Thus we take a glance out of the window and say that the day looks cold, although we well know that we cannot see _cold_. Or we say that the melon sounds green, or the bell sounds cracked, although a _crack_ or _greenness_ cannot be heard. Or we say that the box feels empty, although _emptiness_ cannot be felt. We have come to a.s.sociate cold, originally experienced with days which look like the one we now see, with this particular appearance, and so we say we see the cold; sounds like the one coming from the bell we have come to a.s.sociate with cracked bells, and that coming from the melon with green melons, until we say unhesitatingly that the bell sounds cracked and the melon sounds green.

And so with the various senses. Each gleans from the world its own particular bit of knowledge, but all are finally in a partnership and what is each one's knowledge belongs to every other one in so far as the other can use it.

THE SENSORY PROCESSES TO BE EXPLAINED.--The explanation of the ultimate nature of knowledge, and how we reach it through contact with our material environment, we will leave to the philosophers. And battles enough they have over the question, and still others they will have before the matter is settled. The easier and more important problem for us is to describe the _processes_ by which the mind comes to know its environment, and to see how it uses this knowledge in thinking. This much we shall be able to do, for it is often possible to describe a process and discover its laws even when we cannot fully explain its nature and origin. We know the process of digestion and a.s.similation, and the laws which govern them, although we do not understand the ultimate nature and origin of _life_ which makes these possible.

THE QUALITIES OF OBJECTS EXIST IN THE MIND.--Yet even in the relatively simple description which we have proposed many puzzles confront us, and one of them appears at the very outset. This is that the qualities which we usually ascribe to objects really exist in our own minds and not in the objects at all. Take, for instance, the common qualities of light and color. The physicist tells us that what we see as light is occasioned by an incredibly rapid beating of ether waves on the retina of the eye. All s.p.a.ce is filled with this ether; and when it is light--that is, when some object like the sun or other light-giving body is present--the ether is set in motion by the vibrating molecules of the body which is the source of light, its waves strike the retina, a current is produced and carried to the brain, and we see light. This means, then, that s.p.a.ce, the medium in which we see objects, is not filled with light (the sensation), but with very rapid waves of ether, and that the light which we see really occurs in our own minds as the mental response to the physical stimulus of ether waves. Likewise with color. Color is produced by ether waves of different lengths and degrees of rapidity.

Thus ether waves at the rate of 450 billions a second give us the sensation of red; of 472 billions a second, orange; of 526 billions a second, yellow; of 589 billions a second, green; of 640 billions a second, blue; of 722 billions a second, indigo; of 790 billions a second, violet. What exists outside of us, then, is these ether waves of different rates, and not the colors (as sensations) themselves. The beautiful yellow and crimson of a sunset, the variegated colors of a landscape, the delicate pink in the cheek of a child, the blush of a rose, the shimmering green of the lake--these reside not in the objects themselves, but in the consciousness of the one who sees them. The objects possess but the quality of reflecting back to the eye ether waves of the particular rate corresponding to the color which we ascribe to them. Thus "red" objects, and no others, reflect back ether waves of a rate of 450 billions a second: "white" objects reflect all rates; "black" objects reflect none.

The case is no different with regard to sound. When we speak of a sound coming from a bell, what we really mean is that the vibrations of the bell have set up waves in the air between it and our ear, which have produced corresponding vibrations in the ear; that a nerve current was thereby produced; and that a sound was heard. But the sound (i.e., sensation) is a mental thing, and exists only in our own consciousness.

What pa.s.sed between the sounding object and ourselves was waves in the intervening air, ready to be translated through the machinery of nerves and brain into the beautiful tones and melodies and harmonies of the mind. And so with all other sensations.

THE THREE SETS OF FACTORS.--What exists outside of us therefore is a _stimulus_, some form of physical energy, of a kind suitable to excite to activity a certain end-organ of taste, or touch, or smell, or sight, or hearing; what exists within us is the _nervous machinery_ capable of converting this stimulus into a nerve current which shall produce an activity in the cortex of the brain; what results is the _mental object_ which we call a _sensation_ of taste, smell, touch, sight, or hearing.

2. THE NATURE OF SENSATION

SENSATION GIVES US OUR WORLD OF QUALITIES.--In actual experience sensations are never known apart from the objects to which they belong.

