552 THE HUMAN BODY. 



There are difficulties in the way of the full acceptance of 

 either the Young (often called the Young-Helmholtz) theory 

 or the theory of Hering, and the whole doctrine of color 

 vision is still in a very unsettled state. 



Visual Perceptions. The sensations which light excites 

 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 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 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 on towards the pen it will be 

 touched because with one eye we can estimate direction accu- 

 rately 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 " accommo- 

 date" the eye to see the object distinctly. When we use 

 both eyes our perception of distance is much better; when 

 we look at an object with two eyes the visual axes are con- 

 verged on it, and the nearer the object the greater the con- 

 vergence. 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 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 uncon- 

 sciously bind up perceptions of distance with the sensations 

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

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



