[ 291 ] 



IX. A Determination of the Deflection of Tight by the Suns Gramtational 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.If.S., 



and Mr. C. DAVIDSON. 



(Communicated by the Joint Permanent Eclipse Committee.) 

 Received October 30, Read November 6, 1919. 



[PL ATP: l.] 

 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 Newtonian 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. 



