502 



NOTES TO BOOK IX. 



(DA.) p. 377- THERE appears to be good ground to 

 assent to the assertion of Alhazen's originality by his editor 

 Risner, who says, " Euclideum hie vel Ptolemaicum nihil 

 fere eat." Besides the doctrine of the reflection and refrac- 

 tion of light, the Arabian author gives a description of the 

 eye. He distinguishes three fluids, humor aqueus, crystal- 

 linus, mtreus, and four coats of the eye, tunica adJierens, 

 cornea, uvea, tunica reti similis. He distinguishes also 

 three kinds of vision : " Visibile percipitur aut solo visu, 

 aut visu et syllogismo, aut visu et anticipata notione." 

 He has several propositions relating to what we sometimes 

 call the Philosophy of Vision : for instance this : " E visibili 

 ssepius viso remanet in anima generalis notio," &c. 



(EA.) p. 377. I have already stated (vol. i. p. 117,) 

 that Vitello asserts that his Tables were derived from his 

 own observations. Their near agreement with those of 

 Ptolemy does not make this improbable : for where the 

 observations were only made to half a degree, there was 

 not much room for observers to differ. It is not unlikely 

 that the observations of refraction out of air into water 

 and glass, and out of water into glass, were actually made; 

 while the impossible values which accompany them, of the 

 refraction oat of water and glass into air, and out of glass 

 into water, were calculated, and calculated from an erro- 

 neous rule. 



(FA.) p. 379. Huyghens says of Snell's papers, " Quse 

 et nos vidimus aliquando, et Cartesium quoque vidisse 



