108 



gases and vapours under immense pressure, since other consider- 

 ations preclude the possibility of its being solid, as Herschel 

 thought, or liquid, according to Kirchhoff's views. But we see 

 more than this, for superposed on the brilliant band of colours 

 are hundreds of dark sharp lines as if the image of the slit were 

 wanting in some places. Witliout entering at all into the recip- 

 rocity of radiation and absorption, from the analogy of the 

 darkening of one candle-flame when viewed through another, 

 we shall easily be able to accept the statement that these dark 

 lines are due to the vapours which surround the intensely heated 

 nucleus of the sun, giving a spectrum of bright lines if viewed 

 apart from the sun, but one of relatively dark lines, on the back- 

 ground of the bright continuous spectrum. Thus we have a 

 third sort of spectrum, or that of dark hnes on the bright back- 

 ground, due to absorbing vapours when seen in front of either a 

 solid, or liquid, or even a vapour of the same sort at a higher 

 temperature. The only thing necessary for the production of the 

 dark lines is that the solid, or liquid, or vapour should contain 

 the same radiations as are absorbed, and be at a higher tempera- 

 ture. 



With such knowledge, and with the recently-invented spectro- 

 scope, the observers of 1868 prepared to grapple with the 

 problems presented by the eclipse of that year. Photography 

 was confided to the care of Major Tennant, with the result that 

 a series of six beautiful pictures was obtained, showing among 

 others a wonderful prominence, eighty-eight thousand five 

 hundred miles high, which received the name of the Great Horn. 

 But it was on the spectroscope, now for the first time directed 

 to the eclipsed sun, that the greatest hopes of success were 

 founded ; hopes destined to be most brilliantly realized. A.s soon 

 as the sun was totally obscured, and the rosy-coloured flames 

 burst forth, eager eyes were directed to the new instrument to 

 read the story there told. A spectrum of bright lines ; among 

 which were one in the red, another in the yellow, the famous Dg 

 line, and yet another in the blue. These were observed by Rayet, 

 Herschel, and Tennant, although the positions certified were not 

 exactly accordant, as was hardly to be expected at a first attempt. 

 A spectrum of bright lines ; hence the prominences are due to a 

 glowing gas. What gas ? hydi'ogen was the answer of the tell- 

 tale lines. 



But the results of this eclipse were even of greater value than 

 could have been anticipated. Struck by the vivid brightness of 

 the lines seen in the prominence spectrum, Janssen cried out, as 

 he tells us himself, " Je reverrai ces lignes Id!" Accordingly, 

 on the morrow, he directed his instrument to the sun's limb, 

 and was able at his leisure to examine again the prominence 

 spectrum, equally as well as on the eclipsed sun. The news of 



