The two brief lectures in this volume are each interesting in their own right, although there is little to unify them beyond concern for the most foundational aspects of the science of physics.
The first, on “Ether and the Theory of Relativity,” treats the now-quaint-sounding topic of the luminiferous ether, which seems to have gone the way of phlogiston and other obsolete scientific topics. Surprisingly, Einstein insists that there is a role for the ether concept in relativistic physics! It is, however, a “gravitational ether” from which every mechanical characteristic has been eliminated, and which is therefore just as easily denominated as space. Although this paper avoids mathematical and experimental details, a certain prior familiarity with the history of modern physics is very useful in appreciating it, since Einstein races through an extensive series of theorists in his summary of the evolution of the ether concept.
The second paper begins by offering the distinction between geometry as an axiomatic philosophical undertaking, and the empirical physical science of “practical geometry.” Einstein explains that the effort to adjust physical laws to accommodate known events and behaviors to the axiomatic system of Euclidean geometry was in fact a driving force behind the theory of relativity, even though it eventually became necessary to posit non-Euclidean space as a result of that theory. The later part of the paper is concerned to permit novices to acquire an imaginative appreciation of finite but unbounded spaces–in particular spaces curved through the fourth dimension in a hyperspherical fashion.