16a. Because of this interrelation between the keenness of vision and the arrangement of retinal elements, it has often been concluded that every such element has from the first the property of localizing any stimulus that acts upon it, in that position in space which corresponds to its own projection in the field of vision. In this way the attempt has been made to reduce the property of the visual sense by virtue of which it represents its objects in an external field of vision at some distance from the subject, to a condate energy of the retinal elements or of their central connections in the visual centre in the brain. There are certain pathological disturbances of vision that seem at first sight to confirm this assumption. When some region of the retina is pushed out of place as a result of inflammation underneath, certain distortions in the images, the so-called metamor-phopsia, arise. The extent and direction of these distortions can be fully explained when it is assumed that the displaced retinal elements continue to localize their impressions as they did when in their normal positions. But it is obvious that these distortions of the images when, as in most cases, they appear as continually changing phenomena during the gradual formation and disappearance of the excretion, furnish us with no more evidence for a connate energy of localization in the retina than does the readily observed fact that distorted images of objects are seen when one looks through prismatic glasses. If, on the other hand, a stationary condition is gradually reached, the metamorphopsia disappear, and that, too, not only in cases where it may be assumed that the retinal elements return to their original position, but even in those cases where such a return is entirely improbable on account of the extent of the affection. In cases like the latter, the development of a new connection between the single retinal elements and their corresponding points in the field [p. 120] of vision, must be assumed. This conclusion is supported by observations made with normal eyes on the gradual adaptation to distorted images which are produced by external optical appliances.
If a pair of prismatic glasses are worn before the eyes, marked and disturbing distortions of the images are the regular results. The straight bounding lines appear bent and the forms of the objects are thus distorted. These disturbances gradually disappear entirely if the glasses are worn some time. When the glasses are removed the distortions may appear in the opposite direction. All these phenomena can be understood if we presuppose that the spacial localizations of vision are not original, but acquired.
17. Besides the retinal sensations there are other psychical elements that always take part in the reciprocal spacial arrangement of light-impressions.
The physiological properties of the dye point a priori to the sensations that accompany ocular movements, as such elements. These movements obviously play the same part in the estimation of distances in the field of vision as the tactual movements do in the estimation of tactual impressions.
The grosser conditions of touch are, however, here reproduced in a much more delicate and highly developed form. The eye can be turned in all directions about its centre of rotation, which is fixed in its relation to the head, by means of a most admirably arranged system of six muscles. It is thus well suited to following continuously the bounding lines of objects or to passing each [p. 121] time in the shortest line from a given fixation-point to another. The movements in the directions which correspond to the position of the objects most frequently and closely observed, namely, downward and inward movements, are favored above the others by the arrangement of the muscles. Furthermore, the movements of the two eyes are so adapted to one another through the synergy of their innervation, that normally the two lines of regard are always turned upon the same fixation point. In this way a cooperation of the two eyes is made possible which not only permit a more perfect apprehension of the position of objects in relation to one another, but, more especially, furnishes the most essential means for the determination of the spacial relation of objects to the subject (24 seq).
18. The phenomena of vision teach that the idea of the relative distance of two points from each other is dependent on the motor energy of the eye employed in passing through this distance, just as the discrimination of two distinct points in the field of vision depends on the arrangement of the retinal elements. The motor energy becomes a component of the idea through its connection with a sensation which can be perceived, especially in extensive movements and by comparing ocular movements in various directions. Thus, for example, the upward movement of the eyes is clearly accompanied by more intense sensations than the downward movements; and the same is true of outward movements of one eye as compared with its inward movements.
The influence of sensations of movement on the localization are most apparent in the cases of disturbance arising from partial paralysis of single ocular muscles. These disturbances correspond exactly to the changes in the amount of energy required to move the eye. The general principle [p. 122] of such disorders is that the distance between two points seems greater when they lie in the direction of the more difficult movement.
The more difficult movement has a correspondingly more intense motor sensation, which under normal conditions accompanies a more extensive movement. As a result, the distance passed through appears greater. Furthermore, the same illusion may appear for distances that lie in the same direction, bait have not been actually passed through, for the standard found during a, movement determines the motor impulse in the eye even when it is not moved.