Humboldt, in his Cosmos, gives the following beautiful illustrative proofs of this phenomenon: If, for a moment, we imagine the acuteness of our senses preternaturally heightened to the extreme limits of telescopic vision, and bring together events separated by wide intervals of time, the apparent repose which reigns in space will suddenly vanish; countless stars will be seen moving in groups in various directions; nebulae wandering, condensing, and dissolving like cosmical clouds; the mil...