On the publication of Scheiner's letters Velser transmitted a copy of them to his friend Galileo, with the request that he would favour him with his opinion of the new phenomena. After some delay, Galileo addressed three letters to Velser, in which he combated the opinions of Scheiner on the cause of the spots. These letters were dated the 4th of May, 1612; but though the controversy was carried on in the language of mutual respect and esteem, it put an end to the friendship which had existed between the two astronomers. In these letters, Galileo showed that the spots often dispersed like vapours or clouds; that they sometimes had a duration of only one or two days, and at other times of thirty or forty days; that they contracted in their breadth when they approached the sun's limb, without any diminution of their length; that they describe circles parallel to each other; that the monthly rotation of the sun again brings the same spots into view; and that they are seldom seen at a greater distance than 30° from the sun's equator. Galileo, likewise, discovered on the sun's disc _faculœ_, or _luculi_, as they were called, which differ in no respect from the common ones but in their being brighter than the rest of the sun's surface.[17]
In the last of the letters which our author addressed to Velser, and which was written in December, 1612, he recurs to his former discovery of the elongated shape, or rather the triple structure, of Saturn. The singular figure which he had observed in this planet had entirely disappeared; and he evidently announces the fact to Velser, lest it should be used by his enemies to discredit the accuracy of his observations. "Looking on Saturn," says he, "within these few days, I found it solitary, without the assistance of its accustomed stars, and, in short, perfectly round and defined like Jupiter; and such it still remains. Now, what can be said of so strange a metamorphosis? Are the two smaller stars consumed like the spots on the sun? Have they suddenly vanished and fled? or has Saturn devoured his own children? or was the appearance indeed fraud and illusion, with which the glasses have for so long a time mocked me, and so many others who have often observed with me? Now, perhaps, the time is come to revive the withering hopes of those who, guided by more profound contemplations, have followed all the fallacies of the new observations, and recognised their impossibilities. I cannot resolve what to say in a chance so strange, so new, and so unexpected; the shortness of the time, the unexampled occurrence, the weakness of my intellect, and the terror of being mistaken, have greatly confounded me." Although Galileo struggled to obtain a solution of this mystery, yet he had not the good fortune of succeeding. He imagined that the two smaller stars would reappear, in consequence of the supposed revolution of the planet round its axis; but the discovery of the ring of Saturn, and of the obliquity of its plane to the ecliptic, was necessary to explain the phenomena which were so perplexing to our author.