When we look at any object, AB, for example, we see only one point of it distinctly. In Fig. 5 the point D only is seen distinctly, and every point from D to A, and from D to B, less distinctly. The point of distinct vision on the retina is at _d_, corresponding with the point D of the object which is seen distinctly. This point _d_ is the centre of the retina at the extremity of the line AH_a_, called the optical axis of the eye, passing through the centre of the lens L_h_, and the centre of the pupil. The point of distinct vision _d_ corresponds with a small hole in the retina called the _Foramen centrale_, or _central hole_, from its being in the centre of the membrane. When we wish to see the points A and B, or any other point of the object, we turn the eye upon them, so that their image may fall upon the central point _d_. This is done so easily and quickly that every point of an object is seen distinctly in an instant, and we obtain the most perfect knowledge of its form, colour, and direction. The law of distinct vision may be thus expressed. Vision is most distinct when it is performed by the central point of the retina, and the distinctness decreases with the distance from the central point. It is a curious fact, however, that the most _distinct point d is the least sensitive to light_, and that the sensitiveness increases with the distance from that point. This is proved by the remarkable fact, that when an astronomer cannot see a very minute star by looking at it directly along the optical axis _d_D, he can see it _by looking away from it_, and bringing its image upon a more sensitive part of the retina. But though we see with one eye the direction in which any object or point of an object is situated, we do not see its position, or the distance from the eye at which it is placed. If a small luminous point or flame is put into a dark room by another person, we cannot with one eye form anything like a correct estimate of its distance. Even in good light we cannot with one eye snuff a candle, or pour wine into a small glass at arm’s length. In monocular vision, we learn from experience to estimate all distances, but particularly great ones, by various means, which are called the _criteria of distance_; but it is only with both eyes that we can estimate with anything like accuracy the distance of objects not far from us. The _criteria of distance_, by which we are enabled with one eye to form an approximate estimate of the distance of objects are five in number. 1. The interposition of numerous objects between the eye and the object whose distance we are appreciating. A distance at sea appears much shorter than the same distance on land, marked with houses, trees, and other objects; and for the same reason, the sun and moon appear more distant when rising or setting on the horizon of a flat country, than when in the zenith, or at great altitudes.