274 PREFACE TO THE 



this system is only of value as a convenient way of ex- 

 pressing the result of observation. Such was the view 

 of those who, while they adopted Aristotle's principles, 

 were aware that the astronomical system with which 

 he was satisfied, and of which he has given an account 

 in the twelfth book of the Metaphysics, was wholly in- 

 adequate as a representation of the phenomena. But 

 his more strenuous adherents went further, and followed 

 Averroes in speaking with much contempt of Ptolemy 

 and of his system ; an excess of zeal which Melanc- 

 thon, in the spirit of conciliation which belongs to his 

 gentle nature, has quietly condemned. 1 



Out of this antinomy, if the word may be so used, 

 sprang several attempts to replace the Ptolemaic system 

 by a construction which should be in accordance both 

 with the phenomena and with Aristotle. Of these the 

 best known is the Homocentrica of Fracastorius. As 

 the name implies, all the orbs have on this hypothesis 

 the same centre, and of these homocentric orbs he em- 

 ploys seventy-seven. But a fatal objection to this and 

 all similar attempts is that they can give no explanation 

 of changes in apparent distance. Fracastorius tries to 

 set aside this objection by asserting that although the 

 distance of some of the heavenly bodies from the earth 

 may seem to vary, yet it never does so in reality, the 

 apparent variation being caused by the varying medium 

 through which they are seen. 



Though this explanation is wholly unsatisfactory, the 

 wish to get rid of eccentrics and epicycles was suffi- 

 ciently strong to win for Fracastorius a much more 

 favourable reception than his complex and imperfect 

 hypothesis deserved. He was spoken of as a man who 



1 See Initia Physicae. 



