MOUNT WILSON SOLAR OBSERVATORY. 229 



The complex phenomena which may be involved in the relative 

 displacements of lines in terrestrial and solar spectra, including the 

 effects of pressure and motion of the gases in the solar atmosphere and 

 the possible influences of anomalous dispersion and the Einstein effect, 

 demand investigations of the most rigorous character for their solution. 

 Instrumental errors, physiological complications encountered in the 

 measurement of closely adjoinmg Hnes, and electrical or pressure 

 displacements in the artificial light-sources used for comparison 

 purposes must be run down and eUminated. Much progress has been 

 made in this dnection during the year. The errors involved in the 

 measurement of adjacent lines have been detected (p. 240), and it has 

 been shown clearl}^ that the supposed mutual influence of such Unes, 

 instead of being due to anomalous dispersion in the solar atmosphere, 

 results largely, if not wholly, from physiological causes (p. 241). 



Turning now to inquiries regarding stars and nebulae, we are able to 

 record marked progress. Observations of stellar parallaxes and proper 

 motions, the light changes of certain variable stars, the magnitudes of 

 stars in Kapteyn's Selected Areas, and the radial velocities of stars 

 have been systematically continued, and new methods have been effec- 

 tively applied to the solution of outstanding questions of importance. 



In the study of the structure of the universe, no data are more 

 important than those relating to the distances of the stars. We have 

 recently observed with satisfaction the rise of an active group of 

 investigators of stellar parallaxes, whose cooperative researches are 

 rapidly augmenting the hmited data hitherto available. The use of 

 the 60-inch reflector has rendered it possible for us to contribute to this 

 undertaking, and the exceptional precision of the results will certainly 

 warrant the continuance and extension of the investigation. But this 

 process, in its very nature, is slow and laborious, and a supplementary 

 method, of comparable precision, is greatly to be desired. 



Such a method, mentioned in the last annual report, has now been 

 developed and estabhshed as a fundamental contribution to practical 

 astronomy. Based upon the discovery of a numerical relationship 

 between the absolute magnitudes of stars and the relative intensities of 

 certain lines in their spectra, it is thus directly dependent upon existing 

 methods of parallax determination. But once this relationship has been 

 established, it becomes possible, by means of a few simple measure- 

 ments and calculations, to obtain a star's parallax from a photograph 

 of its spectrum. The method is not as yet applicable to the earlier 

 types of stellar spectra, and its availability is necessarily limited to 

 the range of absolute magnitudes occurring among stars of directly 

 measured parallaxes. Distance does not enter as a limitation, how- 

 ever, except as it determines the apparent magnitude which can be 

 observed spectroscopically with the equipment available. But its 

 simplicity and rapidity of execution will soon increase materially our 



