OF THE UNIVERSE. ROMAN EMPIRE. 193 



and thus to complete at once the yet scarcely sketched map 

 of the earth. 



We have already briefly noticed that Claudius Ptolemseus 

 by his optical researches (which have been preserved to us, 

 although in a very incomplete state, by the Arabians) be- 

 came the founder of a branch of mathematical physics ; 

 which, indeed, according to Theon of Alexandria, ( 303 ) 

 had already been touched upon, so far as relates to the re- 

 fraction of rays, in the Catoptrica of Archimedes. It is a 

 ?ery important step in advance, when physical phenomena, 

 instead of being simply observed and compared with each 

 other, of which we find memorable examples in Grecian 

 antiquity in the pseudo-Aristotelian problems, which are full 

 of matter, and in Roman antiquity in the writings of Seneca, 

 are produced at will under altered conditions, and measured. 

 (304) T} ie process thus referred to characterises Ptolemy's 

 researches on the refraction of rays of light when made to pass 

 through media of unequal density. He caused the rays to pass 

 from air into water and glass, and from water into glass, under 

 different angles of incidence. The results of these " physical 

 experiments" were collected by him into tables. This 

 measurement of a physical phenomenon purposely called 

 forth, of a natural process not reduced to a movement of 

 of the waves of light (Aristotle assumed a movement of the 

 medium intervening between the eye and the object seen), 

 is a solitary occurrence in the period of which we are 

 treating. ( 305 ) In the investigation of inorganic nature, 

 this period offers in addition only a few chemical experiments 

 by Dioscorides, and, as I have elsewhere observed, the 

 technical art of collecting fluids when passing over in distilla- 



