On the Corona of the Sun. 



109 



prominences at the base of the corona. The thin condition of these 

 lines, as well as the breadth of the lines of calcium at H and K, confirms 

 the position which I had ventured to give to the sun relatively to some 

 of the brighter stars, namely as belonging to the least fervid of the 

 white stars, and just above those which begin to show a yellow light. 



There are indeed some stars in the spectra of which the line D 6 

 which is seen in the prominences, and in the lower parts of the corona 

 of the sun, appears as a bright line, but this may be due to gas below 

 any true corona, which may be about these stars. 



There are also the so-called nebulous stars, which are surrounded 

 by an aureole of faint light of measurable angular extent, but it would 

 seem more probable that these belong to, and should be discussed with, 

 the clusters and nebula?, and should not be regarded as exhibiting a 

 corona of the nature of that which surrounds the sun. 



So far then as our present powers of observation go, the corona of 

 the sun stands alone ; it is therefore the more to be regretted that the 

 observations of this object are beset with great and peculiar difficulties. 

 The absorption and scattering of the sun's light by our atmosphere, 

 amounting according to Professor Langley to nearly 40 per cent.,* 

 which are essential to the maintenance of the conditions under which 

 life, as it now exists, is possible upon the earth, comes in, in this case, 

 so seriously to our disadvantage that the corona can be seen for a few 

 minutes only at long intervals. It is only on the rare occasions when 

 the moon coming between us and the sun cuts off the sun's light from 

 the air at the place where the eclipse is total, that we can see the 

 corona through the cone of unilluminated air which is in shadow. On 

 an average once in two years, for from three to six minutes, the corona 

 is visible, and then only over a very narrow strip of the earth's surface. 

 It is not surprising that many attempts have been made to observe the 

 corona without an eclipse. The earlier attempts were based upon the 

 hope 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, the eye might become sufficiently sensitive to perceive 

 the corona. In the later attempts, success lias been sought for from 

 the great diminution of air-glare which takes place at high elevations, 

 when the denser and more dusty parts of the atmosphere are left below 

 the observer. Professor Langley made observations on Mount Etna, 

 and also on Mount Whitney, 15,000 feet high. He says : — " I have 

 tried visual methods under the most favourable circumstances, but 

 with entire non-success." Dr. Copeland, assistant to Lord Crawford, 

 observed at Puno at a height of 12,040 feet. In his report he says :f 

 — " It ought to be mentioned that the appearances produced by the 

 illuminated atmosphere were often of the most tantalising description, 



* " Amer. J. of Science," September, 1884. 

 f Copernicus, vol. iii, p. 212. 



