Nothing is more surprising to the ordinary reflective man than the concept of a finite universe, yet it is to such a concept that the latest development of mathematical science, so admirably expounded in Jeans's Astronomy and Cosmogony, requires us to adapt ourselves. A finite universe is one in which anything moving outwards in a straight line, and maintaining its direction, must after an interval return to its starting-point. This seems a self-contradiction. A straight line, we think, must go on for ever.