OPTICS. 103 



noticed that this flexure is different at different angles from the per- 

 pendicular, and 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 re- 

 fraction 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. 42V), "On y voit des experiences de physique bien 

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



Ptolemy's Optical work was known only by Roger Bacon's refer- 

 ences to it (Opus Majlis, 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 Encyclopcedia Bri- 

 tannica. Ptolemy shows 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 centre " the 

 centre of all the elements, the centre 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 whole of his theory ; and gives a theory more 

 complete than that of any astronomer previous to him. Tycho, for 

 instance, believed that astronomical refraction was caused only by 

 the vapors of the atmosphere, and did not exist above the altitude 

 of 45°. 



Cleomedes, about the time of Augustus, had guessed at Refraction, 

 as an explanation of an eclipse in which the sun and moon are both 

 seen at the same time. " Is it not possible," he says, " that the rav 

 which proceeds from the eye and traverses moist and cloudy air may 

 bend downwards to the sun, even when he is below the horizon ?" And 

 Sextus Empiricus, a century later, says, " The air being dense, by the 

 refraction of the visual ray, a constellation may be seen above the 

 horizon when it is yet below the horizon." But from what follows, it 



