l.\FLUE\'CE OF THE KOxMAX EMPIRE. 193 



posc'd reflections of similar forms on the moon's disk),* should 

 not have relinquished the myth of the unknown south land 

 connecting Cape Prasum with Cattigara and Thinse [Sina- 

 rum ]\Ietro2:)olis), joining, therefore, Eastern Africa with the 

 land of Tsin (China). This myth, which supposes the Indian 

 Ocean to be an inland sea, was based upon views which ma)f 

 be traced from Marinus of Tyre to Hipparchus, Seleucus the 

 Babylonian, and even to Aristotle. t We mus* limit ourselves, 

 in these cosmical descriptions of the progress made in the con- 

 templation of the universe, to a few examples illustrative of 

 the fluctuations of knowledge, by which imperfectly-recognized 

 facts were so often rendered still more obscure. The more the 

 extension of navigation and of inland trade led to a hope that 

 the whole of the earth's surface might become known, the 

 more earnestly did the ever-wakeful imagination of the Greeks, 

 especially in the Alexandrian age under the Ptolemies and 

 under the Roman empire, strive by ingenious combinations to 

 fuse ancient conjectures with newly-acquired knowledge, and 

 thus speedily to complete the scarcely sketched map of the 

 earth. We have already briefly noticed that Claudius Ptole- 

 m.aeus, by his optical inquiries, which have been in part pre- 

 served to us by the Arabians, became the founder of one branch 

 of mathematical physics, which, according to Theon of Alex- 

 andria, had already been noticed, with reference to the refrac- 

 tion of rays of light, in the Catoptrica of Archimedes. t We 

 may esteem it as an important advance when physical phenom- 

 ena, instead of being simply observed and compared together 

 (of which we have memorable examples in Greek antiquity, 

 in the comprehensive pseudo- Aristotelian problems, and in 

 Roman antiquity in the works of Seneca), are intentionally 

 evoked under altered conditions, and are then measured. § 



* Plutarch, De Facie in Orbe Lunce, p. 921, 19 (compare my Examen 

 Crit., t. i., p. 145-191). I have myself met, among higlilj-informed 

 Persians, with a repetition of the hypothesis of Agesianax, according to 

 which, the marks on the moon's disk, in which Plutarch (p. 935, 4) 

 thought he saw " a peculiar kind of shining mountains" (volcanoes ?), 

 were merely the reflected images of terrestrial lands, seas, and isth- 

 muses. These Persians would say, for instance, " What we see through 

 telescopes on the surface of the moon are the reflected images of our 

 own country." 



+ Ptolem., lib. iv., cap. 9 ; lib. vii., cap. 3 and 5. Compare Letronne, 

 .n the Journal des Savans, 1831, p. 476-480, and 545-555 ; Humboldt, 

 Examen Crit., t. i., p. 144. 161, and 329; t. ii., p. 370-373. 



X Delambre, Hist, de V Astronomic Ancienne, t. i., p. liv. ; t. ii., p. 551 

 Theon never makes any mention of Ptolemy's Optics, although he lived 

 fully two centuries after him. 



§ It is often difficult, in reading ancient works on physics, to deoida 

 Vol. II.— I 



