This source presents Albert Einstein's "Relativity: The Special and General Theory," a foundational text on the principles of relativity, translated by Robert W. Lawson. It comprehensively explains the Special Theory of Relativity, discussing concepts like the relativity of simultaneity and the behavior of measuring-rods and clocks in motion, and introduces the Lorentz transformation. The book then expands to the General Theory of Relativity, particularly focusing on gravitation, the equality of inertial and gravitational mass, and the curvature of space-time. Crucially, it details experimental confirmations of these theories, such as the perihelion shift of Mercury and the deflection of light by gravity, before exploring the structure of the universe as a whole.