English: Visual Sensations are referred to a Position in Space.—The localization of a luminous point in space can only be determined by observing its relations to other luminous points with a given position of the head and of the eye. For example, in a perfectly dark room, if we look at a single luminous point, we cannot fix its exact position in space, but we may get some information of a vague character by moving the head or the eye. If, however, a second luminous point appears in the darkness, we can tell whether it is nearer or farther distant, above or below the first. So with regard to other luminous points we observe their reciprocal relations, and thus we localize a number of visual impressions. There are three principal directions in space: the transverse (breadth), the vertical (height) and the sagittal (depth). Luminous points may be localized either in the transverse or vertical directions. Here we have to do simply with localization on a surface. A number of points may be observed simultaneously (as when the eye is fixed) or successively (as when the eye moves). If the movement of the eye be made rapidly, the series of impressions from different points may be fused together, and we are conscious of a line, the direction of which is indicated chiefly by the muscular sensations felt in following it. The case is different as regards points in the sagittal direction. We see only a single point of this line at a time; it may be a transverse series of retinal elements, A B, and each of these formed by a number of smaller elements, 1, 2, 3, 4, situated in the axis of each principal element; it may be, on the other hand, the transverse line a b situated in space and formed by a series of points in juxtaposition. Each of these points will impress a retinal element, and the result will be the perception of a transverse line; but this will not be the same for the points c, d, e, f, g, situated in space in a linear series, in the sagittal direction, only one of those points, c, will impress the corresponding retinal element, and we can see only one point at a time in the line c g. By accommodating successively, however, for the various points at different and considerable distances along the line c g, we may excite retinal elements in rapid succession. Thus, partly by the fusion of the successive impressions on the retina, and partly from the muscular sensations caused by repeated accommodations and possibly of ocular movements, we obtain a notion of depth in space, even with the use of only one eye. It is, however, one of the chief effects of binocular vision to give precision to the notion of space in the sagittal direction.