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PROFESSOR A. FOWLER, F.R.S., OX 



of immediate interest is that the experimental resources of our 

 laboratories are already sufficient for the reproduction of some 

 of the lines which appear in the spectra of stars which are 

 believed to be among the hottest in the heavens. It has not 

 been necessary to discover new elements to match these 

 celestial spectra, but only to develop new lines from old 

 elements by increasing the energy of excitation. 



The lines of proto-helium, as we may now call them, are of 

 further importance as a connecting link between the B stars 

 and the Wolf-Ray et stars, which present us with spectra 

 consisting chiefly of bright lines. The familiar lines of hydrogen, 

 and sometimes those of ordinary helium, are prominent as 

 bright lines in these spectra, and are accompanied by several 

 other lines among which only those of proto-helium have yet 

 been identified. These stars seem to fall naturally just before 

 the B stars in the order of development, and are so placed in 

 the Harvard classification. 



Nearly all the stars which have been spectroscopically 

 examined fall into one or other of the classes which have been 

 described, and in seeking for the antecedents of stars we 

 naturally look to nebulae, which were regarded as representa- 

 tives of the parent masses from which stars had been developed, 

 long before the spectroscope was thought of. 



Nebulas take many varied and beautiful forms, and are 

 spectroscopically divisible into two great classes. One of these 

 classes includes the spiral nebulae, of which several thousands 

 are now known, and the spectra have in recent years been 

 found to consist of dark lines very similar to those of the sun 

 or of the F type of stars. Where to place these nebulae in a 

 scheme of celestial evolution is a difficult problem. The 

 supposition that they represent universes exterior to our own 

 would, perhaps, be in best accordance with the spectra, but, as 

 Herbert Spencer pointed out long ago, their peculiar distribution 

 in the heavens is a serious obstacle to the adoption of this 

 view. Any doubt which might have remained as to their 

 consisting of the same kinds of matter as the sun and stars has 

 now been removed, but why they should all present the same 

 spectrum as star clusters is still mysterious. 



The second class of nebula?, exemplified by the Great Nebula 

 in Orion and the Ring Nebula in Lyra, show spectra consisting 

 of a comparatively small number of bright lines. The chief 

 line in the visible spectrum, in the green, has not yet been 

 found in any terrestrial source, and has been attributed to a 

 hypothetical element, "nebulium." Several other lines are also 



