436 ESTIMATION OF DISTANCE. 
moreover, we are deprived of that power of measuring the 
relative distances of near objects, which we derive from the 
conjoint use of both eyes (§ 563); and thus a well-painted 
picture, still more a photograph, may so strongly suggest the 
idea of projection, in virtue of its exact perspective and its 
contrast of light and shadow, that it is difficult to believe it 
to be a flat surface, even though it be within but arm’s 
length of the eye. 
563. Our idea of the distance of objects is evidently 
acquired by experience. An infant, when a bright object 
is held before its eyes, attempts to grasp it with its little 
hands, but obviously has no certain idea of its situation ; 
and the same is observed in persons who have but recently 
acquired sight. Here, then, the impressions made upon the 
eyes have to be corrected by those received through the touch, 
before the power of judging of distances is acquired; but 
when it has been once acquired, we can accurately estimate 
the relative distances of near objects without using our hands. 
This we do chiefly by the interpretation we have learned to 
make, of the sensations which are occasioned in the muscles 
of the eyes by their various actions. When we fix both our 
eyes upon a distant object, their axes are nearly parallel to 
each other; but when we direct them to a near object, the 
axes of the eyes meet in the point at which we are looking. 
This is very easily seen by watching the eyes of another 
person, when fixed upon an object, first held at arm’s length, 
and then brought nearer and nearer to the middle point 
between the eyes ; the two cornez are at first directed nearly 
straightforwards ; but they gradually turn inwards as the 
object is brought nearer, and at last a very decided inward 
squint is produced, which disappears as soon as the object is 
removed. Thus, for objects within a moderate distance, the 
degree of convergence of the axes of the eyes, and the mus- 
cular sensations thereby produced, afford us sufficient means 
of judgment. 
564. We perceive this in another, as well as in ourselves ; 
for by observing his eyes, we can judge, not only of the 
direction, but of the distance, of the object he is looking at. § 
Thus when a person A sees a friend B looking towards 
him, he can at once tell, by the appearance of B's eyes, 
whether he is looking at him, or at an object nearer or more | 
