A 66 SCIENCE PROGRESS 



this is of extreme interest, for to the observer in the accelerated 

 lift a ray of light (which would be straight to an observer in a 

 uniformly moving system) would appear curved, the deflection 

 from the straight path being opposite to the direction in which 

 the lift is accelerated. If, as Einstein supposes, this will 

 also be the appearance to the observer fixed on the gravitating 

 star, then we reach the important conclusion that the passage 

 of a ray of light near to a gravitating body should produce 

 a bending of the ray towards the body. The curvature to be 

 expected is extremely small — amounting to a change of direc- 

 tion of \"j" in the case of a star seen close to the sun's limb — and 

 it is impossible so far to prove or disprove the hypothesis 

 directly. However, Einstein has succeeded by the develop- 

 ment of the idea, thus roughly sketched, in throwing the laws 

 of motion, of electro-dynamics, and of gravitation into a form 

 which makes the sequence of phenomena entirely independent 

 of any particular framework of reference, and he has been able 

 to predict therefrom a motion of the perihelion of Mercury 

 amounting to 43"per century — just the amount of a motion which 

 has hitherto been regarded as a serious discrepancy between 

 fact and the usual Newtonian theory. Those who are interested 

 will find an elementary account of the matter in Chap. XXII. 

 of Richardson's recent book The Electron Theory of Matter. 

 For those who can follow the mathematical development of the 

 relativity principle, an explanatory paper by Dr. Fokker in 

 the Phil. Mag., Jan. 191 5, will prove useful ; full references 

 to the original literature are given in this paper. Another 

 remarkable consequence from Einstein's theory is pointed out 

 by Fokker in the paper referred to. It was remarked by Mach 

 in his Principles of Mechanics that we cannot speak of mass in 

 an absolute sense — mass is a relative measure of the inertiae 

 of bodies, determined in the case of two bodies by their relative 

 accelerations at an instant when they are mutually acting on 

 each other, and free from other influences. This should not be 

 confused with the idea of gravitating mass, the measure of 

 which makes its appearance in the usual Newtonian law of 

 gravitation. Mach concluded that we are not justified in 

 thinking of the mass as of something absolute belonging to a 

 body, but that it may be due to some inducing influence of 

 bodies one upon the other. If this be so we should expect to 

 find that the inertial mass of a particle is increased by the 



