July 2 1, 1881] 



NATURE 



That was the first experiment with which I am acquainted 

 which enabled us to locate chemical phenomena in any particular 

 part of the sun. 



Now although in the year 1866 a great many people were 

 familiar with the spots on the sun, those who had been favoured 

 by a sight of a total eclipse, and many more who had read the 

 accounts of total eclipses, knew that there \\ as a great deal more 

 of the sun than one generally sees. From the time of Stannyan, 

 who observed the prominences it Berne, down to the year 1S42, 

 let us say, several eclipses had been observed, and very beautiful 

 coloured phenomena had been recorded by different observer?. 

 Red things had been seen projecting round the dark moon during 

 the time of eclipse, and although many held them to Le beautful 

 effects produced by the passage of the moon over the sun, or 

 even clouds in the atmosphere of the moon coloured by the 

 strange way in which the solar light then fell upon them, a 

 larger number of people, on the other hand, insisted that these 

 things must really belong to the sun. Now if that were so, it 

 was perfectly clear that we should not be contented with merely 

 observing the chemical nature of the spots. 



Having the spectroscope, the things which showed thus, and 

 which up to that time had only been observed during eclipses, 

 would be more or less fdt, if they were not absolutely rendered 



visible, by this new instrument ; and for this reason : the 

 things seen round the sun during an eclipse were not there 

 for the instant of the eclipse only : they were .always there : 

 why did we not see them ? The illumination of our own air pre- 

 vented this. What was our own air illuminated by? By the sun- 

 light. Now whereas increasing dispersion does considerably 

 dim a continuous spectrum for the reason that it makes it extend 

 over a larger area on the screen, it does not dim to any great 

 extent the brightness of a line, so that by employing a consider- 

 able number of prisms we ought to be able to abolish the illumina- 

 tion or cur air altogether, and in that way we should no longer be 

 limited to determining merely the chemical nature of the spots, 

 we should be equally able to determine the nature of the sur- 

 rounding solar atmosphere, supposing the jheromena observed 

 during eclipses were really solar, and not lunar or terrestrial. 



I will make an experiment w'ith the electric light. I begin 

 with a bright ccntinuous spectnim. We will charge the cup in 

 the lower pole w ith some vapour which w ill give us a bright line, 

 in addition to the continuous spectrum due to ihe joles, and 

 these two things must fi^jht it out between them. If everything 

 goes well what should happen will be this : by first mounting 

 one prism, then two, and then three, the continuous spectrum 

 will be gradually enfeebled, the line keeping the same luminosity 



Fig. 9. — Eclipse of 1S70. Photograph of the corona taken at Syracu 



during the whole time ; we shall find that relatively the line 

 will be much brighter than the continuous spectrum by the time 

 the experiment is concluded. That was the principle which it 

 was suggested would enable the spectroscope to be used in 

 making what have been called artificial eclipse=. 



Now if we ask what are the phenomena presented by eclipses, 

 the sort of thing the spectroscope is called upon to observe, 

 we shall see the very considerable advantage of the introduction 

 of the new method. In the first place the eclipsres, which are so 

 full of the precious knowledge to be got only at that moment, 

 are almost mstantaneous, so far as each particular phenomenon 

 is concerned ; and, secondly when the duration is say, four or 

 five or six minutes, which is a very considerable time during an 

 eclipse, and which allows a great deal of work to be done; only 

 a very small part of the more interesting regions of the solar 

 atmosphere is uncovered ; one part, of com-se, when the moon 

 is passing over one limb of the sun, and the other when the 

 moon in passing, liberates it, and brings it into light again. 

 What I would draw chief attention to is the lower part of 

 the brilliant portion seen around the dark moon. We shall 

 have to discuss the itpper portion, which is called the coronal 

 atmosphere, or corona, on a later occasion. This mere visual 

 reference, of course, is simply in anticipation of the chemical 



nature of the different strata upon which Twe lave to operate by 

 the spectroscope, and about which I shall have therefore to tell 

 you in that part of the lecture w hich has to do w'ith localisation. 

 We shall thus determine, after what has been already said with 

 regard to KirchholY's hypothesis as to the position of the region 

 where the lines ought to be seen in the corona, whether during 

 an eclipse we get anything like a justification of this hypothesis. 

 This drawing is really a very beautiful reproduction of an eclipse. 

 We have a round dark moon, which in this case is represented as 

 entirely covering the sun ; then these different prominences and 

 luminosities, this wonderful set of streamers, or whatever you 

 like to call them, which seem to veil, or to render less distinct, 

 something else which is lying beyond them. You will see here 

 that some of these prominences are red, and others have a yellow- 

 tinge, and that, quite independent of the colour of the promi- 

 nences, we have the most exquisite coloured effec's. Sometimes 

 the radial structure is not so marked, and reveals indications of 

 structure further away from the sun. You see wonderfully 

 delicate tr.icery, lines being seen now in one part and now in 

 another. In the photograph taken during the eclipse of 1870 

 we see that the luminosity of the solar atmosphere was excess- 

 ively irre;;ular, by which I mean that in one part we get a very 

 considerable excess of light, quite independent of the sharply 



