SCT. XX. THE ETHEREAL MEDIUM. 163 



highly elastic medium or ether, whose particles are ca- 

 pable of receiving 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 interfe- 

 rence 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 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 elevation of another, darkness is the result. 

 At intermediate points betwsen these extremes, the in- 

 tensity of the light corresponds to intermediate differ- 

 ences 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 coexis- 

 tent undulations, is the sum of the disturbances which 

 each undulation would produce separately; conse- 

 quently, the particle will move in the diagonal of a par- 

 allelogram, whose sides are the two undulations. If, 

 therefore, the two undulations agree hi 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 de- 

 duced 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 Buy gens was the first to origi- 

 nate the idea. 



It is supposed that the particles of luminous bodies 



