THE PERCEPTION OF SPACE BY DISPARATE SENSES. 553 the lapse of a few days as of a few minutes. The Eye-memory is slightly superior to the hand-memory ; the arm was not tested. The superiority of sight finds a popular expression in such a phrase as "seeing is believing". The observation of Weber that, in writing letters or imagining them set on the chest, they are naturally inverted, being placed so as to be read by the eye, shows how unconsciously sight rules the other senses. An ingenious- experiment of Helmholtz (Phys. Optik, p. 601) shows that sight must frequently be corrected by motor judgments. Looking; through prismatic spectacles he attempted to guide his finger to an object, and naturally went far off to one side of it. Having learnt to allow for this by practice or by following the finger to the object, he found that the other hand had acquired this facility at the same time, indicating that sight alone was deceived. Chil- dren, in whom the co-ordination of sight and touch is imperfect, can guide things to the mouth more readily than to a seen object. Experiments upon the Blind. By experimenting upon the blind, in whose education sight the space-sense par excellence has had no share, one may be able to detect to what extent their space-conceptions have suffered by this loss, and how far the other senses, by increased practice, have been able to supply the deficiency. Experiments were made upon one subject, blind almost from birth, and the results thus obtained verified upon others. (1) In reproducing the Hand by the Hand or the Arm by the Arm, the error of the blind is slightly greater than that of seeing persons, but in the same direction. (2) If the Hand reproduces the Arm, the error is somewhat greater, if the Arm reproduces the Hand, much greater, than that of seeing persons, the lines being drawn, on the average, less than half their real length. (3) In expressing inches 1 by the Hand, the error is slightly larger ; by the Arm much larger than that of seeing persons. The Motion-inch of the blind is really about inch long. This yields the conclusion that the error of the blind in reproducing one sense by the same or by another sense is quite like that of normal persons, excepting that in the latter case the error is somewhat larger, especially when motion is the expressing sense. It follows, too, that the blind person's notion of inches (especially of the motor inch) is much too small. The seeing person corrects this by sight. When, however, we compare the accuracy with which small differences of length can be recognised by blind and by seeing persons, the effect of practice in the use of the hand and the arm shows a strong superiority in favour of the blind. In the follow- 1 The knowledge of inches was acquired by feeling the intervals between pegs set upon a wooden ruler. 37
