THE common instruments of measurement proposed in theory and employed in practice for the co-ordination of physical events consist of rigid bars and clocks. The limitations of such methods are obvious. The erection of a rigid bar for the direct determination of the distance of the moon from the earth is inconceivable from a practical point of view, while it is a gross absurdity to speak of the measurement of molecular distances by means of rigid bodies. There is only one type of connecting-link across space suitable for co-ordination of events, namely, the light-ray. I here define a system of space-time coordinates which involve only one metrical quantity, the vibration period of an atom.