If we examine the eyes of an infant, a few hours after its birth, we shall discern that it cannot make the smallest use of them; the organ not having acquired a sufficient consistency, the rays of light strike but confusedly upon the retina. Before the sixth week, children turn their eyes indiscriminately upon every object, without appearing to be affected by any, but at about this time they begin to fix them upon the most brilliant colours, and seem peculiarly desirous of turning them towards the light; this exercise does not give any exact notion of objects, but strengthens the eye, and qualifies it for future vision.
The first great error in the sense of seeing, is the inverted representation of objects upon the retina, and, till the sense of feeling has served to undeceive it, the child actually beholds every thing upside down. The second error in early vision is, that every object appears double; from the same object being formed distinctly upon each eye. This illusion like the other, can only be corrected by children from their being in the practice of handling different objects, and from which practice alone it is that they learn things are neither inverted nor double, and custom induces them to believe they see objects in the position the touch represents them to the mind; and therefore, were we denied the sense of feeling, that of seeing would not only deceive us as to the situation, but as to the number of every object around us.
We may easily be convinced that objects appear inverted, (which arises from the structure of the eye) by admitting the light to pass into a darkened room through a small aperture, when the images of the objects without will be represented upon the wall in an inverted position; for, as all the rays which issue from their different points cannot enter the hole in the same extent and position which they had in leaving the object, unless the aperture was as large as the object itself; as every part of the object sends forth its image on all sides; as the rays which from those images flow from all points of the object, as from so many centres, those only can pass through the small aperture which come in opposite directions. Thus the hole becomes a centre for the entire object, through which the rays from the upper, as well as from the lower parts of it, pass in converging directions; and, of consequence, they must cross each other at this centre, and thus represent the objects upon the wall, in an inverted position.