6 4 



NATURE 



[May 1 8. 



this, at mid-height in the sun's atmosphere, we got indi- 

 cations about which there could be no mistake — we were 

 in presence of hydrogen : incandescent hydrogen, be it 

 observed, which plays as important, or indeed a still more 

 important part in the solar air than nitrogen does in our 

 own. Next it was imagined that close to the sun itself 

 there was a vaporous sea containing all the other sub- 

 stances which had been detected by the spectroscope — 

 magnesium, calcium, iron, barium, cobalt, nickel, and ] 

 some twenty other bodies termed "elements" by the 

 chemist, because he cannot reduce them to a condition of 

 greater simplicity. As the sum total of these inquiries, 

 then, we had some such idea of the sun's atmosphere as 

 this : Physically it was incandescent, of enormous extent, 

 very irregular in outline, its extent and outline varying 

 almost every time it could be observed. Chemically it 

 was built up of substances known to terrestrial chemis- 

 try ; it was very simple at top, and very complicated at 

 bottom. This mental image was the joint product of 

 both laboratory and eclipse work. The solar spectrum — 

 that is, the beautiful rainbow ribbon which is produced 

 when light from the sun is made to pass through a prism 

 — enabled us even in our laboratories, without a telescope, 

 to study the chemistry of the sun's atmosphere as a whole, 

 but such work as this localised nothing. Further, the outer 

 atmosphere is so dim as compared with the intensely 

 brilliant interior nucleus, that it, like the stars in the day- 

 time, is put out, and remains invisible so long as the sun 

 itself is in a position to illuminate our upper air. In this 

 we have the use of total eclipses, for at such times the 

 moon prevents the sunlight from falling on our atmo- 

 sphere, and the sun's atmosphere shines out in all its 

 weird splendour, as the stars show themselves when the 

 light of day is withdrawn. It is fair to add, that there is 

 a method which enables us to study the chemistry and 

 even the meteorology of the very brightest portion of the 

 sun's atmosphere, called the chromosphere, without wait- 

 ing for an eclipse, but still, every allowance being made, 

 it should be now clear that to study the physical attributes 

 of the atmosphere as a whole, we are strictly limited to 

 total eclipses. So much, then, for our brief expose. There 

 is still some more ground to be gone over before the 

 question with which we set out is answered. What was the 

 sum total of the work done during the last eclipse — that 

 observed in the United States in 1878, with a wealth of 

 instrumental appliances such as had never been used 

 before? How did it deal with our received notions? 

 Did it endorse them or demolish them ? 



It certainly endorsed them in the main, while it enabled 

 us to accumulate a vast amount of new knowledge on 

 many important points, and showed us how every effort 

 should be made to secure these precious records. Among 

 other things, it intensified the difference between eclipse 

 and eclipse, for the spectroscopic record of the outer 

 corona — as the exterior atmosphere is sometimes called 

 — differed very considerably from the one secured in 

 1871, and it was a noteworthy fact that the eclipse of 

 1 87 1 happened when there were most spots on the sun, 

 while that of 1 878 took place when there were fewest. I 

 said " in the main." But during the eclipse one observa- 

 tion was made, which in the light of former laboratory 

 work suggested that after all there was a rift in the lute, 

 and that our view of the solar economy might be much 

 more wrong than we had any idea of. Since 1878 that 

 same laboratory work has been continued, and a long 

 series of observations of the spectra of sun-spots has been 

 made, and the tendency of all this extra eclipse-work has 

 all been in one direction. We are now face to face with 

 the idea that, in the hottest part of the sun, the tempera- 

 ture is so high that our so-called elementary bodies are 

 broken up into simpler ones, and that the reason that the 

 sun seems to contain so many of our terrestrial elements 

 is simply that both in the sun and in a powerful electric 

 spark the-e bodies are really broken up into their finer 



constituents, the spectral lines of these finer constituents 

 being observed in both cases. Now it is obviously the 

 duty of men of science, if there be any tests of this new 

 view, any crucial observations possible during an eclipse, 

 to apply these tests, to make these observations, as soon 

 as possible — not, of course, to the neglect of the old 

 methods of attack, but, if possible, in addition to them ; 

 and as the problem is one of such general interest, and 

 one which is sure to be keenly debated, as many records 

 independent of personal error or personal bias should be 

 obtained as possible. These permanent records, to which 

 reference has been made, are of course photographs, and 

 here we are brought face to face with another fact ; we 

 have not only a new view to test, but we have new photo- 

 graphic processes to apply to te:>t it, as well as to obtain 

 a series of records comparable with those secured during 

 prior eclipses. We have in this case an instance of the 

 way in which an observation, apparently trivial, is at last 

 seized hold of and made to furnish a stepping-stone for a 

 further advance in scientific inquiry. It is now many 

 years since Faraday, experimenting on gold leaf, which 

 is green when a bright light is observed through it, 

 found that he could change its colour, and he fancied that 

 this might be taken to indicate that the gold in the leaf 

 did not consist of particles all of the same size, but that 

 they existed of almost an infinite series of finenesses. 

 This was in the pre-spectroscopic days. When the 

 spectroscope could be brought to bear, it became apparent 

 that two orders of fineness only were required to produce 

 all the colours observed by Faraday, and Mr. Lockyer 

 soon produced other evidence which went to show that 

 we were here in presence of a general law. From this 

 time we heard the words "blue molecules" and "red 

 molecules" — terms invented to indicate that in the same 

 chemical substances there were some molecules with such 

 physical attributes that they were turned to and could 

 therefore absorb blue light, while others were made active 

 by red light falling upon them. 



Capt. Abney, in a series of painstaking researches, has 

 shown that precisely those salts of silver employed by 

 photographers obey this general law, and hence we can 

 now use blue light and red light indiscriminately, and so, 

 for the first time we can photograph the red end of the 

 spectrum of the sun's external atmosphere. Nor is this 

 all ; other advances in the photographic art enable us 

 now to replace minutes by seconds in the time of exposure ; 

 indeed, in these days of " instantaneous " processes, the 

 difficulty often lies in exposing the plate for a time short 

 enough to the influence of the light. It is as well to insist 

 upon this point, as in the eclipse of next month the totality 

 or period during which the moon entirely com. rs the sun 

 is very short ; but "-hort as it is, it is more than made up 

 for by the increased rapidity of the processes to be em- 

 ployed. Now, the most important phenomena to be 

 recorded, whether by eye or photography, are, first, the 

 spectrum of the lowest stratum of the sun's atmosphere 

 revealed to us at the moment of disappearance and re- 

 appearance of the sun by a sudden flashing out of bright 

 lines ; next, the spectrum of the outer atmosphere, best 

 observable at mid-eclipse ; and then the extent and struc- 

 ture of the atmosphere itself. Now it is imagined, that 

 if the new view to which reference has been trade is 

 correct, the spectrum of the lower siratum will differ from 

 what it is supposed to be, and we say supposed to be, 

 because up to the present time the observations have 

 been of such a general nature th.it it lias been impossible 

 to be quite certain about details. The intention this time 

 is to observe a small portion of the spectrum with great 

 minuteness, so far as the eye observations go, while an 

 attempt will be made to actually photograph the flash of 

 bright lines, and obtain a reference spectrum afterwards 

 by obtaining a photograph of the solar spectrum on the 

 same plate after the eclipse is over. 



Among the mo 5 t interesting observations made during 



