258 PHYSICAL SCIENCE 



Thus the effect of the sun, which for two 

 hundred years and more has been referred to an 

 attractive force between the sun and the planet, 

 can be explained as due to a curvature in space, 

 which makes the natural path of the planet when 

 undisturbed an ellipse instead of a straight line. 

 If a body be falling freely, and so following its 

 natural path, it feels no sensation of force. If 

 it be prevented from falling by a chair or the 

 platform of a weighing machine, it is turned 

 from its natural path and shows the phenomenon 

 we call weight, which may be regarded as due to 

 the upward acceleration impressed on the body 

 by the bombardment of the molecules of the chair 

 or the platform. 



The mathematical theory shows that all the 

 long known facts of gravitation can be deduced 

 equally well from Newton's theory or Einstein's 

 principle. Yet three phenomena have been found 

 in which, when great accuracy of observation is 

 reached, differences should appear and crucial 

 experiments be possible. 



(i) A minute divergence of the planet Mercury 

 from its Newtonian path — a divergence only 

 amounting to 43 seconds of arc in a century — 

 was at once explained by Einstein. 



(2) Both on Newton's theory and on Einstein's, 

 the path of a ray of light, passing near a massive 

 body like the sun, should be bent towards the 

 body, but Newton's deflection is one half that 

 of Einstein. The only way of observing this 

 deflection is to measure accurately the apparent 

 position of the image of a star very near the sun 

 on a photographic plate exposed during an eclipse. 

 This was done in two places, Sobral in Brazil and 



