SENSATION IN GENERAL.
=Incoming nerve-currents are the only agents which normally affect the
brain.= The human nerve-centres are surrounded by many dense wrappings of
which the effect is to protect them from the direct action of the forces
of the outer world. The hair, the thick skin of the scalp, the skull,
and two membranes at least, one of them a tough one, surround the brain;
and this organ moreover, like the spinal cord, is bathed by a serous
fluid in which it floats suspended. Under these circumstances the only
things that can _happen_ to the brain are:
1) The dullest and feeblest mechanical jars;
2) Changes in the quantity and quality of the blood-supply; and
3) Currents running in through the so-called afferent or centripetal
nerves.
The mechanical jars are usually ineffective; the effects of the
blood-changes are usually transient; the nerve-currents, on the
contrary, produce consequences of the most vital sort, both at the
moment of their arrival, and later, through the invisible paths of
escape which they plough in the substance of the organ and which, as we
believe, remain as more or less permanent features of its structure,
modifying its action throughout all future time.
=Each afferent nerve comes from a determinate part of the periphery and
is played upon and excited to its inward activity by a particular force
of the outer world.= Usually it is insensible to other forces: thus the
optic nerves are not impressible by air-waves, nor those of the skin by
light-waves. The lingual nerve is not excited by aromatic effluvia, the
auditory nerve is unaffected by heat. Each selects from the vibrations
of the outer world some one rate to which it responds exclusively. The
result is that our sensations form a discontinuous series, broken by
enormous gaps. There is no reason to suppose that the order of
vibrations in the outer world is anything like as interrupted as the
order of our sensations. Between the quickest audible air-waves (40,000
vibrations a second at the outside) and the slowest sensible heat-waves
(which number probably billions), Nature must somewhere have realized
innumerable intermediary rates which we have no nerves for perceiving.
The process in the nerve-fibres themselves is very likely the same, or
much the same, in all the different nerves. It is the so-called
'current'; but the current is _started_ by one order of outer vibrations
in the retina, and in the ear, for example, by another. This is due to
the different _terminal organs_ with which the several afferent nerves
are armed. Just as we arm ourselves with a spoon to pick up soup, and
with a fork to pick up meat, so our nerve-fibres arm themselves with one
sort of end-apparatus to pick up air-waves, with another to pick up
ether-waves. The terminal apparatus always consists of modified
epithelial cells with which the fibre is continuous. The fibre itself is
not directly excitable by the outer agent which impresses the terminal
organ. The optic fibres are unmoved by the direct rays of the sun; a
cutaneous nerve-trunk may be touched with ice without feeling cold.[2]
The fibres are mere transmitters; the terminal organs are so many
imperfect telephones into which the material world speaks, and each of
which takes up but a portion of what it says; the brain-cells at the
fibres' central end are as many others at which the mind listens to the
far-off call.
=The 'Specific Energies' of the Various Parts of the Brain.=--To a certain
extent anatomists have traced definitely the paths which the sensory
nerve-fibres follow after their entrance into the centres, as far as
their termination in the gray matter of the cerebral convolutions.[3] It
will be shown on a later page that the consciousness which accompanies
the excitement of this gray matter varies from one portion of it to
another. It is consciousness of things seen, when the occipital lobes,
and of things heard, when the upper part of the temporal lobes, share in
the excitement. Each region of the cerebral cortex responds to the
stimulation which its afferent fibres bring to it, in a manner with
which a peculiar quality of feeling seems invariably correlated. This is
what has been called the law of 'specific energies' in the nervous
system. Of course we are without even a conjectural explanation of the
_ground_ of such a law. Psychologists (as Lewes, Wundt, Rosenthal,
Goldscheider, etc.) have debated a good deal as to whether the specific
quality of the feeling depends solely on the _place_ stimulated in the
cortex, or on the _sort of current_ which the nerve pours in. Doubtless
the sort of outer force habitually impinging on the end-organ gradually
modifies the end-organ, the sort of commotion received from the
end-organ modifies the fibre, and the sort of current a so-modified
fibre pours into the cortical centre modifies the centre. The
modification of the centre in turn (though no man can guess how or why)
seems to modify the resultant consciousness. But these adaptive
modifications must be excessively slow; and as matters actually stand in
any adult individual, it is safe to say that, more than anything else,
the _place_ excited in his cortex decides what kind of thing he shall
feel. Whether we press the retina, or prick, cut, pinch, or galvanize
the living optic nerve, the Subject always feels flashes of light, since
the ultimate result of our operations is to stimulate the cortex of his
occipital region. Our habitual ways of feeling outer things thus depend
on which convolutions happen to be connected with the particular
end-organs which those things impress. We _see_ the sunshine and the
fire, simply because the only peripheral end-organ susceptible of taking
up the ether-waves which these objects radiate excites those particular
fibres which run to the centres of sight. If we could interchange the
inward connections, we should feel the world in altogether new ways. If,
for instance, we could splice the outer extremity of our optic nerves to
our ears, and that of our auditory nerves to our eyes, we should hear
the lightning and see the thunder, see the symphony and hear the
conductor's movements. Such hypotheses as these form a good training for
neophytes in the idealistic philosophy!
