June 22, 1882] 



NATURE 



181 



SCIENTIFIC RESULTS OF THE ECLIPSE 



THE following communication appeared in the Daily 

 News of Tuesday from its special correspondent 

 with the English Eclipse Expedition : — 



In my last letter, 1 written as it seems, an age ago, for 

 the incidents since the eclipse have been more or less 

 emotional, I promised in a final one to give the opinions 

 of the astronomers as to the bearing of the work they 

 have been fortunate enough to do in Egypt this year upon 

 the general question of solar inquiry. Hence we have to 

 consider both results and methods, and the latter should 

 include the questions which it seems now most desirable 

 to put to the sun the next time he is eclipsed. First, 

 then, as to results. There seems good ground for sup- 

 posing that the outermost part of the sun's atmosphere 

 suffers changes as does that lower portion in which sun- 

 spots are observed, and that the changes synchronise. 

 Now, in the case of spots, as is generally known, we 

 have a maximum number every eleven years or there- 

 abouts, and in the interval we have a minimum, during 

 which no spots are seen for weeks together. Hence the 

 so-called maximum sun-spot period and minimum sun- 

 spot period. This supposition was first hazarded in 1878, 

 a minimum sun-spot year, as a result of the comparisons 

 of the observations of that year with those obtained in 

 1871, a maximum sun-spot year. In 1871 the corona was 

 most extended away from the equator ; there was no 

 special structure at the poles, and the hydrogen in it was 

 strongly developed and it was very luminous. In 1878 

 it was most extended along the equator ; there was very 

 special structure at both poles, the hydrogen had almost 

 disappeared, and it was faint. Now in 1882, that is 

 eleven years from 1871 and in another maximum sun- 

 spot year, the corona has again put on its condition of 

 eleven years ago. Hence observation has shown that 

 the supposition is so far quite justified by the facts, and 

 accepting this connection as a working hypothesis astro- 

 nomers and physicists have now to try to connect the 

 absence of spots in the lower atmosphere with a condition 

 of things which gives us a great equatorial extension of 

 the atmosphere, and very definite structure at the poles, 

 associated in all probability with a lower temperature, 

 or at all events a greater admixture of cooling material. 



Before the Eclipse Expedition had left England Dr. 

 Siemens had proposed a theory of the solar atmosphere 

 which postulated exactly such conditions as appeared to 

 be revealed in years of least solar activity. The coinci- 

 dence between hypothesis and fact was, to say the least, 

 extremely curious, and there is no doubt that the fact 

 that when the sun is most active the correspondence 

 seems to vanish, will have to be carefully considered. 

 But we have learned more touching the outer atmosphere 

 than its changes. There has been a chemical touch 

 added. When in 1869 its chemical nature was first in- 

 vestigated by means of the spectroscope, it seemed to be 

 built up almost entirely of a substance of which we 

 knew nothing here — a substance revealed by a line in the 

 green part of the spectrum, at 1474 of the scale em- 

 ployed by Kirchhoff for his maps, which were then gene- 

 rally in use. In 1870 hydrogen was added to this unknown 

 substance, if we are to interpret spectroscopic phenomena 

 in the usual manner ; and now again, with the same 

 proviso, calcium has been added ; that is to say, some 

 lines seen in the spectrum of calcium have now been 

 detected in the spectrum of the sun's outer atmosphere. 

 It is now some years since the strange behaviour of 

 calcium when observed in the spectroscope was noticed, 

 and it was the first substance used to point the moral 

 that the spectra of terrestrial substances are sometimes 

 strangely transformed when their lines are examined 

 among those visible in the ordinary spectrum of the sun. 

 Thus the widest lines of all in that spectrum — the lines 

 1 Reprinted in Nature, vol. xxvi. p. 129. 



lettered H and K for purposes of reference — are seen in 

 the spectrum of calcium when high temperatures are 

 employed, though they are absent at low temperatures, 

 when, however, a line in the blue which is but feebly 

 represented among the solar lines is thick and brilliant. 

 The observations of the eclipse in Siam in 1875 strongly 

 suggested that the so-called calcium was really an im- 

 portant constituent of the lower layers, while it is now 

 known that it plays an important part in every spot and 

 prominence ; indeed, in the spectra of sun-spots photo- 

 graphed by Mr. Lockyer at South Kensington, the H and 

 K lines behave differently from all the other lines photo- 

 graphed. But the point of this year's work is that this 

 calcium has been carried very high into the solar atmos- 

 phere, where it exists in such tremendous quantity that 

 the eclipse colouring in all its weirdness can be traced to 

 it, and the proof that this violet light is lighting up our 

 atmosphere more powerfully than any other is found in 

 the fact that in one of the photographs taken on Abney's 

 plates the air between us and the dark moon is shown to 

 be of this colour. This is photographic proof certain and 

 sure, and will remind those learned in these matters of an 

 observation made by Captain Maclear during the eclipse 

 of 1870. It will be seen then that this year's work has 

 left its marks both on the physics and chemistry of the 

 outer atmosphere. We must now descend a little into 

 the lower regions of the solar incandescent air. 



Here we approach a very interesting part of the subject, 

 but one on which it is difficult to say anything without 

 going somewhat into detail. Up till a few years ago the 

 idea that our terrestrial elements, such as iron, hydrogen, 

 and the like, were anything but elements never entered 

 the heads of astronomers as they were daily recording 

 solar phenomena. It was obvious that the .sun was 

 very hot — so hot that it may be said the vapour of 

 iron plays the same part there as the vapour of water 

 plays here ; but the possible result of the high tempera- 

 ture remained practically unconsidered, and our notions 

 of the structure of the solar atmo:phere were influenced 

 by terrestial chemistry. Hence, when it was found that 

 the upper atmosphere consisted mainly of hydrogen, all 

 the lines of the solar spectrum except those due to 

 hydrogen were supposed to owe their origin to absorption 

 of the solar light at very low levels, and close to the sun 

 there was supposed to be a thin stratum, the work of 

 which was so efficient in this direction that it was called the 

 "reversing layer." But after a time, as facts were accu- 

 mulated, the question whether our elements really could 

 and did stand the temperature of the sun without breaking 

 up into something more elementary still was fairly asked ; 

 and as in other cases the question had to be discussed in 

 a scientific manner — that is, cases had to be taken in 

 which the question could be put to the facts in such a way 

 that if the observations were of one kind one view would 

 be strengthened ; if of another kind then the other ex- 

 planation would be more likely the correct one. It was 

 pointed out some time ago that there are two very 

 definite kinds of observations which can be made during 

 eclipses, by which much information might be gained 

 bearing directly upon this question of dissociation — 

 that is to say, the question whether our "elements," as 

 we know them, are or are not capable of existing at solar 

 temperatures. These observations had to do, one with the 

 " reversing layer," the other with the outer atmosphere. 

 The challenge was of the most direct kind touching the 

 reversing layer It went so far as to say that the former 

 observations had been erroneously interpreted. This, 

 however, must not be held to cast doubt upon former 

 observers. The contention was that the former work, 

 dating from 1870, had been of too general a nature, and 

 that when a small part of the field of observation was 

 studied with minute accuracy it would be found that the 

 general statement would be untenable, that general state- 

 ment favouring the view that the elements are still truly 



