SENSATION
Now that these points have been determined, let us proceed to a
general discussion of the whole subject of Sensation.—ARISTOTLE
§ 9. =Sensations from the Skin.=—The skin is part of our organic
birthright. One of the great differences between the living and the
not-living lies in the possession of a skin; stone and iron weather and
rust, but even the naked amœba has its ectosarc, and flowers of tan
their plasmoderm. The skin is also the oldest of the sense-organs, and
the mother of all the rest; how old, we dare hardly guess; but we know
that the chemical elements which make up living tissue took form early
in the history of our planet, earlier than the heavy metals. So it is
natural to begin our survey of sensations by questioning the skin.
The skin is a shifty witness; and to get positive answers, we must
literally cross-examine it; we must go over its surface point by
point and line by line, with all sorts of mechanical and thermal and
electrical and chemical stimuli. The outcome is a little surprising;
we find only =four sensations, pressure, cold, warmth and pain=. The
organs of these sensations are dotted in a sort of irregular mosaic
all over the skin, and the intervening spaces are insensitive. The
organs of pressure, distributed over about 95% of the bodily surface,
are nerve-skeins twined about the roots of the hairs; on the hairless
areas of the body, we find the nerve-skein by itself. The organ of pain
is probably a little brush-like bunch of nerve-fibrils just below the
epidermis. The organs of warmth and cold are certainly distinct; the
sensations are not degrees of one sensation, as the thermometer might
lead us to suppose; but the precise nature of their nerve-endings has
not yet been made out.
You may easily find pressure spots by fastening a short horsehair with
sealing-wax at right angles to the end of a match, and applying the
horsehair point to the back of the hand above a hair-bulb, that is,
just to windward of the issuing hair; dot the horsehair about, here
and there, till the sensation flashes up. You may find cold spots by
passing the blunt point of a lead pencil slowly across the closed
eyelid. Warm spots are more difficult to demonstrate. For pain, take
the shaft of a pin loosely between finger and thumb of the right hand,
and bring the point down sharply on the back of the left hand; you get
two sensations; the first is a pressure, the second—which pricks or
stings—is a pain.
As a rule, these organs are not stimulated separately but in
groups. Itch, for instance, is due to the light stimulation of a
field of pain-endings, and superficial tickle to that of a field of
pressure-organs. The experience of =heat=, curiously enough, is a blend
of warmth and cold; there are no heat spots. It may be observed in this
way: if you apply a surface of increasing warmth to a region of the
skin which has both cold and warm spots, you feel for some time only
the warmth; but when the stimulus has reached a certain temperature,
the cold spots, suddenly and paradoxically, flash out their sensations
of cold; and the blend of warmth and of paradoxical cold is felt
as heat. Cement a smooth copper coin to a handle, and apply it at
gradually increasing temperature to the middle of the forehead just
under the hair; you will presently find the heat. Or if you cannot do
that, note the shiver of cold when you next step into an overhot bath.
When we compare these results with the show that the skin makes as a
sense-organ in everyday life, we can hardly help bringing against it
the charge of dishonesty. The pressure spots give us tickle, contact
or light pressure, and pressure proper; the pain spots, itch, prick or
sting, and pain proper. The cold spots give cold and cool, the warm
spots lukewarm and warm; cold and warm spots together give heat; cold
and pain give biting cold; cold and warm and pain give burning or
scalding heat; and that is all. Yet the skin pretends to tell us of
hard and soft, wet and dry, light and heavy, rough and smooth, yielding
and resistant, sharp and blunt, clammy and greasy, oily and sticky,
stiff and elastic, and so on. Where do we get all these experiences?
§ 10. =Kinæsthetic Sensations.=—We get them, for the most part,
from the cooperation with the skin of certain deeper-lying tissues.
Psychologists have long suspected the existence of a muscle sense. We
now know that sensations are derived, not only from the =muscles=,
but also from the =tendons= and the capsules of the =joints=. These
tissues are, of course, closely bound together, and are all alike
affected by movement of a limb or of the body. Their disentanglement,
from the point of view of sensation, has been a slow and difficult
matter. Psychology has here been greatly aided by pathology; for there
are diseases in which the skin alone is insensitive, in which skin
and muscles alone are insensitive, and in which the whole limb is
insensitive; so that a first rough differentiation is made for us by
nature herself. It is also possible artificially to anæsthetise muscle
and joint; and psychologists have devised various forms of experiment
whereby some single tissue is thrown into relief above the others.
Not only, however, are the sensations of these tissues aroused by
movement; they also form the sensory basis of our perception of the
movement of body and limbs. For this reason they have been named
=kinæsthetic=, or movement-perceiving. They are of the following kinds.
First, we have from the muscles the sensation of physical =fatigue=.
If the skin over a muscle is rendered anæsthetic, and the muscle is
thrown into forced contraction by an electric current, we have, to
begin with, a dull dead pressure; as time goes on, or if the strength
of the current is increased, this pressure becomes dragging, the
sensation of fatigue; and finally it becomes sore and achy, and passes
over into dull pain. From the tendons we get a sensation which, when
we are actively pushing or pulling, we call =effort=, and when we are
passively holding or resisting we call =strain=; it, too, passes over
into pain. Lastly, from the joints we have a =pressure=: something like
the pressure you feel if you smear the right forefinger with vaseline,
and turn it in the loosely closed left hand. Take a piece of elastic
between the forefingers and thumbs; pull it out, and then relax it;
at the moment of relaxation there is a pressure in the finger-joints,
which is the specific joint-sensation.
