Let us first suppose that we look with both eyes, R and L, Fig. 7, upon a luminous point, D, which we see single, though there is a picture of it on the retina of each eye. In looking at the point D we turn or converge the optical axes _d_HD, _d′_H′ D, of each eye to the point D, an image of which is formed at _d_ in the right eye R, and at _d′_ in the left eye L. In virtue of the law of visible direction the point D is seen in the direction _d_D with the eye R, and in the direction _d′_D with the eye L, these lines being perpendicular to the retina at the points _d_, _d′_. The one image of the point D is therefore seen lying upon the other, and consequently seen single. Considering D, then, as a single point of a visible object AB, the two eyes will see the points A and B single by the same process of turning or converging upon them their optical axes, and so quickly does the point of convergence pass backward and forward over the whole object, that it appears single, though in reality only one point of it can be seen single at the same instant. The whole picture of the line AB, as seen with one eye, _seems_ to coincide with the whole picture of it as seen with the _other_, and to appear single. The same is true of a surface or area, and also of a solid body or a landscape. Only one point of each is seen single; but we do not observe that other points are double or indistinct, because the images of them are upon parts of the retina which do not give distinct vision, owing to their distance from the _foramen_ or point which gives distinct vision. Hence we see the reason why distinct vision is obtained only on one point of the retina. Were it otherwise we should see every other point double when we look fixedly upon one part of an object. If in place of _two_ eyes we had a _hundred_, capable of converging their optical axes to one point, we should, in virtue of the law of visible direction, see only _one_ object.