134 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1919. 
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 Professor Einstein" 
in 1911, his argument being based on the principle of equivalence, viz, 
that a gravitational field is indistinguishable 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 radioactivity 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 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 w\as 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,' but the eclipse was an unfavorable 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 
generalized 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 Fraun- 
hofer lines on the sun amounting to about 0.008 A in the violet. Ac- 
cording to Doctor St. John * this displacement is not confirmed. If 
" Annalen der Physik, Vol. XXXV, p. 898. 
8 Observatory, Vol. XLII, p. 298. 
* Astrophysical Journal, Vol. XLVI, p. 249. 
