THE WAVE THEORY. 281 



sound, for instance, passing through the air of this 

 room would have a velocity of about 1 ,100 feet a second, 

 while the particles which constitute the wave, and pro- 

 pagate it at any moment, may only move through in- 

 conceivably small spaces to and fro. Now, in the case 

 of sound, this to-and-fro motion occurs in the direction 

 in which the sound is propagated, and a little reflection 

 will make it clear that no matter how a ray of sound, 

 if we may use the term, is received upon a reflecting 

 surface, it will be reflected equally all round as long 

 as the angle inclosed between the reflecting surface and 

 the ray remains unchanged. In other words, the sound- 

 ray has no sides and no preferences as regards reflec- 

 tion. Now Malus discovered that in certain conditions 

 a beam of light shows such preferences. When caused 

 to impinge upon a plane glass mirror, placed in a cer- 

 tain position, it may be wholly reflected ; whereas when 

 the mirror is placed in the rectangular position it may 

 not be reflected at all. 



Up to the hour when this discovery was made by 

 Malus ligl't had been supposed to be propagated through 

 ether, exactly as sound is propagated through air. In 

 other words, the direction in which the particles of 

 ether were supposed to vibrate to and fro coincided 

 with that of the ray of light. Those who had pre- 

 viously held the undulatory theory were utterly stag- 

 gered by this new revelation, and their perplexity was 

 shared by Young. He was for a time unable to con- 

 ceive of a medium capable of propagating the impulses 

 of light in a way different from the propagation of the 

 impulses of sound. To ascribe to the light-medium 

 qualities which would enable it to differ in its mecha- 

 nical action from the sound-medium was an idea too 

 bold 1 might indeed say too repugnant to the scien- 

 tific mind to be seriously entertained. Yet, deeply 



