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OUR PHYSICAL WORLD 



e strike the mirror at / and g, and are reflected into the eyes 

 of the observer. He sees the image of e at e'. Similarly, he 

 sees c and d at c' and d r respectively. But these imaged eyes 

 appear to face him from back of the mirror. The ear e r of the 

 image is at the right of its eyes, while the ear of the observer e 

 is at the left of his eyes. The eye d f is the right eye of the image, 

 while the corresponding eye, d, of .the observer is his left eye. 



Note that e' appears as far to the rear of the mirror as e is 

 in front of it, because we judge the distance of an object by the 

 angle between the rays of light entering the two eyes from it. 



This angle is evidently the 

 same after reflection from 

 the mirror as when the rays 

 start from e. The eyeballs 

 are turned in their sockets 

 by delicate muscles that are 

 richly supplied with sensi- 

 tive nerves (Fig. 135). So 

 we are able to sense just 

 how much the axes of the 

 two eyeballs converge 

 when we fix our eyes on 

 an object. The axes evi- 

 dently converge strongly 

 when a very near object 

 is examined, e.g., the tip of one's own nose, less strongly as the 

 object is more and more distant. That the two eyes are used 

 in such estimation of the distance of an object is made apparent 

 by a simple experiment. Tie a finger ring to one end of a 

 piece of fine wire or thread. Fasten the other end of the wire 

 to some object, such as an electrolier or a door frame, so the ring 

 hangs freely about breast high. Step away from the ring 2 or 

 3 yards and face its edge. Take a pencil in your hand, close 

 one eye, then walk up to the ring and pass the pencil through 

 it from right to left, with the eye still closed. 



FIG. 135. Section of the eyeball 



