52 GREAT MEN OF SCIENCE 



one of his own works {Dioptrica) the law of refraction dis- 

 covered by Snell, upon which he had also founded the im- 

 portant explanation of refraction in his famous 'Essay on 

 Light.' The same thing here obviously occurred as with 

 Toricelli's letter, only that Snell's name was not mentioned 

 afterwards so prominently as Toricelli's.^ 



Ren6 Descartes, born at Lahaye in Touraine, of an old 

 noble family, was educated in a Jesuit school and then des- 

 tined by his father for the Army. He had a very adventurous 

 life, divided between participation in many campaigns of the 

 Thirty Years War in Dutch, Bavarian and Austrian service, 

 extensive travelling, amusement, and then complete retire- 

 ment; he died at Stockholm at the age of 54. 



His mathematical gifts, which appeared very early, enabled 

 him to become the founder of analytical geometry, the 

 method of representing lines and curves by equations; ar 

 method which has become of the highest importance for all 

 scientific work, and which has enabled mathematics to assist 

 in all investigations of processes taking place in space. 

 He was the author of other mathematical achievements as 

 well. His only direct valuable contribution to scientific 

 investigation is probably his thorough calculation of the rain- 

 bow. He applied Snell's law of refraction for the first 

 time to the course of the ray in the raindrops, and thus 

 found, from a careful calculation of a thousand ray paths, the 

 particular angle, which could not have been found without 

 the law, according to which the rays reflected once and twice 

 in the drop, emerge parallel, and are thus able to aff'ect the 

 eye, whereby they form the first and second rainbow. The 

 formation of the bow by inward reflection in the drops 

 had, however, already been suggested by the little-known 



^ See on this point, particularly as regards the frequent ascription of 

 the law of refraction to Descartes, Mach's Principles of Physical Optics 

 (trans, by J. S. Anderson and A. F. A. Young, 1926). 



