104 PHYSICAL SCIENCES IN ANCIENT GREECE. 



appears doubtful whether he clearly distinguished Refraction and Re- 

 flection. 



In order that we may not attach too much value to the vague ex- 

 pressions of Cleomedes and Sextus Empiricus, 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 really 

 occur whenever the moon is seen in the horizon in the middle of an 

 eclipse :) and that Sextus Empiricus gives his suggestion of the effect 

 of refraction as an argument why the Chaldean astrology cannot be 

 true, since the constellation which appears to be rising at the moment 

 of a birth is not the one which is truly rising. The Chaldeans might 

 have answered, says Delambre, that the star begins to shed its influ- 

 ence, not when it is really in the horizon, but when its light is seen. 

 (Ast. Am. vol. i. p. 231, and vol. ii. p. 548.) 



It has been said that Vitellio, or Vitello, whom we shall hereafter 

 have to speak of in the history of Optics, took his Tables of Refrac- 

 tious from Ptolemy. This is contrary to what Delambre states. He 

 says that Vitello may be accused of plagiarism from Alhazen, and that 

 Alhazen did not borrow his Tables from Ptolemy. Roger Bacon had 

 said {Opus Majus, p. 288), "Ptoleinseus in libro de Opticis, id est, de 

 Aspectibus, seu in Perspective sua, qui prius quam Alhazen dedit hanc 

 sententiam, quam a Ptolemseo acceptam Alhazen exposuit." This 

 refers only to the opinion that visual rays proceed from the eye. But 

 this also is erroneous ; for Alhazen maintains the contrary : " Visio fit 

 radiis a visibili extrinsecus ad visum manantibus." [Opt. Lib. i. cap. 

 5.) Vitello says of his Table of Refractions, " Acceptis instrumenta- 

 litcr, prout potuimus propinquius, angulis omnium refractionum . . . 

 invenimus quod semper iidem sunt anguli refractionum : . . . secun- 

 dum hoc fecimus has tabulas." " Having measured, by means of in- 

 struments, as exactly as we could, the whole range of the angles of 

 refraction, we found that the refraction is always the same for the same 

 angle ; and hence we have constructed these Tables." 



