142 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



SECTION V. 

 Binocular Vision. 



All the phenomena thus far treated are essential to 

 vision. They would still be found if, like the cyclops 

 Polyphemus, we had but one eye in the middle of the 

 forehead. But, in addition to these, there are certain 

 other phenomena which are wholly the result of the use 

 of two eyes as one instrument. These belong to binocular 

 vision. Observe, it is not the mere having of two eyes 

 which gives rise to these phenomena. We might have a 

 hundred eyes and have no binocular phenomena, for 

 each eye may act independently, as is the case in many 

 lower animals. The two eyes must act as one instrument. 



The phenomena now about to be described are far 

 more illusory, more psychical, more difficult to be ob- 

 served. Although we are forming judgments based on 

 them every day of our lives, yet they usually drop out 

 of consciousness, and by many persons are recalled to 

 consciousness with difficulty. For this reason we shall 

 be compelled to treat them much more cursorily than 

 their importance deserves.* 



SINGLE AND DOUBLE VISION. 



Double Vision. — We have two eyes, two retinas, and 

 two fields of view — their spatial representatives — though 

 they indeed partly overlap and form a common field. 

 We have also two retinal images of each object, and two 

 external images, the spatial representatives of the two 

 retinal images. Why, then, do we not see everything 

 double? So indeed we often do, but without observing 

 it. It is necessary first of all to prove this. I do so by 

 some simple experiments. 



* This subject is fully treated in my book Sight. 



