SECT, xx.] THE ETHEREAL MEDIUM. 189 



circumstances whatever ; while, on the contrary, two opposing 

 motions may, and it is impossible not to be struck with the 

 perfect similarity between the interferences of small undula- 

 tions of air or of water and the preceding phenomena. The 

 analogy is indeed so perfect, that philosophers of the highest 

 authority concur in the supposition that the celestial regions 

 are filled with an extremely rare, imponderable, and highly 

 elastic medium or ether, whose particles are capable of receiv- 

 ing the vibrations communicated to them by self-luminous 

 bodies, and of transmitting them to the optic nerves, so as 

 to produce the sensation of light. The acceleration in the 

 mean motion of Encke's comet, as well as of the comet 

 discovered by M. Biela, renders the existence of such a 

 medium almost certain. It is clear that, in this hypothesis, 

 the alternate stripes of light and darkness are entirely the 

 effect of the interference of the undulations ; for, by actual 

 measurement, the length of a wave of the mean red rays of 

 the solar spectrum is equal to the 00000258th part of an 

 inch ; consequently, when the elevation of the waves combine, 

 they produce double the intensity of light that each would 

 do singly ; and when half a wave combines with a whole, 

 that is, when, the hollow of one wave is filled up by the ele- 

 vation of another, darkness is the result. At intermediate 

 points between these extremes, the intensity of the light 

 corresponds to intermediate differences in the lengths of 

 the rays. 



The theory of interferences is a particular case of the 

 general mechanical law of the superposition of small motions ; 

 whence it appears that the disturbance of a particle of an 

 elastic medium, produced by two coexistent undulations, is 

 the sum of the disturbances which each undulation would 

 produce separately ; consequently, the particle will move in 

 the diagonal of a parallelogram, whose sides are the two un- 

 dulations. If, therefore, the two undulations agree in di- 

 rection, or nearly so, the resulting motion will be very nearly 

 equal to their sum, and in the same direction : if they nearly 