=Sensation distinguished from Perception.=--It is impossible rigorously to
_define_ a sensation; and in the actual life of consciousness
sensations, popularly so called, and perceptions merge into each other
by insensible degrees. All we can say is that _what we mean by
sensations are_ FIRST _things in the way of consciousness_. They are the
_immediate_ results upon consciousness of nerve-currents as they enter
the brain, and before they have awakened any suggestions or associations
with past experience. But it is obvious that _such immediate sensations
can only be realized in the earliest days of life_. They are all but
impossible to adults with memories and stores of associations acquired.
Prior to all impressions on sense-organs, the brain is plunged in deep
sleep and consciousness is practically non-existent. Even the first
weeks after birth are passed in almost unbroken sleep by human infants.
It takes a strong message from the sense-organs to break this slumber.
In a new-born brain this gives rise to an absolutely pure sensation. But
the experience leaves its 'unimaginable touch' on the matter of the
convolutions, and the next impression which a sense-organ transmits
produces a cerebral reaction in which the awakened vestige of the last
impression plays its part. Another sort of feeling and a higher grade of
cognition are the consequence. 'Ideas' _about_ the object mingle with
the awareness of its mere sensible presence, we name it, class it,
compare it, utter propositions concerning it, and the complication of
the possible consciousness which an incoming current may arouse, goes on
increasing to the end of life. In general, this higher consciousness
about things is called Perception, the mere inarticulate feeling of
their presence is Sensation, so far as we have it at all. To some degree
we seem able to lapse into this inarticulate feeling at moments when our
attention is entirely dispersed.
=Sensations are cognitive.= A sensation is thus an abstraction seldom
realized by itself; and the object which a sensation knows is an
abstract object which cannot exist alone. _'Sensible qualities' are the
objects of sensation._ The sensations of the eye are aware of the
_colors_ of things, those of the ear are acquainted with their _sounds_;
those of the skin feel their tangible _heaviness_, _sharpness_, _warmth_
or _coldness_, etc., etc. From all the organs of the body currents may
come which reveal to us the quality of _pain_, and to a certain extent
that of _pleasure_.
Such qualities as _stickiness_, _roughness_, etc., are supposed to be
felt through the coöperation of muscular sensations with those of the
skin. The geometrical qualities of things, on the other hand, their
_shapes_, _bignesses_, _distances_, etc. (so far as we discriminate and
identify them), are by most psychologists supposed to be impossible
without the evocation of memories from the past; and the cognition of
these attributes is thus considered to exceed the power of sensation
pure and simple.
='Knowledge of Acquaintance' and 'Knowledge about.'=--Sensation, thus
considered, differs from perception only in the extreme simplicity of
its object or content. Its object, being a simple quality, is sensibly
_homogeneous_; and its function is that of mere _acquaintance_ with this
homogeneous seeming fact. Perception's function, on the other hand, is
that of knowing something _about_ the fact. But we must know _what_ fact
we mean, all the while, and the various _whats_ are what sensations
give. Our earliest thoughts are almost exclusively sensational. They
give us a set of _whats_, or _thats_, or _its_; of subjects of discourse
in other words, with their relations not yet brought out. The first time
we see _light_, in Condillac's phrase we _are_ it rather than see it.
