GALILEO GALILEI 33 



hand he held three pubhc lectures before a large body of 

 hearers. But his clear discourse and his good instruments 

 were powerless against his opponents, who persisted in 

 refusing to see the satellites of Jupiter, or even the telescope. 

 Among his colleagues, his young contemporary in Prague was 

 almost alone in supporting him with comprehending and full 

 admiration. Some educated laymen, however, whose minds 

 were unprejudiced by learning, received a very great impres- 

 sion from Galileo's Messenger, which was widely circulated. 



Galileo himself continued for the time his observations by 

 means of the telescope. His observations of the moons of 

 Jupiter kept him very much occupied; but the determination 

 of their periods of revolution gave great difficulty, since the 

 telescope was not yet adapted for good measurement. He 

 soon recognised that the inmost moon moved fastest and the 

 outermost slowest; his determinations of their periods fol- 

 lowed later. He then discovered the peculiar form of 

 Saturn, which appeared like a body divided into three parts, 

 this being the first indication of the ring system of this planet. 

 He further discovered the phases of Venus, which were like 

 those of the moon; and this proved the planet to be dark in 

 itself and only illuminated by the sun, a fact at that time 

 not merely unknown, but regarded as completely un- 

 tenable. At the same time these phases also proved clearly 

 that Venus revolves around the sun. When stars, dark like 

 the earth, were seen through the telescope, shining in the 

 heavens and moving, it became less and less incredible that 

 the dark earth itself might be also a planet, a star, circling 

 around the sun. Finally, Galileo's observations of the spots 

 and protuberances of the sun showed that this mighty sphere 

 also rotates. For the first time, the motion ascribed to the 

 earth could be directly seen in a heavenly body. 



All this raised Copernicus' picture of the world to such a 

 high degree of certainty for Galileo that he gradually aban- 

 doned the reserve which he had hitherto maintained, and 



Ds 



