Outward Projection of the Visual Image

a. Floating Flakes, — Look toward the sky with your eyelids almost closed and observe a sort of snowfall efFect. Describe it.

Thèse flakes are the shadows, on the retina, of particles floating in the vitreous humor. They are projected asobjects in outer space in accordance with the law of outward projection.!

* Thîs chapter présupposes knowledge of the structure and function of the eye, as outlined in the text-books on psychology.

t" We see indistinct motes floating about in the field of view and slowly gravitating downward. Sometimes they are undulating, transparent tubes, with nucleated cells within; sometimes they are like inextricably tangled threads, or like matted masses of spider's web; sometimes they are slightly darker spots, like faint clouds," (li Conte.)

VISUAL SPACE

6. Shadows of the Retinal Blood-vessels* — Standing in a dark room with one eye closed, wave a candleflame or burning match gently in a small circle close to the side and slightly downward and forward f rom the other eye; look at the opposite wall and you will see a network like Fig. 3.t

Describe size, distance, color if any, stability, etc.

Fig. 3.

The retinal blood-vessels enter at the same point as the optic nerve and spread in a network inside of the retina. As the rods and cônes lie back of this layer, the blood-vessels cast shadows which become visible under the prescribed conditions of illumination. There is nothing on the wall to correspond to this System, yet you see it distinctly ont there in space, in accordance with the law of outward projection.

We are never directly aware of the retinal impression.

* Perform this experiment in the evening, if necessary.

f Do not fatigue the eye. Nothing is gained by straining it. If you do not get the effect at once, it is because it seems unreasonable to look for sueh a thing. Look for a network like Fig. 3, very much enlarged, and you will see it. The experiment shoul^ not require more than a minute or two.

VISUAL SPACE 41

We always see objects out in space. The retinal impression is automatically ref erred to its normal source : that is the law of outward projection. The history of the évolution and development of this tendency constitutes one of the most important chapters in psychology as well as in the theory of knowledge.

The conditions of the above experiments were unusual, if not unnatural, but the mind responded in its habituai way, and this misdirected tendency revealed to us something of the nature of the normal process. The image was laid bare, as it were, by the absence of the object.

The retinal light furnishes us another illustration of this class of entoptic phenomena.* But the best and most serviceable illustration of ail is the after-image, with which we are already familiar. It represents a physiological condition of the retina but is always seen out in space, ne ver within the eye.f

* To observe the retinal light, go into a dark room and cover your eyes so as to shut out ail possibility of objective light. Behold, in a moment you see a gorgeous array of colors in front of the eye. They tend to grow brighter, usually fashion themselves into fantastic designs, and are in a (^ntinual kaleidoscopic commotion. Thèse are nothing but the projection of the local irritation of the retina, chiefly through the circulation of the blood. Thèse retinal lights are the stuff from which many visual dreams are "made".

f Seeing stars" from a blow or fall illustrâtes the same principle. The story is told of a man who was attacked and knocked senseless by a blow on the temple during a pitch-dark night. He accused a neighbor whom he had to confront in court with the évidence, which was essentially this: *'It was pitch-dark, but the moment I felt the blow there was a great flash by the light of which I saw my assailant." This may hâve been naïve testimony: he had seen a light at the proper time; he suspected his neighbor; his conviction was that he had seen his neighbor.

42 VISUAL SPACE

2. The Line of Projection, a. Projection of the After4mage. — Kepeat Exp. 6, Ch. I.

The two spatial factors in the outward projection are distance and direction. We are hère concemed with direction. The law of visible direction may be stated thus: When the rays from any radiant strike the retina, the impression is referred back along the ray-line into space. The retina is a small copy of the plane of projection, a plane across the visual field at right angles to the line of regard. This is illustrated in Fig, 4.

FiG. 4.

