246 FRESNEL. 



strike it singly, while on the contrary no such combina 

 tion can take place when these same rays strike it 

 together, after having pursued (for this condition is 

 necessary) routes differing from one another by quan 

 tities comprised within a certain regular series of num 

 bers ? 



In geometry, in order to demonstrate the inaccuracy 

 of a proposition, we follow it out to all its consequences 

 until there results something which is completely absurd. 

 Ought we not to class in this category a chemical action 

 which is generated, or which disappears, according to 

 the length of route which the reagent has gone through ? 

 Natural phenomena ordinarily present themselves un 

 der very complicated forms, and the true merit of the 

 experimenter consists in disengaging them from a multi 

 tude of accessory circumstances which hinder us from at 

 once seizing their laws. 



If, for example, we had not observed the shadows of 

 bodies except in the open air, if we had never illuminated 

 these bodies by light proceeding from extremely small 

 luminous points, no one would have guessed how many 

 curious subjects of research are offered by a phenomenon 

 so common. But place in the middle of a dark room, 

 and in a beam of homogeneous light, diverging either 

 from a minute hole, or from the focus of a glass lens, any 

 opaque body whatever, and its shadow will show itself 

 marked by a series of contiguous stripes, alternately 

 bright, and completely dark. Substitute white light for 

 the homogeneous beam, and similar stripes vividly col 

 oured will appear to occupy the place of the former. 



Grimaldi first perceived these singular affections of 

 light, to which he gave the name of diffraction. Newton 

 afterwards made them the subject of a special investiga- 



