,i6 



NATURE 



\Auguit 4, 1 88 1 



I translate his words — "Of all the bodies iron has certainly pro- 

 duced the greater number of lines in the solar spectrum. Some 

 of these seem to lie common with thote of calcium." Th.ilen 

 carried on this \\ork, and if one compcires the magnificent 

 table?, which we owe to hi^ untiring skill and industry, one is 

 perfectly astonished to find the number of coincidences which he 

 has so carefully tabulated. 



2. There was another kind of work, a newer kind of work, 

 going on. Observers began to give particular attention to the 

 bright lines of flames, and the lines thickened in i-pots. And 

 here I may limit myself to the general statement that the diver- 

 gence between the .spectra of the different substances as ob.served 

 in the sun and in our laljoraturies vias very much intensified as 

 facts were accumulated. Very many of the lines observed in 

 flames were lines with no terrestrial equivalents, and the spot- 

 spectrum often contained lines much thickened, which Nvere 

 either not represented at all, or only feebly among the Fraun- 

 hofer lines. 



3. Next, among all the metalloids known to chemists only one 

 of them — or one substance classed as such, hydrogen — was present 

 in the solar atmosphere, and that in overwhelming quantity ; 

 whereas the efforts of Angstrom, KirchholT, and othtrs could not 

 trace such substances as o.xygen, chlorine, silicon and other 

 common nietalloidal constituents of the earth's crust. 



/[. Then again, the l.iyer which was produced by''wliat was 

 taken to be gaseous magnesium round the sun, a layer indicated 

 by the brightest member of the b group, '^^ as always higher — 

 always gave us longer lines — than that other layer which was 

 brought under our ken by the bright line D seen in the spectrum 

 of sodium. 



Here was a distinct inversion of the chemical order. The 



S f R /US 



atomic weight of sodium being 23, and of magnesium being 24, 

 the sodium ought to have been higher than the magnesium ; but 

 the contrary was the fact, and that fact still remains after twelve 

 years of observation. 



5. As the \^•ork of tabulating the lines went on, and the more 

 complex outpourings of vapours from the sun's interior were 

 studied, it was found that the lines of iron, calcium, and so forth 

 revealed to us were by no means the brighte^t lines — by no means 

 the most important, or most prominent lines, but lines which 

 really we had very great difficulty in recognising as characteristic 

 of any particular spectrum. There they certainly were, however, 

 mapped as very fine lines by the most industrious observers. 

 Similarly with the spots, there was an absolute inversion of the 

 thickness of the lines of any one substance in the spot. Surely 

 there « as a great screw loose here. 



6. Closely allied to these observations we had another extraor- 

 dinary fact. We could quite understand why in a spot thech.ange 

 of refrangibility of the magnesium lines when there was a 

 storm going on in the sun should be different from the change 

 of refrangibility of, say, the iron lines. The natural explana- 

 tion was, of course, this : you have the magnesium gas going 

 at one rate, the iron gas going at another rate, and that is 

 all there is to be said about it. But it was soon found that the 

 differences which could be tharply seen between the spectrum of 

 a particular mass of magnesium vapour and a particular mass of 

 iron vapour extended to the iron vapour itself. There were 

 just as many variations in the refrangil)ility of the lines of iron 

 itself, for instance, as there were between the lines of iron 

 and other substances : that is to say, we had in the one case 

 magnesium going at one rate and iron !j;oing at another rate ; but 

 when we came to deal with the iron lines alone we found one 



POL /. 6' . 



Fig. 27. — Three chief types of ttellar spect^': 



iron line told us the iron vnponr was going at one rate, and 

 another iron line told us that same iron vapour was goin;; at 

 another rate. It .will be seen at once that there was a great 

 difficulty in that. 



7. Further. The lines on which these obse.vations of the 

 relative motions of the vapour depended were found to go in 

 sets. In a spot, for instance, we would generally Fee movement 

 indicated by one set of iron lines, whereas in a prominence we 

 would always see a different .set— a set in a different part of the 

 spectrum altogether— registering this movement for us. Here 

 again was considerable food for thought. 



That was stated very roundly a good many years ago — in iS6g, 

 I will read what was then written nn this subject : 1 " Alterations 

 of wave-length h.ave been detected in the sodium, magnesium, 

 and iron lines of the spot's spectrum. In the case of the last 

 substance the lines in which the alteration was detected were 

 not those observed when iron, if we accept them to be due t<:) 

 iron alone, is ejected into the chromosphere." 



That caveat with regard to iron arose from the fact that of the 

 460 lines recorded by Kirchhoff in 1869 only three lines of iron 

 had been seen bright in the solar prominences. 



8. Then came a point which has been very slightly alluded to 

 already. How came it that the total chemxal composition of this 

 atmospheue of the sun, which we were taught to look ui>cn as 

 the exemplar of what mu^t have once happened to oiir own 

 planet, varied so enormously from the coaiposition of the crust 

 of our earth ? No oxygen in it, no silicon, no fluorine ; whereas 

 we get abundance of titanium, nickel, and so on. It was difficult 



' Proc. Roy. Soc, vol xviii. p. 74. 



to imagine a stronger difference to exist between any two masses 

 of matter than the chemical constitution of the incandescent 

 sun, and of the earth, \\ hich is now cooling. 



9. There was still another point of view very soon forced 

 upm solar observers by the magnificent success which had 

 attended the labours of Dr. Huggius, Secchi, and other observers 

 in recording the spectra of stars. It was a most interesting 

 inquiry naturally to see whether the stars gave spectra quite like 

 each other, and if it should happen that they did not give spectra 

 hke each other, then the points of difference would be sure to 

 give us some excellent working suggestions. 



Now what are the facts? Here are three typical stellar 

 spectra (Fig. 27), which show us at once that there is a very 

 considerable difference in the phenomena. In the upper part 

 of this diagram we have a star remarkable for the fewness of 

 lines in its spectrum. From one end of the spectrum to the 

 other there are not above halfa-dozen prominent lines. In 

 the next part however we have a star which is remarkably 

 like our own sun, both as regards the number of lines and their 

 arrangement. In the lower part of the diagram, on the other 

 hand, we have a star in which we get flutings instead of lines ; 

 so that we get not only a difference of degree, but a fundamental 

 spectroscopic difference of kind. Now there is a circumstance 

 c mnected with that first star with the simple spectrum very 

 striking to any one in the habit of observing the sun, and it is 

 this : those lines visible in the star, which, be it remembered, 

 had been independently determined to be hotter than our sun, 

 are precisely those lines, and none other, which we see bright 

 on the disk of the sun itself. I have emphasis.'d the fact that 



