A HISTORY OF SCIENCE 



the World," as he was called by his contemporaries. 

 The Almagest of Ptolemy was translated into Latin at 

 his instance, being introduced to the Western world 

 through this curious channel. At this time it became 

 quite usual for the Italian and Spanish scholars to un- 

 derstand Arabic although they were totally ignorant of 

 Greek. 



In the field of physical science one of the most 

 important of the Arabian scientists was Alhazen. His 

 work, published about the year noo A.D., had great 

 celebrity throughout the mediaeval period. The orig- 

 inal investigations of Alhazen had to do largely with 

 optics. He made particular studies of the eye itself, 

 and the names given by him to various parts of the 

 eye, as the vitreous humor, the cornea, and the retina, 

 are still retained by anatomists. It is know r n that 

 Ptolemy had studied the refraction of light, and that 

 he, in common with his immediate predecessors, was 

 aware that atmospheric refraction affects the ap- 

 parent position of stars near the horizon. Alhazen 

 carried forward these studies, and was led through them 

 to make the first recorded scientific estimate of the 

 phenomena of twilight and of the height of the at- 

 mosphere. The persistence of a glow in the atmos- 

 phere after the sun has disappeared beneath the horizon 

 is so familiar a phenomenon that the ancient philoso- 

 phers seem not to have thought of it as requiring an 

 explanation. Yet a moment's consideration makes 

 it clear that, if light travels in straight lines and the rays 

 of the sun were in no wise deflected, the complete 

 darkness of night should instantly succeed to day when 

 the sun passes below the horizon. That this sudden 



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