But all our later optical knowledge is about what this experience gives.
And though we were struck blind from that first moment, our scholarship
in the subject would lack no essential feature so long as our memory
remained. In training-institutions for the blind they teach the pupils
as much _about_ light as in ordinary schools. Reflection, refraction,
the spectrum, the ether-theory, etc., are all studied. But the best
taught born-blind pupil of such an establishment yet lacks a knowledge
which the least instructed seeing baby has. They can never show him
_what_ light is in its 'first intention'; and the loss of that sensible
knowledge no book-learning can replace. All this is so obvious that we
usually find sensation 'postulated' as an element of experience, even by
those philosophers who are least inclined to make much of its
importance, or to pay respect to the knowledge which it brings.
=Sensations distinguished from Images.=--Both sensation and perception,
for all their difference, are yet alike in that their objects appear
_vivid_, _lively_, and _present_. Objects merely _thought of_,
_recollected_, or _imagined_, on the contrary, are relatively faint and
devoid of this pungency, or tang, this quality of _real presence_ which
the objects of sensation possess. Now the cortical brain-processes to
which sensations are attached are due to incoming currents from the
periphery of the body--an external object must excite the eye, ear,
etc., before the sensation comes. Those cortical processes, on the other
hand, to which mere ideas or images are attached are due in all
probability to currents from other convolutions. It would seem, then,
that the currents from the periphery normally awaken a kind of
brain-activity which the currents from other convolutions are inadequate
to arouse. To this sort of activity--a profounder degree of
disintegration, perhaps--the quality of vividness, presence, or reality
in the object of the resultant consciousness seems correlated.
=The Exteriority of Objects of Sensation.=--Every thing or quality felt is
felt in outer space. It is impossible to conceive a brightness or a
color otherwise than as extended and outside of the body. Sounds also
appear in space. Contacts are against the body's surface; and pains
always occupy some organ. An opinion which has had much currency in
psychology is that sensible qualities are first apprehended as _in the
mind itself_, and then 'projected' from it, or 'extradited,' by a
secondary intellectual or super-sensational mental act. There is no
ground whatever for this opinion. The only facts which even seem to make
for it can be much better explained in another way, as we shall see
later on. The very first sensation which an infant gets _is_ for him the
outer universe. And the universe which he comes to know in later life is
nothing but an amplification of that first simple germ which, by
accretion on the one hand and intussusception on the other, has grown so
big and complex and articulate that its first estate is unrememberable.
In his dumb awakening to the consciousness of _something there_, a mere
_this_ as yet (or something for which even the term _this_ would perhaps
be too discriminative, and the intellectual acknowledgment of which
would be better expressed by the bare interjection 'lo!'), the infant
encounters an object in which (though it be given in a pure sensation)
all the 'categories of the understanding' are contained. _It has
externality, objectivity, unity, substantiality, causality, in the full
sense in which any later object or system of objects has these things._
Here the young knower meets and greets his world; and the miracle of
knowledge bursts forth, as Voltaire says, as much in the infant's lowest
sensation as in the highest achievement of a Newton's brain.
The physiological condition of this first sensible experience is
probably many nerve-currents coming in from various peripheral organs at
once; but this multitude of organic conditions does not prevent the
consciousness from being one consciousness. We shall see as we go on
that it can be one consciousness, even though it be due to the
coöperation of numerous organs and be a consciousness of many things
together. The Object which the numerous inpouring currents of the baby
bring to his consciousness is one big blooming buzzing Confusion. That
Confusion is the baby's universe; and the universe of all of us is still
to a great extent such a Confusion, potentially resolvable, and
demanding to be resolved, but not yet actually resolved, into parts. It
appears from first to last as a space-occupying thing. So far as it is
unanalyzed and unresolved we may be said to know it sensationally; but
as fast as parts are distinguished in it and we become aware of their
relations, our knowledge becomes perceptual or even conceptual, and as
such need not concern us in the present chapter.
