SECT. XX. UNDULATOEY THEORY OF LIGHT. 169 



Encke's comet, as well as of the comet discovered by M. Eiela, 

 renders the existence of such a medium certain. It is clear 

 that, in this hypothesis, the alternate stripes of light and dark- 

 ness 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 0'0000258th part 

 of an inch ; consequently, when the elevations 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 corre- 

 sponds 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 co-existent undulations, is the sum of the dis- 

 turbances which each undulation would produce separately ; 

 consequently, the particle will move in the diagonal of a paral- 

 lelogram, whose sides are the two undulations. If, therefore, 

 the two undulations agree in direction, or nearly so, the resulting 

 motion will be very nearly equal to their sum, and in the same 

 direction ; if they nearly oppose one another, the resulting motion 

 will be nearly equal to their difference ; and, if the undulations 

 be equal and opposite, the resultant will be zero, and the particle 

 will remain at rest. 



The preceding experiments, and the inferences deduced from 

 them, which have led to the establishment of the doctrine of the 

 undulations of light, are the most splendid memorials of our 

 illustrious countryman Dr. Thomas Young, though Huygens 

 was the first to originate the idea. 



It is supposed that the particles of luminous bodies are in a 

 state of perpetual agitation, and that they possess the property 

 of exciting regular vibrations in the molecules of the ethereal 

 medium, corresponding to the vibrations of their own molecules ; 

 and that, on account of its elastic nature, one particle of the 

 ether when set in motion communicates its vibrations to those 

 adjacent, which in succession transmit them to those farther off ; 

 so that the primitive impulse is transferred from particle to par- 

 ticle, and the undulating motion darts through ether like a wave 



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