Page:Mind (Old Series) Volume 12.djvu/204

THE PEKCEPTION OF SPACE. (ll.) 191 vastness in the feelings of different parts would have uncor- rected play. Objects gliding from one part of our surface to another would appear to change their size, as in the obser- vations mentioned at the beginning of the paper ; and we should have no certainty as to how much lip was equivalent to so much forehead, how much finger to so much back. But with the power of exploring one part of the surface by another we get a direct perception of cutaneous equivalen- cies. The primitive differences of vastness are overpowered when we feel by an immediate sensation that a certain length of thigh-surface is in contact with the entire palm and fingers. And when a certain motion of the opposite finger-tips draws a line first along this same length of thigh and then along the whole of the hand in question, we get a new manner of measurement, less direct but confirming the equivalencies established by the first. In these ways, by superpositions of parts and by tracing lines on different parts by identical movements, a person deprived of sight can soon learn to reduce all the dimensions of his body to a homoge- neous scale. By applying the same methods to objects of his own size or smaller, he can with equal ease make himself acquainted with their extension stated in terms derived from his own bulk, palms, feet, cubits, spans, paces, fathoms (arm- spreads), &c. In these reductions it is to be noticed that when the resident sensations of largeness of two opposed surfaces conflict, one of the sensations is chosen as the true standard and the other treated as illusory. Thus an empty tooth-socket is be- lieved to le really smaller than the finger-tip which it will not admit, although it may feel larger; and in general it may be said that the hand, as the almost exclusive organ of palpa- tion, gives its own magnitude to the other parts, instead of having its size determined by them. The readjustment of the various retinal space-feelings to a common scale is more complex still. So constantly is the same qualitative impression of colour and form changing its magnitude upon the retina (whilst from incessant reversals of the change and tactile verifications we believe the real size of the object to be unaltered), that we end by ascribing no absolute import whatever to the retinal space-feeling which at any moment we may receive. So complete does this overlooking of retinal magnitude become, that it is next to impossible to compare the visual magnitude subtended by different objects at different distances, without making the experiment of superposition. We cannot say beforehand how much of a distant house or tree our finger will cover. The various answers to the familiar question, How large is