56 SCIENCE AND THE HUMAN MIND 



tical skill, that Galen became famous, and influenced 

 the practice of medicine for fifteen hundred years. 



The chief name which distinguishes the later Graeco- 

 Roman Alexandrian school of science is that of 

 Ptolemy, who taught and made observations there 

 between the years 127 and 151 A.D. His great work 

 on astronomy, usually spoken of by its Arabian name 

 of Almagest, remained the standard treatise until the 

 days of Kepler and Copernicus ; but, in spite of 

 greater fullness of treatment, it does not alter the 

 theories of celestial phenomena already suggested by 

 Hipparchus. Ptolemy, like his master, improved and 

 developed the science of trigonometry, with the view 

 of basing his observations and their results on the 

 " incontrovertible ways of arithmetic and geometry," 

 and laid down the general principle that, in attempting 

 to explain phenomena, it is necessary to adopt the 

 simplest hypothesis that will co-ordinate the facts. 



Ptolemy is perhaps better known as a geographer 

 than as an astronomer, and he exercised an influence 

 in this department of knowledge which only gradu- 

 ally sank into the background under the stimulus of 

 the maritime discoveries of the fifteenth and sixteenth 

 centuries. But, here again, it is difficult to assign 

 the merit of much of the work to the respective 

 shares of Ptolemy himself and his immediate fore- 

 runner, Marinus of Tyre, whose writings have not 

 separately survived. Ptolemy undoubtedly placed 

 geography on a secure footing by insisting that correct 

 observations of latitude and longitude must precede 

 any satisfactory attempts at surveying and map-draw- 

 ing ; but his own materials for carrying out such a 

 design were very inadequate, since there was, indeed, 



