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 cornese 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 Mm, or at an object nearer or more 