This is to say that when we see _yellow_ or _red_ it is always in connection with some surface, or object; when we taste _sour_, this quality belongs to some substance, and so on with all the senses. Yet by sensation we mean only _the simple qualities of objects known in consciousness as the result of appropriate stimuli applied to end-organs_. We shall later see how by perception these qualities fuse or combine to form objects, but in the present chapter we shall be concerned with the qualities only. Sensations are, then, the simplest and most elementary knowledge we may get from the physical world,--the red, the blue, the bitter, the cold, the fragrant, and whatever other qualities may belong to the external world. We shall not for the present be concerned with the objects or sources from which the qualities may come.

To quote James on the meaning of sensation: "All we can say on this point 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 a.s.sociations with past experience. But it is obvious that _such immediate sensations can be realized only in the earliest days of life_."

THE ATTRIBUTES OF SENSATION.--Sensations differ from each other in at least four respects; namely, _quality_, _intensity_, _extensity_, and _duration_.

It is a difference in _quality_ that makes us say, "This paper is red, and that, blue; this liquid is sweet, and that, sour." Differences in quality are therefore fundamental differences in _kind_. Besides the quality-differences that exist within the same general field, as of taste or vision, it is evident that there is a still more fundamental difference existing between the various fields. One can, for example, compare red with blue or sweet with sour, and tell which quality he prefers. But let him try to compare red with sweet, or blue with sour, and the quality-difference is so profound that there seems to be no basis for comparison.

Differences in _intensity_ of sensation are familiar to every person who prefers two lumps of sugar rather than one lump in his coffee; the sweet is of the same quality in either case, but differs in intensity. In every field of sensation, the intensity may proceed from the smallest amount to the greatest amount discernible. In general, the intensity of the sensation depends on the intensity of the stimulus, though the condition of the sense-organ as regards fatigue or adaptation to the stimulus has its effect. It is obvious that a stimulus may be too weak to produce any sensation; as, for example, a few grains of sugar in a cup of coffee or a few drops of lemon in a quart of water could not be detected. It is also true that the intensity of the stimulus may be so great that an increase in intensity produces no effect on the sensation; as, for example, the addition of sugar to a solution of saccharine would not noticeably increase its sweetness. The lowest and highest intensity points of sensation are called the lower and upper _limen_, or threshold, respectively.

By _extensity_ is meant the s.p.a.ce-differences of sensations. The touch of the point of a toothpick on the skin has a different s.p.a.ce quality from the touch of the flat end of a pencil. Low tones seem to have more volume than high tones. Some pains feel sharp and others dull and diffuse. The warmth felt from spreading the palms of the hands out to the fire has a "bigness" not felt from heating one solitary finger. The extensity of a sensation depends on the number of nerve endings stimulated.

The _duration_ of a sensation refers to the time it lasts. This must not be confused with the duration of the stimulus, which may be either longer or shorter than the duration of the sensation. Every sensation must exist for some s.p.a.ce of time, long or short, or it would have no part in consciousness.

3. SENSORY QUALITIES AND THEIR END-ORGANS

All are familiar with the "five senses" of our elementary physiologies, sight, hearing, taste, smell, and touch. A more complete study of sensation reveals nearly three times this number, however. This is to say that the body is equipped with more than a dozen different kinds of end-organs, each prepared to receive its own particular type of stimulus. It must also be understood that some of the end-organs yield more than one sense. The eye, for example, gives not only visual but muscular sensations; the ear not only auditory, but tactual; the tongue not only gustatory, but tactual and cold and warmth sensations.

SIGHT.--Vision is a _distance_ sense; we can see afar off. The stimulus is _chemical_ in its action; this means that the ether waves, on striking the retina, cause a chemical change which sets up the nerve current responsible for the sensation.

The eye, whose general structure is sufficiently described in all standard physiologies, consists of a visual apparatus designed to bring the images of objects to a clear focus on the retina at the _fovea_, or area of clearest vision, near the point of entrance of the optic nerve.

The sensation of sight coming from this retinal image unaided by other sensations gives us but two qualities, _light and color_. The eye can distinguish many different grades of light from purest white on through the various grays to densest black. The range is greater still in color.

