244 CONTEMPORARY SCIENCE 



amounting to 574* of arc per century; and it has been 

 shown that of this a rotation of 532" was due to the direct 

 action of other planets, thus leaving an unexplained rota- 

 tion of 42" per century. Einstein's formulae predicted a 

 rotation of 43", a striking agreement. 



4. In accordance with Einstein's formulae a ray of 

 light passing close to a heavy piece of matter, the sun, for 

 instance, should experience a sensible deflection in to- 

 wards the sun. This might be expected from "general" 

 considerations. A light ray is, of course, an illustration 

 of energy in motion ; energy and mass are generally con- 

 sidered to be identical in the sense that an amount of 

 energy E has the mass E/c 2 where c is the velocity of 

 light ; and consequently a ray of light might fall within 

 the province of gravitation and the amount of deflection to 

 be expected could be calculated by the ordinary formula 

 for gravitation. Another point of view is to consider 

 again the observer inside the compartment falling with the 

 acceleration of the gravitational field. To him the path 

 of a projectile and a ray of light would both appear 

 straight ; so that, if the projectile had a velocity equal to 

 that of light, it and the light wave would travel side by 

 side. To an observer outside the compartment, e.g., to 

 one on the earth, both would then appear to have the 

 same deflection owing to the sun. But how much would 

 the path of the projectile be bent? What would be the 

 shape of its parabola? One might apply Newton's law; 

 but, according to Einstein's formulae, Newton's law should 

 be used only for small velocities. In the case of a ray 

 passing close to the sun it was decided that according to 

 Einstein's formula there should be a deflection of i"-75 

 whereas Newton's law of gravitation predicted half this 

 amount. Careful plans were made by various astrono- 

 mers to investigate this question at the solar eclipse last 

 May, and the result announced by Dyson, Eddington and 

 Crommelin, the leaders of astronomy in England, was 

 that there was a deflection of i".g. Of course the detec- 