_Muscle and joint, then, yield sensations which are like those of
pressure on the skin; and muscle and tendon yield sensations which
are like those of pain from the skin_; it is small wonder that the
skin, the only portion of this whole sensory apparatus that is open to
view, should ordinarily be credited with the entire number. In point
of fact, there are very few of the experiences listed on p. 45 that do
not imply the cooperation of some or all of the deeper-lying organs,
the nerve-spindles of muscle and tendon and the nerve-corpuscles of
the joints. Those that really belong to the skin owe their specific
character to the context in which they are set; they change their
meaning as a particular word changes its meaning from one sentence to
another; think of the horribly clammy feel of a bit of cold boiled
potato as you set your finger on it in the dark, and of its totally
different feel when you have turned the light on and see what it is you
are touching! Wetness, for instance, proves on analysis to be a complex
of pressure and temperature; it is possible, when the observer does
not know the nature of the stimulus, to arouse the feel of wet from
perfectly dry things, such as powder, or cotton wool, or bits of metal;
and it is possible to wet the observer’s hand with water and yet to
arouse the feel only of a dry pressure or a dry warmth or cold.
So our very first adventure in psychology brings out, as clearly as
we need wish, the _difference between science and common sense_.
The skin is really living upon borrowed capital; it has added to
its own sensations those derived from the subjacent tissues; but
common sense, blind to what it cannot see, ascribes to it a ‘sense
of touch’ that includes everything and examines nothing. More than
this, common sense fails to draw the distinction between process and
meaning which we discussed in § 6, and therefore ascribes to the sense
of touch a variety of sensory experience that far outruns the facts.
Hardness and softness and stickiness and oiliness and the rest are,
no doubt, separate and distinct as meanings; but when we analyse the
corresponding experiences, we find only the half-dozen sensations
mentioned above.
§ 11. =Taste and Smell.=—The great physiologist Carl Ludwig once
remarked that smell is the most unselfish of all the senses; it gives
up everything it has to taste, and asks nothing in return. Taste is,
indeed, an inveterate borrower; it borrows from smell and from touch,
very much as the skin borrows from the underlying organs. When we have
a cold in the head, we say that we cannot taste; but how is taste
affected? The truth is that our nose is stopped, and we cannot smell.
If the surface of the =tongue= is explored with various sorts
of stimuli, and the nose is kept out of function by plugging of
the nostrils, we find =four sensations: sweet, bitter, sour, and
salt=. Think, then, how much ‘taste’ there would be in the meats
and vegetables that deck our tables, if the nose were closed and
condiments were not added! The sensation of sweet is strongest at
the tip of the tongue; bitter at the root; sour along the sides; salt
is fairly evenly distributed over all three areas; the middle region
of the tongue is insensitive to taste. The sensory cells are grouped
in flask-shaped structures, the taste-buds or taste-beakers, which
are again gathered together in or about the papillæ of the tongue’s
surface; some of these you can see, as red specks upon the dull pink
mucous membrane, if you look at the tip of your tongue in a glass.
There is only one instance of a blend of tastes; if sweet and salt are
mixed, there appears a new taste, flat or vapid in character. Apart
from these five things—sweet, bitter, sour, salt, vapid,—we ‘taste’
entirely by smell or touch.
=Smell=, on the other hand, =has more sensations than we can count
or name=; more sensations, probably, than all the rest of our senses
put together. We can make out certain great groups of odours: flower,
fruit, spicy, musky, leek, burned, rank, foul, nauseous; we may take
as examples vanilla, orange, cinnamon, sandalwood, onion, toast,
cheese, opium, garbage. Realise that the flower odours comprise the
scents of all the flowers, as well as those of vanilla, tea, hay, and
suchlike things; or that the spicy odours comprise the scents of all
the spices, as well as those of thyme, geranium, bergamot, cedarwood,
and suchlike things; and you will get some idea of the variety of the
world of smell. When we add that odours freely blend or combine to give
new scents, you will understand that the number of smell sensations is
enormous.
The sensory cells are found in two patches of mucous membrane,
each about as big as the little-finger nail, which lie saddle-wise
across the blind top of the nasal cavities. They cannot be stimulated
directly; but particles carried into the outer nostrils by the
breath-stream, or into the inner nostrils by the air-stream thrown
back in the act of swallowing, eddy upward to them and thus arouse
sensation. The second mode of stimulation plays, of course, into the
hands of taste; we think we _taste_ when we swallow; we forget that
we have inner nostrils, though we know very well that we can sniff up
a lotion and bring it down into the back of the mouth. But though the
stimulation is thus indirect, the cells are extraordinarily sensitive;
a mere trace of odorous substance will set up a sensation; and the nose
is also keenly discriminative.
