A HISTORY OF SCIENCE 



problems, but the feature of it which gained it widest 

 celebrity was perhaps that which has to do with ec- 

 centrics and epicycles. This theory was, of course, but 

 an elaboration of the ideas of Hipparchus ; but, owing 

 to the celebrity of the expositor, it has come to be 

 spoken of as the theory of Ptolemy. We have suf- 

 ficiently detailed the theory in speaking of Hippar- 

 chus. It should be explained, however, that, with 

 both Hipparchus and Ptolemy, the theory of epicycles 

 would appear to have been held rather as a working 

 hypothesis than as a certainty, so far as the actuality 

 of the minor spheres or epicycles is concerned. That 

 is to say, these astronomers probably did not conceive 

 either the epicycles or the greater spheres as constitut- 

 ing actual solid substances. Subsequent generations, 

 however, put this interpretation upon the theory, 

 conceiving the various spheres as actual crystalline 

 bodies. It is difficult to imagine just how the various 

 epicycles were supposed to revolve without interfering 

 with the major spheres, but perhaps this is no greater 

 difficulty than is presented by the alleged properties 

 of the ether, which physicists of to-day accept as at 

 least a working hypothesis. We shall see later on how 

 firmly the conception of concentric crystalline spheres 

 was held to, and that no real challenge was ever given 

 that theory until the discovery was made that comets 

 have an orbit that must necessarily intersect the 

 spheres of the various planets. 



Ptolemy's system of geography in eight books, 

 founded on that of Marinus of Tyre, was scarcely less 

 celebrated throughout the Middle Ages than the Al- 

 magest. It contained little, however, that need con- 



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