THE VALUE OF LENSES 257 



bend and scatter light in all directions. Cut glass and toilet 

 articles are made of flint glass because of the brilliant effects 

 caused by its great refractive power, and imitation gems are 

 commonly nothing more than polished flint glass. 



The value of lenses. Place a convex lens near a candle 

 (Fig. 130), and move a paper screen back and forth behjnd the 

 lens. At some position of the screen, a clear enlarged image is 

 seen. Place the candle in a new position and move the screen 



FIG. 130. The relative sizes of image and object depend upon the position of the lens. 



back and forth behind the lens. At some position of the screen 

 a clear, but smaller image is seen. The size and position of 

 the image depend upon the distance of the object from the lens. 

 By means of a lens one can easily form on a visiting card a 

 picture of a distant church steeple. 



If it were not for the fact that a lens can be held at such a 

 distance from an object as to make the image larger than the 

 object, it would be impossible for the lens to assist the watch- 

 maker in locating the small particles of dust which* clog the 

 wheels of the watch. If it were not for the opposite fact 

 that a lens can be held at such a distance from the object as 

 to make an image smaller than the object, it would be impos- 

 sible to have a photograph of a tall tree or building unless the 

 photograph were as large as the tree itself. When a photog- 

 rapher takes a photograph of a person or a tree, he moves his 

 camera until the image formed by the lens is of the desired size. 

 By bringing the camera (really the lens of the camera) near, we 



