r 



April 2 2, [875] 



NA TURE 



491 



were dimmer than the spectrum of the corona is known 

 to be." 



The extreme importance which attaches to the determi- 

 nation of the particular class of spectrum under which 

 that of the corona may be classed was pointed out in 

 Nature, vol. xi. p. 201, and on this point the Times 

 article says : — 



"The most perfect determination would have been 

 accomplished when the peculiarities of the spectrum, of 

 whatever class it might be, with its bright lines or its 

 ' channelled spaces,' had been recorded over a long 

 range. For ttiis purpose the Siam party was provided 

 with a siderostat, a short focus reflector, and a spectro- 

 scopic camera of long focus — that is, a spectroscope in 

 which the ordinary observing telescope had been replaced 

 by a lens of long focus and a photographic camera. If 

 everything had been in order, the air perfectly clear, and 

 the corona very bright, this instrument would have given 

 us the most valuable record of all, as we should have 

 obtained a detailed spectrum of the coronal atmosphere 

 and chromosphere from the Fraunhofer line G to far 

 beyond H, the ordinary limit of visibility. This was the 

 most crucial experiment ; while it was the one least likely 

 to be reahsed, its success would have been of the highest 

 importance, as the chemical as well as the physical con- 

 stitution might have been more or less fully revealed. 

 Next in delicacy to this came a similar arrangement in 

 which the same principles were depended on, but in 

 which, as all the parts were not of quartz and as the focal 

 length of the camera was not so great, equally good 

 results over so large a range were not to be dreamt of. 

 The nature of the spectrum and of some of the con- 

 stituent gases of the solar atmosphere might have been 

 determined in this way, but the information, though equal 

 in quality to that obtained by the instrument to which 

 we have before referred, would have been deficient m 

 quantity. Still, this information might have been ob- 

 tained with a lesb clear air and by less brilliancy in the 

 corona than were necessary for perfect success in the 

 former case. 



" In the prismatic camera, an instrument described 

 at some length in our last article (reprinted in Nature, 

 vol. ,\i. p. 452), we have an instrument which may 

 be held to be certain to give us a valuable result, 

 even if the air be not very clear and if the corona be 

 not very bright. We may say that this was the gross 

 attack upon the chemical nature of the corona, as the 

 siderostat and its accompanying long-focus spectroscope 

 represented the most delicate one. Now this has per- 

 fectly succeeded, and in this lies the extreme importance 

 of the observations made in Siam. For some reason 

 which is not yet clear to us, the m.ore delicate ones have 

 failed. On receipt of the first telegram we attributed this 

 failure to the hazy sky, which would as certainly have cut 

 oft all the violet rays which alone were to be impressed 

 on the photographic plate as the blue rays arc cut off at 

 sunrise, giving us, as the result of the absorption of all the 

 blue light, first the rosy-fingered dawn and then the red sun 

 himself. But from Dr. Schuster's later telegram which we 

 have now received it would appear that some accident 

 had delayed the colonial steamer between Singapore and 

 Siam, and, further, that the observatories which it was 

 hoped would have been built at Chulai Point before the 

 expedition arrived at Singapore had never been built at 

 all ; so that the expedition had to proceed direct to 

 Bangkok, and, as an inevitable consequence, spent in 

 royal receptions the time which was absolutely required 

 for the erection and adjustment of the instruments, with 

 or without observatories over them. 



" Where and how the delay of four days occurred will, 

 of course, be known hereafter, and it is needless to specu- 

 late too closely upon it ; but it is clear that Dr. Schuster 

 is inclined to attribute the incompleteness of the results 

 which his party has attained more to this delay than even 



to the haze. We can well imagine his disappointment in 

 not having the whole story to tell ; but the measure of 

 success his party has achieved is greater than might 

 fairly have been expected from any one expedition, and 

 there is little doubt that the photographs his party has 

 secured will do more to advance solar physics than any 

 permanent records obtained by any former expedition. 

 They are well worth all the time, labour, and thought 

 which have been lavished on the whole attempt. 



" Evidence of the highest value bearing on the general 

 nature of the spectrum of the coronal atmosphere in the 

 blue region has been obtained. It was clear that the 

 minimum of success must enable us to compare the 

 coronal atmosphere as a whole with that part of it which 

 is composed mainly of hydrogen, and if there happened 

 to be a remainder, the chemical nature of that remainder 

 would be demonstrated. Let us explain the sense in 

 which we have used the term ' remainder.' Evidence 

 was collected during the eclipse of 1871 which went to 

 show that above the hydrogen region and that occupied 

 by the brighter layers of that unknown substance which 

 lies outside it, there was matter, at the sun, the light of 

 which was powerful in its action upon a photographic 

 plate, while it was comparatively powerless to act upon 

 the eye. The corona depicted on the photographic plate 

 was vastly different from the corona seen by the eye, but 

 from a very different cause — one depending upon the con- 

 dition of our air, or, at all events, of something between 

 us and the moon. 



" Now, if we assume that there is something at the 

 sun enveloping the hydrogen, this something will be 

 cooler, and we have now an abundance of laboratory 

 experiments to show that the molecular constitution of 

 the vapours of the same chemical element at different 

 temperatures is vastly different ; and, further, that the 

 spectra of these variously constituted molecules are very 

 definite, and, for the same degree of molecular com- 

 plexity, have a strange family likeness to each other. 



" So far as we have gone already, we have never been 

 able to attack those parts of the sun's surroundings where, 

 in consequence of the reduction of temperature, the 

 various affinities of the molecules have begun to come 

 into play, and combinations of molecules with similar or 

 dissimilar molecules must occur. 



" As a consequence of the perfect action of dissociation 

 in the lower layers which has apparently reduced the 

 vapours of all the chemical substances present in the 

 sun's atmosphere to their simplest molecular condition, 

 each vapour in this condition thins out, so to speak, in 

 such a manner that everything represented high up in 

 the atmosphere is more strongly represented low down. 

 But though this is true for a state of things where the 

 molecular constitution is of the simplest, it is quite clear 

 that if we assume an exterior cooler region tilled with 

 molecules of greater complexity in consequence of a 

 reduced temperature, if we can get at this region obser- 

 vationally we shall find that the spectrum which it gives 

 will be confined to the higher levels, and will not be re- 

 presented lower down because the compound molecules 

 which produce it will be broken up by the higher tempera- 

 ture of the subjacent regions. 



" Now, it looks as if this important and anticipated 

 result has been established. In a telegram addressed to 

 the Daily News it is stated that ' the prismatic camera 

 shows the rings with protuberances at the edge of the 

 san, and at least one mare ring towards the ultra-violet 

 without protuberances.' In other words, the molecules 

 which existed higher up, and built up the stratum the 

 spectrum of which consisted of a ring towards the ultra- 

 violet above the prominence-region, were unrepresented 

 below among the simpler molecules the spectra of which 

 consist of rings extending down to, and actually in- 

 cluding, the prominences. 



"We have said this much by way of pointing out one 



