96 THE PATH OF SCIENCE 



of vision that are so common. Spectacles came into use in 

 Italy about the end of the thirteenth century, and it is hard 

 to believe that nothing else of importance was done with 

 lenses until two were combined to form a telescope, nearly 

 three hundred years later. 



The first attempt to discuss the theory of lenses was made 

 by Kepler, who wrote a book on the theory of the telescope. 

 This was just after the publication of the work of Galileo 

 and the discoveries he had made with the instrument. It is 

 interesting that the effect of the revolutionary discovery of 

 the telescope on Kepler was to incite him to a discussion of 

 its theory. One can imagine how different would have been 

 the course of events if Tycho Brahe had lived to learn of the 

 existence of the telescope. The results of Kepler's calcula- 

 tions varied little from the observed facts, but he did not 

 know the law of refraction; that is, the way in which a ray 

 of light is deviated when it passes from air to glass. In spite 

 of this, Kepler's work was undoubtedly very valuable in pro- 

 viding a basis for the design of refracting telescopes. 



The correct statement of the law of refraction was given 

 by Willebrord Snell at the University of Leyden in 1621, but 

 his manuscript was not published at the time; and the law 

 was embodied by Descartes, the great philosopher and mathe- 

 matician, in his book on optics. Descartes, however, prefaced 

 the statement of Snell's law with a mechanical theory of the 

 nature of light, in which he assumed that light traveled more 

 rapidly in denser media. Pierre de Fermat, the French 

 mathematician who formulated the theory of numbers, de- 

 duced the law of refraction from exactly the opposite as- 

 sumption, namely, that light travels more slowly in denser 

 media, and announced the great principle known ever since 

 by his name— that a ray of light originating at a point in one 

 medium will travel to a point in another medium by the 

 path which requires the minimum of time. Of all principles 

 in optics, this has been perhaps the most fruitful. 



As in mechanics, the great scientist who advanced the 

 whole theory of optics was Isaac Newton. Newton showed 



