A HISTORY OF SCIENCE 



large as to exclude the possibility that one man might 

 master it all. So we find a Galileo, for example, 

 making revolutionary discoveries in astronomy, and 

 performing fundamental experiments in various fields 

 of physics. Galileo's great contemporary, Kepler, 

 was almost equally versatile, though his astronomical 

 studies were of such pre-eminent importance that his 

 other investigations sink into relative insignificance. 

 Yet he performed some notable experiments in at 

 least one department of physics. These experiments 

 had to do with the refraction of light, a subject which 

 Kepler was led to investigate, in part at least, through 

 his interest in the telescope. 



We have seen that Ptolemy in the Alexandrian 

 time, and Alhazen, the Arab, made studies of refrac- 

 tion. Kepler repeated their experiments, and, striv- 

 ing as always to generalize his observations, he at- 

 tempted to find the law that governed the observed 

 change of direction which a ray of light assumes in 

 passing from one medium to another. Kepler meas- 

 ured the angle of refraction by means of a simple yet 

 ingenious trough -like apparatus which enabled him 

 to compare readily the direct and refracted rays. He 

 discovered that when a ray of light passes through a 

 glass plate, if it strikes the farther surface of the glass 

 at an angle greate rthan 45 it will be totally refracted 

 instead of passing through into the air. He could not 

 well fail to know that different mediums refract light 

 differently, and that for the same medium the amount 

 of light varies with the change in the angle of incidence. 

 He was not able, however, to generalize his observa- 

 tions as he desired, and to the last the law that gov- 



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