Every point in the plane of projection has a corresponding point on the retina. When the retina is stimulated at Cy let us say, the image is referred back along the rayline C'C and the object is seen in that direction. If an area, e.g. b-e, is stimulated, the object will be seen in the corresponding area e-b' in the plane of projection. The experiment shows that the size of the after-

VISUAL SPACE 43

image varies with the distance of the plane of projection. This law of size of the retinal image may be derived f rom the above law of direction. It is illustrated by the four arrows in Fig. 4, which are ail projections of the same image but at différent distances.*

6. The Size of the Retinal Image. — To détermine the length of the retinal image of your pencil held upright at a distance of 50 centimeters in the Une of regard, proceed as foUows: Let the arrow e'6', Fig. 4, represent the pencil, eb its image upon the retina, and n the nodal point in a simple scheme of the eye. The two triangles eb'n and ebn are similar. The distance an in the normal eye is about 16 millimeters; the distance an is by direction 60 centimeters; you measure the length of the pencil. You then get the proportion e'n : e'V : : en : eb, or x, the length of the retinal image. Assume that the retinal image is on a flat surface in order to simplify the équation.

c. The Inversion of the Retinal Image. — Make a pinhole in a card and hold it toward the light about 10 centimeters from the eye. Looking at the pinhole, hold the head of a pin very close to the eye, and in front of the pupil, and observe that you see the pin inverted back of the card.

Pierce five pinholes close together and proceed as before. Kecord how many pins you see.

*To illustrate how lîttle attention we pay to the analysîs ot perceptions, a professor asked a large class in psychology to estimate how large the image of the moon at the horizon would appear if projected upon a plane at arm's length from the eye. The estimâtes of the diameter were nearly ail too large; as, the size of a dollar, a saucer, or a wagon-wheel, but one man said, "The size of a pea/' and he was right.

44 VISUAL SPACE

Examination of Fig. 4 showed that normally ail raya cross at the nodal point in the eye, and this results in the reversai of the image: what is up in the plane of projection becomes down on the retina; and what is right becomes lef t. This is true, however, only when the object is at such a distance that the rays from it can pass properly through the lens and form an image on the retina. Now, in this experiment, the pin was held too close to the eye to allow a clear retinal image, and it was so close as to cast a good shadow upon the retina. This shadow is not inverted on the retina, but it is projected according to the normal law of visible direction for inverted ima:ges, and the resuit is that it is seen inverted in the plane of projection.

Our normal projection of the retinal image is a habit based, indeed, upon neural mechanism and inherited tendency, y et subject to modification by training. If we should wear, for a month, prism glasses which completely and consistently reversed the whole visual field, we should in that time acquire new habits of projection and be able to harmonize the reversed retinal images with the touch and movement expériences. Spaceperception is always a complex process of association and interprétation, though normally extremely abridged.

d. The Muscular Sensations of Position of the Eye, — RoU a sheet of paper into a tube one inch in diameter. Hold your left hand about 12 centimeters in front of your face ; place the tube in front of the right eye, lean it against the hand and point it toward some distant object. Now look with both eyes (in spite of

VISUAL SPACE 45

the fact that the hand is in the way of the left eye) at the distant object and you will see it and a circular section of its surroundings through a round hole in the palm of your hand.

So far, we hâve considered the retinal image as a basis for the perception of direction. But muscular impressions of position and movement of the eyeball are of no less importance. I^ormally the two cooperate. Thus, in perceiving the direction of a flock of birds in the distance, the head turns in the approximate direction, then the eyeballs tum for finer adjustment and sweep back and forth from one object to the other, measuring the angular différence in direction, and finally the local sign * of the retinal image indicates the relative direction of the différent birds in the flock. The perception of form is, of course, merely the perception of a complex system of directions, f

This experiment illustrâtes the rigidity of thèse muscle-sense marks of direction. The hole in the hand is clean-cut and absolute : it is a very striking illusion. The local sign of the retina is so rigid and the position of the eyeball is sensed so accurately that we see the objects upon which the eyeballs were converged in the true direction in spite of the intervening obstacle.

* Local sîgn is that spécial character of a sensation whereby we are enabled to refer it to a particular place, "that differential quality of a sensation which varies with the part of the sensitive surface stimulated, but not with the nature of the stimulus." (Stout.)

f Normal ly we are not conscious of either location of the retinal image or the sensations of muscular adjustment in the eye. The process of vision has become so abbreviated and automatic that we merely hâve a sort of direct awareness of direction without knowing why or how we become aware of it.

