The extension of the method of science to the study of the astronomical universe as a whole is an impressive and important feature of recent intellectual history. In a field where previously myth and metaphysics ranged at will with speculative abandon but with little in the way of progressively cumulative information or insight, by contrast, scientific cosmology has already produced significant results of an observational sort and opened up vistas of disciplined theoretical vision that are a token of even more (...) extensive disclosures to come. With the aid of high-powered telescopes, observationally grounded descriptive generalizations have been established about the structure, distribution and motions of the bodies constituting the basic astronomical units out of which the universe is composed. Such units are, in contemporary investigations, identified as the extragalactic nebulae. Preliminary surveys revealed a uniformity of distribution of these nebulae throughout space and in all directions. Also original surveys, accomplished primarily by Hubble, suggested an empirical law stating a linear proportionality between the distances to nebulae and the red-shift in their spectra. If such red-shift is interpreted as a velocity effect, it may be taken as marking a mutual recession of nebulae from one another, and leads thereby to the general idea of an “expanding universe.' Meanwhile, theoretical cosmology in the hands of the mathematical physicists has kept pace with observational advances by offering a variety of ”models” or explanatory theoretical devices for the systematic ordering and incorporation of the fruits of observation. Beginning with the ground-breaking paper by Einstein in 1917 where an extension was made of relativistic principles to the cosmologic domain, a variety of further attempts were made along classical relativistic lines by Einstein himself, de Sitter, Robertson, Tolman and Lamaître while somewhat different modes of approach were subsequently opened in several directions by Milne, Eddington, Hoyle, and others. While nothing in the way of a stable or wholly acceptable theory has yet made its appearance, nevertheless fruitful and promising lines of research have thus been inaugurated. (shrink)

In this paper I attempt to trace certain aspects of the evolution of theories of space-time and mechanics as revealed by a brief comparative study of Newtonian theory, Robb's theory, general relativity, and Milne's kinematical relativity. The first object is to emphasise how each theory leaves us in a position in which the succeeding one appears as a perfectly natural next step in the development of ideas. The second object is to show how, in spite of superficial differences in character, (...) the theories all necessarily possess the same general structure constituted by the presence of hypotheses, from which certain general mathematical relations are deduced, which in their turn are used to predict relations between observable quantities. (shrink)