CH. xxxiii. THE POLARIZATION OF LIGHT. 325 



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 which come out of a piece of Iceland spar 

 cannot be made to interfere with or quench each other, as 

 may happen in the case of two ordinary rays. (See p. 315.) 

 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 a natural ray of light is not composed merely of upright 

 waves as in A, Fig. 60, p. 322, but also of waves from side 

 to side as in B, and others at every angle between these two, 

 so that the ray is really round and not flat ; when light is 

 polarized, this complex vibration is destroyed, and the 

 waves of each ray move only in one plane. 



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 vibra- 

 tions (A, Fig. 60). Stop this movement, and then move 

 your hand from side to side ; the waves will now point 

 from wall to wall in horizontal vibrations (B, Fig. 60). If 

 you then move your hand so that it points to the ceiling 

 to your r'ght, 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 ; or, in scientific lan- 

 guage, you cause the string to vibrate in different planes. 



