[Illustration: Fig. 33.--The color circle. R, Y, G and B, stand for

the colors red, yellow, green and blue. The shaded portion corresponds

to the spectrum or rainbow. Complementary colors (see later) lie

diametrically opposite to each other on the circumference.]

A _saturation series_ runs from full-toned or saturated colors to pale

or dull. Since we can certainly say of a pale blue that it is less

saturated than a vivid red, etc., we could, theoretically, arrange our

whole collection of bits of color in a single saturation series, but

our judgment would be very uncertain at many points. The most

significant saturation series confine themselves to a single

color-tone, {208} and also, as far as possible, to a constant

brightness, and extend from the most vivid color sensation obtainable

with this color-tone and brightness, through a succession of less and

less strongly colored sensations of the same tone and brightness, to a

dead gray of the same brightness. Any such saturation series

terminates in a neutral gray, which is light or dark to match the rest

of the particular saturation series.

White, black and gray, which find no place in the color-tone series,

give an intensity series of their own, running from white through

light gray and darker and darker gray to black, and any gray in this

series may be the zero point in a saturation series of any color-tone.

A three-dimensional diagram of the whole system of visual sensations

can be built up in the following way. Taking all the colors of the

same degree of brightness, we can arrange the most saturated, in the

order of their color-tone, around the circumference of a circle, put a

gray of the same brightness at the center of this circle, and then

arrange a saturation series for each color-tone extending from the

most saturated at the circumference to gray at the center. This would

be a two-dimensional diagram for colors having the same brightness.

For a greater brightness, we could arrange a similar circle and place

it above the first, and for a smaller brightness, a similar circle and

place it below the first, and we could thus build up a pile of

circles, ranging from the greatest brightness at the top to the least

at the bottom. But, as the colors all lose saturation when their

brightness is much increased, and also when it is much decreased, we

should make the circles smaller and smaller toward either the top or

the bottom of the pile, so that our three-dimensional diagram would

finally take the form of a double cone, with the most intense white,

like that of sunlight, at the upper point, with dead black at the

lower point, {209} and with the greatest diameter near the middle

brightness, where the greatest saturations can be obtained. The axis

of the double cone, extending from brightest white to dead black,

would give the series of neutral grays. All the thousands of

distinguishable colors, shades and tints, would find places in this

scheme.

[Illustration: Fig. 34.--The color cone, described in the text.

Instead of a cone, a four-sided pyramid is often used, so as to

emphasize the four main colors, red, yellow, green and blue, which are

then located at the corners of the base of the pyramid. (Figure text:

white, black, R, B, G, Y)]

Simpler Forms of the Color Sense

Not every one gets all these sensations. In _color-blindness_, the

system is reduced to one or two dimensions, instead of three. There

are two principal forms of color-blindness: total, very uncommon; and

red-green blindness, fairly {210} common. The totally color-blind

individual sees only white, black, and the various shades of gray. His

system of visual sensations is reduced to one dimension, corresponding

to the axis of our double cone.

_Red-green blindness_, very uncommon in women, is present in three or

four percent of men. It is not a disease, not curable, not corrected

by training, and not associated with any other defect of the eye, or

of the brain. It is simply a native peculiarity of the color sense.

Careful study shows that the only color sensations of the red-green

blind person are blue and yellow, along with white, black and the

grays. His color circle reduces to a straight line with yellow at one

end and blue at the other. Instead of the color circle, he has a

double saturation series, reaching from saturated yellow through

duller yellows to gray and thence through dull blues to saturated

blue. What appears to the normal eye as red, orange or grass green

appears to him as more or less unsaturated yellow; and what appears to

the normal eye as greenish blue, violet and purple appears to him as

more or less unsaturated blue. His color system can be represented in

two dimensions, one for the double saturation series,

yellow-gray-blue, and the other for the intensity series,

white-gray-black.

Color-blindness, always interesting and not without some practical

importance (since the confusions of the color-blind eye might lead to

mistaking signals in navigation or railroading), takes on additional

significance when we discover the curious fact that _every one is

color-blind_--in certain parts of the retina. The outermost zone of

the retina, corresponding to the margin of the field of view, is

totally color-blind (or very nearly so), and an intermediate zone,

between this and the central area of the retina that sees all the

colors, is red-green blind, and delivers only blue and yellow

sensations, along with white, black and gray. Take {211} a spot of

yellow or blue and move it in from the side of the head into the

margin of the field of view and then on towards the center. When it

first appears in the margin, it simply appears gray, but when it has

come inwards for a certain distance it changes to yellow. If a red or

green spot is moved in similarly, it first appears gray, then takes on

a faint tinge of yellow, and finally, as it approaches the center of

the field of view, appears in its true color. The outer zone gets only

black and white, the intermediate zone gets, in addition to these,

yellow and blue, and the central area adds red and green (and with

them all the colors).

[Illustration: Fig. 35.--Color cones of the retina. F is the fovea, or

central area of clearest vision. (Figure text: all colors, white-black &

yellow-blue, white-black)]

Now as to the question of elements, let us see how far we can go,

keeping still to the sensations, without any reference to the

stimulus. If a collection of bits of color is presented to a class of

students who have not previously studied this matter, with the request

that each select those colors that seem to him elementary and not

blends, there is practically unanimous agreement on three colors, red,

yellow and blue; and there are some votes for green also, but almost

none for orange, violet, purple, brown or any other colors. {212}

except white and black. That white and black are elementary sensations

is made clear by the case of total color-blindness, since in this

condition there are no other visual sensations from which white and

black could be compounded, and these two differ so completely from

each other that it would be impossible to think of white as made up of

black, or black of white. Gray, on the other hand, appears like a

blend of black and white. In the same way, red-green blindness

demonstrates the reality of yellow and blue as elementary sensations,

since neither of them could be reduced to a blend of the other with

white or black; and there are no other colors present in this form of

color vision to serve as possible elements out of which yellow and

blue might be compounded. That white, black, yellow and blue are

elementary sensations is therefore clear from the study of visual

sensations alone; and there are indications that red and green are

also elements.

Visual Sensations as Related to the Stimulus

Thus far, we have said nothing of the stimulus that arouses visual

sensations. Light, the stimulus, is physically a wave motion, its

vibrations succeeding each other at the rate of 500,000000,000000

vibrations, more or less, per second, and moving through space with a

speed of 186,000 miles per second. The "wave-length", or distance from

the crest of one wave to the crest of the next following, is measured

in millionths of a millimeter.

The most important single step ever taken towards a knowledge of the

physics of light, and incidentally towards a knowledge of visual

sensations, was Newton's analysis of white light into the spectrum. He

found that when white light is passed through a prism, it is broken up

into all the colors of the rainbow or spectrum. Sunlight consists of a

{213} mixture of waves of various lengths. At one end of the spectrum

are the long waves (wave-length 760 millionths of a millimeter), at

the other end are the short waves (wavelength 390), and in between are

waves of every intermediate length, arranged in order from the longest

to the shortest. The longest waves give the sensation of red, and the

shortest that of violet, a slightly reddish blue.

Outside the limits of the visible spectrum, however, there are waves

still longer and shorter, incapable of arousing the retina, though the

very long waves, beyond the red, arouse the sensation of warmth from

the skin, and the very short waves, beyond the violet, though arousing

none of the senses, do effect the photographic plate. Newton

distinguished seven colors in the visible spectrum, red, orange,

yellow, green, blue, indigo and violet; but there is nothing specially

scientific about this list, since physically there are not seven but

an unlimited number of wave-lengths included in the spectrum, varying

continuously from the longest at the red end to the shortest at the

violet; while psychologically the number of distinguishable colors in

the spectrum, though not unlimited, is at least much larger than

seven. Between red and orange, for instance, there are quite a number

of distinguishable orange-reds and reddish oranges.

If now we ask what differences in the stimulus give rise to the three

kinds of difference in visual sensation that were spoken of

previously, we find that color-tone depends on the wave-length of the

light, brightness on the energy of the stimulus, i.e., on the

amplitude of the vibration, and saturation on the mixture of long and

short wave-lengths in a complex light-stimulus--the more mixture, the

less saturation.

These are the general correspondences between the light stimulus and

the visual sensation; but the whole relationship is much more complex.

Brightness depends, not only on the energy of the stimulus, but also

on wave-length. The {214} retina is tuned to waves of medium length,

corresponding to the yellow, which arouse much brighter sensation than

long or short waves of the same physical energy. Otherwise put, the

sensitivity of the retina is greatest for medium wavelengths, and

decreases gradually towards the ends of the spectrum, ceasing

altogether, as has been said, at wavelengths of 760 at the red end and

of 390 at the violet end.

Saturation, depending primarily on amount of mixture of different

wave-lengths, depends also on the particular wavelengths acting, and

also on their amplitude. So, the red and blue of the spectrum are more

saturated than the yellow and green; and very bright or very dim

light, however homogeneous, gives a less saturated sensation than a

stimulus of medium strength.

Color Mixing

Color-tone depends on the wave-length, as has been said, but this is

far from the whole truth; the whole truth, indeed, is one of the most

curious and significant facts about color vision. We have said that

each color-tone is the response to a particular wave-length. But any

color-tone can be got without its particular wave-length being present

at all; all that is necessary is that wave-lengths centering about

this particular one shall be present. A mixed light, consisting of two

wave-lengths, the one longer and the other shorter than the particular

wave which when acting alone gives a certain color-tone, will give

that same color-tone. For example, the orange color resulting from the

isolated action of a wave-length of 650 is given also by the combined

action of wave-lengths of 600 and 700, in amounts suitably

proportioned to each other.

A point of experimental technique: in _mixing colored lights_ for the

purpose of studying the resulting sensations, we do not mix painter's

pigments, since the physical {215} conditions then would be far from

simple, but we mix the lights themselves by throwing them together

either into the eye, or upon a white screen. We can also, on account

of a certain lag or hang-over in the response of the retina, mix

lights by rapidly alternating them, and get the same effect as if we

had made them strike the retina simultaneously.

By mixing a red light with a yellow, in varying proportions, all the

color-tones between red and yellow can be got--reddish orange, orange

and yellowish orange. By mixing yellow and green lights, we get all

the greenish yellow and yellowish green color-tones; and by mixing

green and blue lights we get the bluish greens and greenish blues.

Finally, by mixing blue and red lights, in varying proportions, we get

violet, purple and purplish red. Purple has no place in the spectrum,

since it is a sensation which cannot be aroused by the action of any

single wave-length, but only by the mixture of long and short waves.

To get all the color-tones, then, we need not employ all the

wave-lengths, but can get along with only four. In fact, we can get

along with three. Red, green and blue will do the trick. Red and green

lights, combined, would give the yellows; green and blue would give

the greenish blues; and red and blue would give purple and violet.

The sensation of white results--to go back to Newton--from the

combined action of all the wave-lengths. But the stimulus _need_ not

contain _all_ the wave-lengths. Four are enough; the three just

mentioned would be enough. More surprising still, two are enough, if

chosen just right. Mix a pure yellow light with a pure blue, and you

will find that you get the sensation of white--or gray, if the lights

used are not strong.

[Footnote: When you mix blue and yellow _pigments_, each absorbs part

of the wave-lengths of white light, and what is left after this double

absorption may be predominantly green. This is absolutely different

from the addition of blue to yellow light; addition gives white, not

green.]

{216}

Lights, or wave-lengths, which when acting together on the retina give

the sensation of white or gray, are said to be _complementary_.

Speaking somewhat loosely, we sometimes say that two _colors_ are

complementary when they mix to produce white. Strictly, the colors--or

at least the color sensations--are not mixed; for when yellow and blue

lights are mixed, the resulting sensation is by no means a mixture of

blue and yellow sensations, but the sensation of white in which there

is no trace of either blue or yellow. Mixing the stimuli which, acting

separately, give two complementary colors, arouses the colorless

sensation of white.

