326 



NINETEENTH CENTURY. 



PT. in. 



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 

 at right angles to the first and along this the other ray goes 

 at a different speed, 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. 60, and it is clear that the waves will be up and 



FIG. 60. 



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. 



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, 

 for the polarization of light is a very difficult subject ; but 

 this was the first step made in it. Fresnel afterwards 

 worked out accurately why, when light is reflected at a 

 certain angle, the vibrations are all made to move in one 



