140 GALILEO AND HIS JUDGES. 



prejudices of men of this stamp, admits (as we have 

 seen) his feelings on this subject, and his suspicions 

 that the new discoveries were to be attributed to 

 optical errors. He was willing to be corrected if 

 mistaken, but such had hitherto been his opinion. 



It was not, however, to be expected that men of 

 sound sense would allow themselves to be misled 

 for any length of time by fallacies such as these. 

 Continued observations carefully made are sure to 

 correct mere optical errors, and after a reasonable 

 interval it must have been evident that the pheno- 

 mena discerned through the telescope were facts that 

 had to be dealt with not phantoms to be ignored. 



Thus, when it was found that the planet Venus 

 presented to the eye phases such as the Moon does, 

 instead of always appearing like a round body, it 

 became evident that she revolved, not as Ptolemy 

 supposed, round the Earth, but round the Sun, an 

 inference subsequently confirmed by the observation 

 of her transits over the Sun's disc. 



This being so, the adherents of Ptolemy had to 

 meet this difficulty : here was a planet much nearer 

 to the Earth than to the Sun,* and yet revolving 

 round the latter in preference to the former. There 

 was clearly, then, some attractive force belonging 

 to the Sun (whatever its nature might be), greater 



* The relative distances could be computed geometrically, even 

 before the absolute distances were known, and in fact were so; 

 Kepler's third law affords a simple rule for calculating them, but 

 they were known even previously. 



