248 MEMORIES OF MY LIFE
their ears with their paws, just as the persons of whom I have spoken often did with their hands.
It was difficult to find a simple machine that would register the length of Reaction Time-that is, the interval between a Stimulus and the Response to it, say between a sharp sound and the pressure of a responding finger on a key., I first used one of Exner's earlier instruments, but it took too much time, so I subsequently made one with a pendulum. The tap that released the pendulum from a raised position made the required sound,-otherwise it made a quiet sight-signal, whichever was wished,-and the responding finger caused an elastic thread parallel to the pendulum and swinging with it to be clutched
and held fast, in front of a scale, graduated to 11,,ths of a second. This acted well ; there was no jar from seizing the elastic thread, and the adjustments gave no trouble.
For testing the Muscular Sense, I used cartridges packed evenly with cotton wool and with shot, so as to be exactly alike on the outsides but of different weights. The weights ran in a regular geometric series, and were broken up into sets of three. Each set lay in a grooved square of wood, in any order ; the test was to arrange them by the sense of their heaviness, in their proper order, as shown by the inscriptions at one end of each. This method acted quickly, because it was easy to judge by the sometimes hesitating, sometimes decided manner in which a particular set was handled, whether or no the differences were clearly perceived, and to substitute others in turn more appropriate to the acuteness of sense of the person tested.