On the Corona of the Sun. 



Ill 



the adjoining parts, in a degree not far removed from the eye's power 

 of distinguishing adjacent areas which differ by a small degree of 

 brightness. 



If, therefore, by any method of observation even a small advantage 

 could be given to the coronal light as compared with the air-glare, 

 and, especially, if, at the same time, we could by any method 

 accentuate the small difference of illumination, a method might be 

 found by which the corona could be observed. 



When the report of the photographs taken during the Egyptian 

 eclipse of 1882 reached this country, I was led to conclude that the 

 coronal light as seeu from the earth was strong in the violet, and 

 probably to some extent also in the ultra-violet part of the spectrum. 



Apart from the question of the greater relative intrinsic intensity 

 of the more refrangible region of the coronal light as a whole, or of 

 any one of its components (its gaseous component gave bright lines 

 in the violet region), there are two considerations which show us that 

 the coronal light should be strong in the violet as compared with the 

 air-glare near the sun. 



The selective absorption of our atmosphere would cause the light 

 scattered by it in the near neighbourhood of the sun to be relatively 

 poor in this part of the spectrum ; but there is a second cause acting 

 in the same direction, which arises from the selective power of absorp- 

 tion of the sun's atmosphere. 



The absorption which the photospheric light suffers from the solar 

 atmosphere has been investigated by Professor Langley, Professor 

 Pickering, and especially with great minuteness by Professor H. C. 

 Vogel. Vogel found that while at the edge of the sun's disk the red 

 light was reduced to 30 per cent, of its value at the centre of the disk, 

 the violet light was reduced to 13 per cent. 



Vogel sums up by saying that if the solar atmosphere were removed, 

 the brightness of the violet part of the sun's light would be increased 

 about three times, but the red light one and a-half times only.* The 

 selective action would doubtless be more strongly marked beyond 

 the visible limit. 



The rapid increase of absorption near the sun's limb, in Vogel's 

 observations, indicates a low and dense solar atmosphere. Professor 

 Langley agrees in this view of the sun's atmosphere. He says, " The 

 portion of the (sun's) atmosphere chiefly concerned in absorption, I 

 have been led to believe, from several considerations, is extremely thin, 

 and I am inclined to think is mainly identical with the reversing layer 

 at the base of the chromosphere." 



Professor Hastings also considers the " layer which produces absorp- 



* " Spectralpbotometriscbe Untersuchungen." " Monatsbericbt der K. Ak. d. 

 Weissenscbaffcen." Berlin, Marz, 1877. Also " Ueber die Absorption der cbemicale 

 wirksamen Strablen in der Atmosphare der Sonne," ibid., Juli, 1872. 



