Astronomical and Nautical Collections, 121 



body which puts it in motion ; and tliat the greater or less 

 amplitude or energy of the oscillations only augments or di- 

 minishes the intensity of the sound, without changing its 

 nature, that is, its tone, or pitch. 



The intensity of the light must depend then on the inten- 

 sity of the vibrations of the ether ; and its nature, that is to 

 say, the sensation of colour that it produces, will depend on 

 the duration of each oscillation, or on the length of the un- 

 dulation, the one of these being proportional to the other. 

 [We find, however, nothing in light of the same colour that 

 is at all analogous to the different register, quality, or timbre 

 of a sound, by which, for instance, the sound of a violin differs 

 from that of a flute in unison with it : the subordinate, or 

 harmonic tones of the sound having nothing in light to cor- 

 respond with them. Tr.] 



The duration of the elementary oscillation remaining the 

 same, the absolute velocity of the ethereal particles, at the 

 corresponding periods of the oscillatory motions, is, as we have 

 seen, proportional to its extent. It is the square of this velo- 

 city, multiplied by the density of the fluid, that represents 

 what is called the living force in mechanics, or otherwise the 

 energy or impetus of the particles, which is to be taken as 

 the measure of the sensation produced, or of the intensity of 

 the light : thus, for example, if in the same medium, the 

 amplitude of the oscillation is doubled, the absolute velo- 

 cities will also be doubled, and the living force, or the inten- 

 sity of the light, will be quadrupled. 



We must, however, take care not to confound this abso- 

 lute velocity of the particles of the fluid with the velocity of 

 the propagation of the agitation. The first varies according to 

 the amplitude of the oscillations ; the second, which is nothing 

 but the promptitude with which the motion is communicated 

 from one stratum to the other, is independent of the inten- 

 sity of the vibrations. It is for this reason, that a weak sound 

 is transmitted by the air with the same velocity as a stronger 

 one ; and that the least intense light is propagated with the 

 same rapidity as the brightest. When we speak of the velo- 

 city of light, we always speak of the velocity of its propaga- 

 tion. Thus, when we say that light passes through 200 thou* 



