Until the great epoch of the discovery of the telescope, at the beginning of the seventeenth century, in the time of Keppler and Galileo, only vague and arbitrary ideas respecting the sun prevailed. The educated, as well as the vulgar, beheld in it merely a globe of fire; the most learned declared that they found in it _pure fire, elementary fire, the principle of light, and of fire_. But as no means existed of examining the surface of the sun, and as his real distance from the earth was either unknown, or very imperfectly understood, a prudent reserve was maintained on this question. The discovery of the telescope immediately placed the astronomers in possession of the celestial realm; it enabled them to sound the depths of space, and to study the apparent configuration of the stars, including the sun himself. A few hours' observation with the astronomical spy-glass, and more was learned of the nature of the sun, than in the two thousand years of more or less philosophical reverie which preceded the discovery of the telescope. With a glass which magnified the apparent diameter of the sun only twenty-sixfold, Galileo, repeating the observations of Fabricius, discovered the spots on the sun. Although Galileo did not use the smoked glasses which have since been found so useful, and although he limited his observations to the horizon, watching the great star at its rising and its setting, or when it was veiled by slight clouds, he studied its spots carefully, and described them faithfully. We may observe that this discovery astonished the philosophers of that period, who were entirely submissive to the authority of Aristotle. The _incorruptibility of the sun_ was held in the schools as a sacred principle, according to Aristotle, and these unfortunate spots perplexed the philosophers. The peripatetics vied with each other in proving to the Florentine astronomer that the purity of the sun was an unassailable principle, and that the spots which he had perceived existed only on his eyes, or on the lens of his glasses. But Galileo had seen correctly, and soon every one could convince himself of the reality of the phenomenon he had proclaimed. Not only do spots exist upon the disc of the sun, but they furnish the only means which we possess of becoming acquainted with the physical and astronomical peculiarities and properties of the great star. The examination of these spots led to the discovery that the sun revolves like the other planets, and that he accomplishes the entire revolution upon his axis in a period of twenty-five days. The sun's days are therefore twenty-five times as long as ours. Here, however, we must remark upon the word _day_. To us, the _day_ signifies the periodical return of the earth to the same point, after a complete revolution upon its axis, with an alternation of light and darkness. It is quite otherwise in the case of the sun, which, being self-luminous in all his parts, can never have any night.