Blue and yellow, then, are complementary. Suppose we set out to find

the complementary of red. Mixing red and yellow lights gives the

color-tones intermediate between these two; mixing red and green still

gives the intermediate color-tones, but the orange and yellow and

yellowish green so got lack saturation, being whitish or grayish. Now

mix red with bluish green, and this grayishness is accentuated, and if

just the right wave-length of bluish green is used, no trace of orange

or yellow or grass green is obtained, but white or gray. Red and

bluish green are thus complementary. The complement of orange light is

a greenish blue, and that of greenish yellow is violet. The typical

green (grass green) has no single wave-length complementary to it, but

it does give white when mixed with a compound of long and short waves,

which compound by itself gives the sensation of purple; so that we may

speak of green and purple as complementary.

What Are the Elementary Visual Sensations?

Returning now to the question of elementary sensations, which we laid

aside till we had examined the relationship of the sensations to the

stimulus, we need to be on our guard against physics, or at least

against being so much impressed with the physics of light as to forget

that we are concerned with the _response_ of the organism to physical

light--a matter on which physics cannot speak the final word.

{217}

Fig. 36.--(After König.) The color triangle, a map of the laws of

color mixture. The spectral colors are arranged in order along the

heavy solid line, and the purples along the heavy dotted line. The

numbers give the wave-lengths of different parts of the spectrum.

Inside the heavy line are located the pale tints of each color,

merging from every side into white, which is located at the point W.

Suppose equal amounts of two spectral colors are mixed: to find from

the diagram the color of the mixture. Locate the two colors on the

heavy line, draw a straight line between these two points, and the

middle of this line gives the color-tone and saturation of the

mixture. For example, mix red and yellow: then the resulting color

is a saturated reddish yellow. Mix red (760) and green (505): the

resulting yellow is non-saturated, since the straight line between

these two points lies inside the figure. If the straight line

joining two points passes through W, the colors located at the two

points are complementary.

Spectral colors are themselves not completely saturated. The way to

get color sensations of maximum saturation is first to stare at one

color, so as to fatigue or adapt the eye for that color, and then to

turn the eye upon the complementary color, which, under these

conditions, appears fuller and richer than anything otherwise

obtainable. The corners, R, G, and B, denote colors of maximum

saturation, and the whole of the triangle outside of the heavy line

is reserved for super-saturated color sensations.

{end 217; text continues from 216}

{218}

Physics tells us of the stimulus, but we are concerned with the

response. The facts of color-blindness and color mixing show very

clearly that the response does not tally in all respects with the

stimulus. Physics, then, is apt to confuse the student at this point

and lead him astray. Much impressed with the physical discovery that

_white_ light is a mixture of all wave-lengths, he is ready to believe

the sensation of white a mixed sensation. He says, "White is the sum

of all the colors", meaning that the sensation of white is compounded

of the sensations of red, orange, yellow, green, blue and

violet--which is simply not true. No one can pretend to get the

sensations of red or blue in the sensation of white, and the fact of

complementary colors shows that you cannot tell, from the sensation of

white, whether the stimulus consists of yellow and blue, or red and

bluish green, or red, green and blue, or all the wave-lengths, the

response being the same to all these various combinations. Total

color-blindness showed us, when we were discussing this matter before,

that white was an elementary sensation, and nothing that has been said

since changes that conclusion.

Consider _black_, too. Physics says, black is the absence of light;

but this must not be twisted to mean that black is the absence of all

visual sensation. Absence of visual sensation is simply nothing, and

black is far from that. It is a sensation, as positive as any, and

undoubtedly elementary.

From the point of view of physics, there is no reason for considering

any one color more elementary than any other. Every wave-length is

elementary; and if sensation tallied precisely with the stimulus,

every spectral color-tone would be an element. But there are obvious

objections to such a view, such as: (1) there are not nearly as many

{219} distinguishable color-tones as there are wave-lengths; (2)

orange, having a single wave-length, certainly appears to be a blend

as truly as purple, which has no single wave-length; and (3) we cannot

get away from the fact of red-green blindness, in which there are only

two color-tones, _yellow_ and _blue_. In this form of color vision

(which, we must remember, is normal in the intermediate zone of the

retina), there are certainly not as many elementary responses as there

are wave-lengths, but only one response to all the longer waves (the

sensation of yellow), one response to all the shorter waves (the

sensation of blue), one response to the combination of long and short

waves (the sensation of white), and one response to the cessation of

light (the sensation of black). These four are certainly elementary

sensations, and there are probably only a few more.

There must be at least two more, because of the fact that two of the

sure elements, yellow and blue, are complementary. For suppose we try

to get along with one more, as _red_. Then red, blended with yellow,

would give the intervening color-tones, namely, orange with reddish

and yellowish orange; and red blended with blue would give violet and

purple; but yellow and blue would only give white or gray, and there

would be no way of getting green. We must admit _green_ as another

element. The particular red selected would be that of the red end of

the spectrum, if we follow the general vote; and the green would

probably be something very near grass green. We thus arrive at the

conclusion that there are six elementary visual responses or

sensations: white and black, yellow and blue, red and green.

It is a curious fact that some of these elementary sensations blend

with each other, while some refuse to blend. White and black blend to

gray, and either white or black or both together will blend with any

of the four elementary colors or with any possible blend of these

four. Brown, for {220} example, is a grayish orange, that is, a blend

of white, black, red and yellow. Red blends with yellow, yellow with

green, green with blue, and blue with red. But we cannot get yellow

and blue to blend, nor red and green. When we try to get yellow and

blue to blend, by combining their appropriate stimuli, both colors

disappear, and we get simply the colorless sensation of white or gray.

When we try to get red and green to blend, both of them disappear and

we get the sensation of yellow.

Theories of Color Vision

Of the most celebrated theories of color vision, the oldest,

propounded by the physicists Young and Helmholtz, recognized only

three elements, red, green and blue. Yellow they regarded as a blend

of red and green, and white as a blend of all three elements. The

unsatisfactory nature of this theory is obvious. White as a sensation

is certainly not a blend of these three color sensations, but is,

precisely, colorless; and no more is the yellow sensation a blend of

red and green. Moreover, the theory cannot do justice either to total

color-blindness, with its white and black but no colors, or to

red-green blindness, with its yellow but no red or green.

The next prominent theory was that of the physiologist Hering. He did

justice to white and black by accepting them as elements; and to

yellow and blue likewise. The fact that yellow and blue would not

blend he accounted for by supposing them to be antagonistic responses

of the retina; when, therefore, the stimuli for both acted together on

the retina, neither of the two antagonistic responses could occur, and

what did occur was simply the more generic response of white.

Proceeding along this line, he concluded that red and green were also

antagonistic responses; but just here {221} he committed a wholly

unnecessary error, in assuming that if red and green were antagonistic

responses, the combination of their stimuli must give white, just as

with yellow and blue. Accordingly, he was forced to select as his red

and green elementary color-tones two that would be complementary; and

this meant a purplish (i.e., bluish) red, and a bluish green, with the

result that his "elementary" red and green appear to nearly every one

as compounds and not elements. It would really have been just as easy

for Hering to suppose that the red and green responses, antagonizing

each other, left the sensation yellow; and then he could have selected

that red and green which we have concluded above to have the best

claim.

A third theory, propounded by the psychologist, Dr. Christine

Ladd-Franklin, is based on keen criticism of the previous two, and

seems to be harmonious with all the facts. She supposes that the color

sense is now in the third stage of its evolution. In the first stage

the only elements were white and black; the second stage added yellow

and blue; and the third stage red and green. The outer zone of the

retina is still in the first stage, and the intermediate zone in the

second, only the central area having reached the third. In red-green

blind individuals, the central area remains in the second stage, and

in the totally color-blind the whole retina is still in the first

stage.

In the first stage, one response, white, was made to light of whatever

wave-length. In the second stage, this single response divided into

two, one aroused by the long waves and the other by the short. The

response to the long waves was the sensation of yellow, and that to

the short waves the sensation of blue. In the third stage, the yellow

response divided into one for the longest waves, corresponding to the

red, and one for somewhat shorter waves, corresponding to the green.

Now, when we try to get a blend of red and green {222} by combining

red and green lights, we fail because the two responses simply unite

and revert to the more primitive yellow response; and similarly when

we try to get the yellow and blue responses together, they revert to

the more primitive white response out of which they developed.

But, since no one can pretend to _see_ yellow as a reddish green, nor

white as a bluish yellow, it is clear that the just-spoken-of union of

the red and green responses, and of the yellow and blue responses,

must take place _below the level of conscious sensation_. These unions

probably take place within the retina itself. Probably they are purely

chemical unions.

[Illustration: Fig. 37--The Ladd-Franklin theory of the evolution of

the color sense. (Figure text: Stage 1--white, Stage 2--yellow blue,

Stage 3--red green blue)]

The _very first_ response of a rod or cone to light is probably a

purely chemical reaction. Dr. Ladd-Franklin, carrying out her theory,

supposes that a light-sensitive "mother substance" in the rods and

cones is decomposed by the action of light, and gives off cleavage

products which arouse the vital activity of the rods and cones, and

thus start nerve currents coursing towards the brain.

In the "first stage", she supposes, a _single_ big cleavage product,

which we may call W, is split off by the action of {223} light upon

the mother substance, and the vital response to W is the sensation of

white.

In the second stage, the mother substance is capable of giving off two

smaller cleavage products, Y and B. Y is split off by the long waves

of light, and B by the short waves, and the vital response to Y is the

sensation of yellow, that to B the sensation of blue. But suppose

that, chemically, Y + B = W: then, if Y and B are both split off at

the same time in the same cone, they immediately unite into W, and the

resulting sensation is white, and neither yellow nor blue.

[Illustration: Fig. 38.--The cleavage products, in the three stages of

the color sense. The "mother substance" is not represented in the

diagram, but only the cleavage products which, according to the

Ladd-Franklin theory, are the direct stimuli for the color sensations.

(Figure text: 1--white, 2--yellow blue, 3--red green blue)]

Similarly, in the third stage, the mother substance is capable of

giving off _three_ cleavage products, R, G and B; and there are three

corresponding vital responses, the sensations of red, green and blue.

But, chemically, R + G = Y; and therefore, if R and G are split off at

the same time, they unite chemically into Y and give the sensation of

yellow. If R, G and B are all split off at the same time, they unite

chemically as follows: R + G = Y, and Y + B = W; and therefore the

resulting sensation is that of white.

This theory of cleavage products is in good general agreement with

chemical principles, and it does justice to all the facts of color

vision, as detailed in the preceding pages. It should be added that

"for black, the theory supposes that, {224} in the interest of a

continuous field of view, objects which reflect no light at all upon

the retina have correlated with them a definite non-light

sensation--that of black." [Footnote: Quotation from Dr.

Ladd-Franklin.]

Adaptation

Sensory adaptation is a change that occurs in other senses also, but

it is so much more important in the sense of sight than elsewhere that

it may best be considered here. The stimulus continues, the sensation

ceases or diminishes--that is the most striking form of sensory

adaptation. Continued action of the same stimulus puts the sense into

such a condition that it responds differently from at first, and

usually more weakly. It is much like fatigue, but it often is more

positive and beneficial than fatigue.

The sense of smell is very subject to adaptation. On first entering a

room you clearly sense an odor that you can no longer get after

staying there for some time. This adaptation to one odor does not

prevent your sensing quite different odors. Taste shows less

adaptation than smell, but all are familiar with the decline in sweet

sensation that comes with continued eating of sweets.