We speak of the seven colors of the spectrum, violet, indigo, blue, green, yellow, orange, and red. But this is not a very serviceable cla.s.sification, since the average eye can distinguish about 35,000 color effects. It is also somewhat bewildering to find that all these colors seem to be produced from the four fundamental hues, red, green, yellow, and blue, plus the various tints. These four, combined in varying proportions and with different degrees of light (i.e., different shades of gray), yield all the color effects known to the human eye. Herschel estimates that the workers on the mosaics at Rome must have distinguished 30,000 different color tones. The _hue_ of a color refers to its fundamental quality, as red or yellow; the _chroma_, to its saturation, or the strength of the color; and the _tint,_ to the amount of brightness (i.e., white) it contains.

HEARING.--Hearing is also a distance sense. The action of its stimulus is mechanical, which is to say that the vibrations produced in the air by the sounding body are finally transmitted by the mechanism of the middle ear to the inner ear. Here the impulse is conveyed through the liquid of the internal ear to the nerve endings as so many tiny blows, which produce the nerve current carried to the brain by the auditory nerve.

The sensation of hearing, like that of sight, gives us two qualities: namely, _tones_ with their accompanying pitch and timbre, and _noises_.

Tones, or musical sounds, are produced by isochronous or equal-timed vibrations; thus _C_ of the first octave is produced by 256 vibrations a second, and if this tone is prolonged the vibration rate will continue uniformly the same. Noises, on the other hand, are produced by vibrations which have no uniformity of vibration rate. The ear's sensibility to pitch extends over about seven octaves. The seven-octave piano goes down to 27-1/2 vibrations and reaches up to 3,500 vibrations.

Notes of nearly 50,000 vibrations can be heard by an average ear, however, though these are too painfully shrill to be musical. Taking into account this upper limit, the range of the ear is about eleven octaves. The ear, having given us _loudness_ of tones, which depends on the amplitude of the vibrations, _pitch_, which depends on the rapidity of the vibrations, and _timbre_, or _quality_, which depends on the complexity of the vibrations, has no further qualities of sound to reveal.

TASTE.--The sense of taste is located chiefly in the tongue, over the surface of which are scattered many minute _taste-bulbs_. These can be seen as small red specks, most plentifully distributed along the edges and at the tip of the tongue. The substance tasted must be in _solution_, and come in contact with the nerve endings. The action of the stimulus is _chemical_.

The sense of taste recognizes the four qualities of _sour_, _sweet_, _salt_, and _bitter_. Many of the qualities which we improperly call tastes are in reality a complex of taste, smell, touch, and temperature.

Smell contributes so largely to the sense of taste that many articles of food become "tasteless" when we have a catarrh, and many nauseating doses of medicine can be taken without discomfort if the nose is held.

Probably none of us, if we are careful to exclude all odors by plugging the nostrils with cotton, can by taste distinguish between sc.r.a.ped apple, potato, turnip, or beet, or can tell hot milk from tea or coffee of the same temperature.

SMELL.--In the upper part of the nasal cavity lies a small brownish patch of mucous membrane. It is here that the olfactory nerve endings are located. The substance smelled must be volatile, that is, must exist in gaseous form, and come in direct contact with the nerve endings.

Chemical action results in a nerve current.

The sensations of smell have not been cla.s.sified so well as those of taste, and we have no distinct names for them. Neither do we know how many olfactory qualities the sense of smell is capable of revealing. The only definite cla.s.sification of smell qualities is that based on their pleasantness or the opposite. We also borrow a few terms and speak of _sweet_ or _fragrant_ odors and _fresh_ or _close_ smells. There is some evidence when we observe animals, or even primitive men, that the human race has been evolving greater sensibility to certain odors, while at the same time there has been a loss of keenness of what we call scent.

VARIOUS SENSATIONS FROM THE SKIN.--The skin, besides being a protective and excretory organ, affords a lodging-place for the end-organs giving us our sense of pressure, pain, cold, warmth, tickle, and itch.

_Pressure_ seems to have for its end-organ the _hair-bulbs_ of the skin; on hairless regions small bulbs called the _corpuscles of Meissner_ serve this purpose. _Pain_ is thought to be mediated by free nerve endings. _Cold_ depends on end-organs called the _bulbs of Krause_; and _warmth_ on the _Ruffinian corpuscles_.

