MOUNT WILSON SOLAR OBSERVATORY.* 



George E. Hale, Director. 



The year has been one of minimum sun-spot activity, but good progress 

 has been made in certain fields of solar research, and other departments of 

 the Observatory's work have advanced in a very satisfactory manner. The 

 completion of the new tower telescope and the addition of important aux- 

 iliary apparatus to the equipment of the 6o-inch reflector have greatly in- 

 creased our instrumental resources, while the erection of a fireproof office 

 building in Pasadena will add to the efficiency of the staff and the safety of 

 plates and records. 



The Observatory has profited more than in any previous year from co- 

 operation with eminent astronomers, who have come to Mount Wilson to 

 conduct investigations for which our instruments are especially fitted. The 

 advantages of our alliance with Professor Kapteyn become increasingly evi- 

 dent. With his counsel, the observing program of the 6o-inch reflector has 

 been planned so as to yield a maximum return, advantageous alike to the 

 problems of stellar distribution and stellar development. 



Among the results of the year's work the following may be mentioned : 



(i) The 150-foot tower telescope and combined spectrograph and spectro- 

 heliograph have been thoroughly tested and found to be very efficient. 



(2) The plane grating has been shown to be fully equal, if not superior, 

 to the concave grating for the determination of the wave-lengths of standard 

 lines. 



(3) A difference in altitude of 5,100 feet (1,555 ^■)y corresponding to a 

 difference in pressure of about one-fifth of an atmosphere, is sufficient to 

 produce easily measurable displacements of certain lines selected for inter- 

 national use as tertiary standards, which should therefore be rejected. 



(4) Except in a single instance, the wave-lengths of the international 

 secondary standards are found to be of the highest accuracy. 



(5) Certain iron lines are greatly displaced to the violet by pressure. 



(6) A continuation of the investigation on the radial flow of vapors in 

 sun-spots gives the following velocities: Calcium (H and K) 1.9 km. per 

 second inward; hydrogen (Ha) 1.5 km. inward; sodium (D^ and Dg) 0.2 

 km. inward; magnesium (&i and 63) 0-4 ^^^- inward; aluminum (A 3961) 

 0.0 km. Various low-level iron lines show outward flow, with velocities 

 ranging from 0.49 to 0.90 km. per second. 



(7) A theoretical investigation has been made on the nature of magnetic 

 fields produced under conditions similar to those existing in sun-spots. 



* Situated on Mount Wilson, California. Grant No. 749. $254,075 for construction, 

 investigations, and maintenance during 1912. (For previous reports see Year Books 

 Nos. 3-10.) 



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