$27 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 char acter 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 (1) 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 num ber 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 im pressions 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 * ff be, on the other hand, the * transverse line b situated , in space and formed by a 2 series of points in juxtaposi tion. Each of these points will impress a retinal ele- A ^ B ment, 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, ., , , . . """ FIG. 29. Diagram illustrating the situated in space in a linear localization f visual perceptions. series, in the sagittal direc tion; only one of those points c will impress the corre sponding retinal element, and we can see only one point at a time in the line eg. 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 direc tion. (d) Visual Sensations are continuous. Suppose the image of a luminous line falls on the retina, it will appear as a line although it is placed on perhaps 200 cones or rods, each of which may be separately excited, so as to cause a difttini t sensation. Again, on the same principle, the im pression of a superficial surface may be regarded as a kind of mosaic, made up of individual portions corresponding to the rods or cones on which the image of the surface falls. But in both cases, the sensation is continuous, so that we see a line or a surface. The individual images are fused together. (2.) Motions derived from Visual Perceptions. When we look at any object, we judge of its size, the direction of its surfaces (unless it be a point), its distance from the eye, its apparent movement or fixedness, and its appearance of solidity. (a) Apparent Size. This, so far as regards a compara tively small object, depends on the size of the retinal C 30. Diagram to illustrate illusions of size and distance. image, as determined by the visual angle. With a very large object, there is an appreciation of size from the mus cular sensations derived from the movements of the eyeball, as we "range" the eye over it. It is * * * * * difficult to appreciate the distance separating two points between which there are other points, as contrasted with an apparently similar distance without intermediate points. For example, the distance A to B appears to be greater than from B to C, in fig. 30. (b) Direction. As the retina is a curved surface, a long straight line, especially when seen from a distance, appears curved. In fig. 31 a curious illusion of direction, first shown by Zoell- ner, is depicted. If these lines be looked at some what obliquely, say from one corner, they will appear to con verge or diverge, and the oblique lines, on each side of the ver tical lines, will appear not to be exactly opposite each other. But the vertical lines are parallel, and the oblique lines are continuous Fia 31.-Zoellner s figure showing an illusion of , , direction, across them. Ihe effect is evidently due to an error of judgment, as it may be controlled by an intense effort, when the lines will bo seen as they really are. (c) Apparent Distance. We judge of distance, as regards large objects at a great distance from the eye (1) from their apparent size, which depends on the dimensions of the visual angle, and (2) from the interposition of other objects between the eye and the distant object. Thus, at sea, we cannot form, without great experience, an accurate estimate of how many miles we are off the coast, and all know how difficult it is to estimate accurately the width of a river. But if objects be interposed between the eye and the distant object, say a few vessels at different distances at sea, or a boat in the river, then we have certain materials on which to form a judgment, the accuracy of which, however, even with these aids, will depend on experience. When we look at a near object, we judge of its distance chiefly by the sense of effort put forth in bring ing the two lines of regard to converge upon it. (d) The Movement of a Body. Ii the eye be fixed, we judge of movement by successive portions of the retina being affected, and possibly also, by a feeling of an alsence of muscular contractions neces sary to move the eye-balls. When the eye moves, so as to "follow" the object, there is a sense of muscular effort, which is increased when, in additio) we require to move the head. (e) The Apparent Solidity of an Object. If we look at an object, say a cube, first with the right eye and then with the left, it will be found that the two images of the object are somewhat different, as in fig. 32. If, then, bv means of a stereoscope, or by holding
/ FIG. 32. Illustrating stereo
scopic vision.