Let us trace the rays of light back to their origin. These encounter first the globes nearest to the sun: the almost imperceptible asteroids, which, on account of the solar splendour, can scarcely be detected, and which, as yet, have received no baptismal names. Yonder beams illuminate other revolving globes,--Mercury, Venus, the Earth. If we here arrest our gaze, we are influenced by a vague and mournful recollection; the Earth was our place of sojourn at an epoch when thought, formerly shackled, had become free. It is thus that a seed, removed from the stem which nourished it, wanders afar to diffuse and perfect its species. Our planet, of an ochreous yellow, relieved with green and white, possesses no special privileges; it shares in the ponderated movements of the spheres; it is neither the largest, nor the smallest, nor the heaviest, nor the lightest, nor the nearest to, nor the farthest from, the sun. And shall Earth alone, of all the planets, nourish that kind of "thinking seed" which we name the human race? It seems improbable. * * * * * But let us return to the great orb of light. It illuminates exactly one half of our earth; the other half lies in shade. And as the earth rotates upon its own axis, every point of its surface is necessarily exposed to the action of the solar rays. This action varies in duration and intensity. All this I was taught, when I was still at school on our revolving planet. I remember, too, as a lesson learned by heart, that, under the Equator, or, more exactly, in 0° 0´ latitude, as well as at the Poles, or under 90° latitude; the duration of the day is equal to that of the night, with this difference, that, while under the Equator, a day of twelve hours alternates invariably with a night of the same duration, at the Poles a day of six months succeeds continually to a night of six months. I also recollect that, at a given moment, namely, at the spring and autumn equinoxes, the duration of the day, over all the terrestrial surface, is equal to that of the night, just as under the Equator; and that, after these two epochs, under the intermediary latitudes, between the Equator and the Poles, the length of the days and their corresponding nights varies according to the seasons; that, in our northern hemisphere, after the spring equinox, the days increase while the nights diminish, to such an extent that, at the summer solstice,--21-22 June,--they attain their _maximum_ of length (and the nights their _minimum_), the reverse taking place during the period that elapses between the autumn equinox and the winter solstice.