All of the cutaneous senses except that for pain are much subject to

adaptation. Continued steady pressure gives a sensation that declines

rapidly and after a time ceases altogether. The temperature sense is

usually adapted to the temperature of the skin, which therefore feels

neither warm nor cool. If the temperature of the skin is raised from

its usual level of about 70 degrees Fahrenheit to 80 or 86, this

temperature at first gives the sensation of warmth, but after a time

it gives no temperature sensation at all; the warmth sense has become

adapted to the temperature of 80 degrees; and now a temperature of 70

will give the sensation of cool. {225} Hold one hand in water at 80

and the other in water at 66, and when both have become adapted to

these respective temperatures, plunge them together into water at 70;

and you will find this last to feel cool to the warm-adapted hand and

warm to the cool-adapted. There are limits to this power of

adaptation.

The muscle sense seems to become adapted to any fixed position of a

limb, so that, after the limb has remained motionless for some time,

you cannot tell in what position it is; to find out, you have only to

move it the least bit, which will excite both the muscle sense and the

cutaneous pressure sense. The sense of head rotation is adaptable, in

that a rotation which is keenly sensed at the start ceases to be felt

as it continues; but here it is not the sense cells that become

adapted, but the back flow that ceases, as will soon be explained.

To come now to the sense of sight, we have _light adaptation, dark

adaptation_, and _color adaptation_. Go into a dark room, and at first

all seems black, but by degrees--provided there is a little light

filtering into the room--you begin to see, for your retina is becoming

dark-adapted. Now go out into a bright place, and at first you are

"blinded", but you quickly "get used" to the bright illumination and

see objects much more distinctly than at first; for your eye has now

become light-adapted. Remain for some time in a room illuminated by a

colored light (as the yellowish light of most artificial illuminants),

and by degrees the color sensation bleaches out so that the light

appears nearly white.

Dark adaptation is equivalent to sensitizing the retina for faint

light. Photographic plates can be made of more or less sensitiveness

for use with different illuminations; but the retina automatically

alters its sensitivity to fit the illumination to which it is exposed.

{226}

Rod and Cone Vision

You will notice, in the dark room, that while you see light and shade

and the forms of objects, you do not see colors. The same is true out

of doors at night. In other words, the kind of vision that we have

when the eye is dark-adapted is totally color-blind. Another

significant fact is that the fovea is of little use in very dim light.

These facts are taken to mean that dim-light vision, or _twilight

vision_ as it is sometimes called, is _rod vision_ and not cone

vision; or, in other words, that the rods and not the cones have the

great sensitiveness to faint light in the dark-adapted eye. The cones

perhaps become somewhat dark-adapted, but the rods far outstrip them

in this direction. The fovea has no rods and hence is of little use in

very faint light. The rods have no differential responsiveness to

different wave-lengths, remaining still in the "first stage" in the

development of color vision, and consequently no colors are seen in

faint light.

Rod vision differs then from cone vision in having only one response

to every wave-length, and in adapting itself to much fainter light. No

doubt, also, it is the rods that give to peripheral vision its great

sensitivity to moving objects.

After-Images

After-images, which might better be called after-sensations, occur in

other senses than sight, but nowhere else with such definiteness. The

main fact here is that the response outlasts the stimulus. This is

true of a muscle, and it is true of a sense organ. It takes a little

time to get the muscle, or the sense organ, started, and, once it is

in action, it takes a little time for it to stop. If you direct your

eyes towards the lamp, holding your hand or a book in front of them as

a screen, remove the screen for an {227} instant and then replace it,

you will continue for a short time to see the light after the external

stimulus has been cut off. This "positive after-image" is like the

main sensation, only weaker. There is also a "negative after-image",

best got by looking steadily at a black-and-white or colored figure

for as long as fifteen or twenty seconds, and then directing the eyes

upon a medium gray background. After a moment a sensation develops in

which black takes the place of white and white of black, while for

each color in the original sensation the complementary color now

appears.

[Illustration: Fig. 39.--The visual response outlasts the stimulus.

The progress of time is supposed to be from left to right in the

diagram. After the stimulus ceases, the sensation persists for a time,

at first as a positive after-image, and then as a negative

after-image, a sort of back swing. (Figure text: stimulus, sensory

response)]

This phenomenon of the negative after-image is the same as that of

color adaptation. Exposing the retina for some time to light of a

certain color adapts the retina to that color, bleaches that color

sensation, and, as it were, subtracts that color (or some of it) from

the gray at which the eyes are then directed; and gray (or white)

minus a color gives the complementary color.

Contrast

Contrast is still another effect that occurs in other senses, but most

strikingly in vision. There is considerable in common between the

negative after-image and contrast; indeed, {228} the negative

after-image effect is also called "successive contrast". After looking

at a bright surface, one of medium brightness appears dark, while this

same medium brightness would seem bright after looking at a dark

surface. This is evidently adaptation again, and is exactly parallel

to what was found in regard to the temperature sense. After looking at

any color steadily, the complementary color appears more saturated

than usual; in fact, this is the way to secure the maximum of

saturation in color sensation. These are examples of "successive

contrast".

"Simultaneous contrast" is something new, not covered by adaptation,

but gives the same effects as successive contrast. If you take two

pieces of the same gray paper, and place one on a black background and

the other on white, you will find the piece on the black ground to

look much brighter than the piece on the white ground. Spots of gray

on colored backgrounds are tinged with the complementary colors. The

contrast effect is most marked at the margin adjoining the background,

and grows less away from this margin. Any two adjacent surfaces

produce contrast effects in each other, though we usually do not

notice them any more than we usually notice the after-images that

occur many times in the course of the day.

The Sense of Hearing

Sound, like light, is physically a wave motion, though the sound

vibrations are very different from those of light. They travel 1,100

feet a second, instead of 186,000 miles a second. Their wave-length is

measured in feet instead of in millionths of a millimeter, and their

vibration frequencies are counted in tens, hundreds and thousands per

second, instead of in millions of millions. But sound waves vary among

themselves in the same three ways that we {229} noticed in light

waves: in amplitude, in wave-length (or vibration rate), and in degree

of mixture of different wave-lengths.

Difference of amplitude (or energy) of sound waves produces difference

of loudness in auditory sensation, which thus corresponds to

brightness in visual sensation. Sounds can be arranged in order of

loudness, as visual sensations can be arranged in order of brightness,

both being examples of intensity series such as can be arranged in any

kind of sensation.

Difference of wave-length of sound waves produces difference in the

_pitch_ of auditory sensation, which thus corresponds to color in

visual sensation. Pitch ranges from the lowest notes, produced by the

longest audible waves, to the highest, produced by the shortest

audible waves. It is customary, in the case of sound waves, to speak

of vibration rate instead of wave-length, the two quantities being

inversely proportional to each other (in the same conducting medium).

The lowest audible sound is one of about sixteen vibrations per

second, and the highest one of about 30,000 per second, while the

waves to which the ear is most sensitive have a vibration rate of

about 1,000 to 4,000 per second. The ear begins to lose sensitiveness

as early as the age of thirty, and this loss is most noticeable at the

upper limit, which declines slowly from this age on.

Middle C of the piano (or any instrument) has a vibration rate of

about 260. Go up an octave from this and you double the number of

vibrations per second; go down an octave and you halve the number of

vibrations. Of any two notes that are an octave apart, the upper has

twice the vibration rate of the lower. The whole range of audible

notes, from 16 to 30,000 vibrations, thus amounts to about eleven

octaves, of which music employs about eight octaves, finding little

use for the upper and lower extremes of the {230} pitch series. The

smallest step on the piano, called the "semitone", is one-twelfth of

an octave; but it must not be supposed that this is the smallest

difference that can be perceived. A large proportion of people can

observe a difference of four vibrations, and keen ears a difference of

less than one vibration; whereas the semitone, at middle C, is a step

of about sixteen vibrations.

_Mixture of different wave-lengths_, which in light causes difference

of saturation, may be said in sound to cause difference of purity. A

"pure tone" is the sensation aroused by a stimulus consisting wholly

of waves of the same length. Such a stimulus is almost unobtainable,

because every sounding body gives off, along with its fundamental

waves, other waves shorter than the fundamental and arousing tone

sensations of higher pitch, called "overtones". A piano string which,

vibrating as a whole, gives 260 vibrations per second (middle C), also

vibrates at the same time in halves, thus giving 520 vibrations per

second; in thirds, giving 780 per second; and in other smaller

segments. The whole stimulus given off by middle C of the piano is

thus a compound of fundamental and overtones; and the sensation

aroused by this complex stimulus is not a "pure tone" but a blend of

fundamental tone and overtones. By careful attention and training, we

can "hear out" the separate overtones from the total blend; but

ordinarily we take the blend as a unit (just as we take the taste of

lemonade as a unit), and hear it simply as middle C of a particular

quality, namely the piano quality. Another instrument will give a

somewhat different combination of overtones in the stimulus, and that

means a different quality of tone in our sensation. We do not

ordinarily analyze these complex blends, but we distinguish one from

another perfectly well, and thus can tell whether a piano or a cornet

is playing. The difference between different instruments, which we

have spoken of as a {231} difference in quality or purity of tone, is

technically known as _timbre_; and the timbre of an instrument depends

on the admixture of shorter waves with the fundamental vibration which

gives the main pitch of a note.

Akin to the timbre of an instrument is the _vowel_ produced by the

human mouth in any particular position. Each vowel appears to consist,

physically, of certain high notes produced by the resonance of the

mouth cavity. In the position for "ah", the cavity gives a certain

tone; in the position for "ee" it gives a higher tone. Meanwhile, the

pitch of the voice, determined by the vibration of the vocal cords,

may remain the same or vary in any way. The vowel tones differ from

overtones in remaining the same without regard to the pitch of the

fundamental tone that is being sung or spoken, whereas overtones move

up or down along with their fundamental. The vowels, as auditory

sensations, are excellent examples of blends, in that, though

compounds, they usually remain unanalyzed and are taken simply as

units. What has been said of the vowels applies also to the

semi-vowels and continuing consonants, such as l, m, n, r, f, th, s

and sh.

Other consonants are to be classed with the noises. Like a vowel, and

like the timbre of an instrument, a noise is a blend of simple tones;

but the fundamental tone in a noise-blend is not so preponderant as to

give a clear pitch to the total sound, while the other tones present

are often too brief or too unsteady to give a tonal effect.

Comparison of Sight and Hearing

The two senses of sight and hearing have many curious differences, and

one of the most curious appears in mixing different wave-lengths.

Compare the effect of throwing two colored lights together into the

eye with the effect of {232} throwing two notes together into the ear.

Two notes sounded together may give either a harmonious blend or a

discord; now the discord is peculiar to the auditory realm; mixed

colors never clash, though colors seen side by side may do so to a

certain extent. A discord of tones is characterized by imperfect

blending (something unknown in color mixing), and by roughness due to

the presence of "beats" (another thing unknown in the sense of sight).

Beats are caused by the interference between sound waves of slightly

different vibration rate. If you tune two whistles one vibration apart

and sound them together, you get a tone that swells once a second;

tune them ten vibrations apart and you get ten swellings or beats per

second, and the effect is rough and disagreeable.

Aside from discord, a tone blend is really not such a different sort

of thing from a color blend. A chord, in which the component notes

blend while they can still, by attention and training, be "heard out

of the chord", is quite analogous with such color blends as orange,

purple or bluish green. At the same time, there is a curious

difference here. By analogy with color mixing, you would expect two

notes, as C and E, when combined, to give the same sensation as the

single intermediate note D. Nothing of the kind! Were it so, music

would be very different from what it is, if indeed it were possible at

all. But the real difference between the two senses at this point is

better expressed by saying that D does not give the effect of a

combination of C and E, or, in general, that no one note ever gives

the effect of a combination or blend of notes higher and lower than

itself. Homogeneous orange light gives the sensation of a blend of red

and yellow; but there is nothing like this in the auditory sphere. In

light, some wave-lengths give the effect of simple colors, as red and

yellow; and other wave-lengths the effect of blends, as greenish

yellow or bluish {233} green; but in sound, every wave-length gives a

tone which seems just as elementary as any other.

