optics. 233 



We can easily see, through a small pin-hole in a 

 piece of paper, all the objects, such as the sky, 

 trees, houses, &c. which we could without the 

 paper. The light proceeding from these objects 

 must pass at the same time through the hole in a 

 great variety of directions, before they arrive at 

 the eye : yet it does not appear that vision is dis- 

 turbed by that means. 



Light, whether it comes from the sun, a candle, 

 or any other luminous body, moves always in 

 straight lines only: of this we may be convinced 

 from several simple observations. If a stick be 

 placed upright before a lighted candle, the line 

 extending from the top of the stick to the end of 

 its shadow, will be a prolongation of that from the 

 top of the stick to the candle. It is also impos- 

 sible to see through a crooked tube. 



When light proceeds in "every direction from a 

 luminous body, as the rays from a candle, its in- 

 tensity is diminished as the square of the distance : 

 that is, if you remove an object to twice the dis- 

 tance from the luminous body, it will be en- 

 lightened only one-fourth as much as before : if to 

 three times the distance, it will be illuminated only 

 one-ninth as much, and so on. 



From certain circumstances in the texture of 

 bodies which cannot yet be explained, some permit 

 the rays of light to pass freely through them : such 

 are called transparent: as glass, water, &c. while 

 others entirely obstruct the light, and are called 

 opaque bodies : as wood, metal, &c. A transparent 

 or pellucid body is called also a medium, and light 

 passing through them is said to be transmitted. 



By a ray of light is meant the motion of a 

 simple particle. 



A pencil of rays is a number of rays diverging 