46 VISUAL SPACE

3. Accommodation. — a. Range : the Near-point of Vision. — Hold the point of a pin close in front of one eye (the other eye covered) and observe that.you cannot see it clearly. Move it back and f orth and find the nearest point at which you can see it without a blur. That point is called the near-point of vision. Kecord the distance from the eye.

If we are to hâve a clear image of the point, the rays of light reflected from that point must come to a focus upon the retina. The near-point marks the limit of neamess for which the lens in the eye can adapt itself . In the normal eye it is about 20 centimeters from the eye. In Fig. 5, p represents the pin-point and r ite

FiG. 6.

image upon the retina. If the pin-point be brought nearer than the near-point of the eye, say to p', the rays from it, if continued, would come to a focus back of the retina at r and would form a diffusion circle where the pencil pr pierces the retina. This diffusion circle corresponds to the blur which you observed when the pin was too near. We can see objects clearly only when they lie at or beyond the near-point.

Near-sighted persons also hâve a far-point beyond which objects blur because the lens cannot adapt itself. to that distance. But, in the normal eye, this point is at infinité distance; i.e., the eye can accommodate for

VISUAL SPACE 47

parallel rays. The range between the near-point and the far-point is called the range of accommodation.

6. Line of Accommodation. — Pierce two pinholes, about 1 millimeter apart, in a card. Stick two pins through a sheet of paper, 20 centimeters apart. Cover one eye and hold the card close in front of the other eye so that the two holes are in a horizontal position and f ail within the circumference of the pupil; hold the paper with the pins pointing upward so that the near pin is 20 centimeters from the eye and the two pin-points

are in the same line of regard. Observe that, when you accommodate for the near pin, it is clear, but the distant one is double ; and when you accommodate for the distant pin, it is clear and the near one is double. Take out the distant pin and move it toward the near one and find how close you must bring it before you can see both clearly with a single accommodation.* Kecord this distance, which is called the line of accommodation. Fig. 6 represents the accommodation for the near pin, and Fig. 7 the distant Write out a full explana-

* Make sure that both holes are in front of the pupil. Work with précision and a purpose and you will not strain the eye!

VISUAL SPACE

tion of each figure, assuming that A and B represent the two pins.

Strictiy the eye can accommodate for only one point in distance at a time. AU points in front of and behind that point must appear blurred. This can be verified roughly by looking systematically with one eye along a row of objects in the Une of regard. Looking through the two pinholes has the advantage of simplifying the situation by producing two images instead of a blur. If there were more holes, there would be more images.

FiG. 7.

Practically, however, the two pins may be separated by a considérable distance and yet both be seen single or clear with the same compromise accommodation because "they seem clear enough." We must therefore speak of a line of accommodation rather than a point. We can see two or more objects clearly at the same time only if they lie within the line of accommodation.

4. Convergence: Double Images. — a, Doubling the Distant Ohject. — Hold two objects such as a pen and a pencil in the same line of regard, the former about 20 and the latter about 40 centimeters away from the eyes. Fixate the point of the pen and, while the eyes are thus

VISUAL SPACE 49

converged, observe that you see two pencils. Close one eye at a time and détermine which of the double images belongs to each eye. Record to which eye each image belongs, the distance between the double images, and the position of the double images with référence to the actual position of the pencil.

Write explanation of Fig. 8, which illustrâtes this.*

FiG. 8.

Our two eyes constitute an effective mechanîsm for the perception of distance or relief. One of the limitations which foUows from the very efficiency of the mechanism is that we can see only one point in distance clearly at a time; every object nearer or farther away than this point must be seen double if attended to.

* The level of the horizontal lîne L-R is somewhat arbitrary, expect that it must lie between p and p'. If the observer had not known the actual distance of the pencil he would probably hâve seen it at about the same distance as the pen; but, knowing the actual distance, there is a tendency to see the double images at approximately the true distance. The level chosen in the diagram represents a compromise.

60 VISUAL SPACE

This experiment bas demonstrated that f act as regards an object beyond the fixation-point The next experiment demonstrates it for objects nearer than the fixationpoint, and the experiment following that proves in a most gênerai way that ail points within the Une of vision, which are nearer or more remote than the fixation-point are seen double.

b. Doubling the Near Object. — With pen and pencil as before, fixate the pencil and, while the eyes are thus converged, observe that you see two pens. Close one eye

FiG. 9.

at a time to identify the double images and record as in Exp. 4 a. Write explanation of Fig. 9, which illustrâtes this.*

c, Doubling of ail Points except the Fixation-point. — Stick a pin into a pencil at the middle, point the pencil

* The double images of objects nearer than the point of fixation are said to be "crossed," while thoee of objects beyond the point of fixation are "uncrossed." Titchener gives the mnemonic: "Remote regard reverses; Nearer, notice, not."

