On page 95 appears a review of a book by Professor E. A. Milne in which is described a new theory of the metrical character of the world and the interpretation, in the light thereof, of many important astronomical phenomena. Although the author states that his object is not to criticize the general form of the principle of relativity, there appears to be a fundamental distinction between the viewpoints of Einstein and Milne which is frequently emphasized and which it is profoundly important to examine as minutely as possible, for if Professor Milne's claim is established the foundations of physics are essentially modified. The purpose of this article is to compare the outlooks implicit in Milne's theory and the theory of relativity, respectively, in order to see how far, if at all, current ideas require alteration. It should, be mentioned that, although Professor Milne speaks of his theory as “the principle of relativity in a much weaker form” (weaker, that is, in its power of restricting natural possibilities, not in its power of representing phenomena) and frequently uses the word “relativity” to describe it, we shall here, to prevent misunderstanding, restrict the meaning of the word to Einstein's theory.