MEDI/EVAL SCIENCE AMONG ARABIANS 



change does not occur, Alluizcn explained as due to the 

 reflection of light by the earth's atmosphi 



Alhazen appears to have conceived the atmosphere 

 as a sharply defined layer, and, assuming that twilight 

 continues only so long as rays of the sun reflected 

 from the outer surface of this layer can reach the 

 spectator at any given point, he hit upon a means 

 of measurement that seemed to solve the hitherto 

 inscrutable problem as to the atmospheric depth. 

 Like the measurements of Aristarchus and Era- 

 tosthenes, this calculation of Alhazen is simple enough 

 in theory. Its defect consists largely in the difficulty 

 of fixing its terms with precision, combined with the 

 further fact that the rays of the sun, in taking the 

 slanting course through the earth's atmosphere, are 

 really deflected from a straight line in virtue of the 

 constantly increasing density of the air near the 

 earth's surface. Alhazen must have been aware of 

 this latter fact, since it was known to the later Alex- 

 andrian astronomers, but he takes no account of it in 

 the present measurement. The diagram will make the 

 method of Alhazen clear. 



His important premises are two : first, the well-rec- 

 ognized fact that, when light is reflected from any 

 surface, the angle of incidence is equal to the angle 

 of reflection; and, second, the much more doubtful 

 observation that twilight continues until such time as 

 the sun, according to a simple calculation, is nineteen 

 degrees below the horizon. Referring to the diagram, 

 let the inner circle represent the earth's surface, the 

 outer circle the limits of the atmosphere, C being the 

 earth's centre, and RR radii of the earth. Then the 



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