INTERPRETATION OF VISUAL SENSATIONS. 431 



556. The picture formed upon the retina closely resembles 

 that which we see in a camera obscura. It represents the 

 outlines, colours, lights and shades, and relative positions, of 

 the objects before us ; but these do not necessarily convey 

 to the mind the knowledge of their real forms, characters, or 

 distances. It would appear, from the actions of the lower 

 animals, that many of them have the power of intuitively or 

 instinctively determining the latter from the former, from the 

 moment when they come into the world ; thus a Fly-catcher 

 just come out of its egg, has been seen to make a successful 

 stroke with its bill at an insect which was near it. If it were 

 not so, those animals which are thrown from the first upon their 

 own resources, would perish almost inevitably ; being starved 

 by want of food during the time required for them to learn 

 how to obtain it. But this is well known not to be the case 

 in regard to Man. The infant is educating his senses long 

 before any indications of mind present themselves. By the 

 combination, especially, of the sensations of sight and touch, 

 he is learning to judge of the shapes and surfaces of objects, as 

 they would be felt, by the appearance they present, to form 

 an idea of their distance, by the mode in which his eyes are 

 directed towards them ( 563), and to estimate their size, by 

 combining the notions obtained through the picture on the 

 retina, with those he acquires by the movement of his hands 

 over their different parts. A simple illustration will show 

 how closely the ideas excited by the two sets of sensations 

 are blended in our minds. The idea of smoothness is one 

 which has reference to the touch, and yet it constantly occurs 

 to us on looking at a surface which reflects light in a particular 

 manner. On lie other hand, the idea of polish is essentially 

 visual, having reference to the reflection of light from the 

 surface of the object ; and yet it would occur to us from the 

 sensation conveyed through the touch, even in the dark. 



557. That this combination is not formed intuitively in 

 Man, but is the result of experience, is particularly evident 

 from cases in which the sense of sight has been wanting 

 during the first years of life, and has then been obtained by 

 an operation. Several such cases are now on record. The 

 earliest, which still remains the most interesting, is one which 

 occurred to Cheselden, a celebrated surgeon at the beginning 

 of the last century. The youth (about twelve years of age), for 