Yet in spite of the tens of thousands of sensations, and in spite of
the extraordinary sensitivity of the cells, we often read that in man
the sense of smell is degenerating! Of this there is not one particle
of evidence. We could not, truly, live by smell, as dogs do; but then
men have never been dogs; and even so there are cases on record—among
the Botocudos of Brazil and the aboriginal tribes of the Malay
peninsula—of savage hunters who track their game by scent. There is
no atom of evidence that, since man was man, his sense of smell has
degenerated. It is true, on the other hand, that =the sense of smell
has fallen into disuse=. The reason is that smell is essentially a
ground sense, as you may convince yourself any summer day that you
lie out on the grass, or any time that you are willing to spend a few
minutes on a dining-room floor; birds in general have a very obtuse
sense of smell, and many of them perhaps lack sensations of smell
altogether. When, then, mankind assumed the upright position, and the
nostrils were lifted several feet above the surface of the ground,
the sense was removed from its normal environment, and fell into
disuse; sight and hearing took its place. But it may still be used.
The late Sir Francis Galton, a cousin of Darwin’s, once made an essay,
for instance, at an =arithmetic by smell=; peppermint stood for one,
camphor for two, carbolic acid for three, and so on. “There was not the
slightest difficulty in banishing all visual and auditory images from
the mind, leaving nothing in consciousness besides real or imaginary
scents. In this way I convinced myself of the possibility of doing
sums in simple addition with considerable speed and accuracy solely by
means of imaginary scents. Subtraction succeeded as well as addition.”
Needless to say, it is not worth our while to do this sort of work;
the very fact that odours have no settled system of names, like cold
or pain, red or blue, shows that they have not been utilized in human
life. It is fair to add, also, that sight and hearing are better suited
than smell to our everyday needs; for smells very soon fade out and
disappear; indeed, if they did not, the work of garbage collectors
or of medical students in the dissecting room would be permanently
disagreeable.
§ 12. =Sensations from the Ear.=—Sensations of hearing fall into two
great groups, tones and noises. When we are speaking of =tones=, we
naturally think of the keyboard of a piano. The piano tones are, in
reality, not simple tones or sensations but compound tones; and we are
able, after a little practice, to break up a compound tone into its
simple constituents. You may get a fair notion of a really simple tone
by blowing gently across the mouth of an empty bottle. The tone is dull
and hollow, as compared with the bright solidity of a piano tone, but
it has also a pleasant mellowness. With these two aids, the bottle tone
and the piano keyboard, we may approach our study of tonal sensations.
Tones have, first of all, the character that we call =pitch=; they lie,
that is, up or down in the scale; they belong to the bass or the treble
or to a middle region. The word ‘pitch’ means height; it is a term
borrowed from perceptions of sight; and we cannot yet say certainly how
it came to be applied to tones. Secondly, tones have the character of
=volume=,—another borrowed word! The highest note on the piano seems
shrunken, narrowed, pointed, as compared with the deepest note in the
bass; and the difference comes out even more clearly with bottle tones.
Thirdly, tones show a sort of recurrence. If you run your finger-nail
quickly up the keyboard in a _glissando_, you perceive a change only
of pitch and volume; but if you play the notes _c_, _d_, _e_ in one
octave and then in another and then in a third, you realise that all
the sequences are alike; we talk, indeed, of playing the _same_ notes
in _different_ octaves. This recurring character of tones is called
=tonality=.
It has recently been stated that tones have a further character, that
of =vocality=. Consider the series of vowels, U, O, A, E, I (voiced
approximately as in the words _moot_, _moat_, _mart_, _mate_, _meet_);
there is no doubt that U suggests a low bottle tone, and I a high
whistle tone. Experiments seem to show that, as we go up the scale, the
tones say M-M, U, O, A, E, I, S-S, F-F, CH (the sound in the Scotch
_loch_); and, curiously enough, that they say these things at intervals
of an octave; so that, when we have found a pure O, we find the pure A
just an octave higher, and the tones that lie between give Oa, OA, oA,
according to their position. The question is still in debate; for these
experiments are opposed by others, and the whole subject of the nature
of vowel-sounds is very thorny. It is quite clear that high and low
tones sound definitely like U and I; but some of the other vowels are
far less distinct; and the point of change from vowel to vowel does not
appear to be as sharp and precise as the first experiments indicated.
On the whole, _we shall do best to suspend judgement_.
There are some ten thousand simple tones in the complete tonal scale;
but the compound tones employed by music are only about a hundred in
number, and are selected from a middle range of hearing. The compound
tone, as we have said, breaks up on analysis into simple partial tones;
the lowest is called the =fundamental=, the others the =overtones=.
It is a remarkable fact that the overtones always stand in a definite
relation to the fundamental. The various musical instruments do not,
however, sound all the overtones alike; their construction favours
some, and weakens or destroys others; and that is the main reason why
we can tell a harp-tone, for instance, from a tone of the same pitch
played on oboe or trumpet. The compound tones thus owe their =colour=
or =timbre=, in the first instance, to _the number and relative
loudness of the overtones which accompany the fundamental_. Timbre has
other factors; but this is the primary source of difference.
_Overtones may readily be heard._ Strike a _c_, very lightly, on the
piano. When it has ceased to sound, strike loudly the _c_ next below;
you can probably, even at the first trial, hear the higher _c_ in the
lower. Now strike very lightly the _g_ next above your higher _c_,
and then the lower _c_ again loudly; you will probably hear the _g_.
