OJV THE SOLAR CORONA. 757 



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

 visual methods under the most favorable 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 appear- 

 ances produced by the illuminated atmosphere were often of the most 

 tantalizing 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 approaches 

 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 withdrawal 

 of the coronal light from behind has made a sensible diminution 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, if our eyes 

 were more sensitive to small differences of illumination of adjacent 

 areas. My friend Mr. John Brett, A. R. A., tells me that he is able to 

 see the corona in a telescope of low power. 



The spectroscopic method by which the prominences can be seen 

 fails because a part only of the coronal light is resolved by the prism 

 into bright lines, and of these lines no one is sufficiently bright and 

 co-extensive with the corona to enable us to see the corona by its 

 light, as the prominences may be seen by the red, the blue, or the green 

 line of hydrogen. 



The corona sends to us light of three kinds : 1. Light which the 

 prism resolves into bright lines, which has been emitted by luminous 

 gas. 2. Light which gives a continuous spectrum, which has come 

 from incandescent liquid or solid matter. 3. Reflected sunlight, which 

 M. Janssen considers to form the fundamental part of the coronal light. 



The problem to be solved was how to disentangle the coronal light 

 from the air-glare mixed up with it, or in other words how to give such 

 an advantage to the coronal light that it might hold its own sufficiently 

 for our eyes to distinguish the corona from the bright sky. 



When the report reached this country in the summer of 1882 that 

 photographs of the spectrum of the corona taken during the eclipse in 

 Egypt showed that the coronal light seen from the earth as a whole 

 is strong in the violet region, it seemed to me probable that if by some 

 method of selective absorption this kind of light were isolated, then 

 when viewed by this kind of light alone the corona might be at a suffi- 

 cient advantage relatively to the air-glare to become visible. Though 



