THE EYE AND THE SENSE OF SIGHT 115 



that we see, but the object itself. Sensation is not in the 

 eye, but in the brain, or in the mind acting through the 

 brain. It is only by study and research that we learn 

 the fact of the inverted image in the eye, and meantime 

 we are accustomed to supplement our visual impressions 

 by the use of our muscles and our organs of touch. The 

 hand interprets the impression on the eye, and we learn 

 to see objects in their true positions. We judge of their 

 positions by the direction from which the light comes to the 

 eye, and of their size by a variety of experiences which 

 complete the impression given by sight. The figure on 

 the retina has little or nothing to do with those judgments. 



155. Seeing with Two Eyes. Two images of one object 

 are formed on the two retinas, and two optic nerves and 

 tracts convey the impression 'to the two opposite sides of 

 the brain. Why, then, do we not see two objects ? Here 

 again we must remember that our perceptions are never 

 simple, due to the action of a single organ and an isolated 

 set of nervous connections. Probably in every act of 

 perception the nervous system acts as a whole through 

 the intricate interlacing of nervous fibers and the close 

 connections of the cells in the various nerve centers. An 

 impression upon one set of end organs is supplemented 

 and corrected by a great number of familiar perceptions 

 of diverse sorts brought before the mind by memory, so 

 that the resulting judgment is an act too intricate and 

 complex to be disentangled. We come to think of the 

 object as we know it from all these combined impressions, 

 and not from a single one of them. Perception is the 

 result of association and experience combined with the 

 physical processes involved. 



156. Advantages of Two Eyes. For perfect vision the 

 retinal images must be formed upon corresponding portions 