Helmholtz, working with thin strings, was able to hear no less than
fifteen overtones with the fundamental.
This blending of the partial tones in a compound tone, to give a single
and unitary impression, is an example of what is called =tonal fusion=.
The best fusion is that of two tones which constitute an octave; here,
indeed, the blend is so close that it is often confused with unison;
a soprano and a bass singer, told to sing in unison, will start off
without hesitation an octave apart. Next after the octave stands the
fifth (_c_ and _g_); boys who think they are whistling the same notes
often whistle, in fact, a fifth apart. Other pairs of tones give lesser
degrees of fusion.
_Tones generate as well as blend._ If you sound together two high
tones, such as you get from a double bicycle whistle, or from small
bottles of different sizes, you hear, besides these tones themselves, a
third tone, very much deeper, larger, more booming; this =differential
tone= is easy to find and, once heard, cannot be mistaken. Only, the
two tones must not be too nearly alike in pitch; for, if they are, you
hear, instead of a differential tone, slow surges or quick rattlings of
sound. Take two bottles of the same size, and mistune one of them by
pouring in small amounts of water; have them blown steadily together;
the course of the =beats=, as they are called, from a slow surge
through a rattle to a harsh blur, may thus be followed.
=Noises=, which form a class of sensations distinct from tones, are
nevertheless aroused by the same sort of stimuli. If a tonal stimulus
is sounded for a very brief time, we hear a dry knock; if a large
number of tonal stimuli are sounded all at once, we hear a buzz or
crash. Noises have =pitch=; the spit of a pistol is higher than the
crack of a rifle, and the sizzle of frying fat is higher than the
murmur of falling rain; but no one has yet established a complete scale
of noise.
_The sensory cells are found in the inner ear_, a tiny structure with
an extremely complicated mechanism. Many different views of its action
have been put forward. That which is most generally accepted was
proposed by the German physicist H. von Helmholtz. _The ear contains
a narrow triangular membrane which carries many thousands of stiffish
cross-fibres_; and the theory is that the air-waves which impinge on
the outer ear play, selectively, upon these fibres; _every air-wave
throws into vibration the fibre which is tuned to respond to it_.
A compound tonal stimulus is thus analysed by the membrane into a
number of simple tonal stimuli, and every simple stimulus excites the
nerve-fibril attached to its particular cross-fibre. This theory
explains our ability to analyse compound tones into their simple
components.
_The ear is, however, more than an organ of hearing._ It includes
organs, of a very ancient type, which help to regulate our balance in
walking, our precision in turning corners or avoiding obstacles, and
so on. Each ear, for instance, has three little organs that resemble
minute spirit-levels, set in the three planes of space, and that give
us the sensation of ‘swimming’ when the head is sharply jerked, and the
sensation of =dizziness= when we twirl on our heels. For the most part
these organs act reflexly, without furnishing sensations; or at any
rate furnish sensations of little strength, and of a pressure-like kind
that blends indistinguishably with the kinæsthetic sensations from the
tissues beneath the skin; but in the cases mentioned the swimmy, dizzy
sensation may be noticed.
§ 13. =Sensations from the Eye.=—You may study tones by help of the
piano and a few medicine bottles; but for the study of lights and
colours you must go beyond household appliances, and secure a fairly
large set of coloured and grey papers; sample-books may be obtained,
very cheaply, from the manufacturers. You will notice, first of all,
that as the world of sounds divides into tones and noises, so does the
world of looks divide into what we have just called colours and lights.
The colourless looks or =lights= may be arranged in a single straight
line that passes from purest white through the greys to deepest black;
they are, as sensations, older than colours, just as noise is older
than tone. =Colours= are more varied. Consider, to begin with, the
character of colour proper or =hue=, that is, the differences of colour
that show in the rainbow. Hues may be arranged, not in one straight
line, but in a square. Setting out, say, from red, you pass through
red-yellow or orange to yellow; that is one straight line; setting
out again from yellow, you pass through yellow-green to green; from
green you pass through green-blue to blue; and finally from blue you
come back, by way of blue-red (violet and purple), to the original
red. Colours have, besides, two further characters, that bring them
into relation with lights. They differ in =tint=, that is, in darkness
or lightness; brown is darker than yellow, sky-blue is lighter than
navy-blue. They differ also in saturation or =chroma=, that is,
in poorness or richness of hue; pinks and yellows look faded and
washed-out as compared with rich reds and blues. _Tint brings colours
into relation with lights_, because, if we can say that a colour is
darker or lighter than a particular grey, we can also find some grey
that matches it in darkness or lightness; and _chroma brings colours
into relation with lights_, in the sense that the better chroma is
farther off from colourlessness (that is, from grey) than the poorer
chroma of the same hue and tint.
_All lights and colours are psychologically simple._ Paints may be
mixed on a palette, and colour-stimuli may be mixed in all sorts of
ways; we learn in physics that white daylight is a mixture of all the
rays that are seen separately in the rainbow. Yet a white, considered
just as a look, is perfectly simple; and the looks of orange and
yellow-green and green-blue are equally simple. There are no compound
colours, to correspond with compound tones. Hence the number of light
and colour sensations is very large, at least ten times as large as the
number of simple tones.
