It appears, on the testimony of Galileo himself, that he taught the Ptolemaic system, out of compliance with the popular feeling, after he had convinced himself of the truth of the Copernican doctrines. In the treatise on the sphere, indeed, which bears his name[7], and which must have been written soon after he went to Padua, and subsequently to 1592, the stability of the earth, and the motion of the sun, are supported by the very arguments which Galileo afterwards ridiculed; but we have no means of determining whether or not he had then adopted the true system of the universe. Although he might have taught the Ptolemaic system in his lectures, after he had convinced himself of its falsehood; yet it is not likely that he would go so far as to publish to the worlds as true, the very doctrines which he despised. In a letter to Kepler, dated in 1597, he distinctly states that he _had, many years ago, adopted the opinions of Copernicus; but that he had not yet dared to publish his arguments in favour of them, and his refutation of the opposite opinions._ These facts would leave us to place Galileo's conversion somewhere between 1593 and 1597; although _many_ years cannot be said to have elapsed between these two dates. At this early period of Galileo's life, in the year 1593, he met with an accident; which had nearly proved fatal. A party at Padua; of which he was one, were enjoying; at an open window; a current of air, which was artificially cooled by a fall of water. Galileo unfortunately fell asleep under its influence; and so powerful was its effect upon his robust constitution; that he contracted a severe chronic disorder, accompanied with acute pains in his body, and loss of sleep and appetite, which attacked him at intervals during the rest of his life. Others of the party suffered still more severely, and perished by their own rashness. Galileo's reputation was now widely extended over Europe; and the archduke Ferdinand (afterwards emperor of Germany), the landgrave of Hesse, and the princes of Alsace and Mantua honoured his lectures with their presence. Prince Gustavus of Sweden also received instructions from him in mathematics, during his sojourn in Italy; and it has been supposed that this was the celebrated Gustavus Adolphus.