§ 17. Sensations from the Eye. — The eye is the most elaborate and the most important of the instru- ments by which we gain knowledge of the outside world. It is a single sense-organ, and all the sensa- tions that come through it are sensations of one kind, — sensations of sight. But the human eye has * evolved '; it is the final product of a long course of development, during which the organ has gradually become more and more delicate. Hence we can distinguish two strata of sight sensations; a lower, primitive layer, which dates as far back as the exist- ence of the organ of sight itself; and a later, more complicated layer, which has appeared more recently. The primitive sensations -are those of black, white Sensations of and grey. We can distinguish a large number of rig ness. ^^ggg brightness sensations^ as they are called. But there can be no doubt that the general difference §17. Se7isatio7is from the Eye 39 between black and white, light and dark, is sensed even by the eye-specks of the jelly-fish. The later sensations are those of colottr.
Colour sensations fall into four series or lines. Sensations of The first runs from red to yellow, through reddish yellow or orange; the second from yellow to green, through yellowish green; the third from green to blue, through greenish blue; the fourth from blue back again to red, through bluish red (violet and purple). All the colours but purple are contained in the rainbow, and in the artificial rainbow, the solar spectrum.
In ordinary conversation we speak of black, white and grey as ' colours.' Notice that they belong to a different group of sensations from the true colours, and that they should be called ^brightnesses.'
The best way to understand the eye is to think of it as The eye a a photographic camera. It has an automatic diaphragm, photographic the ins (the circle that we refer to when we speak of ^ brown' or ^blue' eyes), which regulates the opening of the pupil according to illumination. Behind the iris, in the pupil, is a lens which focusses automatically, — not by coming forwards or retiring inwards, but by altering its curvature. Behind the lens is a dark chamber. The back wall of this chamber is covered by a sensitive film, the nervous network or retina, upon which visual images are formed. The film is self-renewing, so that images can succeed one another upon it very rapidly. The action of hght upon it sets up processes of chemical decomposition, just as in the real photographic plate. (iZ, Lesson IX.; iV., ch. xliii.)
Even if we knew nothing of the eyes of lower animals, Sensations of we should be forced to believe that the brightness sensa- brightness . are older tions are more primitive than those of colour. Objects may than those of be black or white or grey; they need not show the faintest colour.
40 Sensation The system of sight sen- sations.
trace of colour. But we never see a ' pure ' colour; every colour that we know is really a mixture of pure colour with brightness. If you look at a spectrum in very faint light, you do not see any colour in it at all; you see a band of grey. Evidently, then, this grey must be present in the colours when you do see them. Again: people may be perfectly colour-blind, and still see things in the world as black and white and grey. But if people are brightness- blind, if they do not see black and white and grey, they are totally blind and do not see anything. And* again: the retina has a more comphcated structure in the central than in the surrounding parts of its surface. But it is only in the central parts that we see all the colours; as we move out over the outlying parts we gradually lose the colour sense, until finally, at the edges of the retina, we see nothing but brightness.
In order to get an idea of the enormous number of sight sensations, — brightnesses and colours (remember that ' colours ' are really mixtures of pure colour and brightness), — it is worth while to make a diagram.
\ Suppose that we have a square surface (a piece of card or paper), which is tinted a neutral gx^y^ — a grey that lies exactly half-way between dead black and brilliant white. Leaving the grey in the centre, we work outwards towards the edge of the square, mixing in more and more colour as we go. At the four corners we put the four principal coXovlXs,, the end-colours of the four colour series (red, yellow, green and blue); ^^ along the sides come the intermediate col- ours. When the surface of the square is filled in, we have on it all the possible sen- sations which can be built up from neutral grey, — all those which are of the same brightness-value as that grey; beginning with the grey itself, and ending with the purest colours that can be got with this grey in them. Thus, passing from green to the centre we have green, slightly grey green, greyer green, still greyer green,...grey; and similarly with the other colours (Fig. i).
§ I/. Seiisations from the Eye 41 Now we take a second card, tinted a little darker grey, and mix in our colours as before. The corner colours will be differ- ent; red will be getting a tinge of reddish brown, yellow a tinge of brown, green a touch of olive and blue a touch of indigo. Since we cannot distinguish so many shades between this darker grey and reddish brown, etc., as between the neutral grey and red, etc., our square will be a little smaller than the former square.
