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. Every 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 the 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 the mode of propagation of 
waves by transverse vibrations, perhaps it may be desirable to refer 
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