312 NINETEENTH CENTURY. PT. in. 



tions. But in France two men, Fresnel and his friend M. 

 Arago, understood and valued Young's labours as soon as 

 they heard of them, and from that time the three men helped 

 each other in every way without the least jealousy as to who 

 should have the credit of the work. 



Fresnel had puzzled out the question of the ' Interference 

 of Light' before he heard that Young had done so too; and 

 it happened that while Fresnel and Arago were one day 

 making experiments upon the way in which waves of light 

 interfere with each other, they found that the ordinary and 

 extraordinary rays coming out of crystal and Iceland spar 

 would not interfere with or quench each other, as two ordi- 

 nary rays do. (See p. 306.) This led Fresnel to suspect that 

 the waves in the two rays must move in a different manner. 

 He wrote this to Dr. Young, and found that he also had the 

 same idea, and this led to a number of experiments, by 

 which they proved at last that the waves in a natural ray of 

 light do not move merely up and down like waves in a pond, 

 but also from side to side ; and that when light is polarised 

 this complex vibration is destroyed and the waves of each 

 separate ray move only in one direction. 



To understand this, take a piece of string, and, fastening 

 one end to the wall, hold the other end in your hand. If you 

 now move your hand up and down, you will make waves in 

 the string which will point up to the ceiling and down to the 

 ground, making what are called vertical vibrations (A, Fig. 51). 

 Stop this movement, and then move your hand from side to 

 side ; the waves will now point from wall to wall in horizon- 

 tal vibrations (B, Fig. 51). If you then move your hand so 

 that it points to the ceiling to your right, and the floor to 

 your left, you get waves between the two others, and so you 

 can go on varying the position of the waves in all directions ; 