We take a third square, tinted a little lighter grey, and proceed as before. Red now verges to flesh-colour; yellow to straw- colour; green becomes pale green; blue tends towards sky- blue. Our square is again a little smaller than the first was.
So we go on, until our central grey becomes dead black in the one direction and brilliant white in the other: the squares grow f^ smaller and smaller, till at last (at black and white) we have only points, not surfaces at all. Laying the squares together, in the right order, we have a double pyramid (Fig. 2). The line join- ing apex to apex is the black- grey-white line; the square base is surrounded by the purest colours that we can get; the out- side surface shows the browns, olives, pinks, pale greens, etc.; and wherever we cut into the pyramid we have a sensation-line running from a given colour to a given grey.^ When all the sensations are counted up, they amount to more than 30,000.
The explanation of sight sensations, the statement of their Hering's iDodily conditions, is a difficult matter, and the reader must ^|?^^°Y °^ take it largely on trust. The most satisfactory explanation that we have at present we owe to Professor Hering, now professor of physiology in the University of Leipsic. In its latest form it is briefly as follows: vision.
42 Se7isation (i) There are in the retma three different ^visual sub- stances,' three chemical substances that are differently af- fected by hght (/.^., by ether waves).
(2) Each substance is the seat of two chemical processes, ^<?composition and r<fcomposition. The two processes are attended by two different sensations in each case. In one substance, the processes give us white and black; in another, red and green; in the third, yellow and blue. There are thus six different chemical processes that can be set up in the retina; and from the six /r/;^a}^<3j/ sensations accompany- ing them we can get the whole sum of sight sensations. Pale purple, e,g,^ means a mixture of the white-process, the red-process and the blue-process; all three substances are called upon to furnish it.
(3) The black-white substance is affected by every hght stimulus; the other two substances only by certain forms of stimulus.
(4) If red and green light fall upon the same part of the retina, the colours cancel each other, and nothing is left but a sensation of grey. This is because the chemical processes of decomposition and recomposition are antagonistic or op- posite processes; they work against each other in the visual substance. — The same thing is true of yellow and blue.
If black and white fall on the same part of the retina, however, we see a mixture of black and white, a grey; there seems to be no cancelling of black by white, as there is of green by red. Really, black and white do cancel each other in the retina; there is no grey-process there. But the corti- cal cells with which the optic nerve is connected are always in a state of commotion (owing to changes of temperature, etc.), whether there is a stimulus before the eye or not; and this commotion gives us the ^ intrinsic ' or ' subjective ' sight sensation, the sensation of grey. (See F.^ 902.)
Sensations of § 1 8. Sensations from the Ear. — Next in impor- noise and tancc to the cvc Stands the ear. Sensations of heartone. J ing, like those of sight, have evolved or developed; § 1 8. Sensations from tJie Ear 43 and we can distinguish two stages of hearing sensa- tions, (i) sensations of noise and (2) sensations of tone. But we can go back a step farther. All sen- sations of hearing have been in some way developed from sensations of jar or shake, which were not heard at all.
The human ear is extremely complicated; but it has kept some of the primitive shake-organs along- side of the later growth. The shake-organ, as we have it ourselves now, has nothing to do with hear- ing, and must therefore be treated of separately.
ia) The Ear as Organ of Hearijig. — Our sensations of hearing are (i) sensations of simple noise ^ corre- sponding to the brightness sensations of the eye, and (2) sensations of tone^ corresponding to colour sensa- tions. A noise is hard and unmusical; it is set up by a shock or jerk of the air-particles. A tone is smooth and musical; its stimulus is a repeated wave- movement of the air-particles. The pop of a soap- bubble is a noise; the sound that you get by blowing across the mouth of a bottle is a tone.
Although tones and noises sound together far more often than they sound separately, and mix very readily, their mixt- ure is never complete enough to give us a simple sensation, as that of colour and brightness does. The ' tone ' of a vio- lin owes a good deal of its effectiveness to the noise made by scraping the bow over the strings; but we are quite well able to distinguish the scrape from the accompanying musi- cal tone.
