1909] on Solar Vortices and Magnetic Fields. 619 



Some of the photographs strongly remind us of the distribution of 

 iron tilings in a, magnetic field, and suggest that some unknown force 

 is in operation. 



Such was the condition of the subject when the red Ha line of 

 hydrogen was first applied to the photography of the flocculi, on 

 Mount Wilson, in March 1908. The calcium and hydrogen flocculi 

 had been studied for several years, and much had been learned as to 

 their nature and their motions. It had been found, for example, 

 that the calcium flocculi observe the same law of rotation that governs 

 the motions of sun-spots, while the hydrogen flocculi apparently 

 follow a different law, in which the decrease in the angular rotational 

 velocity from the equator toward the poles is much less marked. The 

 latter result is in harmony with the investigations of Adams, whose 

 accurate measures of the approach and recession of the hydrogen at 

 the eastern and western limbs of the sun offer but little evidence of 

 equatorial acceleration on the part of this gas. For this and other 

 reasons it had been concluded that the hydrogen shown in such 

 pliotographs reaches a higher level than the vapours of the bright 

 (Ho) calcium flocculi. The region of the atmosphere previously ex- 

 plored with the spectroheliograph was nevertheless confined (except 

 in the case of eruptions and dark calcium flocculi) to a comparatively 

 low level, lying within a few thousand miles of the photosphere. 

 What might be expected if a still higher region could be satisfactorily 

 photographed in projection on the disk ? 



The red line of hydrogen offered the means of disclosing the phe- 

 nomena of this higher atmosphere. As it may not immediately 

 appear why different lines, caused by the radiation of the same gas, 

 should not give precisely similar photographs, a brief reference to the 

 aspect of a prominence in the red and blue hydrogen lines may be 

 advantageous. Here are two photographs of the same prominence, 

 seen in elevation at the sun's limb, one made with Ha, the other with 

 H8. As the red line is very bright, even in the highest regions, the 

 photograph taken with its aid shows the entire prominence. H8, 

 on the other hand, is relatively weak at the higher levels, and conse- 

 quently only the lower and brighter parts of the prom.inence are well 

 recorded when this line is used. If, now, we suppose ourselves im- 

 mediately above such a prominence, at a point where we observe it 

 in projection against the disk, it is evident that the character of the 

 hydrogen lines must depend upon their brightness at different levels. 

 As we know^ that, speaking generally, absorption is proportional to 

 radiation, the amount of light absorbed in the upper part of the 

 prominence will be much greater for Ha than for H8. Hence the 

 average level represented by the absorption of Ha will be higher than 

 the average level represented by H8, since the higher gases play a 

 more important part in the production of the former line. We may 

 therefore expect that photographs of the sun's disk, taken with the 

 light of Ha, will show the dark areas corresponding to absorption in 



