Castelli writes: "This must now convince the most obstinate." But Galileo, with more experience, replies:--"You almost make me laugh by saying that these clear observations are sufficient to convince the most obstinate; it seems you have yet to learn that long ago the observations were enough to convince those who are capable of reasoning, and those who wish to learn the truth; but that to convince the obstinate, and those who care for nothing beyond the vain applause of the senseless vulgar, not even the testimony of the stars would suffice, were they to descend on earth to speak for themselves. Let us, then, endeavour to procure some knowledge for ourselves, and rest contented with this sole satisfaction; but of advancing in popular opinion, or of gaining the assent of the book-philosophers, let us abandon both the hope and the desire." [Illustration: FIG. 47.--Phases of Venus. Showing also its apparent variations in size by reason of its varying distance from the earth. When fully illuminated it is necessarily most distant. It looks brightest to us when a broad crescent.] What a year's work it had been! In twelve months observational astronomy had made such a bound as it has never made before or since. Why did not others make any of these observations? Because no one could make telescopes like Galileo. He gathered pupils round him however, and taught them how to work the lenses, so that gradually these instruments penetrated Europe, and astronomers everywhere verified his splendid discoveries. But still he worked on, and by March in the very next year, he saw something still more hateful to the Aristotelian philosophers, viz. spots on the sun. [Illustration: FIG. 48.] If anything was pure and perfect it was the sun, they said. Was this impostor going to blacken its face too? Well, there they were. They slowly formed and changed, and by moving all together showed him that the sun rotated about once a month. Before taking leave of Galileo's astronomical researches, I must mention an observation made at the end of 1612, that the apparent triplicity of Saturn (Fig. 46) had vanished. [Illustration: FIG. 49.--A portion of the sun's disk as seen in a powerful modern telescope.]