in the Seventeenth Century. 7 



of velocity parallel to the cloth must be unaffected by the 

 impact; and therefore the projection BE of the refracted ray 

 must be k times as long as the projection BC of the incident 



I 



ray. So if i and r denote the angles of incidence and refraction, 

 we have 



BE BC 



or the sines of the angles of incidence and refraction are in a 

 constant ratio ; this is the law of refraction. 



Desiring to include all known phenomena in .his system, 

 Descartes devoted some attention to a class of effects which 

 were at that time little thought of, but which were destined to 

 play a great part in the subsequent development of Physics. 



The ancients were acquainted with the curious properties 

 possessed by two minerals, amber (riXtKrpov) and magnetic 

 iron ore (77 \iOos Mayv?}r/e). The former, when rubbed, 

 attracts light bodies : the latter has the power of attracting 

 iron. 



The use of the magnet for the purpose of indicating direc- 

 tion at sea does not seem to have been derived from classical 

 antiquity ; but it was certainly known in the time of the 

 Crusades. Indeed, magnetism was one of the few sciences 

 which progressed during the Middle Ages ; for in the thirteenth 

 century Petrus Peregrinus,* a native of Maricourt in Picardy, 

 made a discovery of fundamental importance. 



Taking a natural magnet or lodestone, which had been 

 rounded into a globular form, he laid it on a needle, and marked 



* His Epistola was written in 1269. 



