MOUNT WILSON SOLAR OBSERVATORY. 209 



GENERAL MAGNETIC FIELD OF THE SUN. 



The investigation for determining the incUnation of the sun's magnetic 

 axis and the period of its revolution about the solar axis of rotation has 

 been continued; 632 spectra have been measured by Mr. van Maanen, 

 representing observations on 19 days. The work, which has involved 

 the measurement of 2,132 plates during the last three years, is now 

 Hearing completion. From observations on 63 days, distributed over 

 an interval of 110 days for which plates are available, new values for 

 the inclination and the period have been derived. The reduction of the 

 measures and the necessary computations were made by Miss Wolfe 

 under the superintendence of Mr. Scares. The results are : 



Period = 31'?79=fc0'.'31 Inclination = 6?2±0?4 



Magnetic pole on central meridian, 1914, June 25.31 ±0.42 G. M. T. 



The data give some indication that the inclination is not constant; 

 this, however, may be due to accidental errors or possibly to the influ- 

 ence of spots and those features which have been called "faint markings" 

 by the Greenwich observers, though none of these were seen when the 

 photographs were taken. In general, days on which spots were near 

 the sun's meridian were avoided. To investigate the influence of the 

 "faint markings," some photographs of their spectra have been taken; 

 measures of a few of these show rather strong magnetic fields, of the 

 order of 100 to 150 gausses. It is clear that if many of these local fields 

 were present when the plates were taken for the investigation of the 

 position of the sun's magnetic axis, they may have been a disturbing 

 factor in the derivation of the inclination. It will therefore be neces- 

 sary to see if these markings, first mentioned by the Greenwich obser- 

 vers in January 1915, are visible near the spot minimum or appear only 

 during the time of increasing activity. 



THE EQUIVALENCE PRINCIPLE OF RELATIVITY. 



Mr. St. John's investigations on the accurate determination of the 

 positions of solar and terrestrial lines have been utilized in several 

 specific problems, among them a test of the relativity hypothesis as 

 appUed to the solar atmosphere. 



According to Einstein's generalized equivalence principle of relativitj', 

 the lines in solar and stellar spectra should be displaced to the red when 

 referred to the corresponding terrestrial lines. The calculated dis- 

 placements, equivalent in the case of the sun to the Doppler effect of a 

 velocity of 0.634 km. per sec, are well within the range of solar measure- 

 ments. An extended investigation based upon the behavior of lines in 

 the nitrogen (cyanogen) band at X 3883 was undertaken to determine 

 what consideration should be given to this deduction from the equiva- 

 lence principle in the interpretation of observations of solar spectra. 

 These hues lend themselves to the purpose, as they are not displaced 

 by pressure; freedom from Doppler effect is obtained by making the 



