SEC. 5. VISUAL JUDGMENTS. 



Binocular vision is of use to us inasmuch as the one eye is able 

 to fill up the gaps and imperfections of the other. For example, 

 over and above the monocular filling up of the blind spot, of which 

 we spoke in page 537, since the two blind spots of the two eyes, being 

 each on the nasal side, are not 'corresponding' parts, the one eye 

 supplies that part of the field of vision which is lacking in the other. 

 And other imperfections are similarly made good. But the great use 

 of binocular vision is to afford us means of forming visual judgments 

 concerning the form, size, and distance of objects. 



Judgment of Distance and Size. The perceptions which we 

 gain simply and solely by our field of vision, concern two dimensions 

 only. We can become aware of the apparent size of any part of the 

 field corresponding to any particular object, and of its topographical 

 relations to the rest of the field, but no more. Had we nothing 

 more to depend on, our sight would be almost valueless as far as any 

 exact information of the external world was concerned. By 

 association of the visual sensations with sensations of touch, and 

 with sensations derived from the movements of the eyeballs 

 required to make any such part of the field as corresponds to 

 a particular object distinct, we are led to form judgments, i.e. to 

 draw conclusions concerning the external world by means of 

 an interpretation of our visual perceptions. Looking before us, we 

 say we see a certain object of a certain colour nearly in front of us, 

 or much on our right hand or much on our left ; that is to say, we 

 judge such an object to be in such a position because from 



