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. The _shape_ of objects is learned altogether by the sense of _feeling_. It has before been stated that at the first exercise of vision every thing is a confused mass of different colors, and all appearing to touch the eye. By the aid of the hands the separate existence of different bodies is detected, and the feeling of touch, which once was the sole mode of determining shape, is now associated with a certain form or picture on the eye, so that, in process of time, the eye becomes the principal judge of shape. But, in determining the shape of a thing, an act of judgment is necessary. This may be illustrated by the example of a hoop, which in one position will make a picture in the eye which is circular, in another position the picture of it will be oval, and in another only a straight line. If a person will observe a hoop in these different positions, and then attempt to draw a picture of it, he will be conscious of this varying picture in the eye. Of course, in order to decide the shape of a thing, we must decide its distance, its relative position, and various circumstances which would alter the form of the picture in the eye. It is only by long experience that the infant child gradually acquires the power of determining the shape, size, and distance of objects.