A. Passive Illusions (a) as determined by the Organism.

In dealing with the illusions which are related to certain peculiarities in the nervous organism and the laws of sensibility, I shall commence with those which are connected with certain limits of sensibility.

Limits of Sensibility.

To begin with, it is known that the sensation does not always answer to the external stimulus in its degree or intensity. Thus, a certain amount of stimulation is necessary before any sensation arises. And this will, of course, be greater when there is little or no attention directed to the impression, that is to say, no co-operating central reaction. Thus it happens that slight stimuli go overlooked, and here illusion may have its starting-point. The most familiar example of such slight errors is that of movement. When we are looking at objects, our ocular muscles are apt to execute very slight movements which escape our notice. Hence we tend, under certain circumstances, to carry over the retinal result of the movement, that is to say, the impression produced by a shifting of the parts of the retinal image to new nervous elements, to the object itself, and so to transform a "subjective" into an "objective" movement. In a very interesting work on apparent or illusory movements, Professor Hoppe has fully investigated the facts of such slight movements, and endeavoured to specify their causes.[17]

Again, even when the stimulus is sufficient to produce a conscious impression, the degree of the feeling may not represent the degree of the stimulus. To take a very inconspicuous case, it is found by Fechner that a given increase of force in the stimulus produces a less amount of difference in the resulting sensations when the original stimulus is a powerful one than when it is a feeble one. It follows from this, that differences in the degree of our sensations do not exactly correspond to objective differences. For example, we tend to magnify the differences of light among objects, all of which are feebly illuminated, that is to say, to see them much more removed from one another in point of brightness than when they are more strongly illuminated. Helmholtz relates that, owing to this tendency, he has occasionally caught himself, on a dark night, entertaining the illusion that the comparatively bright objects visible in twilight were self-luminous.[18]

Again, there are limits to the conscious separation of sensations which are received together, and this fact gives rise to illusion. In general, the number of distinguishable sensations answers to the number of external causes; but this is not always the case, and here we naturally fall into the error of mistaking the number of the stimuli. Reference has already been made to this fact in connection with the question whether consciousness can be mistaken as to the character of a present feeling.

The case of confusing two impressions when the sensory fibres involved are very near one another, has already been alluded to. Both in touch and in sight we always take two or more points for one when they are only separated by an interval that falls below the limits of local discrimination. It seems to follow from this that our perception of the world as a continuum, made up of points perfectly continuous one with another may, for what we know, be illusory. Supposing the universe to consist of atoms separated by very fine intervals, then it is demonstrable that it would appear to our sensibility as a continuum, just as it does now.[19]

Two or more simultaneous sensations are indistinguishable from one another, not only when they have nearly the same local origin, but under other circumstances. The blending of partial sensations of tone in a klang-sensation, and the coalescence in certain cases of the impressions received by way of the two retinas, are examples of this. It is not quite certain what determines this fusion of two simultaneous feelings. It may be said generally that it is favoured by similarity between the sensations;[20] by a comparative feebleness of one of the feelings; by the fact of habitual concomitance, the two sensations occurring rarely, if ever, in isolation; and by the presence of a mental disposition to view them as answering to one external object. These considerations help us to explain the coalescence of the retinal impressions and its limits, the fusion of partial tones, and so on.[21]

It is plain that this fusion of sensations, whatever its exact conditions may be, gives rise to error or wrong interpretation of the sense-impression. Thus, to take the points of two legs of a pair of compasses for one point is clearly an illusion of perception. Here is another and less familiar example. Very cold and smooth surfaces, as those of metal, often appear to be wet. I never feel sure, after wiping the blades of my skates, that they are perfectly dry, since they always seem more or less damp to my hand. What is the reason of this? Helmholtz explains the phenomenon by saying that the feeling we call by the name of wetness is a compound sensation consisting of one of temperature and one of touch proper. These sensations occurring together so frequently, blend into one, and so we infer, according to the general instinctive tendency already noticed, that there is one specific quality answering to the feeling. And since the feeling is nearly always produced by surfaces moistened by cold liquid, we refer it to this circumstance, and speak of it as a feeling of wetness. Hence, when the particular conjunction of sensations arises apart from this external circumstance, we erroneously infer its presence.[22]

