96 THE AVAVE THEORY OF LIGHT. 



baffled during the last centuries, but to which skillful contemporaneous 

 physicists have rendered the importance they deserve. 



Whatever may be the opinions granted to the exactness of the 

 deductions of this great philosopher, we must be struck by the bold- 

 ness with which he proclaims the connection of the great cosmical prob- 

 lems and foretells the solutions which actual generations did not yet 

 entire!}' accept, ))ut drew insensibly to. 



In Descartes's view the mechanism of light and that of gravitation 

 are inseparable; the seat of corresponding phenomena is this subtle 

 matter which pervades the universe, and their propagation is per- 

 formed by waves around the acting centers. 



This conception of the nature of light shocked the opinions in vogue; 

 it raised strong opposition. Since the oldest times it was the habit to 

 imagine the luminous ray as the trajectory of rapid projectiles thrown 

 by the radiant source. Their shock on the nerves of the eye produce 

 vision; their resistance or changes of speed, reflection or refraction. 

 The Cartesian theory had, however, some seductive aspects which 

 brought defenders. The waves excited on the surface of still water 

 offer .so clear an image of a propagated motion around a disturbing 

 center! On the other hand, do not the sonorous impressions reach 

 our ear by waves? Our mind feels yet a real satisfaction in thinking 

 that our most sharp and delicate organs are both impressed l)y a 

 mechanism of the same nature. 



Yet a serious dift'erence arose. Sound does not necessarily travel 

 in straight lines as light does. It travels round an}' object opposed to 

 it, and will follow the most circuitous routes with scarcely any loss of 

 strength. Physicists were thus divided into two camps. In one the 

 partisans of emission, in the other those of the wave theory, each sys- 

 tem boasting itself superior, and indeed each being so in certain 

 respects. Other phenomena had to be examined in order to decide 

 between them. 



The chance of discovery brought to view several phenomena which 

 ought to have decided in favor of wave theory, as was proved a century 

 later; but the simplest truth does not prevail without long endeavor. 



A strange compromise was effected l)etween the two systems, helped 

 on l)y a name great among the greatest, and for a century the theory 

 of emission triumphed. 



The tale is a strange one. In IHOl a young scholar, full of eager- 

 ness and penetration, enters Trinity College, Cambridge; his name is 

 Isaac Newton. He has already in his village read Kepler's Optics. 

 Almost inmiediately, and while following Barrow's lectures upon 

 optics, he studies the geometr}^ of Descartes with passionate care; 

 with his savings he buys a prism that he might examine the proper- 

 ties of color and meditate deeply on the causes of gravitation. Eight 

 years later his masters think him worthy to succeed Barrow in the 



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