Page:Mind (Old Series) Volume 11.djvu/550

 THE PERCEPTION OF SPACE BY DISPARATE SENSES. 549 .about the real length. If (1) without looking I take a book between rny thumb and forefinger and mark off by sight a length that seems equal to the thickness felt, and then (2) looking at the thickness of the book, measure the distance between thumb and forefinger that seems equal to the thickness as seen, the mean proportional between the two results will give about the true thickness of the book. C. A third rather peculiar law remains to be noticed. The pro- cesses involved in the above-described experiments can be repre- sented thus : A length presented to the receiving sense makes a certain impression on my brain-centre ; the problem then is to reproduce the objective stimulation which shall give me an equivalent sensation. The two operations being simultaneous, the sensations can be compared and the judgment corrected until they agree. When the receiving and expressing senses are the same, the comparison is between homogeneous sensations, involv- ing one brain-centre ; the operation is easy and the error small. When the expressing sense differs from the receiving sense, hetero- geneous sensations must be compared, involving two brain-centres ; a difficult operation with a large error. The large error seems to be due to a looseness of association between heterogeneous space- centres ; it is a path of high resistance. Why this error is in the direction in which it is, and not in the opposite direction, depends on some fundamental relation of the senses involved, still to be discovered. JFor the present, the fact that the same objective spacial stimulation has a different value for the several space- senses is to be emphasised. Perhaps the following method will shed some little light on the question. Which of the following changes can be made with a minimum alteration of the results : (1) Substitut- ing one receiving sense for another, leaving the expressing sense unchanged ? Or (2) substituting one expressing sense for another, leaving the receiving sense unchanged? If the first, then the error derives its characteristic more from the expressing sense ; if the second, from the receiving sense. In point of fact the generalisations already formulated show that the expressing sense gives the characteristic properties to the curve, for it was seen that with the Eye as expressing sense an underestimation, with the Hand or Arm as expressing sense an exaggeration takes place, it being immaterial whether the eye, the hand or the arm acts as receiving sense. As a peculiar exception l reciprocals. The average reproductions of the exaggerating curves are 185 per cent., 168 per cent, and 144 per cent, respectively ; of the underestimat- ing curves 68 per cent., 65 per cent, and 59 per cent., whose reciprocals are 147 per cent, 154 per cent, and 170 per cent. This justifies the clause "to about the same extent " in Law B. 1 The peculiarity of the exception is that it is a necessary one. Law B requires the curves for reproducing the arm by the hand and the hand by the arm to be on opposite sides of the central line, while Law G requires them to be on the same side ; i.e., whenever expressing by one sense causes the