The most interesting case of illusion connected with the fusion of simultaneous sensations, is that of single vision, or the deeply organized habit of combining the sensations of what are called the corresponding points of the two retinas. This coalescence of two sensations is so far erroneous since it makes us overlook the existence of two distinct external agencies acting on different parts of the sensitive surface of the body. And this is the more striking in the case of looking at solid objects, since here it is demonstrable that the forces acting on the two retinas are not perfectly similar. Nevertheless, such a coalescence plainly answers to the fact that these external agencies usually arise in one and the same object, and this unity of the object is, of course, the all-important thing to be sure of.

This habit may, however, beget palpable illusion in another way. In certain exceptional cases the coalescence does not take place, as when I look at a distant object and hold a pencil just before my eyes.[23] And in this case the organized tendency to take one visual impression for one object asserts its force, and I tend to fall into the illusion of seeing two separate pencils. If I do not wholly lapse into the error, it is because my experience has made me vaguely aware that double images under these circumstances answer to one object, and that if there were really two pencils present I should have four visual impressions.

Once more, it is a law of sensory stimulation that an impression persists for an appreciable time after the cessation of the action of the stimulus. This "after sensation" will clearly lead to illusion, in so far as we tend to think of the stimulus as still at work. It forms, indeed, as will be seen by-and-by, the simplest and lowest stage of hallucination. Sometimes this becomes the first stage of a palpable error. After listening to a child crying for some time the ear easily deceives itself into supposing that the noise is continued when it has actually ceased. Again, after taking a bandage from a finger, the tingling and other sensations due to the pressure sometimes persist for a good time, in which case they easily give rise to an illusion that the finger is still bound.

It follows from this fact of the reverberation of the nervous structures after the removal of a stimulus, that whenever two discontinuous stimulations follow one another rapidly enough, they will appear continuous. This fact is a fruitful source of optical illusion. The appearance of a blending of the stripes of colours on a rotating disc or top, of the formation of a ring of light by swinging round a piece of burning wood, and the illusion of the toy known as the thaumatrope, or wheel of life, all depend on this persistence of retinal impression. Many of the startling effects of sleight of hand are undoubtedly due in part to this principle. If two successive actions or sets of circumstances to which the attention of the spectator is specially directed follow one another by a very narrow interval of time, they easily appear continuous, so that there seems absolutely no time for the introduction of an intermediate step.[24]

There is another limit to sensibility which is in a manner the opposite to the one just named. It is a law of nervous stimulation that a continued activity of any structure results in less and less psychic result, and that when a stimulus is always at work it ceases in time to have any appreciable effect. The common illustration of this law is drawn from the region of sound. A constant noise, as of a mill, ceases to produce any conscious sensation. This fact, it is plain, may easily become the commencement of an illusion. Not only may we mistake a measure of noise for perfect silence,[25] we may misconceive the real nature of external circumstances by overlooking some continuous impression.

Curious illustrations of this effect are found in optical illusions, namely, the errors we make respecting the movement of stationary objects after continued movement of the eyes. When, for example, in a railway carriage we have for some time been following the (apparent) movement of objects, as trees, etc., and turn our eyes to an apparently stationary object, as the carpet of the compartment, this seems to move in the contrary direction to that of the trees. Helmholtz's explanation of this illusion is that when we suppose that we are fixing our eye on the carpet we are really continuing to move it over the surface by reason of the organic tendency, already spoken of, to go on doing anything that has been done. But since we are unaware of this prolonged series of ocular movements, the muscular feelings having become faint, we take the impression produced by the sliding of the picture over the retina to be the result of a movement of the object.[26]

Another limit to our sensibility, which needs to be just touched on here, is known by the name of the specific energy of the nerves. One and the same nerve-fibre always reacts in a precisely similar way, whatever the nature of the stimulus. Thus, when the optic nerve is stimulated in any manner, whether by light, mechanical pressure, or an electric current, the same effect, a sensation of light, follows.[27] In a usual way, a given class of nerve-fibre is only stimulated by one kind of stimulus. Thus, the retina, in ordinary circumstances, is stimulated by light. Owing to this fact, there has arisen a deeply organized habit of translating the impression in one particular way. Thus, I instinctively regard a sensation received by means of the optic nerve as one caused by light.

