116 NOTES TO BOOK II. 



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 vapours 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 ray which proceeds from the 

 eye and traverses moist and cloudy air may bend down- 

 wards 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 constel- 

 lation may be seen above the horizon when it is yet below 

 the horizon." But from what follows, it appears doubtful 

 whether he clearly distinguished Refraction and Reflec- 

 tion. 



In order that we may not attach too much value 

 to the vague expressions of Cleomedes and Sextus Empi- 

 ricus, we may remark that Cleomedes conceives such an 

 eclipse as he describes not to be possible, though he 

 offers an explanation of it if it be : (the fact must occur 

 whenever the moon is seen in the horizon in the middle 

 of an eclipse) : and that Sextus Empiricus gives his sug- 

 gestion of the effect of refraction as an argument why 

 the Chaldean astrology cannot be true, since the con- 

 stellation which appears to be rising at the moment of 

 a birth is not the one which is truly rising. The Chal- 

 deans might have answered, says Delambre, that the star 

 begins to shed its influence, not when it is really in the 

 horizon, but when its light is seen. (Ast. Anc. vol. i. 

 p. 231, and vol. ii. p. 548.) 



