Sixteen Experimental Investigations from the Harvard Psychological Laboratory by Hugo Münsterberg (100 books to read .txt) 📕

open surface, the subject R 3.6, etc.’ Each number is the

average of five judgments. In Table IX. the points were set at

regular intervals. In Table X. the filling was made irregular

by having some points rougher than the others and set at

different intervals.

I can give here only a very brief summary of the results with this

apparatus. In Tables IX. and X. I give a few of the figures which will

show the tendency of the experiments. In these tests a different

length and a different filling were given for each judgment. The

result of the experiments of this group is, first, that the _shorter

filled spaces are judged longer and the longer spaces shorter_ than

they really were. Second, that an increase in the number of points in

the filled space causes no perceptible change in the apparent length.

Third, that when the filling is so arranged as to produce a tactual

rhythm by changing the position or size of every third point, the

apparent length of the space is increased. It will be noticed, also,

that this is just the reverse of the result that was obtained for

passive touch. These facts, which were completely borne out by several

other experiments with different apparatus which I shall describe

later, furnish again a reason why different investigators have

hitherto reported the illusion to exist, now in one direction, now in

the other. Dresslar drew the conclusion from his experiments that the

filled spaces are always overestimated, but at the same time his

figures show an increasing tendency towards an underestimation of the

filled spaces as the distances increased in length. I shall later, in

connection with similar results from other experiments on this

illusion, endeavor to explain these anomalous facts.

In section IV. I mentioned the fact that I found the illusion for

passive touch to be subject to large fluctuations. This is true also

of the illusion for active touch. When the finger-tip is drawn over

the filled, and then out over the open space, the limits between which

the stopping point varies is a much wider range than when the

finger-tip is drawn over two open spaces. In the latter case I found

the variation to follow Weber’s Law in a general way. At first I

thought these erratic judgments were mere guesses on the part of the

subject; but I soon discovered a certain consistency in the midst of

these extreme fluctuations. To show what I mean, I have plotted some

diagrams based on a few of the results for three subjects. These

diagrams are found in Fig. 8. It will be observed that the curve which

represents the collection of stopping points is shorter and higher

where the judgments were on two open spaces. This shows plainly a

greater accuracy in the judgments than when the judgments were on a

filled and an open space, where the curves are seen to be longer and

flatter. This fluctuation in the illusion becomes important in the

theoretical part of my discussion, and, at the risk of apparently

emphasizing unduly an insignificant matter, I have given in Fig. 9 an

exact copy of a sheet of judgments as it came from the apparatus. This

shows plainly how the illusion wears away with practice, when one

distance is given several times in succession. The subject was allowed

to give his judgment on the same distance ten times before passing to

another. A glance at the diagram will show how pronounced the illusion

is at first, and how it then disappears, and the judgment settles down

to a uniform degree of accuracy. It will be seen that the short filled

space is at first overestimated, and then, with the succeeding

judgments, this overestimation is gradually reduced. In the case of

the longer filled distances (which could not be conveniently

reproduced here) the spaces were at first underestimated, and then

this underestimation slowly decreased.

[Illustration: FIG. 8.]

[Illustration: FIG. 9.]

None of the qualitative studies that have hitherto been made on this

illusion have brought to light this significant wearing away of the

illusion.

VII.

I have already spoken of the defects of the apparatus with which the

experiments of the previous chapter were made. I shall now give an

account of some experiments that were made with an apparatus designed

to overcome these difficulties. This is shown in Fig. 10. The block

C was clamped to a table, while the block A could be moved back

and forth by the lever B, in order to bring up different lengths of

filled space for judgment. For each judgment the subject brought his

finger back to the strip D, and by moving his finger up along the

edge of this strip he always came into contact with the first point of

the new distance. The lever was not used in the present experiment;

but in later experiments, where the points were moved under the finger

tip, which was held stationary, this lever was very useful in

producing different rates of speed. In one series of experiments with

this apparatus the filled spaces were presented first, and in another

series the open spaces were presented first. In the previous

experiments, so far as I have reported them, the filled spaces were

always presented first.

[Illustration: FIG. 10.]

In order to enable the subject to make proper connections with the

first point in the filled space, when the open space was presented

first, a slight depression was put in the smooth surface. This

depression amounted merely to the suggestion of a groove, but it

sufficed to guide the finger.

The general results of the first series of experiments with this

apparatus were similar to those already given, but were based on a

very much larger number of judgments. They show at once that the short

filled spaces are overestimated, while the longer spaces are

underestimated. The uniformity of this law has seemed to me one of the

most significant results of this entire investigation. In the results

already reported from the experiments with the former apparatus, I

have mentioned the fact that the judgments upon the distances

fluctuate more widely when one is filled and the other open, than when

both are open. This fluctuation appeared again in a pronounced way in

the present experiments. I now set about to discover the cause of this

variation, which was so evidently outside of the limits of Weber’s

law.

