292 SIR F. W. DYSON, PROF. A. S. EDDINGTON AND MR. C. DAVIDSON ON A 



deviation from the Newtonian law, which on EINSTEIN'S theory causes an excess 

 motion of perihelion of Mercury, becomes magnified as the speed increases, until for 

 the limiting velocity of light it doubles the curvature of the path. 



2. The displacement (2) was first suggested by Prof. EINSTEIN* in 1911, his argument 

 being based on the Principle of Equivalence, viz., that a gravitational field is indis- 

 tinguishable from a spurious field of force produced by an acceleration of the axes of 

 reference. But apart from the validity of the general Principle of Equivalence there 

 were reasons for expecting that the electromagnetic energy of a beam of light would be 

 subject to gravitation, especially when it was proved that the energy of radio-activity 

 contained in uranium was subject to gravitation. In 1915, however, EINSTEIN found 

 that the general Principle of Equivalence necessitates a modification of the Newtonian 

 law of gravitation, and that the new law leads to the displacement (3). 



3. The only opportunity of observing these possible deflections is afforded by a ray of 

 light fro 111 a star passing near the sun. (The maximum deflection by Jupiter is only 

 0"-017.) Evidently, the observation must be made during a total eclipse of the sun. 



Immediately after EINSTEIN'S first suggestion, the matter was taken up by Dr. E. 

 FREUNDLICH. who attempted to collect information from eclipse plates already taken ; 

 but he did not secure sufficient material. At ensuing eclipses plans were made by various 

 observers for testing the effect, but they failed through cloud or other causes. After 

 EINSTEIN'S second suggestion had appeared, the Lick Observatory expedition attempted 

 to observe the effect at the eclipse of 1918. The final results are not yet published. 

 Some account of a preliminary discussion has been given, f but the eclipse was an 

 unfavourable one, and from the information published the probable accidental error is 

 large, so that the accuracy is insufficient to discriminate between the three alternatives. 



4. The results of the observations here described appear to point quite definitely to 

 the third alternative, and confirm EINSTEIN'S generalised relativity theory. As is well- 

 known the theory is also confirmed by the motion of the perihelion of Mercury, which 

 exceeds the Newtonian value by 43" per century an amount practically identical 

 with that deduced from EINSTEIN'S theory. On the other hand, his theory predicts a 

 displacement to the red of the Fraunhofer lines on the sun amounting to about 0-008 A 

 in the violet. According to Dr. ST. JOHN| this displacement is not confirmed. If this 

 disagreement is to be taken as final it necessitates considerable modifications of 

 EINSTEIN'S theory, which it is outside our province to discuss. But, whether or not 

 changes are needed in other parts of the theory, it appears now to be established that 

 EINSTEIN'S law of gravitation gives the true deviations from the Newtonian law both 

 for the relatively slow-moving planet Mercury and for the fast-moving waves of light. 



It seems clear that the effect here found must be attributed to the sun's gravitational 

 field and not, for example, to refraction by coronal matter. In order to produce the 



* ' Annalen der Physik,' vol. XXXV, p. 898. 



t ' Observatory,' vol. XLII, p. 298. 



J ' Astrophysical Journal,' vol. XLVL p. 249. 



