INTERPRETATION OP 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 the 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 
