286 THE HUMAN BODY 



ternal objects. The conceptions which we arrive at in this way are 

 known as visual perceptions. The full treatment of perceptions be- 

 longs to the domain of Psychology, but Physiology is concerned 

 with the conditions under which they are produced. 



The Visual Perception of Distance. With one eye our perception 

 of distance is very imperfect, as illustrated by the common trick of 

 holding a ring suspended by a string in front of a person's face, and 

 telling him to shut one eye and pass a rod from one side through 

 the ring. If a penholder be held erect before one eye, while the 

 other is closed, and an attempt be made to touch it with a finger 

 moved across towards it, an error will nearly always be made. (If 

 the finger be moved straight on towards the pen it will be touched 

 because with one eye we can estimate direction accurately and 

 have only to go on moving the finger in the proper direction till it 

 meets the object.) In such cases we get the only clue from the 

 amount of effort needed to " accommodate " the eye to see the 

 object distinctly. When we use both eyes our perception of dis- 

 tance is much better; when we look at an object with two eyes the 

 visual axes are converged on it, and the nearer the object the 

 greater the convergence. We have a pretty accurate knowledge 

 of the degree of muscular effort required to converge the eyes on 

 all tolerably near points. When objects are farther off, their 

 apparent size, and the modifications of their retinal images brought 

 about by aerial perspective, come in to help.. The relative distance 

 of objects is easiest determined by moving the eyes; all stationary 

 objects then appear displaced in the opposite direction (as for ex- 

 ample when we look out of the window of a railway car) and those 

 nearest most rapidly; from the different apparent rates of move- 

 ment we can tell which are farther and nearer. We so inseparably 

 and unconsciously bind up perceptions of distance with the sensa- 

 tions aroused by objects looked at, that we seem to see distance; 

 it seems at first thought as definite a sensation as color. That it is 

 not is shown by cases of persons born blind, who have had sight 

 restored later in life by surgical operations. Such persons have at 

 first no visual perceptions of distance: all objects seem spread out 

 on a flat surface in contact with the eyes, and they only learn 

 gradually to interpret their sensations so as to form judgments 

 about distances, as the rest of us did unconsciously in childhood 

 before we thought about such things. 



