(4) In its most general form the principle of relativity states that those relations, and those alone, which persist unchanged in form for all possible space-time reference frameworks are the inherent laws of nature. To find such relations Einstein has applied a mathematical method of great power--the calculus of tensors--with extraordinary success. This calculus threshes out the laws of nature, separating the observer's eccentricities from what is independent of him, with the superb efficiency of a modern harvester. The residue is a physical geometry--or geometrical physics--of time-space, in which it appears that the times and spaces contributed by the several observers' reference frameworks are shadows of their own contrivings; while the real, enduring universe is a fourfold order of time and space indissolubly bound together. One observer separates this time-space into his own "time" and "space" in one way, determined by his path through the world of events; another, moving relatively to the first, separates it differently, and what for one is time shades into space for another. This time-space geometry is non-Euclidean. It is "warped" (curved), the amount of warping at any place being determined by the intensity of the gravitational field there. Thus again gravitation is rooted in the nature of things. In this sense it is not a force, but a property of space. Wherever there is matter there is a gravitational field, and hence a warping of space. Conversely, as long ago imagined by Clifford, wherever there is a warping of space, there is matter; and matter is resolved ultimately into wrinkles in time-space. To visualize a warped space, consider a simple analogy. A man walks away from a polished globe; his image recedes into the mirror-space, shortening and thinning as it goes, and thinning (in the direction of motion) faster than it shortens. Everything around him experiences a like effect. If he tries to discover this by a footrule it automatically shortens faster as he turns it into the horizontal position, so his purpose eludes him. The mirror-space is warped in the direction of the image's motion. So is our own. For all bodies, as evidenced by the Fitzgerald contraction, shorten in the direction of motion. And just as the image can never penetrate the mirror-space a greater distance than half its radius, so probably time-space is curved in such a way that our universe, like the surface of a sphere, is finite in extent, but unbounded. XII FORCE VS. GEOMETRY How Einstein Has Substituted the Second for the First in Connection With the Cause of Gravitation BY SAUL DUSHMAN GENERAL ELECTRIC LABORATORIES SCHENECTADY, N. Y.