The organ of vision is the eye; and _the eye is a little photographic
camera_, with shutter, iris-diaphragm, self-adjusting lens, dark
chamber, and self-renewing sensitive film. We are concerned only with
the film, that is, with the =retina= or nervous network that lines the
posterior half of the eyeball. It seems that _the retina is really made
up of three interfused films_; for simplicity’s sake you may consider
them as lying upon one another, just as three saucers might do if you
piled them together. The oldest and largest film, the bottom saucer,
gives us the sensations of _black and white_; the middlemost, somewhat
smaller, gives us _blue and yellow_; and the topmost and smallest
gives us a _purplish-red and a bluish-green_. The existence and size
of the three films can be shown by experiment; for we are all totally
colour-blind at the edge of the field of vision, and are blind to reds
and greens for some distance further in toward the centre. There are
also cases of inherited =colour-blindness=, in which the eye is blind
either for all colours (total colour-blindness) or for red and green
alone (partial colour-blindness); the latter form is fairly common,
as is natural,—for the red-green film, being the last to come, might
be expected to be the first to go. Partial colour-blindness was first
brought to scientific notice by the English chemist John Dalton in
1798. Dalton was a Quaker, but made no objection to wearing the
scarlet gown of a doctor of laws, because, as he said, “to me its
colour is that of nature—the colour of those green leaves”; it is
needless to remark that he did not see green either! The defect is
practically important for pilots and signalmen, who have to distinguish
red and green lights.
_From these three films we get all the lights and colours that we see
in the daytime, with the single exception of_ =neutral grey=; and this
appears to come, not from the eye at all, but from the brain. It may be
seen even when the retina is quite blind, provided that the rest of the
nervous apparatus is in working order; and it may be seen by night as
well as by day; it is mixed, physiologically, with all our sensations
of light and colour, though we cannot by psychological analysis pick
it out from the lights and colours. Strange enough! but we shall
understand better as we go on. The German physiologist Ewald Hering has
shown that _the processes which take place in the films are, in all
probability, chemical processes of an_ =antagonistic= _or_ =reversible=
_kind_; that is why we never see a bluish-yellow, or a greenish-red;
if we throw on the same part of the retina, at the same time, equal
amounts of black and white, or of blue and yellow, or of purplish-red
and bluish-green, the chemical processes go on in opposite directions
and cancel each other, with the result that we see just nothing. This
antagonism can be proved, under the right experimental conditions, for
blue-yellow and for red-green; if these pairs are fittingly thrown
together on the retina we see, in fact, only neutral grey; so that
our seeing of the same grey, when black and white stimuli are acting
together, does not necessarily mean that grey is a retinal mixture of
black and white; the black and white may also cancel each other, and
leave only the brain-grey to be seen.
We have, then, the three films in each eyeball, and we have the
brain-grey behind them. More than this: we have a night or =twilight
eye=. When colours fade out, as twilight deepens, another retinal
film comes into play; the lights that we still see come, not from the
black-white film, but from a fourth film, of the same size, whose only
sensation is _a slightly bluish-white_. Of course, this white is always
mixed, physiologically, with the brain-grey; we never see it by itself;
but we owe to it, among other things, the silvery look of blues in the
twilight. The very centre of the twilight eye is totally blind; if on
a moonless night you want to see a faint star or a distant street-lamp
you must not look directly at it, but just to one side of it.
Children’s fear of the dark is partly due to the fact that they cannot
see what they turn their gaze upon; there had seemed to be something
there, but when they looked at it, it eluded them; and if they think
they see it again, and look in the new direction, again it is gone.
Now suppose that you are looking out, in daylight, over a variegated
landscape. Somewhere or other you see a patch of light grey. You get
this sensation from the black-white film and the brain-grey; the
white-process is stronger than the black-process in the film, and the
excess of white, added physiologically to the brain-grey, shows as
light grey. Or again, you see a patch of dark purple. This sensation
comes from the red-green film (excess of red); from the blue-yellow
film (excess of blue); from the black-white film (excess of black); and
from the brain-grey. All the lights and colours of the landscape can be
accounted for in the same way.
Not quite correctly, however!—there are still other factors at work.
_The film-processes are antagonistic_, for instance, _even when they
go on in different parts of a film_; lights and colours =contrast=
with one another; if you lay a strip of grey paper on red, it looks
greenish; on blue, yellowish; on white, blackish; make the trial with
your own papers. So all the various lights and colours of the landscape
stand out, by contrast, against one another; the eye makes their
differences greater than they ought physically, from the nature of the
stimuli, to appear. _Black, indeed, is wholly a contrast-sensation_;
it has no physical stimulus; and you see deep black only in strong
illumination.
Contrast is effective at once, the moment you cast your eyes on the
landscape. _As time goes on_, however, _the opposed film-processes
tend to settle down into a state of balance or equilibrium_; so that
actually, if you stared at the landscape long enough, without moving
your eyes, you would finally see nothing but the brain-grey. This
levelling down of all lights and all colours toward neutral grey is
called =adaptation=. Stand up two strips of black and white paper,
side by side, and stare at their line of junction for a minute or two;
even in that short time you will find that they tend toward a uniform
grey. If, now, a stimulus to which you are wholly or partly adapted
is suddenly removed, the antagonism of the film-processes shows itself
once more; you see an =after-image=. Lay a disc of red on grey; stare
at it for half a minute; flick it away, keeping the eyes steady,
and look at the grey background; you see a corresponding disc of
green. White leaves a black after-image, black a white; blue a yellow
after-image, and yellow a blue.
