1885.] on the Solar Corona. 203 



The object of our quest is to be found in the glory of radiant 

 beams and bright streamers intersected by darker rifts which appears 

 about the sun at a total solar eclipse. The corona possesses a struc- 

 ture of great complexity, which is the more puzzling in its intricate 

 arrangement because though we seem to have a flat surface before us, 

 it exists really in three dimensions. If we were dwellers in Flatland 

 and the corona were a sort of glorified catherine-wheel, the task of 

 interpretation would seem less difiicult. But as we are looking at an 

 object having thickness as well as extension, the forms seen in the 

 corona must appear to us more or less modified by the effect of 

 perspective. This consideration tells us also that the intrinsic bright- 

 ness of the corona towards the sun's limb is much less than its 

 apparent brightness as seen by us, of which no inconsiderable part 

 must be due to the greater extent of corona in the line of sight as the 

 sun is approached. The corona undergoes great and probably con- 

 tinual change, as the same coronal forms are not present at different 

 eclipses. 



The attempts which have been made from time to time to see the 

 corona without an eclipse have been based mainly upon the hoj)e that 

 if the eye were protected from the intense direct light of the sun, and 

 from all light other than that from the sky immediately about the 

 sun, then the eye might become sufficiently sensitive to perceive the 

 corona. These attempts have failed because it was not possible to 

 place the artificial screen where the moon comes, outside our atmo- 

 sphere, and so keep in shadow the part of the air through which the 

 observer looks. The latest attempts have been made by Professor 

 Laiigley at Mount Whitney, and Dr. Copeland, assistant to Lord 

 Crawford, on the Andes. Professor Langley says, "I have tried 

 visual methods under the most favourable circumstances, but with 

 entire non-success." Dr. Copeland observed at Puno, at a height of 

 12,040 feet. He says : " It ought to be mentioned that the appearances 

 produced by the illuminated atmosphere were often of the most 

 tantalising description, giving again and again the impression that 

 my efforts were about to be crowned with success." 



There are occasions on which the existence of the brighter part of 

 the corona near the sun's limb can be detected without an eclipse. 

 The brightness of the sky near the sun's limb is due to two distinct 

 factors, the air-glare and the corona behind it, which M. Janssen 

 considers to be brighter than the full moon. When Venus comes 

 between us and the sun, it is obvious that the planet as it ai^proaches 

 the sun, comes in before the corona, and shuts off the light which is 

 due to it. To the observer the sky at the place where the planet 

 is appears darker than the adjoining parts, that is to say, the with- 

 drawal of the coronal light from behind has made a sensible diminu- 

 tion in the brightness of the sky. It follows that the part of the 

 sky behind which the corona is situated must be brighter in a small 

 degree than the adjoining parts, and it would perhaps not be too much 

 to say that the corona would always be visible when the sky is clear. 



