34 Journal op^ the jMitciiei.l Society [September 



cc'ivable, yes, Imt is it true? Einstein worked out his predicted devia- 

 tion of a ray of light grazing the sun on the idea that it is true, — 

 that the ray does reall}- suffer a retardation on its way toward the 

 sun proportional to the increasing gravitational field of the sun. 

 This would of course make the part of the wave front closest to the 

 sun move more slowly than the remoter portion, and hence the ray 

 would be swung round through an angle of 1.75 seconds of arc, as 

 already mentioned, and this is more than twice the angle expected 

 from Newton's laws. So here is a clear-cut test; — Newton or Ein- 

 stein, — a deviation of .82 seconds of arc, or a deviation of 1.75 sec- 

 onds. Wliich would it be? The two British eclipse expeditions sent 

 out last summer, one to Sobral, in Brazil, and the other to Prince's 

 Island, west coast of Africa, both detected the deviation, and in the 

 required amount to show the correctness of Einstein's views. 



This was the triumph which put Einstein's name on the front 

 page of every newspaper, and made every physicist in the world prick 

 up his mental ears and begin to study Einstein's work more closely. 



The third test set by Einstein for his theory was this: — his formu- 

 las show that the inside mechanism of an atom moves more slowly 

 in a gravitational field than in free space. Perhaps this is due to the 

 retardation suffered by a particle moving with a prodigious velocity 

 through a gravitational field, as considered above. At any rate, 

 Einstein predicted that light waves coming from a source located 

 in an intense gravitational field would make a spectrum in which the 

 lines would be shifted toward the longer-wave end of the spectrum, 

 which is the red end. 



This effect was looked for without success, lint Einstein remained 

 serene in his conviction that it would be found, and offered to rest the 

 validity of his entire theory upon it. 



Now comes his third triumph, for quite recently two young physi- 

 cists at Bonn not only detected the shift towards the red end of the 

 spectrum, but also discovered the reasons why previous attemi)ts to 

 find it, at Mt. Wilson and elsewhere, were unsuccessful. 



These three triumphant verifications of the Einstein ideas about 

 gravitation" have focussed the eyes of the world upon this modest 

 scholar in Berlin. Born a German Jew, he moved to Switzerland 

 during his boyhood, was educated there, became a Swiss citizen and 

 served for some years in the Swiss patent office. During this time he 

 pursued his mathematical studies, and later taught in the Zurich Poly- 



