THE SPECIAL SENSES 



403 



Images. When a lens is used to form an image, it is 

 always desired that a distinct image be formed on a 



F:: 



Yig. 183. A biconvex lens bending the rays of light so as to bring 

 them to a point, the focus. 



given surface. If a candle is placed in front of a con- 

 vex lens, and a screen is placed back of the lens and 

 moved back and forth, a place will be found at which 

 a sharp but inverted 

 image of the candle 

 is thrown upon the 

 screen. 



Focusing. In the 

 experiment above, it 

 evident that the 



IS 



Fig. 184. Diagram showing the 

 method of the formation of an image 

 by a lens. 



screen must be placed 

 in a certain position if a distinct image is produced. In 

 a camera the image of the object falls upon the sensi- 

 tive plate or film. To secure this for any given object, 

 the lens in the camera must be moved backward and 

 forward until the converging rays are brought to a 

 focus on the plate. 



In the eye, the distance between the lens and the 

 retina can not be changed, consequently the focusing 