There is nothing in auditory sensation to correspond to white, no

simple sensation resulting from the combined action of all

wave-lengths. Such a combination gives noise, but nothing that seems

particularly simple. There is nothing auditory to correspond with

black, for silence seems to be a genuine absence of sensation. There

are no complementary tones like the complementary colors, no tones

that destroy each other instead of blending. In a word, auditory

sensation tallies with its stimulus much more closely than visual

sensation does with its; and the main secret of this advantage of the

sense of hearing is that it has a much larger number of elementary

responses. Against the six elementary visual sensations are to be set

auditory elements to the number of hundreds or thousands. From the

fact that every distinguishable pitch gives a tone which seems as

simple and unblended as any other, the conclusion would seem to be

that each was an element; and this would mean thousands of elements.

On the other hand, the fact that tones close together in pitch sound

almost alike may mean that they have elements in common and are thus

themselves compounds; but still there would undoubtedly be hundreds of

elements.

Both sight and hearing are served by great armies of sense cells, but

the two armies are organized on very different principles. In the

retina, the sense cells are spread out in such a way that each is

affected by light from one particular direction; and thus the retina

gives excellent space information. But each retinal cell is affected

by any light that happens to come from its particular direction. Every

cone, in the central area of the retina, makes all the elementary

visual responses and gives all the possible color sensations; so it is

not strange that the number of visual {234} elements is small. On the

other hand, the ear, having no sound lens, has no way of keeping

separate the sounds from different directions (and accordingly gives

only meager indications of the direction of sound); but its sense

cells are so spread out as to be affected, some by sound of one

wavelength, others by other wave-lengths. The different tones do not

all come from the same sense cells. Some of the auditory cells give

the low tones, others the medium tones, still others the high tones;

and since there are thousands of cells, there may be thousands of

elementary responses.

Theory of Hearing

The most famous theory of the action of the inner ear is the "piano

theory" of Helmholtz. The foundation of the theory is the fact that

the sense cells of the cochlea stand on the "basilar membrane", a

long, narrow membrane, stretched between bony attachments at either

side, and composed partly of fibers running crosswise, very much as

the strings of a piano or harp are stretched between two side bars. If

you imagine the strings of a piano to be the warp of a fabric and

interwoven with crossing fibers, you have a fair idea of the structure

of the basilar membrane, except for the fact that the "strings" of the

basilar membrane do not differ in length anywhere like as much as the

strings of the piano must differ in order to produce the whole range

of notes. Now, a piano string can be thrown into "sympathetic

vibration", as when you put on the "loud pedal" (remove the dampers

from the strings) and then sing a note into the piano. You will find

that the string of the pitch sung has been thrown into vibration by

the action of the sound waves sung against it.

Now suppose the strings of the basilar membrane to be tuned to notes

of all different pitches, within the range of {235} audible

vibrations: then each string would be thrown into sympathetic

vibration whenever waves of its own vibration rate reached it by way

of the outer and middle ear; and the sense cells standing over the

vibrating fibers would be shaken and excited. The theory is very

attractive because it would account so nicely for the great number of

elementary tone sensations (there are over 20,000 fibers or strings in

the basilar membrane), as well as for various other facts of

hearing--if we could only believe that the basilar membrane did

vibrate in this simple manner, fiber by fiber. But (1) the fabric into

which the strings of the membrane are woven would prevent their

vibrating as freely and independently as the theory requires; (2) the

strings do not differ in length a hundredth part of what they would

need to differ in order to be tuned to all notes from the lowest to

the highest, and there is no sign of differences in stretch or in

loading of the strings to make up for their lack of difference in

length; and (3) a little model of the basilar membrane, exposed to

sound waves, is seen to be thrown into vibration, indeed, and into

different forms of vibration for waves of different length, but not by

any means into the simple sort of vibration demanded by the piano

theory. This theory is accordingly too simple, but it probably points

the way towards some truer, more complex, conception.

The fact that there are many elementary sensations of hearing is the

chief reason why the art of tones is so much more elaborate than the

art of color; for while painting might dispute with music as to which

were the more highly developed art, painting depends on form as well

as color, and there is no art of pure color at all comparable with

music, which makes use simply of tones (and noises) with their

combinations and sequences.

{236}

Senses of Bodily Movement

It is a remarkable fact that some parts of the inner ear are not

connected with hearing at all, but with quite another sense, the

existence of which was formerly unsuspected. The two groups of sense

cells in the vestibule--the otolith organs--were formerly supposed to

be the sense organ for noise; but noise now appears to be a compound

of tones, and its organ, therefore, the cochlea. The _semicircular

canals_, from their arrangement in three planes at right angles to

each other, were once supposed to analyze the sound according to the

direction from which it came; but no one could give anything but the

vaguest idea of how they might do this, and besides the ear is now

known to give practically no information regarding the direction of

sound, except the one fact whether it comes from the right or left,

which is given by the difference in the stimulation received by the

two ears, and not by anything that exists in either ear taken alone.

The semicircular canals have been much studied by the physiologists.

They found that injury to these structures brought lack of equilibrium

and inability to walk, swim or fly in a straight course. If, for

example, the horizontal canal in the left ear is destroyed, the animal

continually deviates to the left as he advances, and so is forced into

a "circus movement". They found that the compensatory movements

normally made in reaction to a movement impressed on the animal from

without were no longer made when the canals were destroyed. They found

that something very much like these compensatory movements could be

elicited by direct stimulation of the end-organs in the canals or of

the sensory nerves leading from them. And they found that little

currents of the liquid filling the canals acted as a stimulus to these

end-organs and so aroused the {237} compensatory movements. They were

thus led to accept a view that was originally suggested by the

position of the canals in space.

[Illustration: Fig. 40.--How the sense cells in a semicircular canal

are stimulated by a water current. This current is itself an inertia

back-flow, resulting from a turning of the head in the opposite

direction. (Figure text: water current, nerve to brain)]

Each "semicircular" canal, itself considerably more than a

semicircular tube, opens into the vestibule at each end and thus

amounts to a complete circle. Therefore rotating the head must, by

inertia, produce a back flow of the fluid contents of the canal, and

this current, by bending the hairs of the sense cells in the canal,

would stimulate them and give a sensation of rotation, or at least a

sensory nerve impulse excited by the head rotation.

When a human subject is placed, blindfolded, in a chair that can be

rotated without sound or jar, it is found that he can easily tell

whenever you start to turn him in either direction. If you keep on

turning him at a constant speed, he soon ceases to sense the movement,

but if then you stop him, he says you are starting to turn him in the

opposite {238} direction. He senses the beginning of the rotary

movement because this causes the back flow through his canals; he

ceases to sense the uniform movement because friction of the liquid in

the slender canal soon abolishes the back flow by causing the liquid

to move with the canal; and he senses the stopping of this movement

because the liquid, again by inertia, continues to move in the

direction it had been moving just before when it was keeping pace with

the canal. Thus we see that there are conscious sensations of rotation

from the canals, and that these give information of the starting or

stopping of a rotation, though not of its steady continuance.

Excessive stimulation of the canals gives the sensation of dizziness.

The otolith organs in the vestibule are probably excited, not by

rotary movements, but by sudden startings and stoppings of rectilinear

motion, as in an elevator; and also by the pull of gravity when the

head is held in any position. They give information regarding the

position and rectilinear movements of the head, as the canals do of

rotary head movements. Both are important in maintaining equilibrium

and motor efficiency.

The muscle sense is another sense of bodily movement; it was the

"sixth sense", so bitterly fought in the middle of the last century by

those who maintained that the five senses that were enough for our

fathers ought to be enough for us, too. The question was whether the

sense of touch did not account for all sensations of bodily movement.

It was shown that there must be something besides the skin sense,

because weights were better distinguished when "hefted" in the hand

than when simply laid in the motionless palm; and it was shown that

loss of skin sensation in an arm or leg interfered much less with the

coördinated movements of the limb than did the loss of all the sensory

nerves to the limb.

[Illustration: Fig. 41.--(From Cajal.) A "tendon spindle," very

similar to the muscle spindle spoken of in the text, but found at

the tendinous end of a muscle instead of embedded in the muscle

substance itself, "a" indicates the tendon, and "e" the muscle

fibers; "b" is a sensory axon, and "c" its end-brush about the

spindle. Let the tendon become taut in muscular contraction, and the

fine branches of the sensory axon will be squeezed and so

stimulated.]

Later, the crucial fact was established {239} that sense organs (the

"muscle spindles") existed in the muscles and were connected with

sensory nerve fibers; and that other sense organs existed in the

tendons and about {240} the joints. This sense accordingly might

better be called the "muscle, tendon and joint sense", but the shorter

term, "muscle sense", bids fair to stick. The Greek derivative,

"kinesthesis", meaning "sense of movement", is sometimes used as an

equivalent; and the corresponding adjective, "kinesthetic", is common.

The muscle sense informs us of movements of the joints and of

positions of the limbs, as well as of resistance encountered by any

movement. Muscular fatigue and soreness are sensed through the same

general system of sense organs. This sense is very important in the

control of movement, both reflex and voluntary movement. Without it, a

person lacks information of where a limb is to start with, and

naturally cannot know what movement to make; or, if a movement is in

process of being executed, he has no information as to how far the

movement has progressed and cannot tell when to stop it. Thus it is

less strange than it first appears to learn that "locomotor ataxia", a

disease which shows itself in poor control of movement, is primarily a

disease affecting not the motor nerves but the sensory nerves that

take care of the muscle sense.

{241}

EXERCISES

1. Outline the chapter, rearranging the material somewhat, so as to

state, under each sense, (a) what sense cells, if any, are present

in the sense organ, (b) what accessory apparatus is present in the

sense organ, (c) what stimuli arouse the sense, (d) what are the

elementary responses of the sense, (e) peculiar blends occurring

within the sense or between this sense and another, (f) what can be

said regarding adaptation of the sense, and (g) what can be said

regarding after-images of the sense.

2. Classify the senses according as they respond to stimuli

(a) internal to the body, (b) directly affecting the surface of the

body, (c) coming from a distance.

3. What distinctive _uses_ are made of each sense?

4. Explore a small portion of the skin, as on the back of the hand,

for cold spots, and for pain spots.

5. Try to analyze the smooth sensation obtained by laying the

finger tip on a sheet of paper, and the rough sensation obtained by

laying the finger tip on the surface of a brush, and to describe

the difference in terms of the elementary skin sensations.

6. Is the pain sense a highly developed sense, to judge from its

sense organ? Is it highly specialized? highly sensitive? How does

its peculiarity in these respects fit it for its use?

7. Separation of taste and smell. Compare the taste of foods when

the nostrils are held closed with the taste of the same food when

the nostrils are opened.

8. Make a complete analysis of the sensations obtained from chocolate

ice cream in the mouth.

9. Peripheral vision. (a) Color sense. While your eyes are looking

rigidly straight ahead, take a bit of color in the hand and bring

it slowly in from the side, noticing what color sensation you get

from it when it can first be seen at all, and what changes in color

appear as it moves from the extreme periphery to the center of the

field of view, (b) Form sense. Use printed letters in the same way,

noticing how far out they can be read, (c) Sense of motion. Notice

how far out a little movement of the finger can be seen. Sum up

what you have learned of the differences between central and

peripheral vision. What is the use of peripheral vision?

10. Light and dark adaptation. Go from a dimly lighted place

into bright sunlight, and immediately try for an instant to read

with the sun shining directly upon the page. Remaining in the

sunlight, {242} repeat the attempt every 10 seconds, and notice

how long it takes for the eye to become adapted to the bright

light. Having become light-adapted, go back into a dimly lighted

room, and see whether dark-adaptation takes more or less time than

light-adaptation.