Accordingly, whenever circumstances arise in which a like sensation is produced by another kind of stimulus, we fall into illusion. The phosphenes, or circles of light which are seen when the hinder part of the eyeball is pressed, may be said to be illusory in so far as we speak of them as perceptions of light, thus referring them to the external physical agency which usually causes them. The same remark applies to those "subjective sensations," as they are called, which are known to have as their physical cause subjective stimuli, consisting, in the case of sight, in varying conditions of the peripheral organ, as increased blood-pressure. Strictly speaking, such simple feelings as these appear to be, involve an ingredient of false perception: in saying that we perceive light at all, we go beyond the pure sensation, interpreting this wrongly.

Very closely connected with this limitation of our sensibility is another which refers to the consciousness of the local seat, or origin of the impression. This has so far its basis in the sensation itself as it is well known that (within the limits of local discrimination, referred to above) sensations have a particular "local" colour, which varies in the case of each of the nervous fibres by the stimulation of which they arise.[28] But though this much is known through a difference in the sensibility, nothing more is known. Nothing can certainly be ascertained by a mere inspection of the sensation as to the distance the nervous process has travelled, whether from the peripheral termination of the fibre or from some intermediate point.

In a general way, we refer our sensations to the peripheral endings of the nerves concerned, according to what physiologists have called "the law of eccentricity." Thus I am said to feel the pain caused by a bruise in the foot in the member itself. This applies also to some of the sensations of the special senses. Thus, impressions of taste are clearly localized in the corresponding peripheral terminations.

With respect to the sense of smell, and still more to those of hearing and sight, where the impression is usually caused by an object at a distance from the peripheral organ, our attention to this external cause leads us to overlook in part the "bodily seat" of the sensation. Yet even here we are dimly aware that the sensation is received by way of a particular part of the sensitive surface, that is to say, by a particular sense-organ. Thus, though referring an odour to a distant flower, we perceive that the sensation of odour has its bodily origin in the nose. And even in the case of hearing and sight, we vaguely refer the impressions, as such, to the appropriate sense-organ. There is, indeed, in these cases a double local reference, a faint one to the peripheral organ which is acted on, and a more distinct one to the object or the force in the environment which acts on this.

Now, it may be said that the act of localization is in itself distinctly illusory, since it is known that the sensation first arises in connection with the excitation of the sensory centre, and not of the peripheral fibre.[29] Yet it must at least be allowed that this localization of sensation answers to the important fact that, under usual circumstances, the agency producing the sensation is applied at this particular point of the organism, the knowledge of which point is supposed by modern psychologists to have been very slowly learnt by the individual and the race, through countless experiments with the moving organ of touch, assisted by the eye.

Similarly, the reference of the impression, in the case of hearing and sight, to an object in the environment, though, as we have seen, from one point of view illusory, clearly answers to a fact of our habitual experience; for in an immense preponderance of cases at least a visual or auditory impression does arise through the action on the sense-organ of a force (ether or air waves) proceeding from a distant object.

In some circumstances, however, even this element of practical truth disappears, and the localization of the impression, both within and without the organism, becomes altogether illusory. This result is involved in the illusions, already spoken of, which arise from the instinctive tendency to refer sensations to the ordinary kind of stimulus. Thus, when a feeling resulting from a disturbance in the optic nerve is interpreted as one of external light vaguely felt to be acting on the eye, or one resulting from some action set up in the auditory fibre as a sensation of external sound vaguely felt to be entering the ear, we see that the error of localization is a consequence of the other error already characterized.

