WILLEBRORD SNELL (SNELLIUS) {1591-1626) 

 RENfi DESCARTES (CARTESIUS) (1396-1630) 



Here we have two unlike contemporaries associated, but they 

 approach one another closely in their achievements as men of 

 science. 



Snell was born in Leyden and lived there as professor 

 of mathematics and mechanics, where he was the successor 

 of his father. He made the first measurement of the meri- 

 dian by the method of triangulation, the only one trustworthy 

 as regards accuracy, and used to-day for every kind of sur- 

 veying. It also became of importance in determining the 

 unit of length the metre (one ten-millionth part of the earth's 

 quadrant). He further discovered, in about 1620, the impor- 

 tant law of the refraction of light. Measurements of the angles 

 of refracted rays on entering water or glass had been made 

 earlier, also by Kepler, but the single law which connects 

 the angle of incidence with the angle of refraction had not 

 been discovered, because attention had not been paid to the 

 angle between the ray and the normal to the surface, called 

 to-day the angle of refraction; the more obvious measurement 

 had been made of the angle between the incident and re- 

 fracted ray, which angle depends in a very complicated 

 manner upon the angle of incidence. It cannot be doubted 

 that Kepler, who discovered from Tycho's figures the deeply 

 hidden planetary laws, would also have discovered the law 

 of refraction, if he had devoted his whole attention to it; 

 but the approximation used by him was sufficient to provide 

 him with a basis for his extensive discussion of geometrical 

 optics. But as soon as the law was found with exactness, a 

 knowledge of it must, in view of its great simplicity, have 

 spread very rapidly. Hence we cannot regard it as of im- 

 portance that Snell was prevented by his early death from 

 getting the essay containing this law printed; for Huygens and 

 others had seen the essay, and the former also describes in 



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