11. Color adaptation. Look steadily at a colored surface, and notice

whether the color fades as the exposure continues. Try looking at

the color with one eye only, and after a minute look at the color

with each eye separately, and notice whether the saturation

appears the same to the eye that has been exposed to the color,

and to the eye that has been shielded.

12. Negative after-images. Look steadily for half a minute at a

black cross upon a white surface, and then turn the eyes upon a

plain gray surface, and describe what you see. (b) Look steadily

for half a minute at a colored spot upon a white or gray

background, and then turn the eyes upon a gray background, and

note the color of the after-image of the spot. Repeat with a

different color, and try to reach a general statement as to the

color of the negative after-image.

13. Positive visual after-images. Look in the direction of a bright

light, such as an electric light, holding the hand as a screen

before the eyes, so that you do not see the light. Withdraw the

hand for a second, exposing the eyes to the light, and immediately

screen the eyes again, and notice whether the sensation of the

light outlasts the stimulus.

14. Tactile after-images. Touch the skin lightly for an instant,

and notice whether the sensation ends as soon as the stimulus is

removed. If there is any after-image, is it positive or negative?

15. Tactile adaptation. Support two fingers on the edge of a table,

and lay on them a match or some other light object. Let this

stimulus remain there, motionless, and notice whether the tactile

sensation remains steady or dies out. What is the effect of making

slight movements of the fingers, and so causing the stimulus to

affect fresh parts of the skin?

16. Temperature sense adaptation. Have three bowls of water, one

quite warm, one cold, one medium. After holding one hand in the

warm water and the other in the cold, transfer both simultaneously

to the medium water and compare the temperature sensations got by

each hand from this water. State the result in terms of

adaptation.

17. Overtones. These can be quite easily heard in the sound of a

large bell. What use does the sense of hearing make of overtones?

REFERENCES

For a somewhat fuller discussion of the topic of sensation, see

Warren's _Human Psychology_, 1919, pp. 151-214; and for a much fuller

discussion, see Titchener's _Textbook of Psychology_, 1909, pp.

46-224.

{243}

For a really thorough consideration of the facts and theories of color

vision, see J. Herbert Parsons, _An Introduction to the Study of

Colour Vision_, 1915.

For a more complete statement of the Ladd-Franklin theory, see the

article on "Vision", in Baldwin's _Dictionary of Philosophy and

Psychology_, 1902.

For a recent study that has revolutionized the psychology of the sense

of smell, see _Der Geruch_, by Hans Henning, 1916, or a review of the

same by Professor Gamble in the _American Journal of Psychology_,

1921, Vol. 32, pp. 290-296.

For an extensive discussion of the "Psychology of Sound", sec the book

with this title by Henry J. Watt, 1917.

For a full account of taste, see Hollingworth and Poffenberger's

_Sense of Taste_, 1917.

{244}

CHAPTER XI

ATTENTION

HOW WE ATTEND, TO WHAT, AND WITH WHAT RESULTS

"Attention!" shouts the officer as a preliminary to some more specific

command, and the athletic starter calls out "Ready!" for the same

purpose. Both commands are designed to put the hearer in an attitude

of readiness for what is coming next. They put a stop to miscellaneous

doings and clear the way for the specific reaction that is next to be

called for. They nullify the effect of miscellaneous stimuli that are

always competing for the hearer's attention, and make him responsive

only to stimuli coming from the officer. They make the hearer clearly

conscious of the officer. They arouse in the hearer a condition of

keen alertness that cannot be maintained for more than a few seconds

unless some further command comes from the officer. In all these ways

"attention" in the military sense, or "readiness" in the athletic

sense, affords a good picture of the psychology of attention.

Attention is preparatory, selective, mobile, highly conscious. To

attend to a thing is to be keenly conscious of that thing, it is to

respond to that thing and disregard other things, and it is to expect

something more from that thing.

Attention is, in a word, exploratory. To attend is to explore, or to

start to explore. Primitive attention amounts to the same as the

instinct of exploration. Its natural stimulus is anything novel or

sudden, its "emotional state" is curiosity or expectancy, and its

instinctive reaction consists {245} of exploratory movements. Its

inherent impulse is to explore, examine, or await.

Attention belongs fundamentally among the native forms of behavior.

The child does not have to learn to attend, though he must learn to

attend to many things that do not naturally get his attention. Some

stimuli naturally attract attention, and others attract attention only

because of previous experience and training. In considering the whole

subject of attention, then, we shall in part be dealing with native

responses, and in part with responses that are acquired. But the great

laws of attention, which will come to light in the course of the

chapter, are at the same time general laws of reaction, and belong

under the head of native characteristics.

The Stimulus, or What Attracts Attention

We can attend to anything whatever, but are more likely to attend to

some things than to others. As stimuli for attention, some objects are

much more effective than others, and the question is, in what way one

object has the advantage over another. There are several ways, several

"factors of advantage", we may call them.

_Change_ is the greatest factor of advantage. A steady noise ceases

after a while to be noticed, but let it change in any respect and

immediately it arrests attention. The ticking of the clock is a good

example: as long as it keeps uniformly on, it is unnoticed, but if it

should suddenly beat faster or louder or in a different key, or even

if it should stop altogether, it would "wake us up" with a start. The

change in the stimulus must not be too gradual if it is to be

effective, it must have a certain degree of suddenness. It may be a

change in intensity, a becoming suddenly stronger or weaker; or it may

be change in quality, as in tone, or {246} color, or odor; or it may

be a change in position, a movement in space. When one who is holding

our arm gives it a sudden squeeze to attract our attention, that is a

change of intensity; when we step from the bank into the water, the

sudden change from warmth to cold, that gets our attention without

fail, is a change of quality; and something crawling on the skin

attracts attention by virtue of its motion. Anything moving in the

field of view is also an unfailing stimulus to attention.

_Strength_, or high intensity of a stimulus, is another important

factor of advantage. Other things being equal, a strong stimulus will

attract attention before a weak one. A loud noise has the advantage

over a low murmur, and a bright flash of light over a faint twinkle.

In the case of visible objects, size has about the same effect as

intensity. The large features of the landscape are noticed before the

little details. The advertiser uses large type, and pays for big space

in the newspaper, in the effort to attract the attention of the

reader.

[Footnote: Often he pays more than the space is worth; at least

doubling the size of his "ad" will not, on the whole, double the

amount of attention he gets, or the number of readers whose

attention he will catch. The "attention value" of an advertisement

has been found by Strong to increase, not as fast as the increase in

space, but about as the square root of the space occupied.]

Another similar factor is _repetition_. Cover a billboard with several

copies of the same picture, and it attracts more attention than a

single one of the pictures would. Repeat a "motive" in the decoration

of a building, and it is more likely to be noticed. Repeat a cry or

call several times, and after a while it may be noticed, though not at

first. The "summation of stimuli" has much the same effect as

increasing the intensity of a single stimulus.

If, however, a stimulus is repeated or continued for a long time, it

will probably cease to hold attention, because of its {247} monotony,

or, in other words, because it lacks the element of change.

_Striking quality_ is an advantage, quite apart from the matter of

intensity. Saturated colors, though no stronger in intensity of light

than pale colors, are stronger stimuli for attention. High notes are

more striking than low. Itch, tickle and pain get attention in

preference to smooth touch. "Striking" cannot be defined in physical

terms, but simply refers to the fact that some kinds of stimulus get

attention better than others.

_Definite form_ has the advantage over what is vague. A small, sharply

defined object, that stands out from its background, attracts the eye

more than a broad, indefinite expanse of light such as the sky. In the

realm of sound, "form" is represented by rhythm or tune, and by other

definite sequences of sound, such as occur in the jingles that catch

the little child's ear.

The factors of advantage so far mentioned are native, and a stimulus

possessing one or more of them is a natural attention-stimulus. But

the individual also learns what is worth noticing, and what is not,

and thus forms _habits of attention_, as well as habits of

inattention. The automobile driver forms the habit of attending to the

sound of his motor, the botanist forms the habit of noticing such

inconspicuous objects as the lichens on the tree trunks. On the other

hand, any one forms the habit of not noticing repeated stimuli that

have no importance for him. Move into a house next the railroad, and

at first you notice every train that passes; even at night you awake

with a start, dreaming that some monster is pursuing you; but after a

few days the trains disturb you very little, night or day. The general

rule covering attention habits is this: anything that you have to work

with, or like to play with, acquires the power to attract your

attention, while anything that you do nothing {248} with loses

whatever hold on your attention it may have possessed by virtue of its

intensity, quality, etc.

Besides these permanent habits of attention, there are temporary

adjustments determined by the _momentary interest_ or desire. Stimuli

relevant to the momentary interest have an unwonted hold upon

attention, while things out of line with this interest may escape

attention altogether, even though the same things would ordinarily be

noticed. What you shall notice in the store window is governed by what

you are looking for as much as by the prominence of the object in the

total display. When you are angry with a person, you notice bad points

about him that you usually overlook, and any aroused desire adjusts or

"sets" attention in a similar way. The desire or interest of the

moment _facilitates_ attention to certain stimuli and _inhibits_

attention to others, and is thus an important factor of advantage.

The interest of the moment is often represented by a question. Ask

yourself what spots of red there are in the field of view, and

immediately various red spots jump out and strike the eye; ask

yourself what pressure sensations you are getting from the skin, and

immediately several obtrude themselves. A question sets attention

towards whatever may furnish an answer.

To sum up, we may say that three general factors of advantage

determine the power of any stimulus to attract attention. There is the

native factor, consisting of change, intensity, striking quality, and

definite form; there is the factor of habit, dependent on past

experience; and there is the factor of present interest and desire.

The Motor Reaction in Attention

Attention is obviously a reaction of the individual to the stimulus

that gets his attention; and it is in part a motor {249} reaction. The

movements that occur in attending to an object are such as to afford a

better view of it, or a better hearing of it, or, in general, such as

to bring the sense organs to bear on it as efficiently as possible.

We may distinguish two sorts of motor reaction that occur in

attention: the general attentive attitude, and the special adjustments

of the sense organs. An audience absorbed in a speech or musical

performance gives a good picture of the general _attentive attitude_.

You notice that most people look fixedly towards the speaker, as if

listening with their eyes, and that many of them lean forward as if it

were important to get just as close as possible. All the little

restless movements cease, so that you could "hear a pin drop", and at

the tensest moments even the breath is checked. The attitude of

attention is one of tense immobility, with the whole body oriented

towards the object of attention. When the object of attention is

something not present but thought of, a somewhat similar rigid

attitude is assumed; the body is apt to lean forward, the neck to be

held stiff, and the eyes to "stare at vacancy", i.e., to be fixed on

some convenient object as a mere resting place, while attention is

fixed outside the visual field altogether.

But we spoke of attention as mobile, and it would be strange if its

mobility did not show itself in the motor reaction. It does in fact

show itself in the _sense organ adjustments_ which amount to

exploratory reactions. Attention to an object in the hand is shown by

"feeling of it", to a substance in the mouth by tasting movements, to

an odor by sniffing movements, to a sound by cocking the head and

turning the eyes towards the source of sound. The most instructive of

this type of attention-reactions are those of the eyes. The eye is

focused on the object that arouses attention, the lens being

accommodated for its distance by the action of the little ciliary

muscle inside the {250} eyeball; the two eyes are converged upon the

object, so that the light from it strikes the fovea or best part of

each retina; and the eyes are also turned up, down or sidewise, so as,

again, to receive the light from the object upon the fovea.

