l56 TRANSACTIONS OF SECTION A. 



8. Report of Seismology Committee. — See Reports, p. 35. 



9. Report of Commiltee on Gravity at Sea. — See Reports, p. 83. 



10. Report on Seismology after the War. By Dr. G. W. Walker, 

 F.R.S.—See Reports, p. 32. 



FBIDAY, SEPTEMBER 12. 

 The following Papers and Report were read, and Discussion took place : — 



1. Photographs taken at Principe during the Total Eclipse of the Sun, 

 May 29th. By Professor A. S. Eddingtoin, F.R.S., and E. T. 

 CoTTiNGHAM, followed by a Discussion on Relativity, opened by 

 Professor Eddington, F.R.S. 



Professor Eddington gave an account of the observations which had been 

 made at Principe during the solar eclipse. The main object in view was to 

 observe the displacement (if any) of stars, the light from which passed through 

 the gravitational field of the sun. To establish the existence of such an effect 

 and the determination of its magnitude gives, as is well known, a crucial test of 

 the theory of gravitation enunciated by Einstein. Professor Eddington explained 

 that the observations had been partially vitiated by the presence of clouds, but 

 the plates already measured indicated the existence of a deflection intermediate 

 between the two theoretically, possible values 0-87" and 1-75". He hoped that 

 when the measurements were completed the latter figiire would prove to be 

 verified. Incidentally Professor Eddington pointed out that the presence of 

 clouds had resulted in a solar prominence being photographed and its history 

 followed in some detail ; some very striking photographs were shown. 



Following on thie account Professor Eddington opened the discussion on 

 relativity, and referred again to the bending of the wave front of light to be 

 expected from Einstein's new law when the light passes near a heavy body. It 

 should be possible to test experimentally, this law, which demands that the speed 

 of light varies as 1 — 2 H where fl is the gravitational potential. He showed that 

 whether Einstein's solution of the problem be correct or not, it has at any rate 

 given a new orientation to our ideas of space and time. Sir Oliver Lodge 

 regarded the relativity theory of 1905 as a supplement to Newtonian dynamics 



by the adoption of the "factor (l — -^j and its powers necessitated by experi- 

 mental results ; but he did not consider this dependence of mass and length on 

 velocity as entailing any revolutionary changee of our ideas of space and time, or 

 a* rendering necessary the further complexities of 1915. He compared the diffi- 

 culties involved with the case of measuring temperature, defined in terms of a 

 perfect gas, and made with gases which only approximate to this ideal state. 

 Dr. Silberstein pointed out that Einstein's theory of gravitation predicts three 

 verifiable phenomena, i.e., a shift of spectral lines, the bending of light round 

 the sun and the secular motion of the perihelion of a planet. In the neighbour- 

 hood of a radially symmetric mass, such as our sun, the -line element (hs is given 

 by:- 



The coefficient c-dt^ gives by itself a lengthening of the period of oscillation for 

 a terrestrial observer in the ratio (l-fM/c-r) : 1, demanding a shift of spectral 

 lines of about OIA.U. Secondly., the path of rays of light is obtained by putting 

 ds = o, and the first and second coefficients give jointly a bending which, for rays 

 almost grazing the sun, is 1-75". Thirdly, Keplerian motion is predicted with a 

 progressively moving perihelion which in the case of iMercury turns out to be 

 43" per century. He drew attention to the fact that St. John's results in 1917 