A tone diagram would be a spiral line, like a screw-thread, with the deepest bass tone at the one end, the shrillest treble tone at the other, and the rest arranged in musical order between the two. Round each circle of the spiral are set the tones that we 44 Sensation The ear a piano.
Sensation of giddiness.
can distinguish within the limits of an octave. The Hne must be made spiral, i.e.^ must keep returning as it advances towards the point from which it started, because the tones that bound an octave are more nearly like each other than any other two tones upon the scale; just as the colours that bound the spectrum, red and violet, are more like each other than are any. other two col- ours in the spectral series. On the screw-thread these limiting tones lie directly above and below one another. Music employs only about 90 of the 11,000 tones that we can distinguish. The reasons for this curious fact we shall discuss later (§ 42). — The noise diagram would be a straight and much shorter line; we cannot distinguish nearly so many noises as tones; and there is no recurring likeness of noise to noise, to make the line a spiral.
If the eye is a little camera, the organ of hearing is a tiny piano: a piano with a keyboard for the air to play on, with 11,000 strings behind the keyboard, and with a damper to stop the movement of the strings after they have sounded. (That is why we can speak so quickly; the sound of each word is damped before the next word comes.) When the pianist is an air-wave, we hear a tone; when it is an air- shock, a noise. Generally, several pianists of both kinds are playing together. (ZT., 215 ff.; iV., chs. xxxiv. if.)
{F) The Ear as Organ of Eqiiilibrmm, — The part of the ear which resembles the primitive shake- organ gives us the sensation, not of tone or noise or, indeed, of hearing at all, but of giddiness. Gid- diness means that we have been shaken, our physi- cal balance disturbed, — that we are in danger of falling. Hence though it is very unpleasant, it is also very useful.
We might lose our balance in three ways: by confusing up with down, back with front, right with left. And there are three shake-organs in each ear, which help us to keep our balance steadily in these three directions of space. Nod your head sharply up or down, turn it sharply to right or left, drop it sharply towards the one shoulder or the § 19. Sensations from the Skin 45 other ] in every case you will get a momentary giddiness.
Fig. 3 shows a model of a single shake-organ. The grain of sand, s, is balanced on the hairs coming from a group of cells which are connected with a nerve, n. You can easily see that a shake of the air or water surrounding the ^^^ ^ organ would shift the ' ^ balance of the grain upon the hairs and bend some of the hairs down. In this way an excitation would be set up in the nerve, and carried to the brain.
§ 19. Sensations from the Skin. — The skin is an Sensations of organ of a very different character from the eye or j^emperature the ear. For it is not merely a sense-organ: it has ^^^ p^^^- to do a great deal for the body, besides furnishing sensations for consciousness. Thus it protects the underlying organs from injury, it carries the hair and nails, it contains oil-glands and sweat-glands. But there are in it, notwithstanding, no less than three distinct kinds of sense-organs. One tells us of the weight of objects (sensation of pressure)] another of their temperature (sensations of heat and eold); and a third of the injury they are doing us (sensation of pain).
You cannot get these four sensations from any and every part of the skin; their organs are sprinkled or dotted over its surface. They are all, probably, very old sensations; press- ure and pain, at any rate, are older even than w^hite and black and noise. And their bodily organs are simple; just httle bunches of nerve-fibrils, sometimes lying by themselves, and sometimes twined round the root of a hair or a few cells, in the thickness of the skin. {H., 206 ff.; F., 1037, 1044.)
46 Sensation § 20. Sensations from the Mouth and Nose. — We may treat of these two organs together, because their sensations are intimately blended in everyday experience, and because the office of both of them is to stand guard over digestion, to secure the health of the internal bodily organs. Sensations of {o) The mucous membrane of the mouth is sensi- tive to pressure, heat and cold, and pain. But we also get, from various parts of the cavity of the mouth, the four sensations of taste: sweet, bitter, sour and salt.
It is at first difficult to believe that there are no more than four distinct tastes. But what we call * taste ' in ordi- nary conversation is for the most part a mixture of smell and taste. The reason that we cannot ^ taste' things when we have a cold in the head is that the nasal passages are blocked, so that we cannot snielL A good deal of our daily food is absolutely tasteless.
