When the outer covering of the back part of the eye is removed, the objects which are in front of the eye may be discerned, delicately portrayed in all their perfect colors and proportions, on the retina which lines the interior. It is this impression of light on the optic nerve which gives our ideas of light and colors.
The eye is also formed in such a way that it can alter its shape and become somewhat oblong, while at the same time its lens is projected forward or drawn back. The object of this contrivance is to obtain an equally perfect picture of distant and of near objects.
Our ideas of _shape_ and _size_ at first are not gained by the eye, but by the sense of touch. After considerable experience we learn to determine shape and size by the eye. Experiments made upon persons born blind and restored to sight furnish many curious facts to support this assertion.
When the eye first admits the light, all objects appear to _touch_ the eye, and are all a confused mass of different colors. But by continual observation, and by the aid of the sense of touch, objects gradually are separated from each other, and are then regarded as separate and distinct existences.
The eye is so formed that the picture of any object on the retina varies in size according to its _distance_. Two objects of equal size will make a different picture on the back of the eye, according to the distance at which they are held. The ideas of size at first are regulated by the proportions of this picture in the eye, until by experience it is found that this is an incorrect mode, and that it is necessary to judge of the _distance_ of a body before we can determine its _size_. This accounts for the fact that objects appear to us so different according as we conceive of their distance, and that we are often deceived in the size of bodies because we have no mode of determining their distance.
But it appears also that our ideas of distance are gained, not by the eye alone, but by the eye and the sense of feeling united. A child by the sense of feeling learns the size of his cup or his playthings. He sees them removed, and that their apparent size diminishes. They are returned to him, and he finds them unaltered in size. When attempting to recover them, he finds that when they look very small he is obliged to pass over a much greater distance to gain them than when they appear large, and that the distance is always in exact proportion to their apparent size. In this way, by oft-repeated experiments, the infant reasoner learns to judge both of the size and distance of objects. From this it appears that, in determining the size of an object, we previously form some judgment of its distance, and likewise that, in finding the distance, we first determine the size.