=The Intensity of Sensations.=--A light may be so weak as not sensibly to
dispel the darkness, a sound so low as not to be heard, a contact so
faint that we fail to notice it. In other words, a certain finite amount
of the outward stimulus is required to produce any sensation of its
presence at all. This is called by Fechner the law of the
_threshold_--something must be stepped over before the object can gain
entrance to the mind. An impression just above the threshold is called
the _minimum visibile_, _audibile_, etc. From this point onwards, as
the impressing force increases, the sensation increases also, though at
a slower rate, until at last an _acme_ of the sensation is reached which
no increase in the stimulus can make sensibly more great. Usually,
before the acme, _pain_ begins to mix with the specific character of the
sensation. This is definitely observable in the cases of great pressure,
intense heat, cold, light, and sound; and in those of smell and taste
less definitely so only from the fact that we can less easily increase
the force of the stimuli here. On the other hand, all sensations,
however unpleasant when more intense, are rather agreeable than
otherwise in their very lowest degrees. A faintly bitter taste, or
putrid smell, may at least be _interesting_.
[Illustration: FIG. 1.]
=Weber's Law.=--I said that the intensity of the sensation increases by
slower steps than those by which its exciting cause increases. If there
were no threshold, and if every equal increment in the outer stimulus
produced an equal increment in the sensation's intensity, a simple
straight line would represent graphically the 'curve' of the relation
between the two things. Let the horizontal line stand for the scale of
intensities of the objective stimulus, so that at 0 it has no intensity,
at 1 intensity 1, and so forth. Let the verticals dropped from the
slanting line stand for the sensations aroused. At 0 there will be no
sensation; at 1 there will be a sensation represented by the length of
the vertical _S_¹--1, at 2 the sensation will be represented by
_S_²--2, and so on. The line of _S_'s will rise evenly because by the
hypothesis the verticals (or sensations) increase at the same rate as
the horizontals (or stimuli) to which they severally correspond. But in
Nature, as aforesaid, they increase at a slower rate. If each step
forward in the horizontal direction be equal to the last, then each step
upward in the vertical direction will have to be somewhat shorter than
the last; the line of sensations will be convex on top instead of
straight.
[Illustration: FIG. 2.]
Fig. 2 represents this actual state of things, 0 being the zero-point of
the stimulus, and conscious sensation, represented by the curved line,
not beginning until the 'threshold' is reached, at which the stimulus
has the value 3. From here onwards the sensation increases, but it
increases less at each step, until at last, the 'acme' being reached,
the sensation-line grows flat. The exact law of retardation is called
_Weber's law_, from the fact that he first observed it in the case of
weights. I will quote Wundt's account of the law and of the facts on
which it is based.
"Every one knows that in the stilly night we hear things unnoticed
in the noise of day. The gentle ticking of the clock, the air
circulating through the chimney, the cracking of the chairs in the
room, and a thousand other slight noises, impress themselves upon
our ear. It is equally well known that in the confused hubbub of
the streets, or the clamor of a railway, we may lose not only what
our neighbor says to us, but even not hear the sound of our own
voice. The stars which are brightest at night are invisible by
day; and although we see the moon then, she is far paler than at
night. Every one who has had to deal with weights knows that if to
a pound in the hand a second pound be added, the difference is
immediately felt; whilst if it be added to a hundredweight, we are
not aware of the difference at all....
"The sound of the clock, the light of the stars, the pressure of
the pound, these are all _stimuli_ to our senses, and stimuli whose
outward amount remains the same. What then do these experiences
teach? Evidently nothing but this, that one and the same stimulus,
according to the circumstances under which it operates, will be
felt either more or less intensely, or not felt at all. Of what
sort now is the alteration in the circumstances upon which this
alteration in the feeling may depend? On considering the matter
closely we see that it is everywhere of one and the same kind. The
tick of the clock is a feeble stimulus for our auditory nerve,
which we hear plainly when it is alone, but not when it is added to
the strong stimulus of the carriage-wheels and other noises of the
day. The light of the stars is a stimulus to the eye. But if the
stimulation which this light exerts be added to the strong stimulus
of daylight, we feel nothing of it, although we feel it distinctly
when it unites itself with the feebler stimulation of the twilight.
The poundweight is a stimulus to our skin, which we feel when it
joins itself to a preceding stimulus of equal strength, but which
vanishes when it is combined with a stimulus a thousand times
greater in amount.
