Astronomical and Nautical Collections, 123 



them : but it must not be inferred that the principle of living 

 forces is inapplicable to these phenomena ; it follows, on the 

 contrary, from the most probable idea that can be formed of 

 the mechanical constitution of bodies, that the sum of the 

 living force must remain always the same, as long as the 

 accelerating forces tending to bring the particles to their na- 

 tural positions, remain unchanged, and that the quantity of 

 jiving force which disappears in the state of light, instead of 

 being annihilated, is reproduced in the form of heat. 



In order to obtain a correct idea of the manner in which 

 the oscillation of a small solid body occasions undulations in 

 an elastic fluid, it has been only necessary to consider a 

 complete oscillation of the solid plane, which produces an 

 entire undulation. If we suppose the oscillations of the plane 

 to be continually repeated, we shall have a series of undula- 

 tions instead of a single one : and they will follow each other 

 without intermission, provided that the vibrations of the par- 

 ticle first agitated have been regular. Such a series of re- 

 gular and uninterrupted luminous motions I call a system of 

 undulations. 



It is natural to suppose, on account of the prodigious ra- 

 pidity of the vibrations of light, that the luminous particles 

 may perform a great number of regular oscillations in each 

 of the different mechanical situations in which they are placed 

 during the combustion or the incandescence of the luminous 

 body, although these circumstances may still succeed each 

 other in extremely short periods ; for the millionth part of a 

 second is sufficient to exhibit, for example, 545 millions of 

 undulations of yellow light ; so that the mechanical distur- 

 bances, which derange the regular succession of the vibra- 

 tions of the luminous particles, or which even change their 

 nature, might be repeated a million times in a second without 

 preventing the regular succession of more than 500 millions 

 of consecutive undulations in each state of the particle. We 

 shall soon have occasion to apply this observation to the de-» 

 termination of the circumstances in which the interference 

 of luminous waves is capable of producing sensible effects. 



We have seen that each undulation produced by an oscil- 

 latory motion was composed of two semiundulations, which 



