QUÆRENS. I know that light, whatever it may be, is the agent by which objects are rendered visible to our eyes, that it is not transmitted instantaneously from one point to another, but gradually, like all motion. I know that it flies at the rate of 75,000 leagues a second, that it runs 750,000 leagues in ten seconds, and 4,500,000 each minute. I know that it takes more than eight minutes to cross the distance of 37 millions of leagues which separate us from the Sun. Modern astronomy has made these facts familiar. LUMEN. Do you perfectly realise its undulatory movement? [Sidenote: Undulatory movement of Sound.] QUÆRENS. I think so. I compare it to that of sound, although it be accomplished upon a scale incomparably more vast. By undulation following undulation, sound is diffused in the air. When the bells peal forth their sonorous sound, this is heard at the very moment when the clapper strikes the bell, by those living round the church, but is not heard till one second after, by those living at a distance of 492 yards; two seconds later by those at 765 yards; and three seconds later still, by those at a distance of 1093 yards from the church. Thus sound only gradually reaches one village after another as far as it can go. In the same way light passes successively from one region in space to another at a greater distance, and travels without being extinguished into the far-off realms of Infinity. If we could see from the Earth an event which is being accomplished upon the Moon; for instance, if we had sufficiently good instruments to perceive from here, a fruit falling from a tree on the surface of the Moon, we should not see the fact at the _moment of its occurrence_, but one second and a quarter _after_, because light requires about that time to travel the distance from the Moon to the Earth. Similarly, could we see an event taking place upon a world at ten times greater distance than the Moon, we could not witness it until 13 seconds after it had really happened. If this world were a hundred times farther off than the Moon, we could not see an event until 130 seconds after it had taken place; were it a thousand times more distant, we should not see it until 1300 seconds, or 21 minutes 40 seconds had elapsed. And so on according to the distance. [Sidenote: Time taken by Light in travelling from the Earth to the star Capella.]