COLORIFICATION IN GENERAL. 337 



contained between the line described by the 

 refracted ray, and a line perpendicular to the 

 refracting surface, at the point in which the 

 ray passes through that surface. 



4. The angle of deviation, is that which is 

 contained between the line of direction of an 

 incident ray, and the direction of the same ray 

 after it is refracted. 



Def. 1. A ray, turned back into the same 

 medium, in which it moved before its return, is 

 said to be reflected.- 2. The angle of reflection, 

 is that which is contained between the line 

 described by a reflected ray, and a, line perpen-, 

 dicular to the reflecting surface at the point of 

 reflection. 3. The angle of reflection is equal 

 to the angle of incidence.* Vid. Enfleld. 



* A Latin translation of the Optics of Ptolemy, a work 

 which was supposed to be entirely lost, as we only possessed 

 a few lines of it, which have been transmitted by Bacon, has 

 lately been discovered by Count LAPLACE, in the imperial 

 library at Paris. The translation is by Ammiratus Eugenius 

 Siculus. The first book is wanting, as it was wanting in the 

 original Arabic, from which Ammiratus made his translation. 

 From this work, it appears that Ptolemy was well acquainted 

 with the effects and laws of refraction, and that, in this res- 

 pect, he was even more advanced than Tycho, Kepler, Heve- 

 lius, and all the astronomers till the time of Cassini, who was 

 the first among the moderns that asserted that refraction did 

 not entirely cease up to the zenith. But what is still more 

 curious, and was never in any manner suspected, is that Pto- 



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