Taste At different parts of the tongue and at the back of the mouth we find little bottle-shaped pits. The mouth of the bottle receives the taste stimulus (the sweet, etc., substance). Inside the bottle are the taste cells, from which the nerve runs through the bottom of the bottle to the brain. (ZT., 209.)
Sensations of {b) There are two patches of mucous membrane in smell the two nostrils which give us sensations of smell. We know that there are a great many kinds of smell; and there seem to be groups or classes of smell sensa- tions, like those of tone and noise, or brightness and colour. But we cannot yet say how many there are, or which are the more primitive.
The smell cells carry hairs, which project into the cavity of the nose, and catch the odorous particles as they are carried into the nostrils by breathing. (Z^., 211.)
§21. Sensations from Internal Organs 47 The organ of smell is thus more simple even than that of taste. It may be that, in man, the organ of smell is degen- erating, while that of taste is not changing. This would account for the difficulty that we have in deciding the number of different smells that can be distinguished.
§21. Sensations from Internal Organs. — A last, and by no means unimportant source of sensations is to be found in certain internal bodily organs. With two sets of these, (i) and (2) belov^, we are fairly well acquainted; of the rest, brought together under (3), we know very little.
(i) Bone^ Muscle, Teitdon. — The bones of the body turn in sockets. They are moved by the muscles, which are tied to them by sinews or tendons. We have from the muscles sensations of pi'essnre and pain; from the sinews a new sensation, that of strain; strain. and from the joints or bone-sockets the familiar sen- sation oi pressure.
(2) The Alimentary Canal. — The body is not solid; Hunger, it is pierced by the alimentary canal, whose duty is nausea. to take in food and get rid of waste. From the upper parts of this canal we have three new sensa- tions. The extreme back of the mouth and top of the throat give us thirst; the tube running from mouth to stomach, nausea or sickness; the stomach itself, hunger. No new sensations come from the intestines.
(3) It is probable that the lungs, blood-vessels and stuffiness, bladder furnish new sensations. We have lung sen- ^ ' sations in ' bracing ' and ' stuffy ' feelings; blood- vessel sensations in tingling, itching, and ^ pins and needles '; and bladder sensations in the ' stir up ' of the inside organs that comes, e.g., with the emotion of fear.
48 Sensation The sense-organs dotted over the surface of the joints resemble those found in the skin. On the other hand, the nerves that run to the brain from muscle and tendon start from organs which are pecuHar to these tissues. Neverthe- less, the muscle sensation of pressure is not distinguishable from the skin sensation of pressure. There may possibly be a special sensation of muscular fatigue: but this is very doubtful. The organs of hunger, thirst and nausea are not known, though the sensations can be localised in the mucous membrane of stomach, soft palate and oesophagus respec- tively. {H.^ 176, 203; F.^ 1048, 1059.)
The ' sensation ' of tickling is really a complex of sensa- tions. It contains a light pressure sensation; a sensation of temperature, a thrill of warmth or shiver of cold; a sensa- tion due to change of blood-circulation, of the same kind as tinghng and itching; and, probably, a number of muscular pressures, due to the spasmodic contraction of the muscle- sheet lying just below the part of the skin to which the weak stimulus is applied.
Fig. 4. (which is not quite accurate, from the physical stand- point, but still accurate enough for our present purpose) shows how the sensations set up in muscle and tendon may vary independently of each other. The four diagrams represent the arm, bent at the elbow- joint; the upper and lower arms are held to- gether by a muscle, from either end of which ten- dons run to the bones. In a and b there is the same degree of muscular contraction, though the a :\ -K -E Fig. 4 pull upon the tendons is very different; in c and d the strain is the same, but the muscles are differently contracted.
§ 22. Intensity of Sensations. — So far we have Quality and been considering only one aspect of sensations, their sensations. quality. Quality is what makes one sensation differ- ent from another. All the different tints and hues of colour are qualities of sight sensations; the shades of grey, the differences of tonal pitch, the kinds of smell, are all qualities. A red which differs from another red in hue is a different sensation; a tone which differs from another tone in pitch is a different sen- sation.
