MOUNT WILSON SOLAR OBSERVATORY. l8l 



Spectrum of the Chromosphere. 



The investigation of the spectrum of the chromosphere, for the purpose 

 of determining with the greatest possible precision the wave-length, level, 

 and origin of all the bright lines that can be photographed with the 150- foot 

 tower telescope, is being continued by the Director and Mr. Ellerman, as- 

 sisted in the measurements by Miss Burwell. Unfavorable weather condi- 

 tions and the astigmatism of the mirrors (now overcome) have hindered the 

 observations, and few satisfactory negatives have yet been obtained. The 

 work will necessarily proceed slowly, as photographs showing the maximum 

 number of bright lines can be made only when the atmosphere is exception- 

 ally steady. In connection with this investigation, Mr. King will study in 

 the laboratory the anomalous dispersion of a number of lines which occur 

 in the chromosphere, in order that a further test may be made of the theory 

 of Julius. 



A Sun-spot Hypothesis. 



The following tentative working hypothesis of sun-spots is proposed by 

 the Director merely as a guide to further research. As the result of an 

 eruption, or some other cause tending to produce rapid convection, a gaseous 

 column moves upward from within the sun toward the surface of the photo- 

 sphere. Vortex motion is initiated by the difference in velocity of adjoin- 

 ing surfaces or by irregularities of structure and is maintained by convection. 

 The circulation in the vortex is vertically upward and then outward along 

 the photosphere, as in a terrestrial tornado. Expansion produces cooling 

 at the center of the vortex, and a comparatively dark cloud (the umbra) 

 results. As in Harker's electric-furnace experiments,* a rapid flow of nega- 

 tive ions sets in towards the cooler gases at the center from the hotter gases 

 without. These ions, whirled in the vortex, produce a magnetic field. The 

 descending gases (especially hydrogen) in the higher atmosphere of the sun 

 are drawn in toward the pole of the magnet along the lines of force, as in 

 Birkeland and Stormer's theory of the aurora. This accounts for the con- 

 figuration of the hydrogen {Ha) flocculi, as shown by the spectroheliograph. 



Hitherto we have assumed the spot to be single. The typical spot-group 

 is double, the line adjoining the two spots making only a small angle with 

 the equator. The polarities of the two principal spots (smaller companions 

 are usually present) are opposite, and the hydrogen {Ha) flocculi surround- 

 ing a bipolar group resemble the lines of force about a bar magnet. Two 

 alternative hypotheses may be offered to account for these phenomena. Ac- 

 cording to the first, the lower extremity of the vortex of the primary spot, 

 curved like many terrestrial tornadoes, turns up to meet the photosphere, thus 

 producing a horse-shoe vortex. As in the case of single (unipolar) spots, 

 the hydrogen and other high-level gases flow toward the two members of the 

 group along the lines of force. According to the second view, there are 



* See Nature, July 18, 1912. 



