The correlation of time with its geometrical analogue is of absorbing interest. Representing velocity by the common method of plotting a curve showing positions at various times and marking distances horizontally and times vertically, the velocity of light being 1, $MM'$ and $NN'$ will both represent this velocity. Since this is assumed to be the greatest velocity occurring in nature, all other possible velocities are represented by lines falling within the upper and lower V's. Now this diagram correctly represents two dimensions of Minkowski's Euclidean four-space so, transmuting to real but flat four-space by multiplying times by $\sqrt{-1}$, it is seen that there is a region outside which no effect can be propagated from $O$ since that would involve the existence of a velocity greater than that of light. This region represents the future of $O$. Similarly, $O$ can only be affected by events within the region derived from the downward-opening V, which therefore represents the past of $O$. The region between the two represents events which may be either simultaneous with $O$ or not, according to the velocity of the observer at $O$. Thus in this theory an event dictated by free-will, could affect points in its "future" region, but not in any other, which agrees with experience and shows that the theory is not essentially "determinist." If "free-will" is really free, the future is not yet determined, and the fourspace must be in some way formed by the will as time progresses. The trains of thought inspired by Einstein's postulates have already carried us to a pinnacle of knowledge unprecedented in the history of man. On every hand, as we look out upon the universe from our new and lofty standpoint, unexpected and enthralling vistas open up before us, and we find ourselves confronting nature with an insight such as no man has ever before dared aspire to. It is completely unthinkable that this theory can ever be swept aside. Apart from experimental verifications which, in point of fact, lend it the strongest support, no one could work through the theory without feeling that here, in truth, the inner workings of the universe were laid bare before him. The harmony with nature is far too complete for any doubt to arise of its truth. XVI THE QUEST OF THE ABSOLUTE Modern Developments in Theoretical Physics, and the Climax Supplied by Einstein BY DR. FRANCIS D. MURNAGHAN, JOHNS HOPKINS UNIVERSITY BALTIMORE We shall discuss the more important aspects of the theory popularly known as the "Einstein Theory of Gravitation" and shall try to show clearly that this theory is a natural outcome of ideas long held by physicists in general. These ideas are: (a) The impossibility of "action at a distance;" in other words we find an instinctive repugnance to admit that one body can affect another, remote from it, instantaneously and without the existence of an intervening medium.