CHAPTER IX 



LIGHT AND SOUND 



165. Luminous Bodies. Every object that we see 

 " is seen/ 7 or " is visible," because light from the object 

 enters the eye. When the visible body itself produces the 

 light, the body is said to be self-luminous. Self-luminous 

 bodies are usually very hoi. This is the case with the sun, 

 a lamp flame, an arc electric light. However, most 

 visible bodies are not self-luminous; but they shine, or are 

 seen, by light which they receive, and then reflect to the 

 eye. We say that non-luminous bodies are illuminated 

 by luminous bodies. The moon is a non-luminous body. 

 Moonshine is sunshine reflected to us from the moon's 

 surface. An object seen by moonlight is thus seen by 

 sunlight that has been reflected twice: first from the 

 moon to the object, and then from the object to the eye. 

 When we take a lamp into a dark room, we see the objects 

 hi the room, because they reflect lamplight to the eye. 



166. Transparent and Opaque Bodies. Substances 

 through which objects are seen clearly are said to be 

 transparent. Such are water, glass, and air. A substance 

 that does not permit light to pass through it is said to be 

 opaque. Wood and iron are opaque. Certain substances 

 allow some of the light to pass, but objects cannot be seen 

 distinctly through them; such substances are said to be 



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