CHAPTER VIII. 



LIGHT: ITS SOURCES INTENSITY REFLECTION AND 

 REFRACTION. 



IF we place the hand in the sunlight it soon becomes warm. 

 It also becomes illuminated so that it can be seen, and the skin 

 of the hand at length becomes tanned. Some force has been 

 transmitted to us causing the sensations of heat and of light, 

 and causing also chemical action. What we receive from the sun, 

 whether it affects the sense of touch, or the sense of sight, or 

 whether it promotes chemical action, must be energy. 



These forms of energy are transmitted to us from the sun, 

 through waves of an exceedingly rare and tenuous medium, called 

 ether. This method of transmitting energy is called radiation. 

 All the ether waves from the sun, whatever their intensity, cause 

 an increase of heat in terrestrial objects, but only those of a par- 

 ticular intensity give us light. Probably all the ether waves 

 promote chemical action, but particular waves are more active. 

 Bodies hot enough to emit light waves are said to be incandes- 

 cent. The sun, the fixed stars, and other bodies which are 

 sources of light, are incandescent. Most artificial sources of 

 light depend for their light waves on the incandescence of car- 

 bon. There are some substances which, receiving light waves, 

 absorb their energy without becoming hot, and in turn emit 

 light waves in the dark for several hours after exposure to 

 the light. Such bodies are said to be phosphorescent. Some 

 forms of vegetable and animal life emit phosphorescent light, as 

 certain fungi among plants, and the glowworm and firefly among 

 animals. 



Light waves move in straight lines, as may be seen by ar- 

 ranging screens before a source of light, or by examining a ray 

 of light through the dusty air of a room. A single line of light, 

 (70) 



