Page:Mind (Old Series) Volume 11.djvu/546

 THE PERCEPTION OF SPACE BY DISPARATE SENSES. 545 FIG. 5. CURVES SHOWING THE ERRORS IN REPRODUCING SPACE- IMPKESSIONS. The error is measured by ordinates above or below the broken central line, in percentages as indicated at the sides of the diagram. The abscissae measure in millimetres the original length to be repro- duced, as indicated upon the broken line as well as upon the top and bottom. The curves follow the direction of the arrows. 1 always indicates the Eye, 2 the Hand, and 3 the Arm. 1-1, the curve for receiving the impression and expressing the judgment by the Eye. 2-2, the curve for receiving the impression and expressing the judgment by the Hand. 3-3, the curve for receiving the impression and expressing the judgment by the Arm. 2-1, the curve for receiving by the Hand and expressing by the Eye. 3-1, the Arm " the Eye. 1-2, 3-2, 1-3, 2-3, the Eye the Arm the Eye the Hand the Hand, the Hand, the Arm. the Arm. Each curve represents the average of experiments upon from three to seven individuals and the result of about 1800 observations. The distinguishing characteristic of a curve is its direction ; whether it proceeds from below the line upward, or from above the line downward. Such features as the extent of the error, or the point at which, if at all, the central line is crossed, show great individual differences. It is only when (as in the above described curves) the absolute error is small throughout, that individual differences appear as curves of opposite direction. The average error in the Eye-judgments was only 3 per cent., in the Hand-judg- ments 9 per cent., in the Arm-judgments 10 per cent. When either the Hand or the Arm acted as both the receiving and expressing sense, the two sides of the body were used at once. What now would be the effect of interchanging the hands, mak- ing the receiving hand in the first case the expressing hand in the second ? If it were only to alter slightly the extent of the error, it would indicate an accidental variation or a slight superiority of one hand. If, however, this change were to reverse the former result, it would indicate a lack of symmetrical correlation of the two sides of the body. In point of fact the latter is the result. When the rigid Hand is the expressing hand, the curve agrees with the curve 2-2 (Fig. 5) ; when the left hand is the expressing hand, the curve is irreyular and below the central line. When the left Arm is the expressing arm, the curve agrees with the curve 3-3 (Fig. 5) ; when the right arm is the expressing arm, the curve is irrer/ular and below the line. The next question is, Which of these two curves is to be preferred ? It is answered thus : If instead of using both hands simultaneously, the same hand were used suc- cessively, and it were found that the little difference resulted from interchanging the hands (or arms), the curve for that hand