WRIGHT— RECENT SPECTROSCOPIC OBSERVATIONS. 525 



of stellar evolution, and there is an opinion, which I myself share, 

 that we have not as yet sufficient evidence on which to found a 

 secure conception of cosmogomy ; still any fact is of scientific interest 

 only in so far as it is related to others, and it is therefore necessary 

 to examine all observational material from every reasonable point 

 of view, and to appraise as well as possible its significance. It is 

 for this reason that the present observations, in spite of their 

 meagreness, are examined with respect to their bearing on our 

 present notions of stellar evolution. 



The gaseous nebulae present a bright line spectrum such as is 

 shown in the middle section of Fig. 3. The lines on the extreme right 

 are the so-called first and second nebular lines. They are of unknown 

 chemical origin. The third is due to hydrogen. The fourth strong 

 line is the remarkable one at wave-length 4686 A, regarded, since the 

 advent of the Bohr theory of the atom, as due to the recombination 

 of completely ionized helium. It will, in these remarks, be re- 

 ferred to as the fourth nebular line. The upper spectrum is that 

 of a red, the lower one of a Class O star. The red star has, Hke 

 the nebula, narrow bright lines due to hydrogen and nebulium.* The 

 Class O spectrum is composed of broad bright bands, a few of 

 which correspond in position with the bright lines in the nebula. 

 While having points in common, the three spectra are distinct. 

 There has in past years been much discussion of their possible rela- 

 tionships, but until comparatively recently no certain connection had 

 been established between either of the two outside spectra shown 

 on the screen and the nebular spectrum in the center. The evidence 

 which we have here to discuss is afiforded by an examination of the 

 nuclei or small star-like condensations in the planetary nebulae. 

 The investigation of these minute objects is somewhat exacting in 

 its demands on the resources of observation, for the spectrum must 

 be isolated from that of the rest of the nebula. When this is done 

 they are found in many instances to exhibit the spectra of Class O 

 stars. The diagram (Fig. 4) will indicate the method of making the 

 observations. The sketch on the left represents the telescopic image 



4 The spectrum here shown is that of the variable R Aquarii, already men- 

 tioned as having been found by Merrill to exhibit, temporarily, the lines of 

 nebulium. 



