The 

 relationship of the nebulae to the one group or the other may then, 

 in a sense, be regarded as a criterion to determine between these 

 two conceptions as to the nature of stellar evolution. 



I find it difficult in these remarks to be brief, and at the same 

 time to avoid seeming to imply a degree of definiteness with respect 

 to inferences that may be drawn from the evidence, and in fact, 

 with respect to our conceptions of cosmogony, which would not be 

 justified either by the available observational data nor by the present 

 scientific point of view. I believe that astronomers, particularly 

 observers, are, as a rule, not disposed to dogmatize on the subject 



3 These nebulae like the gaseous or bright-line nebulae favor the Milky 

 Way, and are therefore to be regarded as members of our sidereal system, 

 but our knowledge of their spectra is so limited that it does not afford a 

 secure basis for speculation as to their physical constitution. 



