第42章

Wundt found that twelve impressions could be distinguished clearly as a united cluster, provided they were caught in a certain rhythm by the mind, and succeeded each other at intervals not smaller than 0.3 and not larger than 0.5 of a second.This makes the total time distinctly apprehended to be equal to from 3.6 to 6 seconds.

Dietze gives larger figures.The most favorable intervals for clearly catching the strokes were when they came at from 0.3 second to 0.18 second apart.Forty strokes might then be remembered as a whole, and identified without error when repeated, provided the mind grasped them in five sub-groups of eight, or in eight sub-groups of five strokes each.When no grouping of the strokes beyond making couples of them by the attention was allowed -- and practically it was found impossible not to group them in at least this simplest of all ways --16 was the largest number that could be clearly apprehended as a whole. This would make 40 times 0.3

second, or 12 seconds, to be the maximum filled duration of which we can be both distinctly and immediately aware.

The maximum unfilled, or vacant duration , seems to lie within the same objective range.Estel and Mehner, also working in Wundt's laboratory, found it to vary from 5 or 6 to 12 seconds, and perhaps more.The differences seemed due to practice rather than to idiosyncrasy.

These figures may be roughly taken to stand for the most important part of what, with Mr.Clay, we called, a few pages back, the specious present.

The specious present has, in addition, a vaguely vanishing backward and forward fringe; but its nucleus is probably the dozen seconds or less that have just elapsed.

If these are the maximum, what, then, is the minimum amount of duration which we can distinctly feel?

The smallest figure experimentally ascertained was by Exner, who distinctly heard the doubleness of two successive clicks of a Savart's wheel, and of two successive snaps of an electric spark, when their interval was made as small as about 1/500 of a second.

With the eye, perception is less delicate.Two sparks, made to fall beside each other in rapid succession on the centre of the retina, ceased to be recognized as successive by Exner when their interval fell below 0.044".

Where, as here, the succeeding impressions are only two in number, we can easiest perceive the interval between them.President Hall, who experimented with a modified Savart's wheel, which gave clicks in varying number and at varying intervals, says:

"In order that their discontinuity may be clearly perceived, four or even three clicks or beats must be farther apart than two need to be.When two are easily distinguished, three or four separated by the same interval...are often confidently pronounced to be two or three respectively.

It would be well if observations were so directed as to ascertain, at least up to ten or twenty, the increase required by each additional click in a series for the sense of discontinuity to remain constant throughout."

Where the first impression falls on one sense, and the second on another, the perception of the intervening time tends to be less certain and delicate, and it makes a difference which impression comes first.Thus, Exner found the smallest perceptible interval to be, in seconds: From sight to touch..................0.071

From touch to sight..................0.053

From sight to hearing................0.16

From hearing to sight................0.06

From one ear to another...........0.064 To be conscious of a time interval at all is one thing; to tell whether it be shorter or longer than another interval is a different thing.

A number of experimental data are on hand which give us a measure of the delicacy of this latter perception.The problem is that of the smallest difference between two times which we can perceive.

The difference is at its minimum when the times themselves are very short.Exner, reacting as rapidly as possible with his foot, upon a signal seen by the eye (spark), noted all the reactions which seemed to him either slow or fast in the making.He thought thus that deviations of about 1/100 of a second either way from the average were correctly noticed by him at the time.The average was here 0.1840".Hall and Jastrow listened to the intervals between the clicks of their apparatus.Between two such equal intervals of 4.27" each, a middle interval was included, which might be made either shorter or longer than the extremes."After the series had been heard two or even three times, no impression of the relative length of the middle interval would often exist, and only after hearing the fourth and last would the judgment incline to the plus or minus side.Inserting the variable between two invariable and like intervals greatly facilitated judgment, which between two unlike terms is far less accurate." Three observers in these experiments made no error when the middle interval varied 1/60

from the extremes.When it varied 1/120, errors occurred, but were few, This would make the minimum absolute difference perceived as large as 0.355."

This minimum absolute difference, of course, increases as the times compared grow long.Attempts have been made to ascertain what ratio it bears to the times themselves.According to Fechner's 'Psychophysic Law' it ought always to bear the same ratio.Various observers, however, have found this not to be the case. On the contrary, very interesting oscillations in the accuracy of judgment and in the direction of the error -- oscillations dependent upon the absolute amount of the times compared -- have been noticed by all who have experimented with the question.

Of these a brief account may be given.