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

When the pendulum is not swinging, a hole in the shield lies behind

ON and exactly corresponds with it. Another in the background does

the same. The eye can thus see straight through to the light L.

Each of these three holes has grooves to take an opaque card, x,

y, or z; there are two cards for the three grooves, and they are

pierced with holes to correspond to i and tt of Fig. 4. The

background BB has a second groove to take a piece of milk-glass M.

These cards are shown in Fig. 6 (Plate II.) Card I bears a hole 5

cm. high and shaped like a dumb-bell. The diameter of the end-circles

(e, e) is 1.3 cm., and the width of the handle h is 0.2 cm. Card

T is pierced by two slits EE, EE, each 9 cm. long and 1.3 cm.

high, which correspond to the two ends of the dumb-bell. These slits

are connected by a perforation H, 1.5 cm. wide, which corresponds to

the handle of the dumb-bell. This opening EEHEE is covered by a

piece of ground-glass which serves as a radiating surface for the

light.

[Illustration: Fig. 5.]

The distance EA (Fig. 5) is 56 cm., and PP‘ is 40 cm.; so that the

arc of eye-movement, that is, the angle PEP‘, is very nearly 40°,

of which the 9-cm. opening ON 9° 11’. SS is 2 cm. behind ON, and

BB 2 cm. behind SS; these distances being left to allow the

pendulum to swing freely.

It is found under these conditions that the natural speed made by the

eye in passing the 9-cm. opening ON is very well approximated by the

pendulum if the latter is allowed to fall through 23.5° of its arc,

the complete swing being therefore 47°. The middle point of the

pendulum is then found to move from O to N in 110[sigma][19]. If

the eye sweeps from O to N in the same time, it will be moving at

an angular velocity of 1° in 11.98[sigma] (since the 9 cm. are 9° 11’

of eye-movement). This rate is much less than that found by Dodge and

Cline (op. cit., p. 155), who give the time for an eye-movement of

40° as 99.9[sigma], which is an average of only 2.49[sigma] to the

degree. Voluntary eye-movements, like other voluntary movements, can

of course be slow or fast according to conditions. After the pendulum

has been swinging for some time, so that its amplitude of movement has

fallen below the initial 47° and therewith its speed past the middle

point has been diminished, the eye in its movements back and forth

between the fixation-points can still catch the after-image of i

perfectly distinct and not at all horizontally elongated, as it would

have to be if eye and pendulum had not moved just together. It appears

from this that certain motives are able to retard the rate of

voluntary movements of the eye, even when the distance traversed is

constant.

[19] The speed of the pendulum is measured by attaching a

tuning-fork of known vibration-rate to the pendulum, and

letting it write on smoked paper as the pendulum swings past

the 9-cm. opening.

The experiment is now as follows. The room is darkened. Card T is

dropped into groove z, while I is put in groove y and swings

with the pendulum. One eye alone is used.

Case 1. The eye is fixed in the direction EA. The pendulum is

allowed to swing through its 47°. The resulting visual image is shown

in Fig. 7:1. Its shape is of course like T, Fig. 6, but the part H

is less bright than the rest because it is exposed a shorter time,

owing to the narrowness of the handle of the dumb-bell, which swings

by and mediates the exposure. Sheets of milk-glass are now dropped

into the back groove of BB, until the light is so tempered that

part H (Fig. 7:1) is barely but unmistakably visible as luminous.

The intensity actually used by the writer, relative to that of EE,

is fairly shown in the figure. (See Plate III.)

It is clear, if the eye were now to move with the pendulum, that the

same amount of light would reach the retina, but that it would be

concentrated on a horizontally narrower area. And if the eye moves

exactly with the pendulum, the visual image will be no longer like 1

but like 2 (Fig. 7). We do not as yet know how the intensities of e,

e and h will relatively appear. To ascertain this we must put card

I into groove x, and let card T swing with the pendulum in

groove y. If the eye is again fixed in the direction EA (Fig. 5),

the retina receives exactly the same stimulation that it would have

received before the cards were shifted if it had moved exactly at the

rate of the pendulum. In the experiments described, the handle h of

this image (Fig. 7:2) curiously enough appears of the same brightness

as the two ends e, e, although, as we know, it is stimulated for a

briefer interval. Nor can any difference between e, e and h be

detected in the time of disappearance of their after-images. These

conditions are therefore generous. The danger is that h of the

figure, the only part of the stimulation which could possibly quite

elapse during the movement, is still too bright to do so.

