Another type of instrument is represented by the stopwatch, of which the most perfect form is Hipp's Chronoscope. The hand on the dial measures intervals as short as 1/1000 of a second. The signal (by an appropriate electric connection) starts it; the reaction stops it; and by reading off its initial and terminal positions we have immediately and with no farther trouble the time we seek. A still simpler instrument, though one not very satisfactory in its working, is the 'psychodometer' of Exner & Obersteiner, of which I picture a modification devised by my colleague Professor H. P. Bowditch, which works very well. [Illustration: FIG. 22.--Bowditch's Reaction-timer. _F_, tuning-fork carrying a little plate which holds the paper on which the electric pen _M_ makes the tracing, and sliding in grooves on the base-board. _P_, a plug which spreads the prongs of the fork apart when it is pushed forward to its extreme limit, and releases them when it is drawn back to a certain point. The fork then vibrates, and, its backward movement continuing, an undulating line is drawn on the smoked paper by the pen. At _T_ is a tongue fixed to the carriage of the fork, and at _K_ an electric key which the tongue opens and with which the electric pen is connected. At the instant of opening, the pen changes its place and the undulating line is drawn at a different level on the paper. The opening can be made to serve as a signal to the reacter in a variety of ways, and his reaction can be made to close the pen again, when the line returns to its first level. The reaction time = the number of undulations traced at the second level.] The manner in which the signal and reaction are connected with the chronographic apparatus varies indefinitely in different experiments. Every new problem requires some new electric or mechanical disposition of apparatus.[106] The least complicated time-measurement is that known as _simple reaction-time_, in which there is but one possible signal and one possible movement, and both are known in advance. The movement is generally the closing of an electric key with the hand. The foot, the jaw, the lips, even the eyelid, have been in turn made organs of reaction, and the apparatus has been modified accordingly.[107] The time usually elapsing between stimulus and movement lies between one and three tenths of a second, varying according to circumstances which will be mentioned anon.