As I have already observed, an excitation of a nerve at any other point than the peripheral termination, occurs but rarely in normal life. One familiar instance is the stimulation of the nerve running to the hand and fingers, by a sharp blow on the elbow over which it passes. As everybody knows, this gives rise to a sense of pain at the extremities of the nerve. The most common illustration of such errors of localization is found in subjective sensations, such as the impression we sometimes have of something creeping over the skin, of a disagreeable taste in the mouth, of luminous spots floating across the field of vision, and so on. The exact physiological seat of these is often a matter of conjecture only; yet it may safely be said that in many instances the nervous excitation originates at some point considerably short of its peripheral extremity: in which case there occurs the illusion of referring the impressions to the peripheral sense-organ, and to an external force acting on this.

The most striking instances of these errors of localization are found in abnormal circumstances. It is well known that a man who has lost a leg refers all sensations arising from a stimulation of the truncated fibres to his lost foot, and in some cases has even to convince himself of the non-existence of his lost member by sight or touch. Patients often describe these experiences in very odd language. "If," says one of Dr. Weir Mitchell's patients, "I should say I am more sure of the leg which ain't than the one which air, I guess I should be about correct."[30]

There is good reason for supposing that this source of error plays a prominent part in the illusions of the insane. Diseased centres may be accompanied by disordered peripheral structures, and so subjective sensation may frequently be the starting-point of the wildest illusions. Thus, a patient's horror of poison may have its first origin in some subjective gustatory sensation. Similarly, subjective tactual sensations may give rise to gross illusions, as when a patient "feels" his body attacked by foul and destructive creatures.

It may be well to remark that this mistaken interpretation of the seat or origin of subjective sensation is closely related to hallucination. In so far as the error involves the ascription of the sensation to a force external to the sense-organ, this part of the mental process must, when there is no such force present, be viewed as hallucinatory. Thus, the feeling of something creeping over the skin is an hallucination in the sense that it implies the idea of an object external to the skin. Similarly, the projection of an ocular impression due to retinal disturbance into the external field of vision, may rightly be named an hallucination. But the case is not always so clear as this. Thus, for example, when a gustatory sensation is the result of an altered condition of the saliva, it may be said that the error is as much an illusion as an hallucination.[31]

In a wide sense, again, all errors connected with those subjective sensations which arise from a stimulation of the peripheral regions of the nerve may be called illusions rather than hallucinations. Or, if they must be called hallucinations, they may be distinguished as "peripheral" from those "central" hallucinations which arise through an internal automatic excitation of the sensory centre. It is plain from this that the region of subjective sensation is an ambiguous region, where illusion and hallucination mix and become confused. To this point I shall have occasion to return by-and-by.

I have now probably said enough respecting the illusions that arise through the fact of there being fixed limits to our sensibility. The rationale of these illusions is that whenever the limit is reached, we tend to ignore it and to interpret the impression in the customary way.

Variations of Sensibility.

We will now pass to a number of illusions which depend on something variable in the condition of our sensibility, or some more or less exceptional organic circumstance. These variations may be momentary and transient or comparatively permanent. The illusion arises in each case from our ignoring the variation, and treating a given sensation under all circumstances as answering to one objective cause.

First of all, the variation of organic state may affect our mental representation of the strength of the stimulus or external cause. Here the fluctuation may be a temporary or a permanent one. The first case is illustrated in the familiar example of taking a room to be brighter than it is when emerging from a dark one. Another striking example is that of our sense of the temperature of objects, which is known to be strictly relative to a previous sensation, or more correctly to the momentary condition of the organ. Yet, though every intelligent person knows this, the deeply rooted habit of making sensation the measure of objective quality asserts its sway, and frequently leads us into illusion. The well-known experiment of first plunging one hand in cold water, the other in hot, and then dipping them both in tepid, is a startling example of this organized tendency. For here we are strongly disposed to accept the palpable contradiction that the same water is at once warm and cool.

Far more important than these temporary fluctuations of sensibility are the permanent alterations. Excessive fatigue, want of proper nutrition, and certain poisons are well known to be causes of such changes. They appear most commonly under two forms, exalted sensibility, or hyperÆsthesia, and depressed sensibility, or anÆsthesia. In these conditions flagrant errors are made as to the real magnitude of the causes of the sensations. These variations may occur in normal life to some extent. In fairly good health we experience at times strange exaltations of tactual sensibility, so that a very slight stimulus, such as the contact of the bed-clothes, becomes greatly exaggerated.