It is clear, then, that _the lights and colours of the landscape depend
on many things beside the stimuli there presented_; they depend on
contrast, on the previous adaptation of the eye, on the presence or
absence of after-images. The main reason that we do not notice all
these influences is that we ordinarily view the landscape, not for
itself, but for what it means; it shows us the familiar trees and
stream and houses, and we take their stability for granted. That is
the main reason; it is not the only one. We have said, for instance,
that the normal retina is totally colour-blind along its outer edge,
and partially colour-blind for some distance in toward the centre;
_the edge of the landscape ought therefore to be colourless_, and a
certain outlying portion of it ought to appear simply as blue and
yellow. There is no hint of these differences; and the explanation is
that we are accustomed to turn our eyes directly towards what we want
to see, and therefore to view it with all three of the daylight films;
head and eyes =move= so easily, and we see so much better with the
centre of the retina, that we totally disregard the altered look of
things seen ‘out of the corner of the eye.’ Even if we do not, we are
likely to _remember_ how the things appear in direct vision; we paint
them over, so to speak, with =memory-colours=, colours that represent
their natural or average appearance at the centre of the visual field;
indeed, we may _paint these colours over the whole landscape, and
in that way correct the changes due to contrast or adaptation_. We
always talk of a certain book as brown; we recognise it in all lights,
and in all states of the eye, by its brown colour; we _see_ it, in
memory-colour, as brown; whereas, if that same brown were shown us in
all the different circumstances without our knowing it to be the same,
it might give us sensations of yellow, of pale brown, of deep brown,
of black. These two factors, _movement of the eyes and memory-colour_,
lead us to overlook, in great part, the actual variation of lights and
colours in the landscape.
A final word may be added regarding _the likeness of sight and smell_.
Odours and colours fade out by adaptation; odours, like lights and
colours, contrast, and even cancel one another; and smell-stimuli as
well as sight-stimuli mix to produce new and simple sensations. It is
highly probable that the sensory cells of smell are the seat of only a
few chemical processes, by whose combination all the wealth of odours
is created, just as the cone-cells of the retina are the seat of those
three reversible processes (black-white, blue-yellow, red-green) whose
combination endows us with the variety of daylight vision. We have as
yet, however, no such definite grounds for hypothesis as we have in the
case of sight; we cannot even guess what these processes are, or how
many of them are taking place in the smell-membrane.
§ 14. =Organic Sensations.=—There are still other sensations, coming
to us from the internal bodily organs; from various parts of the
alimentary canal, from the organs of sex, from heart and blood-vessels,
from the lungs, from the sheathing membrane of the bones; but it is
doubtful if they are of new kinds; probably they consist simply of
=pressure=, =cold=, =warmth=, and =pain=. The dull deep-seated pains
that we call =aches= are, perhaps, different from the bright pains of
the skin; but most of the differences among pains, differences that we
express by the terms lancing, throbbing, piercing, stabbing, thrilling,
gnawing, boring, shooting, racking, and so on, are either differences
of time (steady, intermittent) or space (localised, diffused) or degree
(moderate, acute), or else are differences due to the blending of pain
with various other sensations.
The organic sensations, like the kinæsthetic, tend thus to occur
in groups or complexes, and we have as yet no very sure means of
disentangling them. It is, nevertheless, quite clear that in their
case, as in that of the touch-blends, we have to _distinguish between
experience and meaning_. =Hunger= and =nausea= seem, for example,
to be very different; yet the core of both turns out on analysis
to be the same dull pain; and we know that the onset of a bilious
attack is often heralded by an unusually keen appetite, so that the
beginnings of nausea are in fact confused with a growing hunger. The
difference between hunger and nausea is due partly to a difference
in the processes which ordinarily accompany the central pain,—motor
restlessness or lassitude in the case of hunger, and dizziness in
that of nausea; but more especially to a difference of meaning
or interpretation; hunger stands for want of food, and nausea for
indigestion.
We shall see later that _organic sensations play a large part in
emotion, as kinæsthetic sensations do in perception_. Plato set the
‘spirited’ or ‘passionate’ part of the soul in the breast; the Psalms
abound in phrases that suggest the same idea; we speak to-day of the
_heart_ coming up to the mouth, or dropping to the boots. So we read in
the Old Testament that Joseph’s _bowels_ yearned upon his brother, and
in the New Testament of bowels of compassion; and the inner stir that
the writers have in mind is familiar to everybody.
§ 15. =Sensation and Attribute.=—We have been talking all this while
about sensations, but we have not yet said what sensations are. They
make up, as you will have guessed, one class of the mental elements,
the _elementary mental processes_ of § 4, that we reach by analysis
of our complex experiences. They are therefore simple and irreducible
items of the mental world. How shall we define them?