VISUAL SPAOE 61

straight forward and fixate the pin; observe that you see two pencils f orming an X, with the Crossing at the pin. Close one eye at a time and observe that the law of erossed and unerossed images is verified.

Practieally, again, this point of elear vision in convergence is really a Une which we might by analogy call the line of convergence. Experiments with fine points show that within a moderately near range it is really a point, for one of two pin-points will double until they corne together, but as we recède the possible séparation becomes appréciable, although the "line of convergence" is relaitively much shorter than the line of accommodation.

d. Double Images in Indirect Vision. — ^Hold pen and pencil as in Exp. 4 a; fixate the pen (near) and observe how far you can move the pencil to the right f rom the line of vision before one of the double images of the pencil disappears. Kecord five trials.

It becomes more and more difficult to perceive the double images as we pass the object gradually into the indirect field. One of the double images can be seen far beyond where the two can be seen. In this case it is the right-eye image, as can be demonstrated by closing one eye at a time.

e. Doubling Numerous Objeds. — ^Double a strip of paper and stick a row of five pins through it about 1 centimeter apart. Hold the paper in a horizontal position at arm's length and at right angles to the line of regard; fixate the eyes upon the pen-point, held midway between the eyes and the pins, and count the pins. Record number.

62 VISUAL SPACE

AU objects outside of the "Une of convergence'' double when seen with both eyes.* Why, then, do we see single objects in nature and elsewhere when many objects outside of the Une of convergence are seen at the same time? First, we pay attention to only one point at a time ; the eyes shif t with the greatest rapidity from point to point and make snapshots which we are aware of only as a composite view. Second, the utility of disregarding double images has resulted in a natural capacity for "counting'' only the impressions which favor single vision. The fact that the single eye can get a fuUy satisfactory image only at the fovea favors this discarding of other images. And, third, a complex view is never clear.

But why do we see single objects at ail from two images? Like the projection of the single image (indeed it is a part of the process), the identifying of images which fall upon corresponding points of the two retinae has been learned through race and individual expérience in associating sight with touch and movement.

/. Doubling Complex and Large Objects. — ^Hold one or more postage-stamps or other complex objects at arm's length and observe by near fixation, as in Exp. 4 h, or far fixation, as in Exp. 4 a, that the whole objects double. Try larger objects, such as a letter or a wall-picture.

* The indirect field, Exp. 4 d, is no exception, because when one of the double images disappears we see only with one eye.

Of course, the doubling is always in the line of the eyes, normally the horizontal — the whole paper strip doubled, but lengthwise.

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Embody the results of thèse six experiments on double images in binocular vision in one gênerai law and write it ont

5. Relief. — a, Similar Images, — ^Look at Fig. 3, with one eye at a time, and observe that the two images are similar and there is no relief.

6. Disparate Images. — Hold the closed text-book erect and with the back toward you at arm's length ; look at it first with one eye and then with the other and observe that the two images are disparate, i.e., unlike. Look with both eyes and observe that you see the book in relief.

The basis for the perception of relief lies in the disparateness of the two images. Eelief is a mental synthesis based upon two independent séries of sensé data which become harmonized through the relief -interprétation. The mind interprets each image as a différent view of the same object. Look again at the book and observe how the two disparate views supplément each other, blend and are satisfied in the appropriate combination. Knowing the character of the différence between two images and the degree of convergence, we can predict the relief. This is well illustrated in stereoscopic vision.

6. Stereoscopic Vision. — RoU a pièce of paper into a truncated cône and trim it to a diameter of 8 centimeters at the base and 2 centimeters at the frustum, and make it about 6 centimeters high. Pin it together and set it up about 50 centimeters away, with the frustum

64 VISUAL SPACE

f acing you at the level of your eyes. Look at it with the two eyes alternately and observe the disparateness of the two images.

