Page:Mind (Old Series) Volume 11.djvu/542

 THE PERCEPTION OF SPACE BY DISPARATE SENSES. 541 well as by the longest line distinctly visible without motion of the eyeball. More direct methods of testing the relative fineness of these senses and of their memory for absolute lengths were also employed. In several of the operations the two sides of the body were involved, and it became necessary to study the effect of this circumstance. Method and Apparatus. The sense which receives the linear impression is termed the receiving sense ; that which expresses it by a length that is judged to be equal to the first, the expressing sense. If, for example, I look at a line and (without seeing my arm) draw a second line that seems equal in length to the one looked at, I am expressing in terms of motion an impression received in terms of sight, or re- producing sight by motion, the eye by the arm. Unless otherwise stated the operations are to be considered as simultaneous, the receiving and expressing senses acting at the same time, and the attention probably flitting from one to the other. The method and apparatus are equally suitable for successive judgments, which were made in some cases to obviate the use of both sides of the body. From a description of the method by which (1) an impres- sion was received and (2) expressed by each sense, it will be easy to infer the method of reproducing any one of the three senses by itself or by any other. A. Receiving by tJie Eye. A series of lines varying in length from 5 to 120 mm. were drawn upon cardboard and this attached to a horizontal cylinder (Fig. 3). Above this was a screen, through a slit in which any of the lines could be made to appear. The lines were seen perpendicularly to the axis of the eyes, and from above at an angle of about 30. B. Receiving by the Hand. Any one of a series of wooden blocks, 1 inches wide and varying in length from 5 to 120 mm., with slips of glass cemented on each end to ensure equality of surface and temperature, was grasped in a convenient position between the thumb and forefinger of either hand. This gave a definite sensa- tion of a length ' spanned '. To ensure regularity in the opera- tion, the block (D, Fig. 1) was mounted on a stand, and the latter fastened in the grooves of the car (B, Fig. 1). C. Receiving l>y the Arm. The apparatus used (Fig. 4) is called the Motion-triangle, because the amount of motion is regulated by the distance between two ledges set in the form of a triangle, the motion being parallel to its base. The hand grasps a cork carriage (D), through which the point of a glass pencil projects downward, to be stopped at each end of its course by the ledges (B B). Any desirable length of movement is obtained by simply sliding the board (A), upon which the ledges are fixed, in or out, along the grooves (C C), the length being indicated on a scale attached to the board. The base of the triangle is 6 inches