MOUNT WILSON OBSERVATORY. 213 



wave-length determinations of Messrs. St. John and Babcock on the 

 generaUzed theory of relativity. It will be recalled that the careful 

 investigations of Mr. St. John have failed to show a systematic shift 

 of solar lines toward the red of the magnitude predicted by Einstein. 

 Evershed has also failed to find the desired displacement, but the im- 

 portance of the question is so great that no pains should be spared in 

 settling it beyond the peradventure of a doubt. For this reason Messrs. 

 St. John and Babcock have renewed their attack with improved appara- 

 tus, involving many refinements of procedure overlooked by less care- 

 ful spectroscopists, some of whom have found no difficulty in confirm- 

 ing Einstein's prediction. Exceptional weight will certainly be de- 

 served by conclusions based upon the admirable methods described in 

 this report, in which direct spectroscopic determinations of wave-length 

 are checked by wholly independent measures obtained with the aid of a 

 Fabry-Perot interferometer (pp. 259, 260). 



The measured displacement of stars photographed near the sun by 

 the British observers during the total solar ecHpse of May 29, 1919, 

 are of such precision, and in such close correspondence with the de- 

 mands of the theory of relativity, that the validity of the conclusions 

 based upon them does not appear open to question. Confirmatory 

 e\ddence of the same kind is much to be desired, however, and for this 

 reason it is hoped that the ]Michelson interferometer may prove to be 

 suitable for the measm-ement of the small displacement of stars by Jupi- 

 ter called for by the theory. The difficulties arising from atmospheric 

 disturbances may perhaps prove insuperable, but a serious effort will 

 be made to apply the method. 



The solar activity continued to decline during 1919, and at present 

 the sun is remarkably free of spots. The consequent absence of dis- 

 turbing local fields has permitted us to renew our study of the sun's 

 general magnetic field, and ^Nlr. Ellerman has taken a large number of 

 photographs of spectra for this purpose during the summer of 1920. 

 These will usefully supplement the plates secured during a favorable 

 interval in 1916, which Mr. van Maanen's recent measures show to 

 confirm "^dthin narrow limits the period of 31.52 days found for the 

 rotation of the sun's magnetic axis in 1914. No high-latitude spots 

 have yet appeared as forerunners of the next cycle, and we still await 

 the opportmiity to determine whether the reversal of magnetic polarity 

 observed at the last minimum is to be confirmed (p. 222). 



At Mr. Abbot's request, Mr. Nicholson has undertaken a compari- 

 son of the variations in area of the dark hydrogen flocculi (prominences 

 photogi-aphed in projection on the sun) with the variations of the solar 

 constant of radiation. Clayton's recent success in basing weather pre- 

 dictions on changes in the solar radiation directs renewed attention to 

 the importance of finding a corresponding correlation of these short- 

 period fluctuations with other classes of solar phenomena, and Mr. 

 Nicholson will give special attention to this subject. 



