§ 1. Perception of Surface. — There is no difference in principle between the process by which the visual perception of space is developed, and that by which the tactual perception is developed. Both depend on a combination of analysis and synthesis. Active sight corresponds to active touch, and passive sight to passive touch. There is however this important difference, that in the case of sight synthesis and analysis are much more intimately combined. They are for the most part simultaneous rather than successive. We have in the eye an expanded surface sensitive to light, but near the centre of this surface there is one spot in which visual sensation is peculiarly delicate and distinct. Thus there is at any moment of vision a general field of view seen by the eye as a whole, and a limited area within it seen with peculiar clearness and distinctness by the central spot, called from its colour the yellow spot. Within the yellow spot there is a pit or depression called the fovea centralis, and here discrimination is most delicate of all. Now, active sight consists in movements of the eye which successively bring the outlying parts of the field of view within the area of distinct vision. A certain amount and direction of movement is required in order that a stimulation situated in a given position in an outlying part of the retina may be transferred to the yellow spot. Thus by a highly organised system of definite movements the eye is perpetually passing to and fro within the field of vision, bringing its parts successively into the area of distinct vision. The development of the spatial perception is coincident with the perfecting of these movements, and of others connected with the cooperation of the two eyes.

Though the visual and tactual perception of space depend on essentially similar conditions, there are specific peculiarities in the case of sight which require separate treatment. In the first place, visual perception of space cannot be adequately discussed unless we take into account its relation to the tactual experiences which arise in the actual manipulation of objects. The spatial perception is throughout its development determined by practical interest. The object of perception is always ultimately real extension, figure, and magnitude; but these are much more directly and accurately revealed in tactual experience than in visual. Variations of the visual experience are constantly occurring, which imply no variations in the size, figure, and position of the objects seen, but only variations in the position of the body or eyes of the observer. Since what we are interested in is the real spacedeterminations of the objects themselves, we tend to ignore these variations in their relation to the nature of the object. So far as they have meaning for us, they condition the perception of the relative position of the object to the body of the observer. For instance, the area of the retina stimulated by the object varies greatly according to our distance from it. By introspective analysis, we can discern a varying apparent magnitude of the object corresponding to the varying distances. But in ordinary practical experience, these variations are largely ignored. A man crossing the room towards us does not appear to become twice as tall in the process. His distance appears to vary, but his size to remain the same.* In the second place, the eye has means of perceiving the third dimension which are denied to touch. This arises from the fact that the eye is stimulated by objects at a distance from the body. § 2. Visual Perception of the Third Dimension.—(a) As Conditioned by Tactual Experience. The conditions on which the perception of the third dimension by sight depend are manifold and complex. A celebrated theory was advanced by Bishop Berkeley according to which the eye has no power of directly perceiving distance from the body at all. Its apparent power of doing so is merely due to association between visual and tactual experiences. The sight of an object at a distance is supposed to call up a mental representation of the movement of the body required to reach it. This suggested image of tactile and motor experiences constituted, according to Berkeley, the perception of distance. Similarly, the perception of solid shape by the eye was supposed simply to consist in an ideal revival of experiences of active and passive touch. This theory is confuted by a simple appeal to experience. We have, as a matter of fact, a perception of solid figure and of distance, which is essentially visual. None the less, there is in Berkeley's theory an underlying thought which is true and valuable. It cannot be maintained that the visual experience as such remains unchanged, and that tactual elements are superadded to it merely in the way of ideal revival. But it is certain that the tactual perception of extension must play a great part in moulding the visual perception. Tactual and visual perception develop together. The practical interest of sight depends mainly on its power to guide active touch and the practical manipulation of objects.

*This applies to the developed consciousness, but it is probable that the variation in size is a marked feature of the conscious condition before its meaning is apprehended. Persons blind from infancy, who have recovered their sight by an operation, express wonder that a room should be capable of appearing as big as the whole house which contains it, and so on.

The presentation of visual extension depends primarily for its value on its intimate correlation with extension as revealed to touch. There is between them a unity of practical interest, in which the tactual element plays the dominant part. In practice, they are perpetually combined. In exploring a thing by touch, the eye follows the motion of the hand. In so far as sight comes first, it is constantly followed by touch, and is useful only in so far as it guides touch. Now this intimate union cannot exist without mutual modification, and since the tactual experience more directly reveals real extension, the modification of the visual experience will be the more profound. To explain what we mean by this modification, let us take the case of a man handling an object in the dark. As he explores the outlines of the object, he at the same time constructs a visual image of it.*

* The vast majority of mankind does so. There appear however to be exceptions.