148 THE CHEMISTRY OF THE SUN. [CHAP. 



in a much less degree at the positive) extremely intense lines 

 are seen, . . . which, however, are short." 



Thalen observed this localisation to a certain extent, doubt- 

 less on account of the long collimator which he employed. 1 



In 1869, I observed that in a solar storm on the edge of the 

 sun, when -the magnesium lines were visible, the lines of that 

 metal did not all attain the same height. 



Thus of the ~b lines, b l and lr were of nearly equal height, 

 but 6 4 was much shorter. 



Next I found that of the many iron lines observed by 

 Angstrom, only a very few were indicated in the spectrum 

 of the chromosphere when iron vapour was injected into it 

 from below. 



Dr. Frankland and myself were enabled at once to connect 

 these phenomena by reasoning that, as in the case of hydrogen, 

 the spectrum became simpler and the lines thinner where the 

 density was less. 



To test the truth of this assumption by some laboratory 

 experiments, we took the spark in air between two magnesium 

 poles, so separated that the magnesium spectrum did not extend 

 from pole to pole, but was visible only for a little distance, 

 indicated by the atmosphere of magnesium vapour round each 

 pole. We then threw an image of the magnesium pole and 

 surrounding vapour on the slit of the spectroscope exactly as 

 the sun and its surrounding vapours were thrown on the slit in 

 the observatory. 2 



1 He remarks : " II y a aussi des raies brillantes qu'on n' observe que dans des 

 cas exceptionnels, comme, par exemple, quaud la quantite de la substance soumise 

 a 1' experience est tres-abondante ou quand 1'incandescence devient tres-vive, Ces 

 raies qui se presentent ordinairement aux bords du spectre sous la forme de points 

 d'aiguille, meme quand les autres raies du metal forment des lignes continues a 

 travers du spectre, out ete representees surlaplanche par des lignes tres-courtes. " 

 Memoire sur la determination des longueurs d'onde des raies metalliques, p. 1 2, 

 printed in the Nova Ada Regioe Societatis Scientiarum Upsaliensis, ser. iii. vol. 

 vi. Upsala, 1868. 



2 Proc. Roy. Soc. No. 115, 1869. ; 



