144 SCIENCE PROGRESS 



by the more general and at the same time more satisfying hypothesis now 

 known as the Special Principle of Relativity. [Annalen der Physik, 1905.) 



The second paper is that in which the late H. Minkowski showed that the 

 work of Einstein brought the two concepts of space and time into such a 

 close unity that he was able to say in a lecture at Cologne (Sept. 21, 1908), 

 " Henceforth, the old conceptions of space for itself and time for itself are 

 reduced to mere shadows, and only some sort of unity of the two will be found 

 consistent with the facts," This lecture, delivered under the title of " Raum 

 und Zeit," is here produced. 



Finally we have the paper {Annalen der Physik, 4, 49, 19 16, in which 

 Einstein gave the first complete account of his generalised theory of relativity, 

 showing how it made a natural place for the gravitational phenomena which 

 had always eluded the electro-magnetic theory, and how the new theory of 

 gravitation automatically removed the only outstanding discrepancy between 

 theory and observation. 



E. C. 



The Absolute Relations of Time and Space. By A. A. Robb, Sc.D., D.Sc, 

 Ph.D. [Pp. viii+ 80.] (Cambridge: at the University Press, 1921. 

 Price 55. net.) 



This little book is intended to introduce to a larger public the main outline 

 of the fuller discussion of the same matter which the author has published 

 under the title, A Theory of Time and Space, also through the Cam- 

 bridge University Press. Dr. Robb was dissatisfied, as well he might be, 

 with the ordinary physicist's naive view of space and time, a view in most 

 cases quite unconsciously based on the acceptance of Newtonian theory as 

 being the last word in physical science. Another few years of acquaintance 

 with the theory of relativity may well bring about such a change in this 

 respect that Dr. Robb's discussion, instead of appearing abstract and aca- 

 demic, will be taken very much for granted. 



The main purpose of the work is to analyse all temporal and spatial rela- 

 tions in terms of the single idea of one event being before or after another. 

 It is taken for granted that the only purpose of science is "to analyse the 

 relations of order among the instants of which I am directly conscious." 

 Dr. Robb confesses that he started this work through a sense of dissatisfaction 

 with Einstein's standpoint, which seemed to take all meaning out of such a 

 familiar concept as " simultaneity of two events." His own conclusion is that 

 the only really simultaneous events are events which occur at the same 

 place. It must be confessed that Dr. Robb seems to leave even less meaning 

 in the word than do those who treat it as a relative term. Nevertheless, Dr. 

 Robb does us a great service in insisting that it is order relations in an indi- 

 vidual view of the universe which form the subject matter of science, that 

 there is a real difference in those relations between " before " and " after," 

 and that geometry in the physical world is also an empirical science. But the 

 mathematician has not yet succeeded in producing a technique for discussing 

 order relations before introducing the idea of measurement. He has not yet 

 faced up to the fact that measurement is itself merely a statement of 

 certain order relations already perceived to exist. 



Dr. Robb attempts at the conclusion of his book to make a bridge 

 from purely descriptive order relations to the use of co-ordinate numbers ; 

 but he comes here to all the difficult problems of the logical bases of 

 geometry, and there is a sense of insufficiency in his treatment of this 

 part of his argument which is not apparent in the earlier part of the argu- 

 ment. Nevertheless, it would be well if there were more among our physical 

 scientists who had the patience to follow out the consequences of their own 

 premises as logically as Dr. Robb has done. E. C, 



