1889.] Nebulae and Stars with those of Comets and Aurora?. 31 



identical. The flutings of hot carbon, for example, are common to 

 both, as are also the flutings of magnesium, manganese, and lead. 

 The hydrogen line 486 has only been seen in one comet, namely, 

 Comet III, 1880, by Konkoly * 



Other flutings and lines again are special to comets and others to 

 nebulas. Thus, there are practically no indications of hydrogen in 

 comets, although the hydrogen lines are amongst the brightest in 

 nebulae. Again, the lines 447, 479, 495, 509, 554, and 5872 are seen 

 in nebulas, but not in comets. On the other hand, the cool carbon 

 flutings and the fluting at 568 are seen in comets, but not in nebulas. 

 Most of these apparent discrepancies are explained by a consideration 

 of the differences in the conditions of comets and nebulas. It must 

 be remembered that in the case of comets there is an action which 

 repels the vapours produced by collisions, and the vapours first 

 affected will, of course, be those which are least dense. Hydrogen 

 will thus be repelled from the comets, whilst the denser vapours of 

 magnesium and carbon remain. There is then a good reason why 

 hydrogen lines should not be seen in cometary spectra. As there can 

 be no such repulsion in the sparse swarms which constitute nebulas, 

 hydrogen lines are seen in them. 



Two other lines special to nebulas are. 5872 and 447, to which refer- 

 ence has already been made. The evidence tends to show that D 3 

 and / are finer vapours than hydrogen, and hence there is even 

 greater reason for the absence of these lines from cometary spectra, 

 even were the temperature higher, than for the absence of the lines 

 of hydrogen. 



The line at 527 is probably the iron line E ; this was seen in the 

 hotter comets, namely, Comet "Wells and the Great Comet of 1882, so 

 that there is no discordance with regard to the appearance of this 

 line. The other lines special to nebulas are 479, 495, 509, and 554 ; 

 but as no origins for these have yet been determined, it is not possible 

 to explain their absence from cometary spectra. It is not improbable 

 that 554 is an error in measurement for the manganese fluting at 558, 

 the latter having been recorded by Mr. Taylor in the nebula of Orion. 



[On November 25, Mr. Fowler attempted to compare this line, as 

 seen in the planetary nebula Gr.C. 4373, with the manganese fluting, 

 but the line was so faint with the 10-inch that no reliable comparison 

 could be made. The line is certainly not far from the manganese 

 fluting. — December 8.] 



The apparent absence of the cool carbon flutings from nebulas is in 

 all probability due to insufficient observations, as indicated by the 

 discussion of comets. The lowest temperature (magnesium) and the 

 hot carbon stages of comets are both represented in nebulas, and the 



* O'G-yalla Observations, 1881, p. 5. 



