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but zero length is unaltered on Weyl's theory in this manner — 

 is, in fact, unique and requires no route of comparison. It 

 does not remain unaltered in Prof. Eddington's theory, and 

 he states that this was at first his chief obstacle in generalising 

 the theory, as it seemed that the uniqueness of zero length 

 and a unique track for light propagation were bound up together. 

 The real relation between these two is now known. 



As regards the electron, the author succeeds in proving that 

 it is a structure which cannot exist in a resultant field of electro- 

 magnetic force, but can resist differential force of this type. 

 An isolated electron in no external field and at rest remains 

 a " miracle," but it is shown that an accelerated electron in 

 an external electromagnetic field is exactly the same miracle, 

 and nothing more. He concludes, further, that the ordinary 

 mass of an electron is not directly a source of gravitational 

 attraction, and yet it has gravitational mass. The electron is 

 to be regarded as a region of abnormal world curvature, in 

 which the gravitational mass arising is in a constant ratio to 

 the electrical — but not of necessity the same for positive and 

 negative electrons. 



But it is not in our province to deal with these more physical 

 aspects of the theory. Only a further remark or two needs 

 to be addressed to the mathematical readers. Einstein's theory 

 is not to be regarded as a mere approximation, for the discussion 

 of the present theory proves that his postulates and deductions 

 are exact — that, in fact, the natural geometry of the world is 

 that of Riemann and Einstein, and not Weyl or Eddington. 

 For what the two latter are finding is not the geometry of 

 actual space and time, but the geometry of the world-structure 

 which is the common basis of space and time and things. Here 

 we quote the exact words of the author, who clearly feels that 

 this distinction has not hitherto been really appreciated. 



Dr. A. A. Robb, in his new work, The Absolute Relations of 

 Time and Space (Camb. Univ. Press), gives an introduction, in 

 concise form, to his larger treatise, and also some interesting 

 preliminary discussion of the relation of his point of view to 

 that of Einstein. The work is clearly very fundamental, and 

 is not, as he states, strictly a relativity theory at all. Its 

 relation, in one important aspect, to the theory of relativity, is 

 brought out very clearly, and is worthy of a somewhat extended 

 statement in these notes, for the relativity theory alone can 

 apparently never be sufficient in itself without the introduction 

 of a geometrical analysis of the present type, which forms a 

 very severely logical scheme. The theory of time and space is 

 developed in terms of the relations of " before " and " after," 

 and the possibility of doing this is very remarkable. These 

 relations are absolute and not dependent on the particular 