In diseased states of the nervous system these variations of sensibility become much more striking. The patient who has hyperÆsthesia fears to touch a perfectly smooth surface, or he takes a knock at the door to be a clap of thunder. The hypochondriac may, through an increase of organic sensibility, translate organic sensations as the effect of some living creature gnawing at his vitals. Again, states of anÆsthesia lead to odd illusions among the insane. The common supposition that the body is dead, or made of wood or of glass, is clearly referable in part to lowered sensibility of the organism.[32]

It is worth adding, perhaps, that these variations in sensibility give rise not only to sensory but also to motor illusions. To take a homely instance, the last miles of a long walk seem much longer than the first, not only because the sense of fatigue leading us to dwell on the transition of time tends to magnify the apparent duration, but because the fatigued muscles and connected nerves yield a new set of sensations which constitute an exaggerated standard of measurement. A number of optical illusions illustrate the same thing. Our visual sense of direction is determined in part by the feelings accompanying the action of the ocular muscles, and so is closely connected with the perception of movement, which has already been touched on. If an ocular muscle is partially paralyzed it takes a much greater "effort" to effect a given extent of movement than when the muscle is sound. Hence any movement performed by the eye seems exaggerated. Hence, too, in this condition objects are seen in a wrong direction; for the patient reasons that they are where they would seem to be if he had executed a wider movement than he really has. This may easily be proved by asking him to try to seize the object with, his hand. The effect is exaggerated when complete paralysis sets in, and no actual movement occurs in obedience to the impulse from within.[33]

Variations in the condition of the nerve affect not only the degree, but also the quality of the sensation, and this fact gives rise to a new kind of illusion. The curious phenomena of colour-contrast illustrate momentary alterations of sensibility. When, after looking at a green colour for a time, I turn my eye to a grey surface and see this of the complementary rose-red hue, the effect is supposed to be due to a temporary fatigue of the retina in relation to those ingredients of the total light in the second case which answer to the partial light in the first (the green rays).[34]

These momentary modifications of sensibility are of no practical significance, being almost instantly corrected. Other modifications are more permanent. It was found by Himly that when the retina is overexcitable every stimulus is raised in the spectrum scale of colours. Thus, violet becomes red. An exactly opposite effect is observed when the retina is torpid.[35] Certain poisons are known to affect the quality of the colour-impression. Thus, santonin, when taken in any quantity, makes all colourless objects look yellow. Severe pathological disturbances are known to involve, in addition to hyperÆsthesia and anÆsthesia, what, has been called parÆsthesia, that is to say, that condition in which the quality of sensation is greatly changed. Thus, for example, to one in this state all food appears to have a metallic taste, and so on.

If we now glance back at the various groups of illusions just illustrated, we find that they all have this feature in common: they depend on the general mental law that when we have to do with the unfrequent, the unimportant, and therefore unattended to, and the exceptional, we employ the ordinary, the familiar, and the well-known as our standard. Thus, whether we are dealing with sensations that fall below the ordinary limits of our mental experience, or with those which arise in some exceptional state of the organism, we carry the habits formed in the much wider region of average every-day perception with us. In a word, illusion in these cases always arises through what may, figuratively at least, be described as the application of a rule, valid for the majority of cases, to an exceptional case.

In the varieties of illusion just considered, the circumstance that gives the peculiarity to the case thus wrongly interpreted has been referred to the organism. In the illusions to which we now pass, it will be referred to the environment. At the same time, it is plain that there is no very sharp distinction between the two classes. Thus, the visual illusion produced by pressing the eyeball might be regarded not only as the result of the organic law of the "specific energy" of the nerves, but, with almost equal appropriateness, as the consequence of an exceptional state of things in the environment, namely, the pressure of a body on the retina. As I have already observed, the classification here adopted is to be viewed simply as a rough expedient for securing something like a systematic review of the phenomena.