Draw the diameters of the two ends, for each eye separately, as they would be projected on a pane of glass held about 15 centimeters in front of the eyes. When this drawing is seen in a stéréoscope it should give the true skeleton of the cône.*

There are two fundamental conditions of stereoscopic vision: first, that the two views shall differ so as to produce the appropriate disparateness in the two retinal images; and, second, that the two views shall be seen with the two eyes converged upon one point. The drawings provide the first ; the lenses in the stéréoscope, or converging for a point back of the card without the stéréoscope, provide for the second, t

* With a lîttle practice stereoscopîc views may be seen in relief without the use of the stéréoscope by merely converging the eyes upon a point at a suitable distance back of the card. Try it.

fOne of the best elementary treatments of this topic, visual space, is found in Witmer, "Analytical Psychology," Ch. IV.

CHAPTER V AUDITORY SPACE

For Twjo.* Thebe are three aspects of the problem of auditory space; namely, direction, distance, and volume. The présent chapter is devoted to the problem of hearing the direction of sound.

The experiments in this chapter should be helpful in

answering such questions as: Is the ear a space-sense

organ ? How do we perceive the direction of sound by

hearing ? What are some of the laws of localization ? f

Produce the sound, which is to be localized, by snap-

* Thèse experiments must be performed at some other time than during the class period, unless there is opportunity for the class to scatter into différent rooms or out of doors. Two students must work together; the one who manipulâtes the apparatus is called the expérimenter, (E), and the one on whom the experiment is performed is called the observer, (O). Each takes tum as E and 0 for each experiment. E always keeps the record obtained as expérimenter; thus each préserves the record of the other. O should be blindfolded and seated comfortably in such a position that he can hold his head erect and steady in a given position during an experiment.

f There is différence of opinion as to whether or not the ear is a space-sense organ. Those who hold that it is not a spacesense organ base their opinion largely upon two anatomical facts : (1) that the portion of the ear which is the organ of hearing possesses no spread-out surface such that an arrangement of stimulations upon it may represent the spatial relations of the extemal world; and (2) that the ear is unprovided with a muscular apparatus for focusing itself for différent directions. In thèse respects the ear is contrasted with the visual and tactual arrangements for perception of space.

It is also well to bear in mind the chief théories of localiza-

66 AUDITOKY SPACE

ping two coins.* Place one coin on each side of the forefinger and press them together with the thumb and the forefinger until they slide together with a snap. Shift the coins deftly from one hand to the other and avoid swaying movements of the body and rustling of the garments. Stand on the side and reach ont both arms symmetrically. Blindfold and seat the observer.

1. Radially in the Médian Plane, f — Let it be understood that the sound may corne from any of the seven directions 45** apart: up, up-front, front, down-front, down-back, back,' and up-back, ail within the médian plane. Produce the sound three times in each of thèse

tion. Thèse may be divided into five classes:

(a) The intensity theory. The différence in the strength of the sound in the two ears is the basis for the hearing of direction.

(6) The tactual theory. Sound vibrations also give rise to sensations of touch, and we confuse thèse touch sensations with sound sensations.

(c) Semicircular-canal théories. The semicircular canals in the ear are spécial organs for the sensing of direction.

{d) Original space différences in the sensations of the two ears.

(e) The intensity-quality theory. Both quality and intensity aid in the perception of direction.

At the end of this chapter we shall tum back and ask which of thèse théories has been supported.

*The following are better sources of sound if available: (1) a téléphone receiver in circuit with a battery and a mercury key; (2) a "frog snapper" (electric supply houses, 25 cents) ; or (3) a paper clip such as is used in hanging placards.

The apparatus which is used for accurate work in the laboratory is called a sound-perimeter or sound-cage. It is so constructed that the expérimenter can manipulate it from one point in the room without moving around. It enables him to vary the direction, the distance, the kind, the strength, the pitch, and the complexity of the sound; to control thèse conditions; and to make accurate measurements.

fThe médian plane is that vertical plane which, passing through the body, divides it into right and left symmetrical halves.