But a sensation may remain the same sensation, the same pitch or tint or smell, and yet vary in strength or intensity. A pressure may be the press- ure of an ounce or of a pound; it is always pressure, one quality, but its strength differs. The tone that 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 your arm very little or a great deal; the sensation of strain from the tendons of the arm is the same in both cases, but the amount of it is different.
Here a very interesting question arises: the ques- Relation of TTii r«i/ii Stimulus tion whether, if I add to the amount of stimulus (add intensity to to the heaviness of the weight, or the strength of the sound, or the illuminating power of the light) I add in equal measure to the intensity of the correspond- ing sensation. Of course, the strain of carrying three pounds is greater than the strain of carrying one: but is the strain sensation in the first case three times as strong as the strain sensation in the second t The answer to the question is given by what is called Weber's Law.
sensation intensity.
E so Sensation Weber's law.
Its useful- ness.
§ 23. Weber's Law. •— Suppose that I have laid a pound weight in the scale, and measured out a pound of sugar. If I add another pound weight, I must have twice as much sugar to balance the scales; if 1 add a third pound, three times as much sugar; and so on.
Now suppose that I am measuring, not sugar, but the sensation of pressure. A pound weight on the skin gives me a sensation of pressure, P, Two pounds give me a stronger pressure, — let us say, 2 P. Will three pounds give me 3 P?
Experiment says no. If my pressure sensations are to be /^, 2 P, 3 P, 4 P, etc., then the weights used must be i lb., 2 lbs., 4 lbs., 8 lbs., etc. That is: if the third pressure is to be as much stronger than the second as the second was than the first, then the third weight must be proportionately as much larger than the second as the second was larger than the first. To get 2P — P='^P — 2P, we must have sec- ond weight: first = third: second (or, using numbers, The usefulness of this law is clear. When an artist paints a moonlight scene, we recognise it as meant to represent moonlight. Now the painter has no white that is anything like so brilliant as moonlight. What he does, then, is to make the same relative (or proportional) difference between his light and shade that there is between moonlight and shadow in nature. A difference that is relatively the same for stimuli is absolutely the same for sensation; and so we see at once that the picture represents a moonlight effect. It is the same principle that enables us to recognise a musical melody, although it may now be played in quite a different key from that in which we are famiHar with it. — The reasons Q?iestio7is and Exercises 51 for the law are to be looked for in the physiological behav- iour of nervous substance. We cannot enter upon them here.
E. H. Weber (i 795-1 878), after whom the law is named, was professor of physiology in the University of Leipsic. It may interest the reader to know of one of Weber's own experiments: in the text above we have been merely ^supposing.' Weber found, then, in experiments with weights, that it is just as difficult to distinguish between the pressure of 29 and 30 half-ounces as between those of 29 and 30 drach7ns; although the difference of weight in the first case is four times as great as it is in the second (i oz.= 8 dr.). Sameness of difference in sensation means pro- portional sameness of difference between stimuli.
Questions and Exercises (i) Sight. (Try to account for the results of these experiments by applying Hering's explanation to them.)
(a) The laws of colour mixture.
1. Mix on the colour-top two neighbouring colours; red and orange, indigo and violet, etc. Notice that the result is a colour that lies midway for sensation between the two chosen.
2. Mix two complementary or antagonistic colours, to get grey. With the coloured papers (which do not give Hering's colours exactly) you will not get a grey from red and green, or yellow and blue; you must take red and bluish green, orange and blue, yellow and indigo, etc.
3. Mix three colours to get grey: red, yellow, blue; or red, green, violet, etc.
4. Mix all the spectral colours, with purple, to get grey.
(^) The persistence of vision; after-images.
5. Look steadily at a red patch on a white ground.
After 20 sec. remove the red, and you will see a patch of the antagonistic colour (bluish green). Try with all the colours, and with black. Try with white, on a black ground.
52 Sensation {c) Indirect vision.
6. Bandage the left eye, and look steadily with the right at a white point on a black screen. Let an assistant move a coloured patch (held on a black straw) from the white point outwards (to your right) along the screen. You see the white point with the centre, the moving colour with outlying parts of the retina. Notice that the colour changes, as it moves; and that finally you see no colour at all, but simply a black or grey. — Or tack the coloured patch to a wall, some distance to the right of the observing eye. Look straight at the wall- in front of you; and then gradually turn the eye outwards, toward the patch. Notice the change of colour, as the eye moves.