We can define them, in strictness, only by writing down a complete
list of what we have called their characters. _Every sensation shows
itself to us under various aspects_, or, as we are accustomed to say,
possesses a number of =attributes=. We have been dealing, so far, with
the qualitative aspect of sensations. This may itself be single; the
quality of lights is just their lightness or darkness; or it may be
manifold; the quality of colours can be properly described only if
we take account of hue, tint, and chroma; that of tones only if we
take account of pitch, volume, and tonality, perhaps also of vocality.
=Quality= is the natural thing to start out from, because it is what
interests us most in everyday life, and has therefore been named; so
that, when we speak of sensations, we speak of them by their qualities.
There are, however, several other attributes; sensations possess
=intensity=, and =vividness=, and =duration=, and some of them possess
=extension=. We shall discuss these aspects later on.
_Does it seem strange, now, that an elementary hit of experience
should turn so many sides to the observer?_ Think then of chemistry,
and of the chemical elements. Sodium is a chemical element; but it has
many aspects or properties; physically regarded, it is soft, it is
fusible, it volatilises at high temperatures; chemically, it combines
with oxygen, it decomposes water, it is univalent, it has a low atomic
weight, it is electropositive, and so forth. Sodium cannot be reduced,
chemically, to anything simpler than itself, but it is nevertheless
many-sided. The same thing is true of sensations.
So a complete list of the aspects or attributes of sensation is as
near as we can come to a definition. But since that sort of statement
is clumsy; since we cannot make it complete till we have observed the
sensations under all their possible aspects; and since we know that
mental processes are correlated with processes in the nervous system;
we may adopt another plan, and _define sensation by reference to the
special bodily organ with which it is connected_. Sensations are then
elementary mental processes that come to us by way of skin, muscle,
ear, and the rest of the sense-organs.
§ 16. =The Intensity of Sensation.=—A sensation may remain the same
in quality, and yet vary in strength or =intensity=. A pressure may be
the pressure of an ounce or of half-a-pound; it is always pressure, the
same quality, but its intensity differs. The tone you get by blowing
across the mouth of a bottle may be loud or faint, though it is still
the same pitch, the same tone. The weight you carry may strain the arm
very little or a great deal; the sensation of strain from the tendons
is the same in both cases, but its intensity is different.
The study of this attribute of sensations has led to _the discovery
of a psychological law_, which has much practical importance. Suppose
that we are working with intensities of noise, the noise made by the
drop of an ivory ball upon an ebony block. Suppose that, by varying the
height from which the ball falls, we have found a series of intensities
of sensation _a_, _b_, _c_, _d_, _e_, which may be represented by the
numbers 1, 2, 3, 4, 5; a series, that is, in which the difference
between the two noises _a_ and _b_ is equal in sensation to the
difference between _b_ and _c_, or between _c_ and _d_, or between _d_
and _e_. That sounds a little difficult; but the series may really
be established without much trouble. Now, what about the stimuli,
the heights of fall? Must the ball drop twice as far for _b_ as for
_a_, three times as far for _c_ as for _a_, and so on? No: _equal
differences in intensity of sensation do not correspond with equal
differences in intensity of stimulus. Equal differences in intensity of
sensation correspond rather with_ =relatively= _equal difference in the
intensity of stimulus_. In other words,
the sensation-series 1 2 3 4 5 corresponds with
a stimulus-series of the type 1 2 4 8 16;
or, mathematically expressed, _an arithmetical series of intensities
of sensation is correlated with a geometrical series of intensities
of stimulus_. In the instance given, the exponent of the geometrical
series is 2; but that is only an imaginary instance; in the case of
=noise= the actual exponent is 4/3, so that
the sensation-series 1 2 3 4 5 corresponds with
the stimulus series 1 4/3 16/9 64/27 256/81;
or, if we take units of some sort, such as millimetres of height of
fall,
the sensation-series 1 2 3 4 5 corresponds with
the stimulus-series 81 108 144 192 256.
This law of correlation was first formulated by the German physiologist
E. H. Weber in 1834 as follows: “in comparing objects and observing
the distinction between them, we perceive, not the difference between
the objects, but the ratio of this difference to the magnitude of the
objects compared.” Weber speaks of objects, because he was thinking
of experiments that he had made with weights; he should have said
sensations. His law holds, over a middle range of intensities of
sensation, for =lights=, =sounds=, =pressures=, =various kinæsthetic
complexes=, and =odours=. Its validity in the fields of taste and
temperature is doubtful.
It is because of Weber’s law that we are able to ignore the manifold
changes of illumination to which we are exposed in the course of the
daylight hours; that the painter, who cannot at all reproduce by his
pigments the absolute intensities of light in nature, can nevertheless
give us a recognisably true copy of any natural scene; and that a large
block of seats in the concert-room, at a moderate distance from the
stage, can all be sold at the same price and all have equal advantages
for hearing. You will readily find other instances of its working, if
you are clear as regards the principle involved; namely, that the less
you have of anything, the less need be added, and the more you have,
the more must be added, to make an appreciable difference; or, on the
negative side, that you are not likely to notice any difference in your
surroundings, so long as the relations of the stimuli remain unchanged.
So Weber’s law furnishes yet another reason for the apparent stability
of the landscape that we discussed on p. 63.
