12 PRp:SrDENT's ADDRESS — SECTION A. 



observed displacements of the individual stars show an effect varying 

 inversely as the distance of perihelion, in accordance with theory. 

 This last fact supports the view that light is subject to gravitation, 

 but does not discriminate between the Einstein and Newton Theories. 

 It should be noticed that if we revert to our original co-ordinates the 

 theoretical deflection is still im/R ; for the substitution of r — in 

 for /• does not change the form of the curve at infinity, and consequently 

 leaves the direction of the asymptotes unchanged. 



It appears then that the Einstein Theory is adequate to explain 

 the motion of Mercury and the deflection of light by a gravitational 

 field. The op])onents of the theory have put forward alternative 

 explanations, the most plausible of which is that the intra-Mercurial 

 gaseous matter apparent in the zodiacal light and the solar corona 

 causes the two effects, the first by gravitation, the second by refraction. 

 Dr. Jeffreys has discussed this suggestion at some length in the Monthly 

 Notices of the Eoyal Astronomical Society. He comes to the con- 

 clusion that the amount of such matter present is wholly inadequate 

 to produce the effects. In the same paper he discusses the Einstein 

 Theory, starting from the experimental evidence and the assumption 

 that the path of a particle or light pulse is given by hKfds = where 

 K is some function of x, y, z, t, and els'- some quadratic function of their 

 differentials. He shows that K is irrelevant and the equation of the 

 path is given by hfds = 0, and establishes the fact that the equations 

 are necessarily co-variant. Then, assuming symmetry in the case 

 of the iso/lated attracting centre, he uses the experimental results to 

 find the co-efficients in ds'^ and arrives at the result that the form 

 deduced by Einstein from the Principle of Equivalence is the only one 

 possible. 



Einstein has suggested a third experimental test of his Theory. 

 He predicts that the spectral lines of light from a solar source will 

 show, when compared with lines from a similar terrestrial source, a 

 displacement towards the red of an amount equivalent to the Doppler 

 effect corresponding to a velocity of '634 kilometres per second. On 

 the experimental evidence available it would, jDcrhaps, be rash either 

 to affirm or deny the existence of this displacement. It is certain that, 

 whether the Einstein effect exists or not, it is masked by various other 

 effects due to solar physical conditions. It has been claimed that 

 the observations show a general tendency towards a shift in the Einstein 

 direction, but the magnitudes of the effects vary so greatly that this 

 cannot be taken as evidence in support of the Einstein Theory. Solar 

 physicists ajjpear reluctant to commit themselves to a definite opinion, 

 but they seem to agree in asserting that the existence of the Einstein 

 effect is not by any means definitely established. The only positive 

 statement in favour of the shift that I have been able to find is contained 

 in a letter from K. W. Lawson to Nature of 29th January, 1920. He 

 says that Einstein has written to him to the effect that two young 



