530 ^ THE HUMAN BODY. 



still to be accounted for on physiological grounds, as due 

 to conditions of the nervous system. 



Visual Perceptions. The sensations which light exci-tes 

 in us we interpret as indications of the existence, form, and 

 position of external objects. The conceptions which we 

 arrive at in this way are known as visual perceptions. 

 The full treatment of perceptions belongs to the domain of 

 Psychology, but Physiology is concerned with the condi- 

 tions 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 pen* 

 holder 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 out 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 modifi- 

 cations their retinal images experience 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 example 

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

 nearest most rapidly; from the different apparent rates of 

 movement we can tell which are farther and nearer. We 

 so inseparably and unconsciously bind up perceptions of dis 

 tance with the sensations aroused by objects looked at, that 



