24. The simplest case of a relation between an impression and the subject, that can appear in a visual idea, is evidently that in which the impression is reduced to a single point. If a single point of light is presented in the field of vision, both lines of regard are turned upon it as a result of the reflex impulse exerted by the stimulus (p. 129), in such [p. 131] a way that in both eyes the images fall upon the retinal centres. At the same time the organs of accommodation are adapted to the distance of the point. The point thus represented on the centres of both retinas is seen as single and as situated in a certain particular direction and at a certain particular distance from the ideating subject.
The subject is represented, as a rule, by a point that may be defined as the middle point of the straight line connecting the centres of rotation of the two eyes. We will call this the point of orientation for the field of vision, and the straight line drawn from it to the intersection of the two lines of regard, that is to the external fixation-point, the line of orientation. When a point in space is fixated, there is always a fairly exact idea of the direction of the line of orientation.
This idea is produced by the inner tactual sensations connected with the position of the two eyes. Such sensations are very noticeable because of their intensity, when the eyes are rotated much out of the central position.
They are just as perceptible for a single eye, so that localization in direction is as perfect in monocular as in binocular vision. In the former case, however, the line of orientation generally coincides with the line of regard. [ 3 ]
25. The idea of the distance of the objects from the subject, or of the absolute length of the line of orientation, is much more indefinite than that of its direction. We are always inclined to ideate this distance shorter than it really is, as may be shown by comparing it with a standard [p. 132] placed somewhere in the field of vision perpendicular to the line of orientation. In this way we find that the distance on the standard judged to be equal to the line of orientation, is always much shorter than the real length of this line. The difference between the two increases further away the point of fixation moves, that is, the longer the line of orientation becomes. The only sensational components that can produce this idea of distance, are those inner tactual sensations connected with the position of the two eyes, that arise particularly from the convergence of the lines of regard and give somewhat of a measure for the absolute extent of this convergence. In fact, it is possible to observe sensations when the convergence is changed; mainly from the inner angle of the eye when the degree of convergence is increased, from the outer, when it is decreased. The sum of all the sensations corresponding to a given position of convergence distinguishes it completely from all other positions.
26. It follows that an idea of a definite, absolute length of the line of orientation can be developed only through the influences of experience, where in addition to the sensational elements a great many associations also have a part. This explains why these ideas always remain indefinite and why they are sometimes aided, sometimes interfered with by other components of visual ideas, especially by the size of the retinal images of familiar objects. On the other hand, we have a relatively fine measure in the sensations of convergence for differences in the distances of objects seen, that is, for the relative changes which the length of the line of orientation undergoes when the fixation-point approaches or recedes. For positions in which the lines of regard are nearly parallel, changes in convergence may be perceived that correspond to an angle of vision of 60" or 70". When the convergence increases, this least perceptible change in convergence also [p. 133] increases considerably, but, in spite of that, the corresponding differences in the length of the line of orientation become smaller and smaller. Thus the purely intensive sensations which accompany movements of convergence, are translated directly into ideas of changes in the distance between the fixation-point and the point of orientation of the subject.
This translation of a certain particular sensational complex into an idea of distance, is not due to any connate energy, but to a particular psychical development, as is shown by a great number of experiences which point to such a development. Among these is the fact that the apprehension both of absolute distances and of differences in distance, is greatly improved by practice. Children are generally inclined to localize very distant objects in the immediate neighborhood: they grasp at the moon, at the slater on the tower, etc. In the same way, it has been observed that the congenitally blind are, immediately after an operation, entirely unable to distinguish near and far.
27. It is of importance for the development of this discrimination between far and near, that under the natural conditions of vision not mere isolated points are presented, but extended three-dimensional objects, or at least a number of points at different depths, to which we assign relatively different distances their respective lines of orientation.