ASTRONOMY 527 



ASTRONOMY. By H. Spencer Jones, M.A., B.Sc, Chief Assistant, 

 Royal Observatory, Greenwich. 



The Relativity Displacement of Spectral Lines. — One of the 

 crucial tests of Einstein's Theory of Relativity depends upon 

 the establishment of the existence of a slight displacement 

 towards the red of the spectral lines of a massive body such 

 as a star as compared with those from a terrestrial source. 

 At present, the investigation of this test must be confined to 

 the solar spectrum, as the wave-lengths of stellar lines are 

 not known with the required accuracy. Even in the case of 

 the Sun, it is a matter of extreme difficulty to establish beyond 

 possibility of doubt whether the required displacement is or 

 is not absent. This difficulty arises from the smallness of the 

 effect which it is required to determine and from the presence 

 of a number of other disturbing causes such as pressure dis- 

 placements, etc., whose magnitudes are comparable with that 

 of the Einstein effect. Nevertheless, several claims to have 

 established the existence of the displacements required by 

 Einstein have been made, and some of the supporters of the 

 theory have jumped to the conclusion that the matter can 

 be regarded as settled. In particular, the experiments of 

 Grebe and Bachem in Germany were held by many in that 

 country to have proved that there is a displacement of the 

 solar lines of the amount required by Einstein. 



It is well, therefore, that these claims should be examined 

 impartially and the present state of the situation made clear. 

 This has been done by St. John, whose conclusions appear 

 in the last report of the Mount Wilson Observatory. The 

 following is a summary of St. John's survey : 



Perot found that there was a sun-arc displacement of the 

 amount required by theory. His investigation was based on 

 the line X4197, the unsymmetrical head of the second band of 

 cyanogen. In obtaining his result he applied corrections for 

 an assumed downward movement in the solar atmosphere 

 and for a negative pressure shift of the cyanogen band lines, 

 the latter being approximately equal in magnitude to the 

 shift required by relativity. Dr. Birge, using two lines in the 

 \3883 cyanogen band, and applying corrections for an upward 

 movement in the solar atmosphere, but assuming that there 

 was no pressure shift, also finds a displacement in accordance 

 with the theory. Grebe and Bachem also used the cyanogen 

 lines, but assumed no radial movement and no pressure shift ; 

 they, however, applied a correction for a supposed asymmetry 

 of the arc lines. They also deduced approximately the 

 Einstein effect. At Mount Wilson it is found that the pressure 

 shift of the cyanogen band is much smaller than assumed by 

 P^rot : it is also concluded, from a detailed study of the band, 



