PRESIDENTIAL ADDRESS. 6 



see that the vibration is transmitted along the string, each point of 

 the string in turn taking up the motion and swinging from side 

 to side. Fig. i also shows a wave of this kind in eight different 

 stages of its progress. If we look at the string as a whole, we 

 shall see the appearance of waves travelling a/ong it, although if 

 we look at any particular knot we see that it moves backwards and 

 forwards perpendicularly to the string. We here have vibration 

 going on in a direction perpendicular to the line along which it is 

 being propagated. 



Fig. I. — Light-Waves. 

 If light consists of vibrations such as these, there must be 

 something to vibrate. We can understand a piece of jelly or 

 india-rubber vibrating from side to side in this way ; but nothing 

 of the na.ture of a fluid such as air could do so, much less a 

 vacuum, and light caji travel through a vacuum. To account for 

 light-waves, it is customary to suppose space to be filled with a 

 kind of substance called the ether^ which allows ordinary matter 



