THE SPECIAL SENSES 



263 



OBJECT 



EYE 



pulling its tubes in or out. When we look from a distant 

 to a near object with an opera glass, we change the focus 

 by turning the adjusting screw. 



A tiny muscle called the ciliary, or hairlike, muscle does 

 for the eye what the adjusting screw does for the opera 

 glass. As it contracts and relaxes, the elastic lens, held 

 in place by a delicate membrane, becomes more rounded 

 or flatter, ac- 

 cording as we 

 may wish to look 

 at nearer or more 

 distant objects. 



In other words, 

 our eyes adjust, 

 or accommodate, 

 themselves to 

 the varying dis- 

 tances of objects, 

 just as the photographer pushes in or out the lens of his 

 camera so as to bring it nearer to or farther from the 

 surface which receives the image (Fig. 162). 



Experiment 77. To illustrate accommodation. Standing near a 

 source of light, close one eye and hold up both forefingers not quite 

 in a line, keeping one finger about six or seven inches from one eye, 

 and the other forefinger about sixteen or eighteen inches from the 

 other eye. Look at the near finger ; a distinct image is obtained of 

 it, while the far one is blurred or indistinct. Look at the far image ; it 

 becomes distinct, while the near one becomes blurred. Observe that in 

 accommodating for the near object one is conscious of a distinct effort. 



363. Formation of an Image on the Retina. The rays of 

 light are thus brought to a focus on the sensitive retina. 

 The iris has regulated with wonderful skill the proper 



FIG. 163. BLACKBOARD SKETCH. 



Diagram illustrating the Manner in which the Image 

 of an Object is inverted on the Retina. 



