RECENT ADVANCES IN SCIENCE 373 



How, then, is the result to be interpreted ? There seems 

 little doubt but that the deflection which has been obtained 

 is very closely double that which would have been anticipated 

 on Newton's law of gravitation, supposing light to possess 

 inertia. This suggests that Newton's law of gravitation is 

 merely a first approximation. There is nothing very surpris- 

 ing in this suggestion, although the popular press has made 

 much of it ; we have already become accustomed to the idea 

 that Newtonian mechanics is but a first approximation, re- 

 quiring modification when velocities comparable with the 

 velocity of light are concerned. It is not, then, altogether 

 surprising that the law of gravitation may also be modified for 

 very high velocities, though it would not be possible a priori 

 to say what the modification would be. 



The theory of Einstein has naturally commanded attention 

 as it predicted the result which has apparently been obtained. 

 The difficulty of understanding what is involved in the theory, 

 owing to the apparent impossibility of separating the basic 

 ideas involved in it from the mathematics with which they 

 are bound up, has perhaps not unnaturally caused some 

 scepticism as to whether the theory provides the true explana- 

 tion of the result. Our conservative ideas rebel against the 

 belief that a fundamental law of nature can only be stated 

 by invoking the aid of the calculus of variations. May it 

 not be, however, that, if only other axes of reference were 

 chosen, a simpler law would result ? As a very elementary 

 illustration, the expression of phenomena occurring on a 

 rotating body assumes a simpler form if axes rotating with the 

 body are chosen. Einstein's theory is a theory of space and 

 time and not a theory of gravitation, and the distinction is an 

 important one. It is an extension of the older relativity theory 

 which developed out of the attempts to explain the negative 

 results of all experiments seeking to obtain evidence of a 

 motion of the earth relative to the aether. It has been stated 

 that the result now obtained, justifying Einstein's theory, 

 has disproved the existence of the aether. This is incorrect. 

 If the aether is not necessary on the generalised theory of rela- 

 tivity, then it was not necessary on the older theory. In fact, 

 many scientists, including the present writer, considered that 

 the older relativity theory had made the conception of the 

 aether unnecessary. The theory of Einstein has extended the 

 conceptions of the older theory and, in addition to the rela- 

 tivity of velocities, has, as it were, through the introduction of 

 the equivalence hypothesis, introduced the conception of the 

 relativity of accelerations. It is this added feature which 

 leads to the results affecting gravitation, but an explanation 

 as to the nature of gravitation is not given. 



