TRANSVERSAL VIBRATIONS. 167 



pose the direction of the vibrations to be perpendicular to 

 that of the ray : then it is obvious that if that direction be 

 vertical, for example, while the ray advances horizontally, 

 the ray will bear a relation to the parts of space above and 

 below , different from that which it bears to those parts which 

 are on the right hand and on the left. Such is, in fact, the 

 mode of vibration which is now assumed to belong to the 

 ether, in the wave-theory, the ethereal molecules being sup- 

 posed to vibrate in the plane of the wave ; and we shall find 

 that, with the help of this assumption, all the complicated 

 phenomena of polarization and double refraction are explained 

 in the fullest and most complete manner. 



The principle of transversal vibrations, as it is called, seems 

 to have first occurred to Hooke, and was announced, in 1672, 

 in his Micrographia. Young and Fresnel arrived at the same 

 principle independently ; and the latter has reared upon its 

 basis the noblest fabric which has ever adorned the domain 

 of physical- science, Newton's system of the universe alone 

 excepted. 



(180) In order to conceive the manner in which an un- 

 dulation may be propagated by transversal vibrations, let us 

 imagine 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 made to vibrate, by moving 

 the hand up and down, each particle of the cord will, in suc- 

 cession, be thrown into a similar state of vibration ; and a 

 series of waves will be propagated along it with a uniform 

 velocity. The vibrations of each succeeding particle of tHe 

 cord, being similar to that of the first, will all be performed 

 in the same plane, and the whole will represent the state of 

 the ethereal particles in a polarized ray. 



Now if, after a certain number of vibrations in the verti- 

 cal plane, the extremity of the cord be made to vibrate in 



