MOUNT WILSON SOLAR OBSERVATORY.* 



George E. Hale, Director. 



The continuation and extension of the researches described in my last an- 

 nual report have led to the following results : 



(i) The comparative study of the spectra of the limb and center of the sun 

 favors the conclusion that the relative displacements of the lines near the 

 limb (after eliminating the Doppler effect) are due to pressure. Further lab- 

 oratory work is required before a final decision can be reached. 



(2) The angular velocity of the solar rotation, as determined from the mo- 

 tions of the calcium (Hg) flocculi on spectroheliograph plates, decreases from 

 the equator toward the poles. The mean velocity at any latitude is very 

 nearly the same as that of the sun-spots. 



(3) The angular velocity of the solar rotation, as determined from the 

 motion in the line of sight of the hydrogen represented by dark lines in the 

 solar spectrum, is more rapid at the equator than the angular velocity of the 

 spots and calcium (Hg) flocculi, and decreases less rapidly toward the poles. 



(4) The hydrogen (H8) flocculi, as measured on spectroheliograph plates, 

 move at the equator with about the same angular velocity as the calcium 

 (H2) flocculi, and show no evidence of retardation toward the poles. This 

 result was obtained before the discovery of solar vortices, and must be tested 

 by measurements of the motions of Ha flocculi lying outside of the vortices. 



(5) Investigations with an electric furnace have shown that such changes 

 of relative intensity as are exhibited by the lines of iron, titanium, and other 

 substances in sun-spots are produced by lowering the temperature of the fur- 

 nace. This confirms our conclusion that the temperature of these vapors in 

 sun-spots is low^er than in the reversing layer outside of spots. 



(6) The flutings of calcium hydride, as observed in another form of elec- 

 tric furnace, have been identified with flutings in our photographs of sun-spot 

 spectra. This is a further confirmation of the above conclusion. 



(7) Through the use of the Ha line of hydrogen, it has become possible to 

 photograph a hitherto unexplored region of the solar atmosphere. The photo- 

 graphs show that sun-spots are surrounded, and probably produced, by exten- 

 sive vortices, which draw the hydrogen, and doubtless other high-level gases, 

 into the spots at great velocity. 



* Address : Observatory Office, Pasadena, California. Grant No. 480. $85,000 for 

 construction, investigations, and maintenance. (For previous reports see Year Books 

 Nos. 3, 4, 5, and 6.) 



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