The Senses and the Nervous System 91 



open the left and close the right eye. You still see the finger 

 (in fact, even more distinctly, since it is nearer to the center 

 of your visual field). This proves that the finger in that posi- 

 tion is in the common field of vision, and that when both eyes 

 are open both see it and send messages to both sides of your 

 brain. But when the left eye is closed, and the right eye alone 

 sees it, it is registering only on the right side of the right 

 retina, and the message is being sent only to the right side 

 of your brain (see Diagram M). Is the final impression on 

 your consciousness when the image is being registered on 

 only one side of your brain different from when it is being 

 registered on both? Essentially, noj but there is one dif- 

 ference which is important, and that is that when both eyes 

 see the object you are able to make a much quicker and more 

 accurate estimate of its distance, especially if there is no 

 light and shade, and no other object in view, to guide your 

 judgment. And here we have the answer to why the fish 

 turns head on to an object it wishes to examine: in order to 

 estimate its distance. The large portions of each eye through 

 which come the single images produced by things to the right 

 and left of the common field serve to keep the fish informed 

 of what is going on over a wide area — perhaps to warn of 

 approaching danger; but he must bring an object into the 

 common field if he wants to obtain his best estimate of its 

 distance — as would be the case with something he wanted 

 to pounce on and eat. 



The next question is, if seeing a thing with both eyes im- 

 proves judgment of distance, why have not the rays and the 

 flatfishes taken advantage of the opportunity offered by their 

 peculiar bodily shape to locate their eyes in such a position as 

 to have a large common field? (See Figure 29.) Instead of 

 this, each eye, in many of them at least, is surrounded by 

 a socket or mounted on a stalk in such a way as to be cut off 

 from what the other eye can see. And, as if to emphasize 



