general theory there is no postulate of the constancy of velocity of a light-signal and accordingly no method of assigning a time to events corresponding to that adopted in the special theory. In this latter theory attention was confined to material systems moving with uniform velocity with respect to each other and it developed that the velocity of light was the ultimate velocity faster than which no system could move--a result surprising and a priori rather repugnant. It is merely a consequence of our mode of comparing times of events; if some other method--thought transference, let us say--were possible the velocity of this would be the "limiting velocity." In conclusion we should remark that the postulated equivalence of "gravitational" and "centrifugal" forces demands that anything possessed of inertia will be acted upon by a gravitational field and this leads to a possible identification of matter and energy. Further our guiding idea (a) will prompt us to say, following the example of Faraday in his electrical researches, that the geodesics of a gravitational space have a physical existence as distinct from a mere mathematical one. The four-dimensional space we may call the ether, and so restore this bearer of physical forces to the position it lost when, as a three-dimensional idea in the Special Relativity Theory, it had to bear an identical relation to a multitude of relatively moving material systems. The reason for our seemingly paradoxical title for an essay on Relativity will be clear when it is remembered that in the new theory we consider those space-time properties which are absolute or devoid of reference to any particular material reference-frame. Nevertheless, although the general characteristics of the theory are thus described, without reference to experiment, when the theory is to be tested it is necessary to state what the four coordinates discussed actually are--how they are determined by measurement. It is our opinion that much remains to be done to place this portion of the subject on a satisfactory basis. For example, in the derivation of the nature of the gravitational space, surrounding a single attracting body, most of the accounts use Cartesian coordinates as if the space were Euclidean and step from these to polar coordinates by the formulæ familiar in Euclidean geometry. But these details are, perhaps, like matters of elegance, if we shall be allowed to give Einstein's quotation from Boltzmann, to be left to the "tailor and the cobbler." XVII THE PHYSICAL SIDE OF RELATIVITY The Immediate Contacts Between Einstein's Theories and Current Physics and Astronomy BY PROFESSOR WILLIAM H. PICKERING HARVARD COLLEGE OBSERVATORY, MANDEVILLE, JAMAICA