In an age when the existence of a measurable light-velocity seems to belong to the realm of facts long since experimentally proved; when science has begun to measure the universe, using the magnitude of this velocity as a constant, valid for the whole cosmos; and when entire branches of science have been founded on results thus gained, it is not easy, and yet it cannot be avoided, to proclaim that neither has an actual velocity of light ever been measured, nor can light as such ever be made subject to such measurement by optical means - and that, moreover, light, by its very nature, forbids us to conceive of it as possessing any finite velocity.