Lumen. Do you perfectly realise its undulatory movement?
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.
Time taken by Light in travelling from the Earth to the star Capella.
Lumen. Exactly, and you are aware that the luminous ray sent to the Earth by the star Capella takes seventy-two years in reaching it. It follows, therefore, that if we only receive the luminous ray to-day, which left its surface seventy-two years ago, the denizens of Capella see only that which happened on the Earth seventy-two years ago. The Earth reflects in space the light that it gets from the Sun, and from a distance, appears as brilliant as Venus and Jupiter appear to you, planets lighted by the same Sun that lights the Earth. The luminous aspect of the Earth, its photograph, journeys in space at the rate of 75,000 leagues a second, and only reaches Capella after seventy-two years of incessant travel. I recall these elementary principles in order that you may have them thoroughly fixed in your memory; you will then be able to comprehend, without difficulty, the facts which have happened to me during my ultra-terrestrial life since our last interview.
Quærens. These principles of optics are, to my mind, clearly established. The day after your death in October 1864, when, as you have confided to me, you found yourself rapidly transported to Capella, you were astonished to arrive there at the moment when the philosophical astronomers of the country were observing the Earth in the year 1793, and witnessing one of the most significant acts of the French Revolution. You were not less surprised to see yourself again as a child, running about in the streets of Paris. Then, leaving Capella and coming nearer to the Earth, you arrived at the zone where that part of the terrestrial photography passed before your vision, which showed you your infancy, and you saw yourself at six years of age, not in memory, but in reality. Out of all your previous revelations, this is the one I had the most difficulty in believing—I mean, in grasping its meaning.
Lumen. That which I now wish to make you comprehend is stranger still. But it was first necessary for you to admit that one, before I could adequately reveal to you this one.
Retrospective survey of life on Earth.