THE SUN'S CORONA AND HIS SPOTS. 23 



such a corona as was shining in July, 1 878. For the power of 

 the spectroscope to show objects which under ordinary con- 

 ditions are invisible, depends on the separation of rays of 

 certain tints from the rays of all the colours of the rainbow, 

 which make up solar light ; and as the corona in July, 1878, 

 shone with all the colours of the rainbow, and not with 

 certain special tints, the power of the spectroscope would be 

 thrown away on a corona of that kind. All that we can 

 ever hope to do is to discern the gaseous corona when, as 

 in 1871, it is well developed, by spectroscopic appliances 

 more effective for that purpose than any which have hitherto 

 been adopted ; for all which have as yet been adopted have 

 failed. 



Now, the difficulty of the problem will be recognised 

 when we remember that the strongest tints of the corona's 

 light the green tint classified as 1474 Kirchhoff has been 

 specially but ineffectually searched for in the sun's neigh- 

 bourhood with the most powerful spectroscopic appliances 

 yet employed in the study of the coloured prominences. In 

 other words, when the light of our own air over the region 

 occupied by the corona has been diluted as far as possible 

 by spectroscopic contrivances, the strongest of the special 

 coronal tints has yet failed to show through the diluted 

 spectrum of the sky. Again, we have even stronger evi- 

 dence of the difficulty of the task in the spectroscopic 

 observations made by Respighi during the eclipse of 1871. 

 The instrument, or I should rather, perhaps, say the ar- 

 rangement, which during mid totality showed the green 

 image of the corona to a height of about 280,000 miles, did 

 not show any green ring at all at the beginning of totality. 

 In other words, so faint is the light of the gaseous corona, 

 even at its brightest part, close to the sun, that the faint 

 residual atmospheric light which illuminates the sky over 

 the eclipsed sun t the beginning of totality sufficed to 

 obliterate this part of the coronal light. 



Whether with any combination specially directed to 

 meet the difficulties of this observation, the gaseous corona 



