NOTES TO BOOK II. 115 



there is an elaborate collection of measures of the flexure 

 at different angles, made by means of an instrument 

 devised for the purpose. There is also a collection of 

 similar measures of the refraction when the ray passes 

 from air to glass, and when it passes from glass to water. 

 This part of Ptolemy's work is, I think, the Oldest 

 extant example of a collection of experimental measures, 

 in any other subject than astronomy ; and in astronomy, 

 our measures are the result of observation, rather than 

 of experiment. As Delambre says (Astron. Anc. vol. ii. 

 p. 427.) "On y voit des experiences de physique bien 

 faites, ce qui est sans example chez les anciens." 



Ptolemy's Optical work was known only by Roger 

 Bacon's references to it (Opus Majus, p. 286, &c.) till 

 1816 : but copies of Latin translations of it were known 

 to exist in the Royal Library at Paris, and in the 

 Bodleian at Oxford. Delambre has given an account of 

 the contents of the Paris copy in his Astron. Anc. ii. 414. 

 and in the Connoissance des Temps for 1816; and Prof. 

 Rigaud's account of the Oxford copy is given in the 

 article Optics, in the Encyclopedia Britannica. Ptolemy 

 shews great sagacity in applying the notion of Refraction 

 to the explanation of the displacement of astronomical ob- 

 jects which is produced by the atmosphere, Astronomical 

 Refraction, as it is commonly called. He represents the 

 visual ray as refracted in passing from the ether, which is 

 above the air, into the air ; the air being bounded by a 

 spherical surface which has for its center "the center of all 

 the elements, the center of the earth;" and the refraction 

 being a flexure towards the line drawn perpendicular to 

 this surface. He thus constructs, says Delambre, the 

 same figure on which Cassini afterwards founded the 



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