(^) Contrast.
7. Lay a red patch on a white sheet of paper, and cover both with white tissue-paper. The white surface will seem slightly tinged with bluish green.
8. Cut discs like that of Fig. 5 (where the white parts stand for white, the black for black, and the shaded parts for some colour) ^^^- 5 and spin them on the colour-top.
If the colour is orange, you see a yellowish sur- face with a bluish ring; if it is green, a pale greenish surface with a purplish ring; etc.
Contrast is due, on Hering's explanation, to the physiological unity of the retina. If ^^composition is set up at one point of the retina, ^'^composition is set up in the neighbouring parts, and vice versa. If we look at a blue patch on a grey ground, we have, as direct effect of the stimulus, a sensation of blue; as its indirect effect, a sensation of yellow-grey over the parts of the grey ground that adjoin the blue patch. — Other writers regard contrast as a case of apperceptive illusion (see below, p. 117). In all probability, however, Bering's view is correct.
Questions and Exercises 53 (2) Hearing.
{a) Discrimination of tones.
9. Take six short pieces of gutta-percha tubing, and soften one end of each piece in warm water. Pinch this end together, so that the opening is a mere sHt. Wire the pieces to the necks of six bottles so that you get their tones by blowing into the tubing. Tune them (by pouring water in, as required) to two consecutive notes on the piano: three of them, say, to the ^, and three to the ^-sharp of the middle octave. This tuning must be done by some one with musical experi- ence and a good musical ear.
Two of the bottles, a c and a ^-sharp, you set aside as standards. Now take a c bottle, and pour in a very little water; thus raising its pitch till its tone is just perceptibly higher than that of the standard c. Then take the remaining c bottle, and raise its pitch in the same way, till it sounds just perceptibly higher than the c that you have already raised. Go on in this way with the two bottles, till one of them sounds the same tone as the ^-sharp standard. Note how many tones you can discriminate between the c and the ^-sharp. — At the first trial you should get at least 8 intermediate tones, and with practice many more.
Repeat the experiment with the ^-sharp bottles, working downwards (by pouring water out) to- wards c. Note how many tones you can hear between the (:-sharp and the c.
If you regard the experiment, in this form, as too laborious, use one of a set of Quincke's tubes (Fig. 7). The cork in the lower tube lowers the tone an octave. (A^., p. 366.) Water may be Fig. 6 n 54 Sensation poured in, as required. Blow evenly and steadily, and not too strongly.
(Jf) Quality of noise.
10. Procure a number of drug- gist's sample phials, of different sizes. Cork them firmly, and arrange them in the order of size. Pull the corks out sharply, in succession. You get ^ pops ' of different quality.
Or blow a large and a small soap-bubble, and note the difference in their snap. The small bubble bursts with a sharp pop, the large one p with a thud. These are qualities of noise; there is no tone present.
The experiment is more striking if you use, instead of air, a mixture of hydrogen and air. In that case you touch the bubbles olT with a match. Be careful that they are well away from the air-hydrogen mixture before you ignite them.
(3) Pressure and Temperature from the Skin.
1 1. Mark out with ink a square centimetre of skin on the back of the hand. Work over this lightly, in all directions, with a piece of pointed pith or cork. Move the point slowly. As it travels, sensations of pressure will flash out at the pressure-spots; be- tween the spots you will have no pressure sensa- tion. Make a map of the spots on a square cm. of cross-section paper, putting down an ink-dot for every spot.
12. Work over the same place with a pointed metal tube filled first with hot and then with cold water. Sensations of heat and cold will flash out at the temperature spots in the same way; between the spots you will have no temperature sensation. Make maps of the hot and cold spots. Compare the three maps and see which has the most spots, and on what patterns the spots are arranged. — Does introspection show any differ- ences between the sensations of pressure,, heat and cold, over and above their qualitative differences?
Questions and Exercises 55 (4) Muscle, Tendon and Joint.
13. Hold your arm out straight, and fold it slowly in towards your chest. Notice the pressures in the muscles of the fore and upper arms and in the elbow joint.
14. Clench your fist, slowly. Distinguish the pressures