MOUNT WILSON SOLAR OBSERVATORY * 



Walter S. Adams, Acting Director. 



The investigations completed during the past year have comprised numer- 

 ous studies in solar spectroscopy, direct photography, and photometric and 

 spectroscopic observations of stars and nebulae with the 6o-inch reflector, 

 and a wide variety of laboratory researches. The year has seen the com- 

 mencement of several extensive investigations, particularly in stellar spec- 

 troscopy and photometry, which will require several years for completion, 

 but which should prove of high value in connection with certain of the 

 problems of stellar distribution. The fact that the solar activity is still near 

 its minimum, and that consequently very few sun-spots have appeared during 

 the past year, necessarily has prevented any large amount of work upon the 

 nature of the magnetic field in sun-spots. With increasing solar activity 

 this woric will be carried on with the aid of the 150-foot tower telescope. 



Among the results of the investigations completed during the year we may 

 refer to the following: 



(i) The determination of the probable system of circulation of calcium 

 vapor in sun-spots and the regions surrounding them. 



(2) The fact that calcium vapor has a radial motion inward across the 

 penumbrse of sun-spots. This is occasionally found combined with a rotary 

 motion resulting in the formation of vortical structure similar to that found 

 in Ha flocculi photographed with the spectroheliograph. 



(3) In addition to calcium vapor, which is always found flowing into sun- 

 spots, a study of the lines of other elements indicates a motion generally 

 inward on the part of hydrogen and frequently inward on the part of mag- 

 nesium and sodium. This is true of the high-level or, in general, stronger 

 lines of these elements. For the low-level lines the reverse is true, the 

 motion being outward (the Evershed effect), and between the two are found 

 lines which indicate essentially no motion. The relative level of the gases 

 producing different spectrum lines may perhaps be determined in this way. 



(4) Measurements upon spectra of the two opposite edges of the sun in 

 three different regions of the spectrum have given values for the rotational 

 velocity at the equator and at 15° of latitude which are slightly less than i 

 per cent smaller than the values found in 1908. Between 30° of latitude and 

 the pole the values are almost identical. It is doubtful whether this small 

 difference represents a real variation in the sun's rate of rotation in these 

 latitudes. 



* Situated on Mount Wilson, California. Grant No. 682. $106,055 for construction, 

 investigations, and maintenance during 1911. (For previous reports see Year Books 

 Nos. 3-9.) 



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