"We may therefore lay it down as a general rule that a stimulus, in
order to be felt, may be so much the smaller if the already
preëxisting stimulation of the organ is small, but must be so much
the larger, the greater the preëxisting stimulation is.... The
simplest relation would obviously be that the sensation should
increase in identically the same ratio as the stimulus.... But if
this simplest of all relations prevailed, ... the light of the
stars, e.g., ought to make as great an addition to the daylight as
it does to the darkness of the nocturnal sky, and this we know to
be not the case.... So it is clear that the strength of the
sensations does not increase in proportion to the amount of the
stimuli, but more slowly. And now comes the question, in what
proportion does the increase of the sensation grow less as the
increase of the stimulus grows greater? To answer this question,
every-day experiences do not suffice. We need exact measurements,
both of the amounts of the various stimuli, and of the intensity of
the sensations themselves.
"How to execute these measurements, however, is something which
daily experience suggests. To measure the strength of sensations
is, as we saw, impossible; we can only measure the difference of
sensations. Experience showed us what very unequal differences of
sensation might come from equal differences of outward stimulus.
But all these experiences expressed themselves in one kind of fact,
that the same difference of stimulus could in one case be felt, and
in another case not felt at all--a pound felt if added to another
pound, but not if added to a hundredweight.... We can quickest
reach a result with our observations if we start with an arbitrary
strength of stimulus, notice what sensation it gives us, and then
_see how much we can increase the stimulus without making the
sensation seem to change_. If we carry out such observations with
stimuli of varying absolute amounts, we shall be forced to choose
in an equally varying way the amounts of addition to the stimulus
which are capable of giving us a just barely perceptible feeling of
_more_. A light to be just perceptible in the twilight need not be
near as bright as the starlight; it must be far brighter to be just
perceived during the day. If now we institute such observations for
all possible strengths of the various stimuli, and note for each
strength the amount of addition of the latter required to produce a
barely perceptible alteration of sensation, we shall have a series
of figures in which is immediately expressed the law according to
which the sensation alters when the stimulation is increased...."
Observations according to this method are particularly easy to make in
the spheres of light, sound, and pressure. Beginning with the latter
case,
"We find a surprisingly simple result. _The barely sensible
addition to the original weight must stand exactly in the same
proportion to it_, be the _same fraction_ of it, no matter what the
absolute value may be of the weights on which the experiment is
made.... As the average of a number of experiments, this fraction
is found to be about ⅓; that is, no matter what pressure there may
already be made upon the skin, an increase or a diminution of the
pressure will be _felt_, as soon as the added or subtracted weight
amounts to one third of the weight originally there."
Wundt then describes how differences may be observed in the muscular
feelings, in the feelings of heat, in those of light, and in those of
sound; and he concludes thus:
"So we have found that all the senses whose stimuli we are enabled
to measure accurately, obey a uniform law. However various may be
their several delicacies of discrimination, _this_ holds true of
all, that _the increase of the stimulus necessary to produce an
increase of the sensation bears a constant ratio to the total
stimulus_. The figures which express this ratio in the several
senses may be shown thus in tabular form:
Sensation of light 1/100
Muscular sensation 1/17
Feeling of pressure, }
" " warmth, } 1/3
" " sound, }
"These figures are far from giving as accurate a measure as might
be desired. But at least they are fit to convey a general notion of
the relative discriminative susceptibility of the different
senses.... The important law which gives in so simple a form the
relation of the sensation to the stimulus that calls it forth was
first discovered by the physiologist Ernst Heinrich Weber to obtain
in special cases."[4]
=Fechner's Law.=--Another way of expressing Weber's law is to say that to
get equal positive additions to the sensation, one must make equal
_relative_ additions to the stimulus. Professor Fechner of Leipzig
founded upon Weber's law a theory of the numerical measurement of
sensations, over which much metaphysical discussion has raged. Each just
perceptible addition to the sensation, as we gradually let the stimulus
increase, was supposed by him to be a _unit_ of sensation, and all these
units were treated by him as equal, in spite of the fact that _equally
perceptible_ increments need by no means appear _equally big_ when they
once are perceived. The many pounds which form the just perceptible
addition to a hundredweight feel bigger when added than the few ounces
which form the just perceptible addition to a pound. Fechner ignored
this fact. He considered that if _n_ distinct perceptible steps of
increase might be passed through in gradually increasing a stimulus from
the threshold-value till the intensity _s_ was felt, then the sensation
of _s_ was composed of _n_ units, which were of the same value all along
the line.[5] Sensations once represented by numbers, psychology may
become, according to Fechner, an 'exact' science, susceptible of
mathematical treatment. His general formula for getting at the number of
units in any sensation is _S_ = _C_ log _R_, where _S_ stands for the
sensation, _R_ for the stimulus numerically estimated, and _C_ for a
constant that must be separately determined by experiment in each
particular order of sensibility. The sensation is proportional to the
logarithm of the stimulus; and the absolute values, in units, of any
series of sensations might be got from the ordinates of the curve in
Fig. 2, if it were a correctly drawn logarithmic curve, with the
thresholds rightly plotted out from experiments.
