Page:The wonders of optics (1869).djvu/39

 unreal. In this chapter we shall only speak of those errors of the eye of which we have actually lost all cognizance, so effectually has our judgment succeeded in counteracting their influence.

We all know that the first thing a child does with its eyes, even when it is only five or six weeks old, is to turn them towards the most brilliant object within its reach. Instinctively and without being aware of it, the child's eye seems to seek the light. The whole of nature, from the lowest plant to the baby in the cradle, appears more or less endowed with this instinct of turning towards the light.

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