Case 2. The cards are replaced in their first positions, T in groove

z, I in groove y which swings. The subject is now asked to make

voluntary eye-sweeps from P to P’ and back, timing his moment of

starting so as to bring his axis of vision on to the near side of

opening ON at approximately the same time as the pendulum brings I

on the same point. This is a delicate matter and requires practice.

Even then it would be impossible, if the subject were not allowed to

get the rhythm of the pendulum before passing judgment on the

after-images. The pendulum used gives a slight click at each end of

its swing, and from the rhythm of this the subject is soon able to

time the innervation of his eye so that the exposure coincides with

the middle of the eye-movement.

[Illustration: PSYCHOLOGICAL REVIEW. MONOGRAPH SUPPLEMENT, 17. PLATE III.

Fig. 7.

HOLT ON EYE-MOVEMENT.]

It is true that with every swing the pendulum moves more slowly past

ON, and the period of exposure is lengthened. This, however, only

tends to make the retinal image brighter, so that its disappearance

during an anæsthesia would be so much the less likely. The pendulum

may therefore be allowed to ‘run down’ until its swing is too slow for

the eye to move with it, that is, too slow for a distinct,

non-elongated image of i to be caught in transit on the retina.

With these eye-movements, the possible appearances are of two classes,

according to the localization of the after-image. The image is

localized either at A (Fig. 5), or at the final fixation-point (P

or P’, according to the direction of the movement). Localized at

A, the image may be seen in either of two shapes. First, it may be

identical with 1, Fig. 7. It is seen somewhat peripherally, judgment

of indirect vision, and is correctly localized at A. When the

subject’s eye is watched, it is found that in this case it moved

either too soon or too late, so that when the exposure was made, the

eye was resting quietly on one of the fixation-points and so naturally

received the same image as in case 1, except that now it lies in

indirect vision, the eye being directed not toward A (as in case 1)

but towards either P or P’.

Second, the image correctly localized may be like 2 (Fig. 7), and then

it is seen to move past the opening ON. The handle h looks as

bright as e, e. This appearance once obtained generally recurs

with each successive swing of the pendulum, and scrutiny of the

subject’s eye shows it to be moving, not by separate voluntary

innervations from P to P’ and then from P’ to P, but

continuously back and forth with the swing of the pendulum, much as

the eye of a child passively follows a moving candle. This movement is

purely reflex,[20] governed probably by cerebellar centers. It seems

to consist in a rapid succession of small reflex innervations, and is

very different from the type of movement in which one definite

innervation carries the eye through its 42°, and which yielded the

phenomena with the perimeter. A subject under the spell of this reflex

must be exercised in innervating his eye to move from P to P’ and

back in single, rapid leaps. For this, the pendulum is to be

motionless and the eye is not to be stimulated during its movement.

[20] Exner, Sigmund, _Zeitschrift f. Psychologie u. Physiologie

der Sinnesorgane_, 1896, XII., S. 318. ‘Entwurf zu einer

physiologischen Erklärung der psychischen Erscheinungen,’

Leipzig u. Wien, 1894, S. 128. Mach, Ernst, ‘Beiträge zur

Analyse der Empfindungen,’ Jena, 1900, S. 98.

These two cases in which the image is localized midway between P and

P’ interest us no further. Localized on the final fixation-point,

the image is always felt to flash out suddenly in situ, just as in

the case of the ‘correctly localized’ after-image streaks in the

experiments with the perimeter. The image appears in one of four

shapes, Fig. 7: 2 or 3, 4 or 5.

First, the plain or elongated outline of the dumb-bell appears with

its handle on the final fixation-point (2 or 3). The image is plain

and undistorted if the eye moves at just the rate of the pendulum,

elongated if the eye moves more rapidly or more slowly. The point that

concerns us is that the image appears with its handle. Two

precautions must here be observed.

The eye does not perhaps move through its whole 42°, but stops instead

just when the exposure is complete, that is, stops on either O or

N and considerably short of P or P’. It then follows that the

exposure is given at the very last part of the movement, so that the

after-image of even the handle h has not had time to subside. The

experiment is planned so that the after-image of h shall totally

elapse during that part of the movement which occurs after the

exposure, that is, while the eye is completing its sweep of 42°, from

O to P, or else from N to P’. If the arc is curtailed at point

O or N, the handle of the dumb-bell will of course appear. The

fact can always be ascertained by asking the subject to notice very

carefully where the image is localized. If the eye does in fact stop

short at O or N, the image will be there localized, although the

subject may have thoughtlessly said before that it was at P or P’,

the points he had nominally