THE ECLIPSE OF 1871. 123 



brightness as from the region of actually glowing 

 hydrogen. 1 



A difficulty remains in the fact that the spectrum 

 of iron contains upwards of four hundred and fifty 

 bright lines, and that the green line in question is not 

 y even the most conspicuous of these. Nor, indeed, is it 

 absolutely certain that this particular line, though 

 always seen in the spectrum of iron, belongs actually 

 to that metal. At present, however, the most probable 

 conclusion appears to be that which has been presented 

 above ; and we may suppose either that the other lines 

 of iron are really present, but too faint for recognition, 

 or that their absence is due to the special circumstances 

 under which iron exists in the upper regions of our own 

 air and in the rare hydrogen atmosphere of the sun. 



In our journey outwards from the sun's light surface, 

 we have now approached the inner boundary of the 

 most interesting of all the solar surroundings, the 

 outer radiated corona, the reality of which had been so 

 long disputed. Eespecting this appendage occupying 

 a space enormously greater than any structure known 



1 It should be explained that if an electrical discharge passes from 

 iron to iron through hydrogen, the observed spectrum is a combination 

 of the iron spectrum and the hydrogen spectrum. Now the actual 

 brightness of light in this case is not inferior to that of hydrogen 

 glowing with intensity of heat ; but the total quantity of light is less 

 than that which would be obtained if the whole of the hydrogen in the 

 tube were so glowing. In like manner, the supposed electrical dis- 

 charges in the sun's hydrogen atmosphere would produce a light as 

 intense in itself as that of the sierra ; but as the discharges would 

 cause portions only of the inner corona to flow with this light, the total 

 luminosity would be far inferior to the luminosity of the sierra, where 

 all the hydrogen is aglow with its own heat. 



