To be more explicit with regard to the conditions of judgment; two intervals were presented to the subject in immediate succession.  That is, the second stimulation marked the end of the first interval and the beginning of the second.  The first interval was always the standard, while the second, or compared interval, varied in length, as determined by the series of cards, and the subject was requested to judge whether it was equal to, or longer or shorter than the standard interval.

In all of the work under Group 1, and the first work under Group 2, the standard interval employed was 5.0 seconds.  This interval was selected because the minimum variation possible with the pendulum apparatus (1/2 sec.) was too great for the satisfactory operation of a shorter standard, and it was not deemed advisable to keep the subject’s attention on the strain for a longer interval, since 5.0 sec. satisfied all the requirements of the experiment.

In all work here reported, the cork tip on the tapping instrument was circular in form, and 1 mm. in diameter.  In all, except one experiment of the second group, the areas stimulated were on the backs of the fingers, just above the nails.  In the one exception a spot on the forearm was used in conjunction with the middle finger.

In Groups 1 and 2 the intensity of stroke used was just sufficient to give a sharp and distinct stimulation.  The intensity of the stimulation was not of a high degree of constancy from day to day, on account of variations in the electric contacts, but within each test of three stimulations the intensity was constant enough.

In experiments under Group 3 two intensities of strokes were employed, one somewhat stronger than the stroke employed in the other experiments, and one somewhat weaker—­just strong enough to be perceived easily.  The introduction of the two into the same test was effected by the use of an auxiliary loop in the circuit, containing a rheostat, so that the depression of the first key completed the circuit as usual, or the second key completed it through the rheostat.

At each test the subject was warned to prepare for the first stimulation by a signal preceding it at an exact interval.  In experiments with the pendulum apparatus the signal was the spoken word ‘now,’ and the preparatory interval one second.  Later, experiments were undertaken with preparatory intervals of one second and 1-4/5 seconds, to find if the estimation differed perceptibly in one case from that in the other.  No difference was found, and in work thereafter each subject was allowed the preparatory interval which made the conditions subjectively most satisfactory to him.

Ample time for rest was allowed the subject after each test in a series, two (sometimes three) series of twenty to twenty-four tests being all that were usually taken in the course of the hour.  Attention to the interval was not especially fatiguing and was sustained without difficulty after a few trials.