51 HISTORY OF OPTICS. 



CHAPTER II 



DISCOVERY OF THE LAW OF REFRACTION. 



E have seen in the former part of this history that the Greeks 

 had formed a tolerably clear conception of the refraction as well 

 as the reflection of the rays of light ; and that Ptolemy had measured 

 the amount of refraction of Mass and water at various angles. If we 



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give the names of the angle of incidence and the angle of refraction 

 respectively to the angles which a ray of light makes with the line 

 perpendicular to surface of glass or water (or any other medium) 

 within and without the medium, Ptolemy had observed that the angle 

 of refraction is always less than the angle of incidence. He had sup- 

 posed it to be less in a given proportion, but this opinion is false ; and 

 was afterwards rio-htlv denied by the Arabian mathematician Alhazen. 



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The optical views which occur in the work of Alhazen are far sounder 

 than those of his predecessors ; and the book may be regarded as the 

 most considerable monument which we have of the scientific genius 

 of the Arabians ; for it appears, for the most part, not to be borrowed 

 from Greek authorities. The author not only asserts (lib. vii.), that 

 refraction takes place towards the perpendicular, and refers to experi- 

 ment for the truth of this : and that the quantities of the refraction 

 differ according to the magnitudes of the angles which the directions 



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of the incidental rays (rjrimce Ihiece) make with the perpendiculars to 

 the surface ; but he also says distinctly and decidedly that the angles 

 of refraction do not follow the proportion of the angles of incidence. 



[2nd Ed.] [There appears to be good ground to assent to the 

 assertion of Alhazen's originality, made by his editor Eisner, who 

 says, "Euclideum hie vel Ptolemaicum. nihil fere est." Besides the 

 doctrine of reflection and refraction of light, the Arabian author gives 

 a description of the eye. He distinguishes three fluids, humor aqueus, 

 crystallinus, vitreus, and four coats of the eye, tunica adherens, cornea, 

 ui'ea, tunica reti similis. lie distinguishes also three kinds of vision: 

 " Visibile percipitur aut solo visu, aut visu et syllogismo, aut visu et 

 anticipate notione." He has several propositions relating to what we 

 sometimes call the Philosophy of Vision : for instance this : " E visi 

 bili sa?pius viso remanet in anima gencralis notio," etc.] 