This last class of eye movements is specially instructive and shows

specially well the mobility of attention. Let a bright or moving

object appear somewhere in the field of view--immediately the eyes

turn towards it with a quick jump, fixate it for a few seconds and

then jump elsewhere unless the object is found to be specially

significant. Watch the eyes of one who is looking at a picture or

scene of any sort, and you will see his eyes jumping hither and

thither, as his attention shifts from one part of the scene to

another. Ask him to abstain from this jumpy movement and let his eyes

"sweep over" the scene, and he will confidently try to follow your

instructions, but if you watch his eyes you will find them still

jumping. In fact, "sweeping the glance" is a myth. It cannot be done.

At least, there is only one case in which it can be done, and that is

when there is a moving object to look at. Given an object moving at a

moderate speed across the field of view, and the eyes can follow it

and keep pace with it pretty accurately. But without the moving object

as stimulus, the eyes can only execute the jump movement. There are

thus two types of exploratory eye movement: the "jump" in passing from

one object to another, and the "pursuit movement" in examining a

moving object.

In reading, the eye moves by a series of short jumps from left to

right along the first line of print, makes a long jump back to the

beginning of the second line and another series of short jumps along

that line, and so on. To appreciate the value of this jerky movement,

we need to understand that each short jump occupies but a thirtieth to

a fiftieth {251} of a second, while the "fixation pauses" between

jumps last much longer, with the result that over ninety per cent. of

the time spent on a line of print is fixation time, and less than ten

per cent, is occupied in jumping from one fixation to the next. Now,

it has been found that nothing of any consequence is seen during the

eye jumps, and that the real seeing takes place only during the

fixations. The jump movement, therefore, is simply a means of passing

from one fixation to another with the least possible loss of time.

The eye sees an object distinctly only when at rest with respect to

the object. If the object is still, the eye must be still to see it

distinctly, and to see its different parts must fixate one after the

other, jumping from one part to another. But if the object is in

motion, the eye may still be able to see it distinctly by means of the

pursuit movement, which is a sort of moving fixation.

The Shifting of Attention

Eye movement affords a good picture of the mobility of attention.

Ordinarily the eye shifts frequently from one part of the field of

view to another. When simply exploring a scene, it shifts about in

what seems an indiscriminate way, though really following the

principle of deserting each object as soon as it has been examined,

and jumping to that other object which next has the advantage on

account of movement, brightness, color, definite form, or habit of

attention. In reading, however, the eye is governed by a definite

interest, and moves consecutively along the series of words, instead

of shifting irregularly about the page.

A moving object, or an object that is doing something, or even a

complex object that presents a number of parts to be examined in turn,

can hold the eye for some time. But it is almost impossible to hold

the eye fixed for any length of time on a simple, motionless,

unchanging object.

{252}

Attention is mobile because it is exploratory; it continually seeks

something fresh for examination. In the presence of a complex of

sights and sounds and touch stimuli, it tends to shift every second or

two from one part of the situation to another. Even if you are lying

in bed with your eyes closed, the movement of attention still appears

in the rapid succession of thoughts and images, and some shift usually

occurs as often as once a second.

A few simple experiments will serve to throw the shifting of attention

into clearer relief. Look fixedly at a single letter written on a

blank sheet of paper, and notice how one part after another of the

letter stands out; notice also that attention does not stick

absolutely to the letter, since thoughts obtrude themselves at

intervals.

[Illustration:

O O O

O O

O O O

O O

O O O

Fig. 42.--A dot figure, from Sanford. Look steadily at it.]

Or, make a "dot figure", composed of six or eight or more dots

arranged either regularly or irregularly, and look steadily at the

collection. Probably you will find that the dots seem to fall into

figures and groups, and that the grouping changes frequently.

Objectively, of course, the dots are grouped in one way as much as

another, so that any particular grouping is your own doing. The

objective stimulus, in other words, is capable of arousing several

grouping reactions on your part, and does arouse different reactions

one after another

Shifting also appears in looking at an {253} "ambiguous figure", drawn

so as to represent equally well a solid object in either of two

different positions. The transparent cube, showing near and far edges

alike, is a good example. Look steadily at such a drawing, and the

cube will appear to shift its position from time to time. Numerous

such figures can be constructed; the most celebrated is the ambiguous

staircase. Look steadily at it, and suddenly you see the under side of

a flight of stairs, instead of the upper; and if you keep on looking

steadily, it shifts back and forth between these two positions.

[Illustration: Fig. 43.--The ambiguous cube figure.]

[Illustration: Fig. 44.--The ambiguous staircase figure.]

A still more striking case of shifting goes by the name of "binocular

rivalry", and occurs when colors or figures that we cannot combine

into a single picture are presented, {254} one to one eye, and the

other to the corresponding part of the other retina. Hold red glass

close in front of one eye and blue before the other, and look through

both at once towards a bright background, and you will see red part of

the time and blue part of the time, the two alternating as in the case

of ambiguous figures.

[Illustration: Fig. 45.--Another ambiguous figure, which can be seen

in three ways.]

The stereoscope is a great convenience in applying inconsistent

stimuli to the two eyes, and by aid of this instrument a great variety

of experiments can be made. It is thus found that, if the field before

one eye is a plain color, while the other, of a different color, has

any little figure on it, this figure has a great advantage over the

rival plain color and stays in sight most of the time. Anything moving

in one field has a similar advantage, and a bright field has the

advantage over a darker one. Thus the same factors of advantage hold

good in binocular rivalry as in native attention generally.

A different kind of shifting appears in what is called "fluctuation of

attention". Make a light gray smudge on a white sheet of paper, and

place this at such a distance that the gray will be barely

distinguishable from the white {255} background. Looking steadily at

the smudge, you will find it to disappear and reappear periodically.

Or, place your watch at such a distance that its ticking is barely

audible, and you will find the sound to go out and come back at

intervals. The fluctuation probably represents periodic fatigue and

recovery at the brain synapses concerned in observing the faint

stimulus.

Shiftings of the fluctuation type, or of the rivalry type either, are

not to be regarded as quite the same sort of thing as the ordinary

shiftings of attention. The more typical movement of attention is

illustrated by the eye movements in examining a scene, or by the

sequence of ideas and images in thinking or dreaming. Rivalry and

fluctuation differ from this typical shifting of attention in several

ways:

(1) The typical movement of attention is quicker than the oscillation

in rivalry or fluctuation. In rivalry, each appearance may last for

many seconds before giving way to the other, whereas the more typical

shift of attention occurs every second or so. In fact, during a

rivalry or fluctuation experiment, you may observe thoughts coming and

going at the same time, and at a more rapid rate than the changes in

the object looked at. Attention does not really hold steady during the

whole time that a single appearance of an ambiguous figure persists.

(2) Rivalry shifts are influenced very little, if at all, by the

factor of momentary desire or interest, and are very little subject to

control.

(3) In rivalry, the color that disappears goes out entirely, and in

looking at a dot figure or ambiguous figure you get the same effect,

since the grouping or appearance that gives way to another vanishes

itself for the time being. But when, in exploring a scene with the

eyes, you turn from one object to another, the object left behind

simply retires to the background, without disappearing altogether;

and, {256} in the same way, when attention shifts from one noise to

another, the first noise does not lapse altogether but remains vaguely

heard. Or when, in thinking of a number of people, one after another

comes to mind, the first one does not go out of mind altogether when

attention moves to the next, but remains still vaguely present for a

few moments.

Laws of Attention and Laws of Reaction in General

Shifting occurs also in reflex action. Let two stimuli be acting at

once, the one calling for one reflex and the other for the opposed

reflex (as flexion and extension of the same limb), and the result is

that only one of these reactions will occur at the same time, the

other being completely inhibited; but the inhibited reflex gets its

turn shortly, provided the two stimuli continue to act, and, in fact,

the two reactions may alternate in a way that reminds us of binocular

rivalry or ambiguous figures. Three fundamental laws of reaction here

come to light.

(1) The _law of selection_: of two or more inconsistent responses to

the same situation (or complex of stimuli), only one is made at the

same time.

(2) The _law of advantage_: one of the alternative responses has an

initial advantage over the others, due to such factors as intensity

and change in the stimulus, or to habits of reaction.

(3) The _law of shifting_: the response that has the initial advantage

loses its advantage shortly, and an alternative response is made,

provided the situation remains the same.

These three laws hold good of reactions at all levels, from reflex

action to rational thinking.

The mobility of attention obeys these same laws; only, attention is

livelier and freer in its movements than reflex action or than the

shifting in rivalry. Attention is more mobile and less bound to rigid

rules.

{257}

Sustained Attention

The mobility of attention is only half the story. When we speak, for

instance, of a student as having good powers of attention, we are not

thinking of mobility but rather of the opposite.

Eye movement, which we employed before as a picture of the movement of

attention, affords also a picture of sustained attention. Remember how

the eye moves in reading. Every second it shifts, but still it keeps

to the line of print. Just so, attention keeps moving forward in the

story we are reading, but sticks to the story. The more absorbed we

are in the story, the more rapidly we read. Attention is sustained

here, and still it moves. Sustained attention is not glued to one

point, by any means, but is simply confined to a given object or

theme, within which its motion may be as lively as ever.

What is it, then, that sustains attention? Evidently it is the factor

of present desire or interest, already mentioned. It is a

reaction-tendency, aroused to activity by some stimulus or other,

unable to reach its goal instantly, but persisting in activity for a

while and facilitating responses that are in its line, while

inhibiting others. Such a tendency facilitates response, i.e.,

attention, to certain stimuli, and inhibits attention to others, thus

causing them to be overlooked and neglected.

For the student, the ideal attention-sustainer is an interest in the

matter presented. If, however, he cannot get up any absorbing interest

in the subject-matter at once, he may generate the necessary motive

force by taking the lesson as a "stunt", as something to be mastered,

a spur to his self-assertion. In the old days, fear was often the

motive force relied upon in the schoolroom, and the switch hanging

{258} behind the efficient teacher's desk was the stimulus to

sustained attention. There must be _some_ tendency aroused if

attention is to be sustained. The mastery impulse is certainly

superior to fear for the purpose, but better than either is a genuine

interest in the subject studied.

In order to get up a genuine interest in a subject--an objective or

inherent interest--it is usually necessary to penetrate into the

subject for some little distance. The subject may not appeal to any of

our native impulses, or to any interest that has been previously

acquired, and how then are we to hold attention to it long enough to

discover its inherent interest? Curiosity will give us a start, but is

too easily satisfied to carry us far. Fear of punishment or

disapproval, hope of reward or praise, being put on our mettle, or

realizing the necessity of this subject for our future success, may

keep us going till we find the subject attractive in itself.

So, when the little child is learning to read, the printed characters

have so little attractiveness in themselves that he naturally turns

away from them after a brief exploration. But, because he is scolded

when his mind wanders from those marks, because other children make

fun of his blunders, because, when he reads correctly, he feels the

glow of success and of applause, he does hold himself to the printed

page till he is able to read a little, after which his interest in

what he is reading is sufficient, without extraneous motives, to keep

his nose between the covers of the story book more, perhaps, than is

good for him. The little child, here, is the type of the successful

student.

Attention to a subject thus passes through three stages in its

development. First comes the instinctive exploratory sort of

attention, favored by the native factors of advantage. Next comes the

stage of forced attention, driven by {259} extraneous motives, such as

fear or self-assertion. Finally arrives the stage of objective

interest. In the first and last stages attention is spontaneous, in

the middle stage forced. The middle stage is often called that of

voluntary attention, since effort has to be exerted to sustain

attention, while the first and last stages, being free from effort,

may be called involuntary.

Distraction

Distraction is an important topic for consideration in connection with

sustained attention. A distraction is a stimulus that attracts

attention away from the thing to which we mean to attend. There are

always competing stimuli, and the various factors of advantage,

especially desire or interest, determine which stimulus shall get

attention at any moment.

