IN his Friday evening discourse at the Royal Institution on Jan. 22, Sir Arthur Eddington discussed the expansion of the universe. Outside our own galaxy of stars there exists a vast number of external galaxies, each containing many millions of stars, which appear as nebulae. These are to be found running away from us almost unanimously; and the farther away they are, the faster they recede. This effect has been observed up to a distance of more than a hundred million light-years; the speed there reaches 20,000 km.—a sec.as fast as an alpha particle. It looks at first as though the nebulæ must have a particular aversion to us, but it is not difficult to see that the recession is the effect of a general expansion of the universe, and is not especially aimed at us. An effect of this kind has been anticipated theoretically. Einstein's law of gravitation contains a term representing repulsive force, which is ordinarily minute and negligible; but at very great distances the repulsion—becomes large and overmasters the ordinary gravitational attraction, so that very remote objects tend to scatter apart. The theory, however, does not predict the magnitude of this ‘cosmical repulsion’, and hitherto it has only been possible to evaluate it from direct astronomical observations of the nebulæ. Sir Arthur is convinced, however, that precisely the same cosmical term is concerned in the theory of the atom and supplies the standard which determines, for example, the radius of an atom. So that out of the theory of the atom (without any astronomical observations) we can predict the rate of recession of the nebulæ or alternatively, astronomical observation of the distances and velocities of the nebulæ is a method of determining the masses of the electron and proton. Sir Arthur stated that, in his opinion, this astronomical phenomenon of the expanding universe is the main clue by which we can ultimately unravel the mechanism of the atom.