Now there is a theory which would regard the above statement as a complete account of the binocular perception of solid figures. Distance from the area of distinct vision, behind or before it, is supposed to find its full and ultimate explanation in the disparateness of the position of like impressions in the two retinas, the degree of distance corresponding to the degree of disparateness. This view seems plausible if we consider visual perception in its fully developed form. It does not appear that any other conditions can be operative when the two slides of the stereoscope are lit by a momentary illumination which allows no time for movements of the eyes. But when movements of the eyes are thus excluded, the stereoscopic effect is compara tively dim and imperfect. It must also be borne in mind that when in ordinary vision we steadily fixate a single point in the field of view, and attend to objects before or behind it, we obtain double images rather than a perception of distance from the point fixated. These facts point to active exploration by movements of the eye as an important factor in the perception of the third dimension. An even more powerful reason for introducing this factor is the general analogy of the way in which the spatial perception develops. The apprehension of tactual space develops through a cooperation of active and passive touch. What we know about children and about persons blind from early infancy who have recovered their sight by an operation, shows that the same is true of sight. In a case of operation for congenital cataract,* a boy could not count even as few as two objects by means of passive sight, although he had learnt to count by means of touch. When two objects were placed before him, and he was called on to say how many they were, using sight only, he could do so only by fixing his eyes on each of them in turn. At the outset, it was necessary for him to point to each of them successively with the finger. Pointing without touching was sufficient. At a later stage he was able to count merely by fixing his glance on each object in turn. This he did at first not by movements of the eye, but by lateral movements of the head. It was not till much later that he learnt to count a number of objects at a single glance. In all cases of this kind, the perception of distance in the third dimension develops very gradually. At the outset, the patient appears to have only the analogies of his tactual experience to guide him.
* Uhthoff, "Sehenlernen blindgeborener und spater mit Erfolg operierter Menschen," in Zeitschrift fur Psychologie und Physiologie der Sinnesorgane, Bd. xiv., Heft 3 und 4.
We may then assume that active as well as passive vision is required for the development of the perception of the third dimension. In principle, this development takes place in a way precisely analogous to the development of the spatial perception in general. When the eyes are fixed on any point in the field of view, those parts of the field which lie behind or before this point are perceived by means of disparate retinal impressions. If and so far as the disparateness does not give rise to double images, it gives rise to a peculiar modification of visual sensation, varying concomitantly with the nature and degree of the disparateness. Thus there are differences in the passive sensibility of the retina corresponding minutely with the varying distances of other objects from the object which is at any moment fixated by the two eyes.
Thus we have given in the way of synthetic or passive sensibility the material for the perception of the third dimension. But this synthetic and passive experience can only acquire spatial order in which its parts become positions separated and connected by distances, when active sight successively explores the data simultaneously given to passive sight. Active sight takes the form of increasing or decreasing convergence of the two eyes. When the eyes are turned inwards, so that the lines of vision* converge, objects nearer than the point first fixed by the eyes, which have for that reason previously produced disparate impressions on the retina, come to produce impressions on corresponding points. Decreasing convergence has the same effect for objects lying beyond the point originally fixated. This process is perpetually going on in every moment of waking life; and it is perpetually required for practical adjustment to the environment. Hence the two cooperative factors, active or analytic and passive or synthetic vision, must combine to form a total disposition, which is excited as a whole by each of them. In this way each acquires spatial significance which it would not have in isolation from the other. The peculiar qualitative differences due to varying disparateness of the retinal impressions become perceptions of relative distance from the point on which the eyes are fixed and the combined movement of the two eyes becomes for consciousness a movement over a tract of space.
* The line of vision is an imaginary straight line connecting the fovea and any point to which the gaze is directed.
In this way we may account for the perception of relative distance from the point on which the eyes are fixed at any moment. But the question still remains, How is the distance of the fixationpoint itself determined? Part of the answer has already been given by implication. Whatever determines the relative distance of other points from the fixationpoint must also determine the relative distance of the fixationpoint from these other points. Thus all objects intervening between the body and the eye contribute to fix the absolute distance from the body of the point distinctly seen. But besides this another factor is no doubt operative in a greater or less degree,—the sensations due to the varying position of the eyes themselves. There are no jointsensations because the muscles of the eye do not work on joints. But this defect is compensated by the tactile experiences due to the movement of the eye in its socket; and muscular sensations proper are probably contributory factors. Thus the varying degrees of convergence will be marked by varying tactual and motor sensations in the eyes. These will also help to mark varying direction and extent of movement. But it should not be forgotten that the movements of the eyes, whether in the way of convergence or otherwise, are optical as well as motor experiences. They are accompanied by displacement of impressions of the retina. In converging movements, disparate impressions are in process of becoming correspondent, and vice versa. It has been urged by Professor Hering and others that this purely optical process admits of greater delicacy of discrimination and therefore is a more important factor in our experience of movement and position of the eyes, than the motor sensations themselves. The question is still under debate, but the probability is that Hering is right.