l.'J;^0\ The Theory of Relativity 33 



This prediction was made in 1917, two years before the eclipse, 

 and Einstein was a professor in a German university, shut in by the 

 war, and so helpless to test the observation himself, but the English 

 promptly began to make plans for sending out not one but two eclipse 

 expeditions to test the theory. Let us examine the point in question. 

 For years it has been customary to regard light as having mass, for 

 we reasoned as follows : Light is a form of energy ; being of the 

 kinetic type it is expressed mathematically in terms of its velocity 

 and its mass, or at least something that takes the place of mass and 

 acts like it. Now if light possesses mass and velocity it must possess 

 momentum, and ought to exert a push when it falls on a body. This 

 light pressure, or radiation pressure, was predicted by Maxwell, and 

 its value calculated by him, but it was only experimentally confirmed 

 forty years later by Nichols and Hull and Lebedew. It is the radia- 

 tion pressure from the sun which causes a comet 's tail to stream behind 

 the comet when approaching the sun, and to stream ahead of the 

 comet when receding from the sun. 



The question next arises, does light possess weight? Is the mass 

 of light the kind which is acted on by gravitation? If so, we can 

 consider a beam of light passing near the sun as a comet, and using 

 the regular comet formula we can regard the speed of light as the 

 speed of the comet, and as usual insert the proper value of nearest 

 approach to the sun. From this formula we can find the angle be- 

 tween the asymptotes of the cometary orbit, and hence find the angle 

 of deviation of the ray which we should expect on the basis of New- 

 tonian mechanics. The result is an angle of .82 seconds of arc. Now 

 in the case of ordinary comets we have always found that their speed 

 increases while they are approaching the sun, and decreases after they 

 have swung round it on their long journey back into the depths of 

 space, so Newton's idea was that this acceleration should be expected 

 of any comet moving with any speed, and acted on by the sun's gravi- 

 tation, because force always produces acceleration in a mass free to 

 move. 



And just here is one of the most interesting of Einstein's dis- 

 coveries. It is shown from his formulas that if a body is approaching 

 the sun with a velocity less than about 100,000 miles a second, it will 

 be accelerated in its motion, but if it has a velocity greater than this 

 amount it will actually he retarded as it moves toward the sun. Such 

 a thing is inconceivable on the basis of Newtonian mechanics. Incon- 