Questions and Exercises
(1) Mark out, by indelible ink, a sq. cm. upon the outer surface of the
forearm. Make upon transparent paper three maps of the area, marking
hairs, veins, etc. Work over the area (_a_) with the horsehair, for
pressure spots; (_b_) with a warmed carpenter’s spike, for warm spots;
and (_c_) with a cooled spike, for cold spots. Enter the spots, as you
find them, on the maps; remember to dot the hair down for pressure, but
to draw the spike slowly and evenly along the skin for temperature. Lay
the three maps together, and note the distribution and the relative
number of the spots.
(2) After shampooing, the scalp is sensitive and irritable under the
brush. Why?
(3) When you are writing with a pencil, or prodding in a pool with a
stick, the sensations seem to come from the end of the pencil or stick.
What organs are involved? And why should the sensations be localised as
they are? Try to think out some experimental means of attacking this
question.
(4) What sensations do you get in the act of yawning? What in that of
swallowing? What unusual sensations do you have, from the face, after
you have been running hard?
(5) How do sour and sweet in the mouth affect the sense of touch? Make
solutions, in varying strengths, of sugar and of the juice of some very
sour fruit; leave plenty of time between observations.
(6) Prepare some bits of apple, onion, and raw potato. Close your eyes
and hold your nose; then pick up these morsels at random, and chew
them. Can you tell the difference? How?
(7) Is there any evidence of taste contrast?
(8) Secure adaptation to the scent of camphor; breathe regularly, and
note the length of time necessary for the odour to disappear. Now smell
at vanilla, heliotrope, absolute alcohol. Do you smell them? Try
to account for the result, arguing by analogy from what you know of
colours.
(9) The next time that you listen to an orchestra, pick out the tones
of the various instruments, and try to describe their timbre; do not
be afraid to string adjectives together, but be sure that you hear
what you put down. Later, look up in a reference-book the composition
of these various compound tones, and see if there is any correlation
between your description and the number and loudness of the overtones.
(10) If you drop a block of wood on a desk, the sound is simply noisy.
If the same block forms part of a xylophone scale, and is struck with
the wooden hammer, it gives a tone. How is this?
(11) When you next go to a reception, stand outside the main rooms for
a minute, and try to determine the pitch of the buzz of voices; try to
sing the pitch yourself. Is the buzz tonal or merely noisy?
(12) When you are listening to beats, do you hear one beating tone, or
both the primary tones beating? If one tone only, is it identical with
either of the primaries?
(13) Test the law of visual antagonism by getting the after-images of a
number of colours.
(14) To prove normal colour-blindness, get a small square of red glass;
stand before a window, with your left eye closed and your right eye
fixed upon some distant point; bring the red glass slowly into the
field, with the left hand, and note its changes.
(15) Can you suggest experiments for working out in detail the laws of
visual contrast? Try to think what sort of things would be likely to
enhance or to reduce the contrast-effect.
(16) Could a man go through life, and take an ordinary place in
society, without knowing that he was colour-blind? Give your reasons.
(17) Blue and yellow are antagonistic; yet blue and yellow paints,
mixed on the palette, give green. How is this?
(18) Dalton says: “In lecturing on optics I got six ribands,—blue,
pink, lilac,—and red, green, and brown,—which matched very well,
and told the curious audience so. One gentleman came up immediately
afterwards and told me he perfectly agreed with me; he had not remarked
the difference by candlelight.” How could these triads have been
confused? and would the candlelight make any difference?
References
A more detailed treatment of sensation is given in the author’s
Text-book of Psychology, 1910, 46 ff., 201 ff. The reader may further
consult: J. H. Parsons, An Introduction to the Study of Colour
Vision, 1915; H. L. F. von Helmholtz, On the Sensations of Tone as a
Physiological Basis for the Theory of Music, translated by A. J. Ellis,
1895; C. S. Myers, A Text-book of Experimental Psychology, pt. i.,
1911, chs. 2-8, 18, 19; G. T. Ladd and R. S. Woodworth, Elements of
Physiological Psychology, 1911, pt. ii., chs. 1-3; W. Wundt, Lectures
on Human and Animal Psychology, 1896, Lects. 2-7; various articles in
Dictionary of Philosophy and Psychology, ed. by J. M. Baldwin, vols.
i., ii., 1901-2; the chapters on sensation in E. A. Schäfer, Text-book
of Physiology, ii., 1900, and W. H. Howell, A Text-book of Physiology,
1908; E. Mach, Contributions to the Analysis of the Sensations, trs. by
C. M. Williams, 1910; E. B. Titchener, Experimental Psychology, II.,
ii., 1905, Introduction.
The special references to _smell_ will be found in E. B. Tylor,
Anthropology, 1881, ch. ix., 207; W. W. Skeat and C. O. Blagden, Pagan
Races of the Malay Peninsula, i., 1906, 200; F. Galton, Psychological
Review, i., 1894, 61 ff.; and those to _Dalton_ in W. C. Henry, Memoirs
of the Life and Scientific Researches of John Dalton, 1854, 24, 49,
172, 187. For the term _kinæsthesis_ see H. C. Bastian, The Brain as an
Organ of Mind, 1885, 543.