20, and, on the
other hand movements in the rhythm of 200 almost as accurate as those
of 140 to the minute. Thus we have a lower limit below which decrease
of rapidity does not increase the accuracy any further, and an upper
limit beyond which a further increase of rapidity brings no
additional deterioration. The mistakes of the unskilled left hand
increase still more rapidly than the number of movements. If the eyes
are closed, the rapid movements are usually too long and the slow ones
too short.

An investigation in the Harvard laboratory varied this problem in a
direction which brings it still nearer to technical conditions of
industry. Our central question was whether the greatest exactitude of
rhythmical movement is secured at the same rapidity for different
muscle groups.[36] We studied especially rhythmical movements of hand,
foot, arm, and head, and studied them, moreover, under various
conditions of resistance. The result from 340,000 measured movements
was the demonstration that every muscle group has its own optimum of
rapidity for the greatest possible accuracy and that the complexity of
the movement and the resistance which it finds has most significant
influence on the exactitude of the rhythmical achievement. If we
abstract at first from the fluctuations around the average value of a
particular group of movements and consider only this average itself in
its relation to the starting movement which it is meant to imitate, we
find characteristic tendencies toward enlargement or reduction
dependent upon the rapidity. The right foot, for instance, remained
nearest to the original movement at a rapidity of 80 motions in the
minute, while the head did the same at about 20. For a hand movement
of 14 centimeters, the most favorable rapidity was 120 repetitions in
the minute, while for a hand movement of 1 centimeter the average
remained nearest to the standard at about 40 repetitions. The mean
variation from time average is the smallest for the left foot at 20 to
30 movements, for the right at 160 to 180, for the head at 40, for the
larger hand movement at 180, and so on. Investigations of this kind
have so far not affected industrial life in the least, but it seems
hardly doubtful that a systematic study of the movements necessary for
economic work will have to pass through such strictly experimental
phases. The essential point, however, will be for the managers of the
industrial concerns and the psychological