PHYSICS OF THE NINETEENTH CENTUR Y. LIGHT. 455 



diffraction. In 1823 he was unanimously elected a member of that 

 academy, and two years afterwards the Royal Society of London ap- 

 pointed him one of their foreign associates. In 1827 the same body 

 awarded him the Rumford Gold Medal. A few days after he had 

 received this scientific distinction he passed away, at Ville d'Avray. 



It is the glory of Fresnel that he established the undulatory theory on 

 the sure basis of quantitative determinations. His investigations con- 



ducted him to mathematical formulae which expressed the laws of the 

 phenomena in all their generality, and the consequences deduced from 

 these mathematical expressions led the way to new and unexpected 

 discoveries. Fresnel's experimental arrangements were as effective as 

 his mathematical analysis was far-reaching. His famous experiment 

 of the inclined mirrors is perhaps the most direct method of producing 

 the phenomena of interference, and of studying their significance. In 

 this experiment a light falling upon a screen, say, of white paper, is 

 turned to darkness, not by interposing an opaque body, or turning 

 aside the rays of light, but actually by the addition of more light-rays. 

 That is, light added to light is capable, under certain conditions, of 

 producing darkness. In order that the reader may with the more 

 certainty conceive the manner in which this apparently paradoxical 



