This scholarly introduction to Einstein's theory of gravitation traces the ideas and observations that made relativity necessary, from the behavior of light to the failures of classical mechanics. With Albert Einstein's endorsement, and framed by H. H. Turner's introduction, Henry L. Brose's translation, and Professor Eddington's support, it offers readers with some grounding in mathematics and physics a clear path into one of science's great revolutions. Beginning with light as the medium through which we know the universe, the book uses vivid analogies, stellar aberration, the Michelson-Morley experiment, and the boat-in-a-stream image to question the ether and explain why the speed of light forced a new mechanics. It then contrasts Galilean relativity and Newtonian absolutes with Einstein's special theory, where uniformly moving frames are equivalent but Lorentz transformations replace older assumptions about space and time. From there, the general theory recasts gravity as the geometry of space-time itself, linking inertial and gravitational mass and extending the discussion into cosmology, where the universe may be finite yet unbounded. Astronomical tests, especially Mercury's orbit, the solar spectrum, and the bending of starlight near the sun, culminate in the drama of the 1919 eclipse expedition and the dawn of a new era in physics. This edition has been fully restored with modern typesetting, custom cover design.