Page:Mind (Old Series) Volume 12.djvu/38

 THE PEKCEPTION OF SPACE. (l.) 25 The moment we attend to them they grow distinct through one of these motions which leads to the idea prevalent among uninstructed persons that we see distinctly all parts of the field of view at once. The result of this incessant tracing of radii is that whenever a local-sign P is awakened by a spot of light falling upon it, it recalls forthwith, even though the eyeball be unmoved, the local-signs of all the other points which lie between P and the fovea. It recalls them in imaginary form, just as the normal reflex movement would recall them in vivid form ; and with their recall is given a consciousness more or less faint of the whole line on which they lie. In other words, no ray of light can fall on any retinal spot without the local-sign of that spot revealing to us, by recalling the line of its most habitual associates, its direction and distance from the centre of the field. The fovea acts thus as the origin of a system of polar co-ordinates, in relation to which each and every retinal point has through an incessantly repeated process of association its distance and direction determined. Were P alone illumined and all the rest of the field dark we should still, even with motion- less eyes, know whether P lay high or low, right or left, through the ideal streak, different from all other streaks, which P alone has the power of awakening. 1 So far all has been plain sailing, but now our course begins to be tortuous. When P recalls an ideal line leading to the fovea the line is felt in its entirety and but vaguely ; whilst P, which we supposed to be a single star of actual light, stands out in strong distinction from it. The ground of the distinction between P and the ideal line which it terminates is manifest P being vivid while the line is faint ; but why should P hold the particular position it does, at the end of the line, rather than anywhere else for example, in its middle ? That seems something not at all manifest. 1 Notice that all these tracing motions, as we describe them, are supposed to awaken sensibility by the lines they draw on the sensitive surfaces, by moving these over objective points, lines which for an instant are felt through their whole extent. They are not supposed to be per- ceived by the muscular organs, as so much space moved through, along which the surface-sensations are distributed like beads upon a string. We shall later see reason to think that all the muscular sensations have a certain largeness ; they never can give rise in the mind to anything as distinct as the feeling' of a line, with its direction and length. Only a sensitive surface is competent to that. Most English psychologists, how- ever, assume that when muscles contract their sensation is that of the line traversed by the extremity which they move. Undoubtedly muscular contractions do break space up for us into lines ; they dissect it in a way impossible without their aid, but only because they draw lines for us upon our sensitive surfaces.