232 FRESNEL. 



santly, they mix with the air, which becomes a vehicle 

 for them, and diffuses them in every direction. A grain 

 of musk, whose subtle emanations penetrate through all 

 parts of a vast surrounding circuit loses its power from 

 day to day ; it ends by being entirely dissipated and 

 totally disappearing. 



It is not the same with a sounding body. Eveiy one 

 knows that a distant bell, whose sound strikes faintly on 

 our ear, nevertheless does not send to us a single mole- 

 cule of metal ; that it can resound without interruption 

 for successive centuries without losing any of its weight. 

 When the clapper strikes it, its sides vibrate, they undergo 

 an oscillatory motion which communicates itself immedi- 

 ately to the neighbouring portions of the air, and thence 

 by degrees to the whole atmosphere. These atmospheric 

 vibrations constitute sound. 



Our organs, whatever may be their nature, cannot be 

 put in relation with distant bodies, except in one or the 

 other of these two ways : thus either the sun emits inces- 

 santly, as odorous bodies do, material particles from all 

 points of his surface with a velocity of 77,000 leagues in 

 a second, and these are minute solar fragments which by 

 penetrating into the eye produce vision ; — or else that 

 luminary, in this respect like a bell, excites simply an 

 undulatory movement in a medium extremely elastic, fill- 

 ing all space, and these vibrations proceed to agitate our 

 retinas as the sonorous undulations affect the membrane 

 of the tympanum. 



Of these two explanations of tlie phenomena of light, 

 one is called the theory of emission, the other is known 

 under the name of the system of waves.* We find long 



* To assist the general conception of tlie mode of propagation of 

 waves by transverse vibrations, perhaps it may be desirable to refer 



