EINSTEIN'S LAW OF GRAVITATION 245 



tion of such a minute deflection was an extraordinarily 

 difficult matter, so many corrections had to be applied to 

 the original observations ; but the names of the men who 

 record the conclusions are such as to inspire confidence. 

 Certainly any effect of refraction seems to be excluded. 



It is thus seen that the formulae deduced by Einstein 

 have been confirmed in a variety of ways and in a most 

 brilliant manner. In connection with these formulae one 

 question must arise in the minds of everyone: by what 

 process, where in the course of the mathematical develop- 

 ment, does the idea of mass reveal itself? It was not in 

 the equations at the beginning and yet here it is at the 

 end. How does it appear? As a matter of fact it is first 

 seen as a constant of integration in the discussion of the 

 problem of the gravitational field due to a single particle ; 

 and the identity of this constant with mass is proved whei\ 

 one compares Einstein's formulae with Newton's law which 

 is simply its degenerated form. This mass, though, is the 

 mass of which we become aware through our experiences 

 with weight; and Einstein proceeded to prove that this 

 quantity which entered as a constant of integration in his 

 ideally simple problem also obeyed the laws of conserva- 

 tion of mass and conservation of momentum when he in- 

 vestigated the problems of two and more particles. 

 Therefore Einstein deduced from his study of gravita- 

 tional fields the well-known properties of matter which 

 form the basis of theoretical mechanics ; i.e., he established 

 the identity of gravitational and inertial mass. A further 

 logical consequence of Einstein's development is to show 

 that energy has mass, a concept with which everyone 

 nowadays is familiar. 



The description of Einstein's method which I have given 

 so far is simply the story of one success after another; 

 and it is certainly fair to ask if we have at last reached 

 finality in our investigation of nature, if we have attained 

 to truth. Are there no outstanding difficulties ? Is there 

 no possibility of error? Certainly, not until all the pre- 