Fechner's psycho-physic formula, as he called it, has been attacked on
every hand; and as absolutely nothing practical has come of it, it need
receive no farther notice here. The main outcome of his book has been to
stir up experimental investigation into the validity of Weber's law
(which concerns itself merely with the just perceptible increase, and
says nothing about the measurement of the sensation as a whole) and to
promote discussion of statistical methods. Weber's law, as will appear
when we take the senses, _seriatim_, is only approximately verified. The
discussion of statistical methods is necessitated by the extraordinary
fluctuations of our sensibility from one moment to the next. It is
found, namely, when the difference of two sensations approaches the
limit of discernibility, that at one moment we discern it and at the
next we do not. Our incessant accidental inner alterations make it
impossible to tell just what the least discernible increment of the
sensation is without taking the average of a large number of
appreciations. These _accidental errors_ are as likely to increase as to
diminish our sensibility, and are eliminated in such an average, for
those above and those below the line then neutralize each other in the
sum, and the normal sensibility, if there be one (that is, the
sensibility due to constant causes as distinguished from these
accidental ones), stands revealed. The methods of getting the average
all have their difficulties and their snares, and controversy over them
has become very subtle indeed. As an instance of how laborious some of
the statistical methods are, and how patient German investigators can
be, I may say that Fechner himself, in testing Weber's law for weights
by the so-called 'method of true and false cases,' tabulated and
computed no less than 24,576 separate judgments.
=Sensations are not compounds.= The fundamental objection to Fechner's
whole attempt seems to be this, that although the outer _causes_ of our
sensations may have many parts, every distinguishable degree, as well as
every distinguishable quality, of the _sensation itself_ appears to be a
unique fact of consciousness. Each sensation is a complete integer. "A
strong one," as Dr. Münsterberg says, "is not the multiple of a weak
one, or a compound of many weak ones, but rather something entirely new,
and as it were incomparable, so that to seek a measurable difference
between strong and weak sonorous, luminous, or thermic sensations would
seem at first sight as senseless as to try to compute mathematically the
difference between salt and sour, or between headache and toothache. It
is clear that if in the stronger sensation of light the weaker sensation
is not _contained_, it is unpsychological to say that the former differs
from the latter by a certain _increment_."[6] Surely our feeling of
scarlet is not a feeling of pink with a lot more pink added; it is
something quite other than pink. Similarly with our sensation of an
electric arc-light: it does not contain that of many smoky tallow
candles in itself. Every sensation presents itself as an indivisible
unit; and it is quite impossible to read any clear meaning into the
notion that they are masses of units combined.
There is no inconsistency between this statement and the fact that,
starting with a weak sensation and increasing it, we feel 'more,'
'more,' 'more,' as the increase goes on. It is not more of the same
_stuff_ added, so to speak; but it is more and more _difference_, more
and more _distance_, from the starting-point, which we feel. In the
chapter on Discrimination we shall see that Difference can be perceived
between simple things. We shall see, too, that _differences themselves
differ_--there are _various directions of difference_; and along any one
of them a series of things may be arranged so as to increase steadily in
that direction. In any such series the end differs more from the
beginning than the middle does. Differences of 'intensity' form one such
direction of possible increase--so our judgments of more intensity can
be expressed without the hypothesis that more units have been added to a
growing sum.
