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 uramum 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 from 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. 
Feeundlich, who attempted to collect information from eclipse plates already taken; 
but he did not secm’e 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 prelinfinary discussion has been given,]* 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 discrinfinate between the tliree alternatives. 
4. The results of the observations here described appear to point quite definitely to 
the tliird 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. JohnJ 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 Newtoman 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. 
X ‘ Astrophysical Journal,’ vol. XLVI, p. 249. 
