IX. 



LIGHT. 



ECTION I, 



THE NATURE, VELOCITY AND INTENSITY 

 OF LIGHT. 



579. What is Light? Light is that mode of 

 motion which is capable of affecting the optic 

 nerve. Tlie only physical difference between light 

 and radiant heat is one of wave length. 



(a.) We have seen that the vibrations of air particles in a sound 

 wave are to and fro in the line of propagation. In the case o^' 

 radiant heat and light, the ether particles vibrate to and fro across 

 the line of propagation. Vibrations in a sound wave are longitudi- 

 nal; those of a heat or light wave are transversal. 



580. Luminous and Non-Luminous Bodies. 



Bodies that emit light of their own generating, as the 

 sun or a candle, are called luminous. Bodies that merely 

 diffuse the light that they receive from other bodies are 

 said to be non-luminous or illuminated. Trees and plants 

 are non-luminous. 



(a.) Visible bodies may be luminous or illuminated, but in either 

 case they send light in every direction from every point in their 

 surfaces. In Fig. 271 we see represented a few of the infinite 

 number of lines of light starting from A, B and C, three of the 



