168 TRANSVERSAL VIBRATIONS : 



another plane, and then in another, and so on, in rapid 

 succession, each particle of the cord will, after a time pro- 

 portional to its distance from the extremity, assume in suc- 

 cession all these varied vibrations; and the whole cord, 

 instead of taking the form of a plane curve, as in the last 

 case, will be thrown into a species of helical curve, depending 

 on the nature of the original disturbance. Such is the con- 

 dition of the ethereal particles in a ray of common, or un- 

 polarized light. 



"When, therefore, we admit a connexion to subsist among 

 the particles of the ether, such as that which holds among 

 the particles of the cord, there -is no difficulty in conceiving 

 how a vibration may be propagated in a direction perpendi- 

 cular to that in which it is executed. It is true, the particles 

 of the ether are not chained together by cohesive forces, like 

 those of the cord ; but the attractive forces which subsist 

 among them are of the same kind, and may be shown to pro- 

 duce a similar effect. In fact, let us conceive the ether to 

 be composed of separate molecules, which act on one another 

 according to some law varying with the distance. When any 

 row or line of such molecules is similarly displaced, through a 

 space which is small compared with the separating intervals, 

 the molecules of the succeeding row will be moved in the 

 same direction by the forces developed with the change of 

 distance ; so that the vibrations of the particles composing 

 the first row will be communicated to those of the second, and 

 thus the vibratory motion will be propagated in a direction 

 perpendicular to that in which it takes place. The rapidity 

 of the propagation will depend on the magnitude of the force 

 developed by the displacement. 



To account for the fact, that there are no sensible vibra- 

 tions in a direction normal to the wave, we have only to sup- 

 pose the repulsive force between the molecules to be very 

 great, or the resistance to compression very considerable. 



