An answer to this question has been suggested by various psychologists. The eye, by an instinctive movement, turns so as to bring every impression upon that point of the retina, near its centre, which has the acutest sensibility. A series of muscular sensations therefore always follows upon the conspicuous excitement of any outlying point. The object, as the eye brings it to the centre of vision, excites a series of points upon the retina; and the local sign, or peculiar quality of sensation, proper to each of these spots, is associated with that series of muscular feelings involved in turning the eyes. These feelings henceforth revive together; it is enough that a point in the periphery of the retina should receive a ray, for the mind to feel, together with that impression, the suggestion of a motion, and of the line of points that lies between the excited point and the centre of vision. A network of associations is thus formed, whereby the sensation of each retinal point is connected with all the others in a manner which is that of points in a plane. Every visible point becomes thus a point in a field, and has a felt radiation of lines of possible motion about it. Our notion of visual space has this origin, since the manifold of retinal impressions is distributed in a manner which serves as the type and exemplar of what we mean by a surface. _Values of geometrical figures._ Sec. 21. The reader will perhaps pardon these details and the strain they put on his attention, when he perceives how much they help us to understand the value of forms. The sense, then, of the position of any point consists in the tensions in the eye, that not only tends to bring that point to the centre of vision, but feels the suggestion of all the other points which are related to the given one in the web of visual experience. The definition of space as the possibility of motion is therefore an accurate and significant one, since the most direct and native perception of space we can have is the awakening of many tendencies to move our organs.