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 amoeba 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.