Two unabridged addresses summarizing aspects of Einstein's Special and General Theories of Relativity comprise this succinct volume. The first, Ether and Relativity, was delivered in 1920 and examines the properties demanded of the ether of space by the theory of relativity. Geometry and Experience, delivered the following year, describes the limits within which the Euclidean system can be held to be approximately true in relation to the concept of a finite universe. Written clearly and concisely, these lectures are a fascinating read for both scientist and layman. Many of the earliest books, particularly those dating back to the 1900s and before, are now extremely scarce and increasingly expensive. We are republishing these classic works in affordable, high quality, modern editions, using the original text and artwork.