CH. xxxii. VERTICALS HORIZONTAL VIBRATIONS. 313 



or, in scientific language, you cause the string to vibrate in 

 a different plane. 



Now Young and Fresnel proved that a natural ray of 

 light is composed of all these different waves moving at 

 the same time, some up and down, some from side to 

 side, and some between the two. But when the light 

 passes through a piece of Iceland spar, there are two and 

 only two ways in which the waves can move : one up and 

 down — and along this path one ray of light goes ; the 

 other from side to side — and along this the other ray goes, 

 and so they become divided. 



You can imitate this by passing your string through a 

 card with a straight slit in it. Place the card upright, as 

 at A, Fig. 51, and it is clear that the waves will be up 





Diagram illustrating the passage of Waves of Light through a Crystal. 

 A, Waves moving in a vertical plane, b, Waves moving in a horizontal plane. 



and down ; place it sideways, as at b, and the waves will 

 be from side to side. These two positions of the card 

 imitate the two paths of a ray of light through a crystal, 

 and they show how the difference in the peculiarities of 

 the divided rays is caused by their moving in a different 

 plane. 



We cannot follow this out more completely in a history, 

 15 



