CHAPTER XI 



LIGHT 



IN Chapter IX the student has had his attention directed to 

 this subject in considering energy. Radiation was there regarded 

 as a form of energy, and was seen to present itself under several 

 different aspects. Sound was instanced as an example of radia- 

 tion which, resulting in the first place from a sounding body, is 

 transmitted through the surrounding medium, air, as a vibration 

 of its particles. The vibrating source of sound sets the neigh- 

 bouring air particles oscillating backwards and forwards, and 

 this one, in its turn, sets its neighbour in motion in the same 

 way all along the line of travel of the sound disturbance. But 

 after the passage of the disturbance the air returns to its 

 previous condition and undergoes no permanent change. Such 

 a mode of propagation is spoken of as wave-motion. 



In this chapter, however, we shall chiefly concern ourselves 

 with those radiations which include light and are conveyed 

 through the other medium which has been already referred to. 

 It is known as the ether and pervades all space ; it exists as well 

 throughout the mass of all material bodies. These radiations are 

 also propagated by waves, though in a different manner from 

 that in which sound travels in the air. They may be regarded 

 as ether- waves. 



Waves. Before the nature of wave-movements in the ether 

 can be understood, it will be necessary to learn what is meant by 

 a wave, as well as those terms which are used in speaking about 

 them, and this cannot be better done than by beginning with 

 waves in water. Everybody has started these by dropping a 

 stone into a still pond. Where the stone is dropped the water 

 is pushed down into a hollow cavity which, as it is watched, 



