LUMEN. The calculation is very easy. The interval which separates them is of necessity that which light travels in a hundred years. Now, at the rate of 75,000 leagues per second, you see at once that it travels 4,500,000 leagues in a _minute_, 270,000,000 leagues in an _hour_, 6,480,800,000 leagues in a _day_, 2,366,820,000,000 in a _year_, allowing for leap-years; consequently, the result would be that the interval between two points of departure at the distance of a _century_ from one another, is nearly 236 _billions_ 682 thousand millions of leagues. Here, then, I say we have a series of terrestrial photographs, imprinted in space, at corresponding distances, one after another. Let us now suppose that between each of these centennial pictures we should find annual pictures, between each of which the distance is preserved in accordance with the time that light travels in a year, which I have just given you; then between each of the annual pictures we have those of every day, and as each day contains the photographs of each hour, every hour the photographs of its minutes, and every minute of its seconds, all succeeding one another, according to their respective distances apart--we shall have in a ray of light, or rather in a jet of light, composed of a series of distinct pictures in juxtaposition, the aerial register of the history of the Earth. [Sidenote: Psychical optics.] When the spirit travels in this ethereal ray of pictures with a swiftness greater than that of light, it sees in succession, backwards, the ancient pictures. When it arrives at the distance at which the aspect of events that set out in 1767 is to be seen, it has already retraced a hundred years of terrestrial history. When it reaches the point where the aspect of 1667 has arrived, it retraces two centuries. When it attains to the photograph of 1567, it has seen, again, three centuries, and so on successively. I told you in the beginning that I directed my course toward a group of stars situated at the left of Capella. This group proved to be at an incomparably greater distance than that star, although from the Earth it appeared to be close beside it, because the two visual rays are near one another. This apparent proximity is solely due to the perspective. In order to give you an idea of the remoteness of this far-off universe, I may tell you that it is not less vast in size than the Milky Way. One may then ask to what distance should the Milky Way be transported to reduce it to the apparent size of this nebula. My learned friend Arago made this calculation, of which you must be aware, as he repeated it every year in his course of lectures at the Observatory, that have been published since his death. It would be necessary to suppose the Milky Way to be transported to a distance equal to 334 times its own length. Now, as light takes 15,000 years to traverse the Milky Way from one end to another, it follows that it cannot take less than 334 times 15,000