A DETERMINATION OF THE DEFLECTION OF LIGHT BY 
THE SUN'S GRAVITATIONAL FIELD, FROM OBSERVA- 
TIONS MADE AT THE TOTAL ECLIPSE OF MAY 29, 1919/ 
By Sir F. W. Dyson, F. R. S., astronomer royal, Prof. A. S. Eddington, F. R. S., 
and Mr. C. Davidson. 
[With 1 plate.] 
I. PURPOSE OF THE EXPEDITIONS. 
1. The purpose of the expeditions was to determine what effect, if 
an}?^, is produced by a gravitational field on the path of a ray of 
light traversing it. Apart from possible surprises, there appeared to 
be three alternatives, which it was especially desired to discriminate 
between — 
(1) The path is uninfluenced by gravitation. 
(2) The energy or mass of light is subject to gravitation in 
the same way as ordinary matter. If the law of gravitation is 
strictly the Newtonian law, this leads to an apparent displace- 
ment of a star close to the sun^s limb amounting to 0.87'' 
outward. 
(3) The course of a ray of light is in accordance with 
Einstein's generalized relativity theory. This leads to an ap- 
parent displacement of a star at the limb amounting to 1.75" 
outward. 
In either of the last two cases the displacement is inversely pro- 
portional to the distance of the star from the sun's center, the dis- 
placement under (3) being just double the displacement under (2). 
It may be noted that both (2) and (3) agree in supposing that 
light is subject to gravitation in precisely the same way as ordinary 
matter. The difference is that, whereas (2) assumes the Newtonian 
law, (3) assumes Einstein's new law of gravitation. The slight 
deviation from the Newtonian law, which on Einstein's theory causes 
' Reprinted by permission from Philosophical Transactions of the Royal Society of 
London, Series A, vol. 220, pp. 291-333. 
12573°— 21 10 133 
