Page:Mind (Old Series) Volume 11.djvu/541

 540 J. JASTEOW : Mr. Spencer and especially Prof. Bain lay most stress on the influence of the muscular sense in forming space conceptions. 1 In order to approach the problem by experimental methods it will be necessary to define accurately such terms as sight, touch, motion. The following classification, though provisional and imperfect, will perhaps be found convenient. We can obtain the notion of extension I. By the stimulation of a definite portion of a sensitive surface, (1) Of the retina (where the distance of the stimulating object must be inferred) ; (2) Of the skin, (a) By the application of a pair of points, leaving the intermediate skin unstimulated, or (a') stimulating it by the application of a straight edge, (b) By the motion of a point along the skin (see MIND 40, pp. 557 ff.)j [(a) and (b) may be contrasted as simultaneous and successive.] II. By the perception of distance between two movable parts of the body, e.g., between thumb and forefinger ; III. By the free motion of a limb, e.g., the arm. 2 The operations to be known as reproducing judgments by the Eye, the Hand and the Arm, are respectively judging lengths by fixating the eyes upon them without motion of the eyeball, a form of I. (1) ; judging distances between thumb and forefinger, a form of II.; and judging distances by guiding a pencil over them with a free arm-movement, a form of III. The problem was to compare the judgments of linear extension made by these three senses, and to determine their relative accuracy. The method consisted in presenting a definite length to one of these senses of the subject, who was then required to adjust a second length equal to the first by the use of the same or of another sense. The judgments were confined to lengths between 5 and 120 mm. The lower limit is set by the incon- venience of seeing, drawing and measuring such small lines ; the upper by the greatest ' span ' between thumb and forefinger, as 1 For an account of this whole subject see Common Sensiblcs by Dr. Theodor Loewy, Leipzig, 1884 ; and X'njht and Touch by T. K. Abbott 2 To this list ought perhaps to be added the perception of distance by a passive moving of the body, as in riding. (See Much, Bewywigs-Empfin- dungen.) In all the above processes variations may be made. In I. (1) motion of the eyeball may be admitted or excluded; direct or indirect vision with one or with both eyes. In II. one part may lie movable, the other not, as in estimating distances between the linger and 1'orcliead. In the varia- tion used, the sensations are due to the tensing of muscles and skin.