ATJDITORY SPACE 6.7

directions, at a distance of about 50 centimeters, measuring from the center of the head. Distribute the trials approximately as they might run by chance. Require O to say in which of the seven directions he hears the Sound. * Record each answer under the appropriate heading in a prepared tabular form. Figure how many answers out of the 21 are right, and how many are 45**, 90°, 135°, and 180° wrong respectively.

Exp. 1 might well hâve been entitled the "inability to localize sound." The observer is very much surprised and discouraged when he sees his record.

There are two factors in the record, the number of correct localizations and the magnitude of the incorrect localizations, to be considered. As there are 21 trials distributed equally among 7 points, three of the localizations would be right by pure chance. The trials are not sufficiently numerous to enable us to apply the laws of chance in prédiction, but extensive experiments show that a few more localizations than can be accounted for by chance, will be right, probably less than 10 per cent On that basis, the observer might hâve four or five correct localizations in this experiment.f

*It is of the greatest importance that 0 should hâve no other means of detecting direction than by hearing. E must therefore take every possible précaution to avoid giving any suggestion or due by word, situation, or movement.

Of course, the expérimenter will be shrewd enough to give no intimations to the observer about his errors during the progress of the experiment.

f If the observer gave more than one-fourth of the localizations correctly, it is probable that this was due to failure on the part of the expérimenter to snap the coins exactly in the médian plane, or to eliminate accessory sounds from his own movements, breathing, etc. Creaking sounds from the coat-sleeve are often taken as a eue.

68 AUDITOKY SPACE

The other f actor, the degree of error in the misplacements, îs equally significant. TJnder strict expérimental conditions the observer is almost as liable to make an error of 180° as of 45° misplacement*

If the same stimulus is used, a person can improve in this localization ; but the improvement will be almost entirely lost as soon as the kind of sound and the strength of sound are varied.

It is a striking fact that the observer has undue confidence in his ability. When he said "up-front'' he heard the sound distinctly there, although it may hâve come from any other point. This illusion of certainty is characteristic of ail our hearing of direction. We continually either misjudge the direction, or learn the direction through other means than hearing; yet we hâve a distinct feeling that we hâve heard the direction, t

2. Horizontally in Front. — ^Measure the discriminative sensibility for direction of sound in a horizontal plane in front, at the level of the ears. Proceed as follows: Mark a stick about 50 centimeters long into 3centimeter steps, beginning at the middle, and marking

* This inability to localize sounds in the médian plane was discovered by Lord Rayleigh in 1875. The inability is peculiar to this plane, and therefore has great signiôcance as a test of théories.

f Sitting near the central aisle of the Fifth Avenue Cathedral, New York, and looking straight forward, it is practically impossible to tell whether the organ-tones issue from the front or back of the cathedral.- If we could see the organ, the matter would be différent. One of the reasons that this inability does not disturb us much is that, when there is no corrélation in the other sensés or in reason, we remain uncorrected in the illusion of having heard the direction rightly.

AUDITORY SPACE 69

symmetrically in both directions. Seat O as in Exp. 1. Hold the stick 1 meter from the center of his head, directly in front, in a horizontal position, at right angles to the médian plane, and at the level of his ears. Sound, in quick succession, two clicks — the first, or standard, sound directly in front (at the middle or 0° of the stick) and the second, or compared, click on either side of the standard, the order of sides to be practically such as would foUow from chance. Eequire O to say whether the second sound was to his right or lef t. Start with a distance of 3 centimeters between the standard and compare sounds and repeat the trials until O makes a mistake or has ten successive answers right. If a mistake is made, try with a distance of 6 centimeters in the same way. Continue thus, trying succes- '=jive larger steps, increasing each time by 3 centimeters, until you hâve reached one for which O gives ten successive correct answers. Eecord the number of right answers for each step.

3. Horizontally at the Back. — ^Make the same measurement as in Exp. 2, for the symmetrical position at the back of O. In this and the foUowing experiments, E should retain the same position, and O should turn around in order to hâve the résonance in the room constant.

4. Horizontally at the Side, — Make the same measurement as in Exp. 2 for a point in the horizontal plane at the level of the ears and 1 meter directly to the right of the center of O's head. Lèt O answer "forward" or "backward."

AUDITORY SPACE

5. Vertically at the Side. — From the same position as in Exp. 4, but in the vertical plane, make the same

measurement as in Exp.

"down."