428 



NATURE 



[July 31, 1919 



penumbrae of sun-spots, where they are drawn out 

 nearly parallel to the solar surface. It was further 



obtained for the cross-sections at different 

 levels with the spectroheliograph. Photographs in 



Fig. 3. — Bi-polar spot group. (<i) Slit on portion of continuous spectrum ; (b) slit on centre of Ha, 13-ft. spectroheliograph. 



supposed by Secchi that these photospheric I low-level calcium (Hj or Kj) correspond with a level 

 granules were closely related to the radial fila- | somewhat below that observed visually at the 



sun's limb, and here the granules 



^- 



A 



^.. K 





r 



^ 







.•^.^s 



fy,U<' 







-Record of magnetic pola 



ies and field-strengths in different parts of a spot group, 

 August 10, 1917. 



mentary structure of the chromosphere, and this 

 relation is strongly supported by the results 

 NO. 2596, VOL. 103] 



are relatively small. At the 

 higher level represented by the 

 H.2 and Kg lines, the structure is 

 generally similar to that of the 

 photosphere, but the average size 

 of the small, bright flocculi is 

 rather greater than that of the 

 granules found by Langley. At 

 the still higher levels depicted 

 when the second slit is set on the 

 centre of Ha, the corresponding 

 granules are dark, and the 

 smallest of them are about 2" in 

 diameter, or twice the size of the 

 smallest calcium flocculi. The 

 filaments thus appear to expand 

 as they rise, and Prof. Hale con- 

 siders that these observations 

 support the view that the photo- 

 sphere and the gaseous atmo- 

 sphere above it are formed of 

 columns of hot gases, rising by 

 convection from the interior of 

 the sun. In order to explain the 

 change from the continuous 

 spectrum of the photosphere to 

 the bright-line spectrum of the 

 chromosphere, it seems necessary 

 to suppose that precipitation 

 of refractory materials occurs 

 at the photospheric level, or 

 that the conditions may be 



such as to cause the gases to emit a continuous 



spectrum. 