TABLE XI.

I. II.

Subjects. R. B. A. R. B. A.

2= 3.1 3.2 3.7 2.7 2.5 3.1

3= 4.5 4.4 4.1 4.1 4.0 3.6

4= 5.3 5.0 4.3 4.2 4.6 4.6

5= 6.0 5.1 5.8 5.9 5.2 4.3

6= 6.8 5.6 6.2 6.9 5.3 6.0

7= 7.4 7.2 6.9 7.6 7.3 6.8

8= 8.1 8.4 7.3 8.3 9.7 7.8

9= 9.3 9.0 8.5 9.5 8.9 8.7

Filled 10= 10.1 10.0 8.1 10.3 10.0 9.2

Spaces. 11= 10.5 9.3 9.7 10.6 8.7 9.6

12= 11.7 10.6 10.6 11.8 9.7 10.2

13= 12.3 10.9 10.9 11.1 10.2 9.6

14= 12.2 11.5 12.2 10.4 9.6 11.3

15= 13.6 12.3 11.9 13.1 10.1 9.6

16= 14.1 13.5 14.1 12.3 13.2 13.3

17= 14.9 12.9 14.6 14.1 12.6 13.7

18= 15.0 15.3 14.9 15.0 15.3 13.8

19= 15.2 14.6 15.2 14.1 13.9 14.2

20= 17.1 16.5 15.7 16.1 16.4 14.7

The first line of group I. reads: ‘When the finger-tip was

passed over a filled space of 2 cm., the subject R measured

off 3.1 cm. on the open space, the subject B 3.2 cm., and

the subject A 3.7.’ In group II., the numbers represent the

distance measured off when both spaces were unfilled.

In my search for the cause of the variations reported previously I

first tried the plan of obliging the subject to attend more closely to

the filled space as his finger was drawn over it. In order to do this,

I held a piece of fine wire across the line of the filled space, and

after the subject had measured off the equal open space he was asked

to tell whether or not he had crossed the wire. The wire was so fine

that considerable attention was necessary to detect it. In some of the

experiments the wire was inserted early in the filled space, and in

some near the end. When it was put in near the beginning, it was

interesting to notice, as illustrating the amount of attention that

was being given to the effort of finding the wire, that the subject,

as soon as he had discovered it, would increase his speed, relax the

attention, and continue the rest of the journey more easily.

The general effect of this forcing of the attention was to increase

the apparent length of the filled space. This conclusion was reached

by comparing these results with those in which there was no compelled

attention. When the obstacle was inserted early, the space was judged

shorter than when it came at the end of the filled space. This shows

very plainly the effect of continued concentration of attention, when

that attention is directed intensely to the spot immediately under the

finger-tip. When the attention was focalized in this way, the subject

lost sight of the space as a whole. It rapidly faded out of memory

behind the moving finger-tip. But when this concentration of attention

was not required, the subject was able to hold together in

consciousness the entire collection of discrete points, and he

overestimated the space occupied by them. It must be remembered here

that I mean that the filled space with the focalized attention was

judged shorter than the filled space without such concentration of

attention, but both of these spaces were judged shorter than the

adjacent open space. This latter fact I shall attempt to explain

later. Many other simple devices were employed to oblige the subject

to fix his attention on the space as it was traversed by the finger.

The results were always the same: the greater the amount of attention,

the longer the distance seemed.

In another experiment, I tried the plan of tapping a bell as the

subject was passing over the filled space and asking him, after he had

measured off the equivalent open space, whether the sound had occurred

in the first half or in the second half of the filled space.

When the finger-tip was drawn over two adjacent open spaces, and

during the first a bell was tapped continuously, this kind of filled

space was underestimated if the distance was long and overestimated if

the distance was short. So, too, if a disagreeable odor was held to

the nostrils while the finger-tip was being drawn over one of the two

adjacent open spaces, the space thus filled by the sensations of smell

followed the law already stated. But if an agreeable perfume was used,

the distance always seemed shorter than when an unpleasant odor was

given.

In all of these experiments with spaces filled by means of other than

tactual sensations, I always compared the judgment on the filled and

open spaces with judgments on two open spaces, in order to guard

against any error due to unsymmetrical, subjective conditions for the

two spaces. It is difficult to have the subject so seat himself before

the apparatus as to avoid the errors arising from tension and flexion.

In one experiment, a piece of plush was used for the filled space and

the finger drawn over it against the nap. This filled space was judged

longer than a piece of silk of equal length. The sensations from the

plush were very unpleasant. One subject said, even, that they made him

shudder. This was of course precisely what was wanted for the

experiment. It showed that