Page:Mind (Old Series) Volume 11.djvu/544

THE PERCEPTION OF SPACE BY DISPARATE SENSES. 543 Fig. 2. The Span-triangle, to be placed upon the car B (Fig. 1). A, A', carriages moving on rollers along the rods B and against the glass edge D ; E, a pair of rods projecting through the carriages A A' and guided in the centre by a wire, seen near the apex ; F, a bent wire recording on the scale H (Fig. 1). Fig. 3. For receiving by the Eye. By revolving the cylinder the lines are seen at A, through the slit B in the screen. Fig. 4. The Motion-triangle. A, a board movable along the grooves C and C, and carrying a scale ; B B, two wooden ledges set in the form of a triangle ; D, the carriage moving along the glass slide E, and its project- ing point striking against B B, seen below, A'. Expressing by the Eye. Parallel to the base of a triangle (also 6 by 20 inches) were drawn a series of lines each differing in length from its neighbour by 1 mm. Each line could be referred to by a letter marked at one end ; and the operation consisted in selecting a line that seemed equal to the impression received. This may be called the Sight-triangle. As this method proved to be somewhat difficult and restricted the judgment, especially in tbe case of short lengths, it was supplemented by marking off on ruled paper as much of the ruled line as seemed equal to the impression received. The hand did not move over the line, but indicated by a stroke the desired length. B'. Expressing by the Hand. The apparatus (Fig. 2) is called the Span-triangle, and like the others is 6 by 20 inches in size. It consists of an upper part (D) and a lower part (B), joined at the apex and at the base, with an interval of about f inch between the two. A pair of brass carriages (A, A'), triangular in shape and complementary to half of the large triangle, move on rollers along the rods (B) and the glass slip (D). A pair of rods (E) projecting on either side (through the interval between the two parts) is guided by a wire stretched from the apex to the middle of the base of the triangle, and keeps the carriages parallel and opposite to one another. An indicator (F) records the distance between the carriages at any point, upon a scale (H, Fig. 1). The carriages are grasped between the thumb and forefinger and adjusted by an easy motion up or down. C'. Expressing by the Arm. A duplicate of the Motion-triangle above described is used, and, when the subject receives with one arm and expresses with the other, is regulated by the operator at the command of the subject. A handier method (especially applicable in successive judgments) is shown in Fig. 1'. It con- sists in drawing, with eyes closed, a line (E) along a straight edge (F). By moving the board upon which the paper is attached after each line, a series of lines is obtained. Results. In judging that a length perceived by the Eye is equal to another length perceived either by the Eye, Hand or Arm, there will be an error. The problem consists in tracing the nature and extent of this error.