The auditory stimulus, a click, adjustable to any part of the series, was made as follows: A wooden shaft, mounted on the same axle as the discs, and beyond the discs from the observer, could be rotated freely around the axle when the nut securing it was loosened. This shaft extended beyond the edge of the disc. It carried a copper wire which was in contact with the axle. A mercury cup was placed on the table, upon which the machine rested, in such position that the copper tip passed through the mercury when the discs rotated. It was thus a very simple matter to connect an electric sounder so that it would click every time the circuit was made by the copper passing through the mercury. And, by the adjustment of the wooden shaft, the click was readily placed anywhere in the visual series.
As already suggested above, the length of interval between members of the visual series, and also the time between clicks, seem to be important factors in determining the amount, and perhaps also the direction of the displacement. Bessel found his personal equation was considerably diminished when he used a clock marking half-seconds instead of one marking seconds. Wolf also diminished his error by using a clock beating one hundred times a minute instead of one beating seconds, which he was accustomed to use. Wundt found his customary negative displacement on the pendulum apparatus (coördinating the sound with a position of the index earlier than that with which it was actually simultaneous) disappeared when he had members of the visual series one thirty-sixth second apart and the auditory stimuli one second apart. It seemed important at the outset, therefore, to determine, if possible, the effects of each of these factors.
BOTH INTERVALS PROGRESSIVELY VARIED
In each experiment the observer was allowed to observe as many complications (coincidences of click and letter) as he desired, in order to assure himself of his judgment. The experimenter counted and recorded the number observed in each experiment. Experiments were made in series of ten. Six different combinations of intervals were used in this first group of experiments. The auditory intervals (time between successive clicks) and visual intervals (time between successive members of the visual series) are given at the tops of the columns in Table I. This table is a summary presentation of the results of this group. There were three observers. During each hour of experimentation with a given observer, at least one series with each of the first four time-interval combinations was tried out. "Aver. num. Trials" means the average number of complications observed in the whole number of tests averaged. "Num. Series av." means the number of series of ten experiments each averaged to give the displacement results below. "Aver. Error" is the average of all the displacements of the auditory impression, irrespective of the direction of the displacement. "Mean Displacement" is the actual mean displacement as obtained by dividing the algebraic sum of all displacements, positive and negative, by the number of experiments. The plus sign indicates a positive displacement, and the minus sign, a negative. Negative and positive are here used in the sense customary in similar experiments,—namely, the click, being heard as simultaneous with a visual impression which actually came before it, was said to be displaced negatively, and the click, being heard as simultaneous with a visual impression coming in fact later than it did, was said to be displaced positively. Average errors and mean displacements are given in the table in thousandths of seconds. Observers were asked to locate the click in the visual series in terms of one tenth the distance or time between the letters.
TABLE I
| Aud. Interval (sec.) | 1.28 | 2.56 | 4.04 | 8.40 | 1.28 | 2.02 | |
| Vis. Interval (sec.) | .040 | .080 | .120 | .260 | .080 | .120 | |
| Obs. | |||||||
| B | Av. num. Trials | 13.9 | 5.8 | 3.8 | 2.1 | 9.8 | 13.9 |
| Num. Series av. | 8 | 13 | 13 | 8 | 2 | 2 | |
| Aver. Error (sec.) | .056 | .064 | .077 | .164´ | .045 | .067 | |
| Mean Displac'mt (sec.) | +.045 | -.040 | -.067 | -.152 | +.045 | +.067 | |
| Bo | Av. num. Trials | 9.4 | 4.1 | 3.0 | 2.0 | 5.5 | 3.5 |
| Num Series av. | 6 | 10 | 11 | 8 | 3 | 2 | |
| Aver. Error. (sec.) | .114 | .060 | .054 | .049 | .05 | .082 | |
| Mean Displac'mt (sec.) | +.114 | +.045 | +.033 | .000 | +.045 | +.082 | |
| M | Av. num. Trials | 6.3 | 3.1 | 2.5 | 2.2 | 5.3 | 4.4 |
| Num. Series av. | 9 | 12 | 12 | 10 | 3 | 2 | |
| Aver. Error (sec.) | .09 | .07 | .076 | .110 | .067 | .172 | |
| Mean Displac'mt (sec.) | +.089 | -.058´ | -.058 | -.104 | +.062 | +.168 |
Transcriber Note
(sec.) moved into row heading from main body of the table.
Obs B row, 13.9 sec., taken out sec.