394 L. T. STEVENS : making, at the same time, a sharp click. What influence the introduction of muscular force into the experiment would exert upon the time-sense, beyond that of fatigue, is not definitely known ; and the same can be said for the influence of sensory impressions. But it was desirable to exclude, as far as was possible, all probably complicating conditions from the method, and, consequently, for the Morse's key was substituted the com- pound lever, which worked noiselessly and with the least possible resistance. The individual under experiment tapped the lever synchronously with the beats of a metronome. When he had become perfectly familiar with the given interval, the drum was set in motion and the first round of the tracing was taken, with the metronome still beating ; the latter was then stopped, while the person kept on tapping the lever at the same rate. The average of the inter- vals recorded in the first line of the tracing was the standard time ; the rest of the tracings gave successive reproductions of this standard. At first, the duration of each experiment was from two to three minutes, but this brought on fatigue so rapidly that it was impos- sible to obtain more than four or five experiments at one sitting. It was, therefore, reduced to one minute, under which condition ten or even more could be readily made at one time. In the dis- cussion of the results, the reproductions for the first minute only will be considered in those cases where the experiments extended beyond that time. Between two successive experiments there was an intermission of at least three minutes ; in the majority of the series its length was from five to ten minutes. In the majority of experiments the standard intervals ranged between '36 sec. and 1'5 sees.; there are several observations for 27 sec., and only one for 2 - 9 sees. Experiments, to the number of 135 in all, were performed upon seven different individuals. Of these, 114 point to this funda- mental principle : That there is an interval of time (the value of which varies between -53 and -87 sec.) which can be reproduced with considerable accuracy ; but with all other intervals an error is made, which is plus for those above and minus for those below the so-called indifference-point. These will be spoken of as the " regular " experiments. The remaining 21 follow no recognised law, and, in contradis- tinction to the 114, will be called the " irregular " experiments. A. Regular Experiment*. The value of the standard interval being from -53 to -87 sec., and the duration of an experiment one minute, each experiment consists in reproducing the standard interval from about 40 to
