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CHAPTER II. 

 DISCOVERY OF THE LAW OF REFRACTION. 



TTTE have seen in the former part of this history 

 f V that the Greeks had formed a tolerably clear 

 conception of the refraction as well as the reflexion 

 of the rays of light; and that Ptolemy had mea- 

 sured the amount of refraction of glass and water 

 at various angles. If we give the names of the 

 angle of incidences and the angle of refraction re- 

 spectively 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 inci- 

 dence. He had supposed it to be less in a given 

 proportion, but this opinion is false ; and was after- 

 wards rightly denied by the Arabian mathematician 

 Alhazen. 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 ap- 

 pears, 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 experiment for the 





