We can judge the _distance_ of a near object very accurately by noting the degree to which the two eyes have to be turned in in order to see it clearly with both. We are quite unconscious of this means of making the judgment; all we know is that we can tell. It is easy to prove that it depends on the two eyes by closing one and trying to make movements that depend on accurate knowledge of distance. A good example is threading a needle sideways. With both eyes open this can be done fairly easily, but with one shut it cannot be done at all, except by chance. Objects so far away that the eyes are not turned in perceptibly in looking at them are judged as to distance wholly on the basis of their size. It is clear that the actual size of any image on the retina will depend in part on how large the object is, and in part on how far away it is. If an object that we know to be large casts a small image on the retina, we conclude that it is far away. It follows that unless there are some familiar objects in view, judgments of distance are not at all trustworthy. A good illustration is in looking up a bare hillside, and trying to estimate the distance to the top. If, while this estimation is being made, a man or horse suddenly comes into view at the top, the man or horse will nearly always appear unexpectedly large, showing that the top of the hill is not actually as far away as it was judged to be. The possession of two eyes instead of one is an advantage to us in another way, in addition to helping in the estimation of near distances. This is in making objects appear solid, or in other words, in helping the estimation of depth. When we look about us we have no difficulty in realizing that some objects are near and others far, and that the objects themselves have some parts that are nearer to us than other parts. A great many things assist us in this realization. First and foremost comes that which is known in art as perspective, namely the tendency of distant objects or distant parts of objects to appear smaller than those that are near. This can best be illustrated in the case of parallel lines extending away from the eye, as when one stands on a straight railroad track and looks along it. Although we know perfectly well that the rails are the same distance apart all along, if we were to believe our eyesight implicitly we should think that they came gradually together. It is on account of this matter of perspective that drawings of solid objects must show supposedly parallel sides nearer together at the far end than at the near. Besides perspective there are the shadows to be taken into account. Only solid objects cast shadows, so if we see a shadow apparently cast by any object we naturally conclude that it is solid. Both perspective and shadows can be and are used by artists in making drawings and paintings look real. In fact, they have almost no other means of doing this. As we all know,