Cutaneous or skin sensation may arise from either _mechanical_ stimulation, such as pressure, a blow, or tickling, from _thermal_ stimulation from hot or cold objects, from _electrical_ stimulation, or from the action of certain _chemicals_, such as acids and the like.

Stimulated mechanically, the skin gives us but two sensation qualities, _pressure_ and _pain_. Many of the qualities which we commonly ascribe to the skin sensations are really a complex of cutaneous and muscular sensations. _Contact_ is light pressure. _Hardness_ and _softness_ depend on the intensity of the pressure. _Roughness_ and _smoothness_ arise from interrupted and continuous pressure, respectively, and require movement over the rough or smooth surface. _Touch_ depends on pressure accompanied by the muscular sensations involved in the movements connected with the act. Pain is clearly a different sensation from pressure; but any of the cutaneous or muscular sensations may, by excessive stimulation, be made to pa.s.s over into pain. All parts of the skin are sensitive to pressure and pain; but certain parts, like the finger tips, and the tip of the tongue, are more highly sensitive than others. The skin varies also in its sensitivity to _heat_ and _cold_. If we take a hot or a very cold pencil point and pa.s.s it rather lightly and slowly over the skin, it is easy to discover certain spots from which a sensation of warmth or of cold flashes out. In this way it is possible to locate the end-organs of temperature very accurately.

[Ill.u.s.tration: FIG. 17.--Diagram showing distribution of hot and cold spots on the back of the hand. C, cold spots; H, hot spots.]

THE KINaeSTHETIC SENSES.--The muscles, tendons, and joints also give rise to perfectly definite sensations, but they have not been named as have the sensations from most of the other end-organs. _Weight_ is the most clearly marked of these sensations. It is through the sensations connected with movements of muscles, tendons, and joints that we come to judge _form_, _size_, and _distance_.

THE ORGANIC SENSES.--Finally, to the sensations mentioned so far must be added those which come from the internal organs of the body. From the alimentary ca.n.a.l we get the sensations of _hunger_, _thirst_, and _nausea_; from the heart, lungs, and organs of s.e.x come numerous well-defined but unnamed sensations which play an important part in making up the feeling-tone of our daily lives.

Thus we see that the senses may be looked upon as the sentries of the body, standing at the outposts where nature and ourselves meet. They discover the qualities of the various objects with which we come in contact and hand them over to the mind in the form of sensations. And these sensations are the raw material out of which we begin to construct our material environment. Only as we are equipped with good organs of sense, especially good eyes and ears, therefore, are we able to enter fully into the wonderful world about us and receive the stimuli necessary to our thought and action.

4. PROBLEMS IN OBSERVATION AND INTROSPECTION

1. Observe a schoolroom of children at work with the aim of discovering any that show defects of vision or hearing. What are the symptoms? What is the effect of inability to hear or see well upon interest and attention?

2. Talk with your teacher about testing the eyes and ears of the children of some school. The simpler tests for vision and hearing are easily applied, and the expense for material almost nothing. What tests should be used? Does your school have the test card for vision?

3. Use a rotator or color tops for mixing discs of white and black to produce different shades of gray. Fix in mind the gray made of half white and half black; three-fourths white and one-fourth black; one-fourth-white and three-fourths black.

4. In the same way mix the two complementaries yellow and blue to produce a gray; mix red and green in the same way. Try various combinations of the four fundamental colors, and discover how different colors are produced. Seek for these same colors in nature--sky, leaves, flowers, etc.

5. Take a large wire nail and push it through a cork so that it can be handled without touching the metal with the fingers. Now cool it in ice or very cold water, then dry it and move the point slowly across the back of the hand. Do you feel occasional thrills of cold as the point pa.s.ses over a bulb of Krause? Heat the nail with a match flame or over a lamp, and perform the same experiment. Do you feel the thrills of heat from the corpuscles of Ruffini?

6. Try stopping the nostrils with cotton and having someone give you sc.r.a.ped apple, potato, onion, etc., and see whether, by taste alone, you can distinguish the difference. Why cannot sulphur be tasted?

CHAPTER VII

PERCEPTION

No young child at first sees objects as we see them, or hears sounds as we hear them. This power, the power of perception, is a gradual development. It grows day by day out of the learner's experience in his world of sights and sounds, and whatever other fields his senses respond to.