In the excited insane condition known as "mania" or the "manic state",

the patient is excessively distractible. He commences to tell you

something, all interest in what he has to say, but, if you pull out

your watch while he is talking, he drops his story in the middle of a

sentence and shifts to some remark about the watch. He seems to have

no impulse persistent enough to hold his thoughts steady. There are

contrary insane conditions in which it is almost impossible to

distract the patient from his own inner broodings, so much is he

absorbed in his own troubles.

Distraction is a favorite topic for experiment in the laboratory. The

subject is put to work adding or typewriting, and works for a time in

quiet, after which disturbances are introduced. A bell rings, a

phonograph record is played, perhaps a perfect bedlam of noise is let

loose; with the curious result that the subject, only momentarily

distracted, accomplishes more work rather than less. The distraction

has acted as a stimulus to greater effort, and by this effort {260} is

overcame. This does not always happen so in real life, but it shows

the possibilities of sustained attention.

There are several ways of overcoming a distraction. First, greater

energy may be thrown into the task one is trying to perform. The extra

effort is apt to show itself in gritting the teeth, reading or

speaking aloud, and similar muscular activity which, while entirely

unnecessary for executing the task in hand, helps by keeping the main

stream of energy directed into the task instead of toward the

distracting stimuli. Effort is necessary when the main task is

uninteresting, or when the distraction is specially attractive, or

even when the distraction is something new and strange and likely to

arouse curiosity. But one may grow accustomed or "adapted" to an

oft-recurring distraction, so as to sidetrack it without effort; in

other words, a habit of inattention to the distracting stimulus may be

formed. There is another, quite different way of overcoming a

distraction, which works very well where it can be employed, and that

is to couple the distraction to the main task, so as to deal with both

together. An example is seen in piano playing. The beginner at the

piano likes to play with the right band alone, because striking a note

with the left hand distracts him from striking the proper note with

the right. But, after practice, he couples the two hands, strikes the

bass note of a chord with the left hand while his right strikes the

other notes of the same chord, and much prefers two-handed to

one-handed playing. In short, to overcome a distraction, you either

sidetrack it or else couple it to your main task.

Doing Two Things at Once

The subject of distraction brings to mind the question that is often

asked, "Can any one do two things at once?" In this form, the question

admits of but one answer, for we {261} are always doing at least two

things at once, provided we are doing anything else besides breathing.

We have no trouble in breathing and walking at the same time, nor in

seeing while breathing and walking, nor even in thinking at the same

time. But breathing, walking, and seeing are so automatic as to

require no attention. The more important question then, is whether we

can do two things at once, when each demands careful attention.

The redoubtable Julius Caesar, of happy memory, is said to have been

able to dictate at once to several copyists. Now, Caesar's copyists

were not stenographers, but wrote in long-hand, so that he could speak

much faster than they could write. What he did, accordingly, was

undoubtedly to give the first copyist a start on the first letter he

wished to send, then turn to the second and give him a start on the

second letter, and so on, getting back to the first in time to keep

him busy. Quite an intellectual feat, certainly! But not a feat

requiring absolutely simultaneous attention to several different

matters. In a small way, any one can do something of the same kind. It

is not impossible to add columns of numbers while reciting a familiar

poem; you get the poem started and then let it run on automatically

for a few words while you add a few numbers, switch back to the poem

and then back to the adding, and so on. But in all this there is no

doing of two things, attentively, at the same instant of time.

You may be able, however, to combine two acts into a single

coördinated act, in the way just described under the head of

distraction, and give undivided attention to this compound act.

The Span of Attention

Similar to the question whether we can attentively perform more than a

single act at a time is the question of {262} how many different

objects we can attend to at once. The "span of attention" for objects

of any given kind is measured by discovering how many such objects can

be clearly seen, or heard, or felt, in a single instant of time.

Measurement of this "span" is one of the oldest experiments in

psychology. Place a number of marbles in a little box, take a single

peek into the box and see if you know how many marbles are there. Four

or five you can get in a single glance, but with more there you become

uncertain.

In the laboratory we have "exposure apparatus" for displaying a card

for a fifth of a second or less, just enough time for a single glance.

Make a number of dots or strokes on the card and see whether the

subject knows the number on sight. He can tell four or five, and

beyond that makes many mistakes.

Expose letters not making any word and he can read about four at a

glance. But if the letters make familiar words, he can read three or

four words at a glance. If the words make a familiar phrase, he gets a

phrase of several words, containing as many as twenty letters, at a

single glance.

Expose a number of little squares of different colors, and a

well-trained subject will report correctly as many as five colors,

though he cannot reach this number every time.

Summary of the Laws of Attention

Bringing together now what we have learned regarding the higher and

more difficult forms of attention, as revealed by sustained attention

and work under distraction, by the span of attention and by trying to

do two things at once, we find the previously stated three laws of

attention further illustrated, and a couple of new laws making their

appearance.

(1) The _law of selection_ still holds good in these more {263}

difficult performances, since only one attentive response is made at

the same instant of time. Automatic activities may be simultaneously

going on, but any two attentive responses seem to be inconsistent with

each other, so that the making of one excludes the other, in

accordance with the general law of selection.

What shall we say, however, of reading four disconnected letters at

the same time, or of seeing clearly four colors at the same time?

Here, it would seem, several things are separately attended to at

once. The several things are similar, and close together, and the

responses required are all simple and much alike. Such responses,

under such very favorable conditions, are perhaps, then, not

inconsistent with each other, so that two, three, or even four such

attentive responses may be made at the same time.

(2) The _law of advantage_ holds good, as illustrated by the fact that

some distractions are harder to resist than others.

(3) The _law of shifting_ holds good, as illustrated by the constant

movement of attention, even when it is "sustained", and by the

alternation between two activities when we are trying to carry them

both along simultaneously.

(4) The _law of sustained attention_, or of _tendency_ in attention,

is the same old law of tendency that has shown itself repeatedly in

earlier chapters. A tendency, when aroused to activity, facilitates

responses that are in its line and inhibits others. A tendency is thus

a strong factor of advantage, and it limits the shifting of attention.

(5) A new law has come to light, the _law of combination_, which reads

as follows: _a single response may be made to two or more stimuli_;

or, _two or more stimuli may arouse a single joint response_.

Even though, in accordance with the law of selection, only one

attentive response is made at the same time, more than {264} one

stimulus may be dealt with by this single attentive response. Groups

of four dots are grasped as units, familiar words are grasped as

units. Notice that these units are our own units, not external units.

Physically, a row of six dots is as much a unit as a row of four, but

we grasp the four as a unit in a way that we cannot apply to the six.

Physically, six letters are as much a unit when they do not form a

word as when they do; but we can make a unitary response to the six in

the one case and not in the other. The response is a unit, though

aroused by a number of separate stimuli.

The law of combination, from its name, is open to a possible

misconception, as if we reached out and grasped and combined the

stimuli, whereas ordinarily we do nothing to the stimuli, except to

see them and recognize them, or in some such way respond to them. The

combination is something that happens _in us_; it is our response. If

the expression were not so cumbersome, we might more accurately name

this law that of "unitary response to a plurality of stimuli".

Sometimes, indeed, we do make an actual motor response to two or more

stimuli, as when we strike a chord of several notes on the piano. The

law of combination still holds good here, since the movements of the

two hands are coördinated into a single act, which is thought of as a

unit ("striking a chord"), attended to as a unit, and executed as a

unit. Such coördinated movements may be called "higher motor units",

and we shall find much to say regarding them when we come to the

subject of learned reactions. The law of combination, all in all, will

be found later to have extreme importance in learned reactions.

Passing now to another side of the study of attention, we shall

immediately come across a sixth law to add to our list.

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Attention and Degree of Consciousness

Up to this point, the introspective side of the psychology of

attention has not been considered. One of the surest of all

introspective observations belongs right here, to the effect that we

are more conscious of that to which we are attending than of anything

else. Of two stimuli acting at once upon us, we are the more conscious

of that one which catches our attention; of two acts that we perform

simultaneously, that one is more conscious that is performed

attentively.

We need not be entirely unconscious of the act or the stimulus to

which we are not attending. We may be dimly conscious of it. There are

degrees of consciousness. Suppose, for example, you are looking out of

the window while "lost in thought". You are most conscious of the

matter of your thoughts, but conscious to a degree of what you see out

of the window. Your eyes are focused on some particular object

outside, and you are more conscious of this than of other objects seen

in indirect vision, though even of these last you are not altogether

unconscious. Consciousness shades off from high light to dim

background.

The "field of attention" is the maximum or high light of

consciousness; it comprises the object under attentive observation,

the reaction attentively performed. The "field of consciousness"

includes the field of attention and much besides. It includes objects

of which we are vaguely aware, desires active but not clearly

formulated, feelings of pleasantness or unpleasantness, of tension,

excitement, confidence, etc.

Apparently the field of consciousness shades off gradually into the

field of unconscious activity. Some physiological processes go on

unconsciously, and very habitual movements may be almost or entirely

unconscious. The boundary {266} between what is vaguely conscious and

what is entirely unconscious is necessarily very vague itself, but the

probability is that the field of consciousness is broader than we

usually suspect, and that many activities that we ordinarily think of

as unconscious, because we do not observe them at the time nor

remember them later, lie really near the margin of the field of

consciousness, but inside that field. "Unconscious motives", such as

spite or pride often seem to be, are probably vaguely conscious rather

than unconscious. We shall return to the fascinating topic of the

unconscious at the close of the book.

Degree of consciousness does not always tally with intensity of

sensation or energy of muscular action. You may be more conscious of a

slight but significant sound than of much louder noises occurring at

the same time. You may be more conscious of a delicate finger movement

than of a strong contraction of big muscles occurring at the same

time. Degree of consciousness goes with degree of mental activity. Of

all the reactions we are making at the same time--and usually there

are several--the most active in a mental way is the most conscious.

The slight sound arouses intense mental response because it means

something of importance--like the faint cry of the baby upstairs,

noticed instead of the loud noises of the street. The delicate finger

movement aims at some difficult result, while the big muscles may be

doing their accustomed work automatically.

It is not always the most efficient mental process that is most

conscious; indeed, practising an act makes it both more efficient and

less conscious. It is, rather, the less efficient processes that

require attention, because they require mental work to keep them going

straight.

Our sixth law of attention, emerging from this introspective study, is

naturally of a different style from the remainder of the list, which

were objectively observed; yet it {267} is no less certain and perhaps

no less significant. It may be called:

(6) The _law of degrees of consciousness_, and thus stated: _An

attentive response is conscious to a higher degree than any

inattentive response made at the same time_. An inattentive response

may be dimly conscious or, perhaps, altogether unconscious. The less

familiar the response, and the higher it stands in the scale of mental

performances, the more attentive it is, and the more conscious.

The Management of Attention

Attentive observation is more trustworthy than inattentive, and also

gives more facts. Attentive movement is more accurate than

inattentive, and may be quicker as well. Attentive study gives quicker

learning than inattentive, and at the same time fixes the facts more

durably.

Shall we say, then, "Do everything attentively"? But that is

impossible. We sense so many stimuli at once that we could not

possibly attend to all of them. We do several things at once, and

cannot give attention to them all. A skilful performance consists of

many parts, and we cannot possibly give careful attention to all the

parts. Attention is necessarily selective, and the best advice is, not

simply to "be attentive", but to attend to the right things.

In observation, the best plan is obviously to decide beforehand

exactly what needs to be observed, and then to focus attention on this

precise point. That is the principle underlying the remarkably sure

and keen observation of the scientist. Reading may be called a kind of

observation, since the reader is looking for what the author has to

tell; and the rule that holds for other observation holds also for

reading. That is to say that the reader finds the most when he knows

just what he is looking for. We can learn {268} something here from

story-reading, which is the most efficient sort of reading, in the

sense that you get the point of the story better than that of more

serious reading matter, the reason being that attention is always

pressing forward in the story, looking for something very definite.

