532 SCIENCE PROGRESS 



the ideas involved to give the theory a wide degree of accept- 

 ance. The Physical Society of London has published a Report 

 on the Relativity Theory of Gravitation, by Prof. A. S. Eddington, 

 F.R.S. (London : Fleetway Press, Ltd., 91 pp. 6s. net), in which 

 may be found an admirable summary of the ideas underlying 

 the new theory, of their mathematical formulation, and of the 

 results which are deducible therefrom. This is the best con- 

 nected summary of the theory which has appeared in English 

 and can be recommended to those interested in the subject. 



The theory is already faced with difficulties. One of its 

 deductions requires a displacement of the solar lines towards 

 the red by an amount 0*008 tenth-metres, corresponding to a 

 Doppler displacement of 0-63 km. per sec. We have previously 

 referred to the result obtained by Dr. St. John at the Mount 

 Wilson Observatory (Science Progress, 13, 15, 191 8), who 

 from a measurement of certain lines in the solar spectrum con- 

 cluded that " within the limits of error, there is no evidence 

 in these observations of a displacement to longer wave-length 

 either at the centre or the limit of the sun, of the order of 

 coo 8 A., as required by the principle of relativity." Owing to 

 the importance of confirming or refuting this adverse con- 

 clusion, further evidence on this matter was of fundamental 

 importance. Some results have recently been obtained by 

 J. Evershed of the Kodaikanal Observatory {Observatory, 41, 371, 

 191 8, and 42, 51, 1919), and these are also adverse to the theory. 

 Mr. Evershed found that certain lines of cyanogen showed a 

 displacement in the sun as compared with the arc of -f '005 A. 

 at the centre of the disc, or + 'Oo8 A. at the limb. These lines 

 are not subject to a pressure shift, and the displacement might 

 therefore be attributable either to motion in the line of sight 

 or to a gravitational effect, as predicted by Einstein's theory. 

 The former objection seemed improbable, as it would appar- 

 ently require the earth to be controlling the motion and driving 

 away the gas at all points on the sun's limb. On the other 

 hand the displacement was of approximately the amount 

 required by Einstein's theory : nevertheless, certain dis- 

 crepancies were found in the displacements at various parts 

 of the sun's disc, the magnitudes of which differed, not only 

 inter se, but also from the predicted amount. In order further 

 to test the theory therefore, Mr. Evershed conceived the beauti- 

 ful idea of obtaining the spectrum of the back of the sun, by 



