Then the world came upon some startling facts. In 1905 a paper appeared by Professor Albert Einstein which asserted that the explanation of certain remarkable discoveries in physics gave us a new conception of this strange four-dimensional manifold in which we live. Thus, the great difference between the space and time of philosophy and the new knowledge is the objective reality of the latter. It rests upon an amazing sequence of physical facts, and the generalized theory, which appeared several years later, founded as it is upon the abstruse differential calculus of Riemann, Christoffel, Ricci and Levi-Civita, emerges from its maze of formulas with the prediction of real phenomena to be sought for the in the world of facts. We shall, therefore, approach the subject from this objective point of view. Let us go to the realm of actual physical events and see how the ideas of relativity gradually unfolded themselves from the first crude wonderings of science to the stately researches that first discovered the great ocean of ether and then penetrated in such a marvelous manner into some of its most mysterious properties. THE ELECTROMAGNETIC THEORY OF LIGHT Suppose that we go out on a summer night and look into the dark depths of the sky. A thousand bright specks are flashing there, blue, red, yellow against the dark velvet of space. And as we look we must all be impressed by the fact that such remote objects as the stars can be known to us at all. How is it that light, that curious thing which falls upon the optic nerve and transmits its pictures to the brain, can ever reach us through the black regions of interstellar space? That is the question which has for its answer the electromagnetic theory of light. The first theory to be advanced was Newton's "corpuscular" theory which supposed that the stars are sending off into space little pellets of matter so infinitesimally small that they can move at the rate of 186,000 miles a second without injuring even so delicate a thing as the eye when they strike against it. But in 1801, when Thomas Young made the very important discovery of interference, this had to give way to the wave theory, first proposed by Huyghens in the 17th century. The first great deduction from this, of course, was the "luminiferous ether," because a wave without some medium for its propagation was quite unthinkable. Certain peculiar properties of the ether were at once evident, since we deduce that it must fill all space and at the same time be so extremely tenuous that it will not retard to any noticeable degree the motion through it of material bodies like the planets.