You want to know how the hero gets out of the fix he is in, and you

press forward and find out with great certainty and little loss of

time. The best readers of serious matter have a similar eagerness to

discover what the author has to say; they get the author's question,

and press on to find his answer. Such readers are both quick and

retentive. The dawdling reader, who simply spends so much time and

covers so many pages, in the vague hope that something will stick,

does not remember the point because he never got the point, and never

got it because he wasn't looking for it.

In skilled movement, or skilled action of any sort, the best rule is

to fix attention on the end-result or, if the process is long, on the

result that immediately needs to be accomplished. "Keep your eye on

the ball" when the end just now to be achieved is hitting the ball.

Attention to the details of the process, though necessary in learning

a skilled movement, is distracting and confusing after skill has been

acquired. The runner does not attend to his legs, but to the goal or,

if that is still distant, to the runner just ahead of him.

Theory of Attention

The chief facts to take account of in attempting to form a conception

of the brain action in attention are mobility, persistence in spite of

mobility, and focusing.

The mobility of attention must mean that brain activities are in

constant flux, with nerve currents continually shooting hither and

thither and arousing ever fresh groups of neurones; but sustained

attention means that a brain {269} activity (representing the desire

or interest or reaction-tendency dominant at the time) may persist and

limit the range of the mobile activities, by facilitating some of

these and inhibiting others.

The "focusing" of mental activity is more difficult to translate into

neural terms. The fact to be translated is that, while several mental

activities may go on at once, only one occupies the focus of

attention. This must mean that, while several brain activities go on

at once, one is superior in some way to the rest. The superiority

might lie in greater intensity of neurone action, or in greater

extent; that is, one brain activity is bigger in some way than any

other occurring at the same time--bigger either because the neurones

in it are working more energetically or because it includes a larger

number of active neurones.

But why should not two equally big brain activities sometimes occur at

the same moment, and attention thus be divided? The only promising

hypothesis that has been offered to explain the absence of divided

attention is that of "neurone drainage", according to which one or the

other of two neurone groups, simultaneously aroused to activity,

drains off the energy from the other, so putting a quietus on it.

Unfortunately, this hypothesis explains too much, for it would make it

impossible for minor brain activities to go on at the same time as the

major one, and that would mean that only one thing could be done at a

time, and that the field of consciousness was no broader than the

field of attention. On the whole, we must admit that we do not know

exactly what the focusing of attention can mean in brain terms.

{270}

EXERCISES

1. Outline the chapter, in the form of a number of "laws", putting

under each law the chief facts that belong there.

2. See if you can verify, by watching another person's eyes, the

statements made on page 250 regarding eye movements.

3. Choose a spot where there is a good deal going on, stay there

for five minutes and jot down the things that attract your

attention. Classify the stimuli under the several "factors of

advantage".

4. Mention some stimulus to which you have a habit of attention,

and one to which you have a habit of inattention.

5. Close the eyes, and direct attention to the field of cutaneous

and kinesthetic sensations. Do sensations emerge of which you are

ordinarily only dimly conscious? Does shifting occur?

6. Of the several factors of advantage, which would be most effective

in catching another person's attention, and which in holding his

attention?

7. How does attention, in a blind person, probably differ from that

of a seeing person?

8. Doing two things at once. Prepare several columns of one-place

numbers, ten digits in a column. Try to add these columns, at the

same time reciting a familiar poem, and notice how you manage it,

and how accurate your work is.

9. Consider what would be the best way to secure sustained

attention to some sort of work from which your mind is apt to wander.

REFERENCES

Walter B. Pillsbury gives a full treatment of the subject in his book

on _Attention_, 1908, and a condensed account of the matter in Chapter

V of his _Essentials of Psychology_, 2nd edition, 1920.

Another full treatment is that of Titchener, in his _Textbook of

Psychology_, 1909, pp. 265-302.

On the topic of distraction, see John J. B. Morgan's _Overcoming of

Distraction and Other Resistances_, 1916.

{271}

CHAPTER XII

INTELLIGENCE

HOW INTELLIGENCE IS MEASURED, WHAT IT CONSISTS IN AND EVIDENCE OF ITS

BEING LARGELY A MATTER OF HEREDITY

Before leaving the general topic of native traits and passing to the

process of learning or acquiring traits, we need to complete our

picture of the native mental constitution by adding intelligence to

reflex action, instinct, emotion, feeling, sensation and attention.

Man is an intelligent animal by nature. The fact that he is the most

intelligent of animals is due to his native constitution, as the fact

that, among the lower animals, some species are more intelligent than

others is due to the native constitution of each species. A rat has

more intelligence than a frog, a dog than a rat, a monkey than a dog,

and a man than a monkey, because of their native constitutions as

members of their respective species.

But the different individuals belonging to the same species are not

all equal in intelligence, any more than in size or strength or

vitality. Some dogs are more intelligent than others, and the same is

notably true of men. Now, are these differences between members of the

same species due to heredity or environment? This question we can

better approach after considering the methods by which psychologists

undertake to measure intelligence; and an analysis of these methods

may also serve to indicate what is included under the term

"intelligence".

{272}

Intelligence Tests

Not far from the year 1900 the school authorities of the city of

Paris, desiring to know whether the backwardness of many children in

school resulted from inattention, mischievousness and similar

difficulties of a moral nature, or from genuine inability to learn,

put the problem into the hands of Alfred Binet, a leading psychologist

of the day; and within a few years thereafter he and a collaborator

brought out the now famous Binet-Simon tests for intelligence. In

devising these tests, Binet's plan was to leave school knowledge to

one side, and look for information and skill picked up by the child

from his elders and playmates in the ordinary experience of life.

Further, Binet wisely decided not to seek for any _single_ test for so

broad a matter as intelligence, but rather to employ many brief tests

and give the child plenty of chances to demonstrate what he had

learned and what he could do. These little tests were graded in

difficulty from the level of the three-year-old to that of the

twelve-year-old, and the general plan was to determine how far up the

scale the child could successfully pass the tests.

These were not the first tests in existence by any means, but they

were the first attempt at a measure of general intelligence, and they

proved extraordinarily useful. They have been added to and revised by

other psychologists, notably by Terman in America, who has extended

the scale of tests up to the adult level. A few samples from Terman's

revision will give an idea of the character of the Binet tests.

From the tests for three-year-olds: Naming familiar objects--the

child must name correctly at least three of five common objects that

are shown him.

Six-year test: Finding omissions in pictures of faces, from which

the nose, or one eye, etc., is left out. Four such pictures are

shown, and three correct responses are required to pass the test.

Eight-year test: Tell how wood and coal are alike; and so with three

other pairs of familiar things; two out of four correct responses

are required to pass the test.

{273}

Twelve-year test: Vocabulary test--rough definitions showing the

child's understanding of forty words out of a standard list of one

hundred.

The question may be raised, "Why such arbitrary standards-three out of

five required here, two out of four there, forty out of a hundred the

next time?" The answer is that the tests have been standardized by

actual trial on large numbers of children, and so standardized that

the average child of a given age can just barely pass the tests of

that age.

Intelligence is measured by Binet on a scale of _mental age_. The

average child of, let us say, eight years and six months is said to

have a mental age of eight years and six months; and any individual

who does just as well as this is said to have this mental age, no

matter what his chronological age may be. The average child of this

age passes all the tests for eight years and below, and three of the

six tests for age nine; or passes an equivalent number of tests from

the total series. Usually there is some "scatter" in the child's

successes, as he fails in a test here and there below his mental age,

and succeeds here and there above his mental age, but the failures

below and the successes above balance each other in the average child,

so that he comes out with a mental age equal to his chronological age.

[Footnote: The Binet scale, it must be understood, is an instrument

of precision, not to be handled except by one who has been

thoroughly trained in its use. It looks so simple that any student

is apt to say, "Why, I could give those tests!" The point is that he

couldn't--not until he knew the tests practically by heart, not till

he had standardized his manner of conducting them to agree perfectly

with the prescribed manner and till he knew how to score the varying

answers given by different children according to the scoring system

that goes with the tests, and not till, by experience in handling

children in the tests, he was able to secure the child's confidence

and get him to do his best, without, however, giving the child any

assistance beyond what is prescribed. Many superior persons have

looked down on the psychological examiner with his (or her)

assortment of little tests, and have said, "Certainly no special

training is necessary to give these tests. You simply want to find

out whether the child can do these stunts. I can find out as well as

you." They miss the point altogether. The question is not whether

the child can do these stunts (with an undefined amount of

assistance), but whether he _does_ them under carefully prescribed

conditions. The child is given two, three or four dozen chances to

see how many of them he will accept; and the whole scale has been

standardized by try-out on many children of each age, and so adapted

that when given according to instructions, it will give a correct

measure of the child's mental age. But when given by superior

persons in ignorance of its true character, it gives results very

wide of the mark. So much by way of caution.]

{274}

If a child's mental age is the same as his chronological age, he is

just average, neither bright nor dull. If his mental age is much above

his chronological, he is bright; if much below, dull. His degree of

brightness or dullness can be measured by the number of years his

mental age is above or below his chronological age. He is, mentally,

so many years advanced or retarded.

Brightness or dullness can also be measured by the _intelligence

quotient_, which is employed so frequently that it is customarily

abbreviated to "IQ". This is the mental age divided by the

chronological, and is usually expressed in per cent. The IQ of the

exactly average child, of any age, is 1, or 100 per cent. The IQ of

the bright child is above 100 and of the dull child below 100. About

sixty per cent. of all children have an IQ between 90 and 110, twenty

per cent, are below 90 and twenty per cent, above 110. The following

table gives the distribution in somewhat greater detail:

IQ below 70, 1%

IQ 70-79, 5%

IQ 80-89, 14%

IQ 90-99, 30%

IQ 100-109, 30%

IQ 110-119, 14%

IQ 120-129, 5%

IQ over 129, 1%

---

100

{275}

For convenience, those with IQ under 70 are sometimes labeled

"feeble-minded", and the others, in order, "borderline", "low normal",

"average" (from 90 to 110), "superior", "very superior", "exceedingly

superior"; but this is arbitrary and really unscientific, for what the

facts show is not a separation into classes, but a continuous

gradation from one extreme to the other. The lower extreme is near

zero, and the upper extreme thus far found is about 180.

While the mental age tells an individual's intellectual level at a

given time, the IQ tells how fast he has progressed. An IQ of 125

means that he has picked up knowledge and skill 25 per cent. faster

than the average individual--that he has progressed as far in four

years as the average child does in five, or as far in eight as the

average does in ten, or as far in twelve as the average does in

fifteen. The IQ usually remains fairly constant as the child grows

older, and thus represents his rate of mental growth. It furnishes a

pretty good measure of the individual's intelligence.

Performance Tests

Since, however, the Binet tests depend greatly on the use of language,

they are not fair to the deaf child, nor to the child with a speech

defect, nor to the foreign child. Also, some persons who are clumsy in

managing the rather abstract ideas dealt with in the Binet tests show

up better in managing concrete objects. For all such cases,

_performance tests are useful. Language plays little part in a

performance test_, and concrete objects are used. The "form board" is

a good example. Blocks of various simple shapes are to be fitted into

corresponding holes in a board; the time of performance is measured,

and the errors (consisting in trying to put a block into a differently

shaped hole) are also counted. To the normal adult, this task seems

too simple {276} to serve as a test for intelligence, but the young

child finds it difficult, and the mentally deficient adult goes at it

in the same haphazard way as a young child, trying to force the square

block into the round hole. He does not pin himself down to the one

essential thing, which is to match blocks and holes according to

shape.