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IX. A Determination of the Deflection of Liflit h\j the Suns Granitational Field, 
from Observations 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. 
{Communicated by the Joint Permanent Eclipse Committee.) 
Received October 30,—Read November 6, 1919. 
[Plate 1.] 
Contents. 
Page 
I. Purpose of the Expeditions.291 
II. Preparations for the Expeditions . . . ..293 
III. The Expedition to Sobral.296 
IV. The Expedition to Principe.312 
V. General Conclusions.330 
I. Purpose of the Expeditions. 
1. The purpose of the expeditions was to determine what effect, if any, 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 Ne’wtonian law, this leads to 
an apparent displacement of a star close to the sun’s limb amounting to 0"'87 
outwards. 
(3) The course of a ray of light is in accordance with Einstein’s generalised relativity 
theory. This leads to an apparent displacement of a star at the limb amounting 
to l"-75 outwards. 
In either of the last two cases the displacement is inversely proportional to the distance 
of the star from the sun’s centre, the displacement 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 gravita¬ 
tion 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 
VOL. CCXX.-A 579 . 2 s [Published April 27, 1920. 
