From the time that children begin to distinguish objects, their eyes are liable to be affected by two causes of error. Before being able to judge of the position of things surrounding them, they see everything upside down; they consequently acquire a false impression of the position of objects. The next cause of error that is likely to mislead them is the fact of their seeing everything double, a separate image of everything being formed on each eye; and it can only be by the experience gained through the sense of touch that they can acquire the knowledge necessary to rectify these errors, and see those objects single which appear to them double. This error of sight, as well as the first one, is set right so easily in the end, that although in reality we see everything double and upside down, we imagine that we see them single, and in their proper positions, a state of things brought about entirely through another sense exercising its power over our judgment; and it is hardly too much to say that, if that sense were deprived of the power of feeling, our eyes would deceive us, not only as to the number, but the position of the objects within our view. It is very easy to convince ourselves that we really see objects double, although we imagine them to be only single. We have only to look at the same object first with the right eye, and we shall see it directly against some portion of the wall of the room in which we are sitting; then looking at it with the left eye, we shall see that it covers a different part of the wall. This experiment is easily tried, and is very convincing. Thus we see that an image is formed on both eyes, and we consequently see the object, whatever it may be, repeated twice. By degrees, however, the eyes gain the power of converging their axes on objects at different distances, so that they fall on similar portions of each retina, and so convey a single impression to the brain. Thus, for instance, if we look at a pencil held up at arm’s length, and then, without changing the position of the eyeball, look at some distant object, we shall see it double. Let us, however, converge the eyes upon it, and the two images unite. Reverse the experiment by now looking at the pencil without converging the eyes upon it, and we shall see that object double in its turn. The same thing happens if we push aside one of the eyes with the finger while looking at any object. During severe illness it often happens that the patient from extreme weakness loses the power of convergence, and consequently sees every thing double, and we continually see children’s faces wearing a most distressing appearance through having temporarily lost the power of moving the muscles of the eye. It is a common expression to use in speaking of drunken people, that they see double, but the saying, unlike many others, is no metaphor; when a man gets drunk he loses his power over the muscles of his eye, just as he does over those that sustain his