1870.] DOUGLAS — ON OBSERVATIONS OF THE SUN. 139 



but at the equator one-fourtli of the sun's diameter in height ; 

 diminishing in intensity from within outwards. The rays, which 

 to the eye seemed distinct and in constant motion, like cilia, form 

 in the photograph, of necessity, from the length of exposure, an 

 unbroken sheet of light. Prof. Hume describes the structure of 

 the corona as " fibrous, slightly crooked, or twisted, somewhat 

 like a cirrous cloud, and of silvery whiteness." The dim haze 

 seen round the moon in other photographs is probably also 

 produced by the corona, as the chromosphere would give a better 

 defined outline. 



"We saw that the Indian observers disagreed as to the spectros- 

 copic character of its light, M. Rayet finding no spectrum, and 

 Major Tenuant a continuous one. Prof. Young thinks it gave a 

 faint, continuous spectrum, and that three of the lines, viz. : 1250, 

 1350 and l-iTO, w^hich he found in the protuberance spectrum, 

 extend into the corona, and that these three are the lines which 

 Prof. Winlock detected in the spectrum of the aurora borealis. 

 Prof. Young is, however, not confident of the accuracy of his 

 observation, and thinks it possible that the three lines in question 

 may extend only beyond the more visible parts of the protuber- 

 ance into that hazy region which the photographs dimly reveal, 

 as if it were a shadow thrown by the flame. These three lines 

 are not exactly coincident with any known lines, though they 

 vary very little from three iron lines. 



Prof. Harkness is not doubtful of the accuracy of his observa- 

 tion. He found the corona to yield a faint continuous spectrum, 

 with one bright line, whose position is given in the table above. 

 He remarks : — " The brightness of the continuous portion was 

 about equal to — perhaps slightly less than — that of the spectrum 

 which I get from the moon in the same instrument; and I am 

 perfectly convinced that there was no absorption lines. I looked 

 particularly for them, and the light was sufiiciently intense, and 

 the slit sufiiciently narrow, for me to have seen them if they had 

 been present. The bright line was tolerably conspicuous, but it 

 did not stand out so glaringly as the bright line in the promi- 

 nences. So far as a single observation can be depended upon, it 

 seems to me that this one tends to prove that the corona is a 

 highly-rarified, self-luminous atmosphere surrounding the sun, and 

 that it is composed principally of iron in the state of incandescent 

 vapor. Probably the selective absorption of the continuous 

 portion of the spectrum is not sufiiciently strong to do more than 



