MOUNT WILSON OBSERVATORY. 189 



GENERAL MAGNETIC FIELD OF THE SUN. 

 Three series of plates for determining the constants of the sun's general 

 magnetic field were made in 1914, 1916, and 1920. A fourth series has 

 been completed this year by Ellerman. The series consists of photographs, 

 obtained at the 150-foot tower telescope, of the spectrum in the region X5247 

 to X5329 in the third order of the 75-foot spectrograph with a nicol prism and 

 compound quarter-wave plate over the slit. Six exposures at different 

 latitudes on the central meridian were made on about 40 days at regular 

 intervals during two complete rotation periods of the sun. This series, in 

 connection with those obtained previously, will be used for the determination 

 of a more exact value of the period of revolution of the magnetic pole, and for 

 a study of possible variations in the constants of the sun's magnetic field 

 and their relationship to the phase of sun-spot activity. 



SUN-SPOT SPECTRA. 



The measurement of the wave-length of lines on photographs of sun-spot 

 spectra taken with the 75-foot spectrograph through a nicol prism and 

 quarter-wave plate has been continued by Miss Mayberry in connection with 

 the investigations of Hale on the strength of the magnetic fields in spots and 

 the displacement of the p-component. The region from X3900 to X6600 has 

 been completed and more than 11,000 lines have been measured. The dis- 

 cussion of this great amount of material and the comparison of the results 

 with those obtained with the registering microphotometer in the case of close 

 pairs of fines will be the subject of future studies, 



SOLAR ROTATION. 



The investigation of the period of the sun's rotation through the displace- 

 ment of the spectral lines has been continued by St. John, assisted by Miss 

 Ware. Especial attention has been given to points in high latitudes. The 

 observations have all been made with the 150-foot tower telescope, and mainly 

 in portions of the spectrum in which atmospheric lines are available for the 

 detection of local solar disturbances. The results show no evidence of a 

 progressive change in the equatorial rotation period from year to year. 



WAVE-LENGTHS OF LINES IN THE SOLAR SPECTRUM. 



St. John has completed the determination, by means of grating spectro- 

 graphs, of the wave-lengths upon the international system of several hundred 

 lines in the solar spectrum in the region between X3650 and X6750. A com- 

 parison of the results with those obtained by Babcock with the interferometer 

 in the region to the red of X4500 shows such close agreement that full confi- 

 dence may be placed in the values for the entire list. 



CENTER AND LIMB DISPLACEMENTS. 

 A study of the displacements of lines of the solar spectrum at the limb forms 

 a part of the work which is being carried on by St. John with the 150-foot 

 tower telescope and by Babcock with the Snow telescope and 30-foot spectro- 

 graph. From measurements of a selected list of lines supplemented by a few 

 results from the interferometer, Babcock finds a value of 0.0054 a at X5400 for 

 the mean increase in wave-length at the limb as compared with the center. 

 This displacement seems to increase slowly with the wave-length and depends 

 upon the intensity of the line. The results are in good agreement with those 

 of St. John. 



