44 REPORT — 1902. 



of the probable errors in measuring wave-lengths so faint as most of them 

 are, but there is no such doubt about the wave-lengths of the rays in solar 

 protuberances measured by Deslandres and Hale. Stassano found that 

 these rays, forty-four in number, lying between the Fraunhofer line F 

 and 3148 in the ultra-violet agree very closely with rays which Professor 

 Liveing and myself measured in the spectra of the most volatile atmo- 

 spheric gases. It will be remembered that one of the earliest suggestions 

 as to the nature of solar prominences was that they were solar auroras. 

 This supposition helped to explain the marvellous rapidity of their 

 changes, and the apparent suspension of brilliant self-luminous clouds at 

 enormous heights above the sun's surface. Now the identification of the 

 rays of their spectra with those of the most volatile gases, which also 

 furnish many of the auroral rays, certainly supports that suggestion. A 

 stronger support, however, seems to be given to it by the results obtained 

 at the total eclipse of May 1901, by the American expedition to Sumatra. 

 In the ' Astrophysical Journal ' for June last is a list of 339 lines in the 

 spectrum of the corona photographed by Humphreys, during totality, 

 with a very large concave grating. Of these no fewer than 209 do not 

 differ from lines we have measured in the most volatile gases of the 

 atmosphere, or in krypton or xenon, by more than one unit of wave- 

 length on Armstrong's scale, a quantity within the limit of probable 

 error. Of the remainder, a good many agree to a like degree with argon 

 lines, a very few with oxygen lines, and still fewer with nitrogen lines ; 

 the characteristic green auroral ray, which is not in the range of 

 Humphreys' photographs, also agrees within a small fraction of a unit of 

 wave-length with one of the rays emitted by the most volatile atmospheric 

 gas. Taking into account the Fraunhofer lines H, K, and G, usually 

 ascribed to calcium, there remain only fifty-five lines of the 339 un- 

 accounted for to the degree of probability indicated. Of these consider- 

 ably more than half are very weak lines which have not depicted 

 themselves on more than one of the six films exposed, and extend but a 

 very short distance into the sun's atmosphere. There are, however, seven 

 which are stronger lines, and reach to a considerable height above the 

 sun's rim, and all have depicted themselves on at least four of the six 

 films. If there be no considerable error in the wave-lengths assigned (and 

 such is not likely to be the case), these lines may perhaps be due to some 

 volatile element which may yet be discovered in our atmosphere. How- 

 ever that may be, the very great number of close coincidences between 

 the auroral rays and those which are emitted under electric excitement 

 by gases of our atmosphere almost constrains us to believe, what is indeed 

 most probable on other grounds, that the sun's coronal atmosphere is 

 composed of the same substances as the earth's, and that it is rendered 

 luminous in the same way — namely, by electric discharges. This conclusion 

 has plainly an important bearing on the explanation which should be 

 given of the outburst of new stars and of the extraordina,ry and rapid 

 changes in their spectra. Moreover, leaving on one side the question 