=The so-called 'Law of Relativity.'=--Weber's law seems only one case of
the still wider law that the more we have to attend to the less capable
we are of noticing any one detail. The law is obvious where the things
differ in kind. How easily do we forget a bodily discomfort when
conversation waxes hot; how little do we notice the noises in the room
so long as our work absorbs us! _Ad plura intentus minus est ad singula
sensus_, as the old proverb says. One might now add that the homogeneity
of what we have to attend to does not alter the result; but that a mind
with two strong sensations of the same sort already before it is
incapacitated by their amount from noticing the detail of a difference
between them which it would immediately be struck by, were the
sensations themselves weaker and consequently endowed with less
distracting power.
This particular idea may be taken for what it is worth.[7] Meanwhile it
is an undoubted general fact that the psychical effect of incoming
currents does depend on what other currents may be simultaneously
pouring in. Not only the _perceptibility_ of the object which the
current brings before the mind, but the _quality_ of it, is changed by
the other currents. "Simultaneous[8] sensations modify each other" is a
brief expression for this law. "We feel all things in relation to each
other" is Wundt's vaguer formula for this general 'law of relativity,'
which in one shape or other has had vogue since Hobbes's time in
psychology. Much mystery has been made of it, but although we are of
course ignorant of the more intimate processes involved, there seems no
ground to doubt that they are physiological, and come from the
interference of one current with another. A current interfered with
might naturally give rise to a modified sensation.
Examples of the modification in question are easy to find.[9] Notes make
each other sweeter in a chord, and so do colors when harmoniously
combined. A certain amount of skin dipped in hot water gives the
perception of a certain heat. More skin immersed makes the heat much
more intense, although of course the water's heat is the same. Similarly
there is a _chromatic minimum_ of size in objects. The image they cast
on the retina must needs excite a sufficient number of fibres, or it
will give no sensation of color at all. Weber observed that a thaler
laid on the skin of the forehead feels heavier when cold than when warm.
Urbantschitsch has found that all our sense-organs influence each
other's sensations. The hue of patches of color so distant as not to be
recognized was immediately, in his patients, perceived when a
tuning-fork was sounded close to the ear. Letters too far off to be read
could be read when the tuning-fork was heard, etc., etc. The most
familiar examples of this sort of thing seem to be the increase of
_pain_ by noise or light, and the increase of _nausea_ by all
concomitant sensations.
=Effects of Contrast.=--The best-known examples of the way in which one
nerve-current modifies another are the phenomena of what is known as
'simultaneous color-contrast.' Take a number of sheets of brightly and
differently colored papers, lay on each of them a bit of one and the
same kind of gray paper, then cover each sheet with some transparent
white paper, which softens the look of both the gray paper and the
colored ground. The gray patch will appear in each case tinged by the
color _complementary_ to the ground; and so different will the several
pieces appear that no observer, before raising the transparent paper,
will believe them all cut out of the same gray. Helmholtz has
interpreted these results as being due to a false application of an
inveterate habit--that, namely, of making allowance for the color of the
medium through which things are seen. The same _thing_, in the blue
light of a clear sky, in the reddish-yellow light of a candle, in the
dark brown light of a polished mahogany table which may reflect its
image, is always judged of its own proper color, which the mind _adds_
out of its own knowledge to the appearance, thereby correcting the
falsifying medium. In the cases of the papers, according to Helmholtz,
the mind believes the color of the ground, subdued by the transparent
paper, to be faintly spread _over_ the gray patch. But a patch to _look_
gray through such a colored film would have really to _be_ of the
complementary color to the film. Therefore it _is_ of the complementary
color, we think, and proceed to _see_ it of that color.
This theory has been shown to be untenable by Hering. The discussion of
the facts is too minute for recapitulation here, but suffice it to say
that it proves the phenomenon to be physiological--a case of the way in
which, when sensory nerve-currents run in together, the effect of each
on consciousness is different from that which it would be if they ran
in separately.
'_Successive contrast_' differs from the simultaneous variety, and is
supposed to be due to fatigue. The facts will be noticed under the head
of 'after-images,' in the section on Vision. It must be borne in mind,
however, that after-images from previous sensations may coexist with
present sensations, and the two may modify each other just as coexisting
sensational processes do.
Other senses than sight show phenomena of contrast, but they are much
less obvious, so I will not notice them here. We can now pass to a very
brief survey of the various senses in detail.