PROPAGATION OF LIGHT. 13 



inclined to the direction of the original motion. Now, if 

 there be any difference between the intensity of the direct and 

 lateral propagation, this difference may be ever so great ; i. e. 

 the ethereal medium may be so constituted that the intensity 

 of the laterally-propagated vibration shall be insensible. 



But the solution of the difficulty rests upon more solid 

 grounds than analogy. A more minute examination of 

 the nature and laws of vibratory motion has, in fact, shown 

 this to be the case, as respects the luminiferous waves. It 

 has been proved, that whatever be the intensity of the partial 

 waves of the ether, which are propagated laterally round any 

 interposed obstacle, the total light resulting from their joint 

 action must degrade rapidly. And the luminous fringes which 

 have been observed within the shadows of bodies do, in fact, 

 represent the intensities resulting from these lateral waves, 

 when submitted to the most rigid mathematical calculation. 



(16) Let us now proceed to consider, somewhat more mi- 

 nutely, the nature of a wave and its mode of propagation. 



Let us conceive, then, a cord stretched in a horizontal 

 position, one end being attached to a fixed point, and the 

 other held in the hand. If the latter extremity be agitated, 

 by the motion of the hand up and down, a series of waves 

 will be propagated along the cord, each of which will advance 

 uniformly. Here it is evident that each particle of the cord 

 has merely a vibratory motion in a vertical direction. But 

 as this vibratory motion is communicated from each particle 

 to the next, along the whole length of the cord, it will 

 follow that some of the particles reach their highest posi- 

 tion when others are in the lowest; while other particles, 

 intermediate to these, are neither in their highest nor their 

 lowest position, but in some intermediate stage of their vibra- 

 tion. Thus, while each particle moves only to and fro verti- 

 cally, an undulation or wave is propagated horizontally along 



