Page:Mind (Old Series) Volume 11.djvu/552

 THE PEKCEPTION OF SPACE BY DISPAEATE SENSES. 551 senses are the same, the error of the Eye is smaller than that of the Hand, and of the latter slightly smaller than that of the Arm ; (2) when the receiving and expressing senses are different, the same holds true, if we regard the expressing sense as the charac- teristic one; (3) in expressing and judging inches, the order of accuracy is also Eye, Hand, Arm; the two last being nearly alike. Prof. Bowditch and Mr. Southard 1 have compared Sight and the Motion-sense by placing a small disc upon a table and requiring the subject (with eyes closed) to guide a pencil to the disc. In one set of experiments the knowledge of the position of the disc was obtained by glancing at it, in another set by placing it in position with the hand. They find that the error, i.e., the distance between the disc and the point upon which the pencil was placed, is least when the eye in direct vision is the sense used, and greatest when the opposite hand is used, it being intermediate if the disc and the pencil be placed by the same hand. The effect of the time-interval between placing and finding the disc is about the same in each method, the most accurate adjustments being made with an interval of two seconds. The following experiments were designed to obtain the ratio of erroneous judgments made in deciding which of two slightly different lengths was the longer, when the decision was made by the eye, by the hand and by the arm. For testing Sight two groups of lines, with three lines in each group, were drawn and fastened to the cylinder (Fig. 3) to be viewed successively through the slit in the screen. The lengths of the first lines in the groups were 25 mm. and 100 mm. respectively, the second differing from the first by Jg- and the third by yi^ of the entire length. For testing the sense of Span two precisely similar groups of blocks were constructed and mounted on the car (B, Fig. 1), to be felt successively by thumb and forefinger. For testing the Motion- sense a delicate Motion-triangle was devised and the points recorded on the scale at which the desired differences were reached. An observation was made as follows : For example, we are testing the eye, using the larger lengths (100 mm.) and a. difference of J^. The subject then either (1) sees a line 100 mm. long, then one 98 mm. long, and again one 100 mm. long, or (2) sees a line 98 mm. long, then one 100 mm. long, and again one 98 mm. long. The first and third lengths are always alike, the middle length is either shorter as in the first case, or longer as in the second, and the subject is to decide whether the middle length was shorter or longer. By having two changes one an in- crease, the other a decrease in each experiment, the well-known difference in sensibility for these two kinds of change is avoided ; while the chance of a correct answer is increased. In addition to its correctness or falsity, the confidence felt in the judgment, given was recorded on a scale of 4 degrees, in which 3 denoted 1 Journal of Physiology, iii. 3.