A. Passive Illusions (b) as determined by the
Environment.

In the following groups of illusion we may look away from nervous processes and organic disturbances, regarding the effect of any external stimulus as characteristic, that is, as clearly marked off from the effects of other stimuli, and as constant for the same stimulus. The source of the illusion will be looked for in something exceptional in the external circumstances, whereby one object or condition of an object imitates the effect of another object or condition, to which, owing to a large preponderance of experience, we at once refer it.

Exceptional Relation of Stimulus to Organ.

A transition from the preceding to the following class of illusions is to be met with in those errors which arise from a very exceptional relation between the stimulus and the organ of sense. Such a state of things is naturally interpreted by help of more common and familiar relations, and so error arises.

For example, we may grossly misinterpret the intensity of a stimulus under certain circumstances. Thus, when a man crunches a biscuit, he has an uncomfortable feeling that the noise as of all the structures of his head being violently smashed is the same to other ears, and he may even act on his illusory perception, by keeping at a respectful distance from all observers. And even though he be a physiologist, and knows that the force of sensation in this case is due to the propagation of vibrations to the auditory centre by other channels than the usual one of the ear, the deeply organized impulse to measure the strength of an external stimulus by the intensity of the sensation asserts its force.

Again, if we turn to the process of perceptional construction properly so called, the reference of the sensation to a material object lying in a certain direction, etc., we find a similar transitional form of illusion. The most interesting case of this in visual perception is that of a disturbance or displacement of the organ by external force. For example, an illusory sense of direction arises by the simple action of closing one eye, say the left, and pressing the other eyeball with one of the fingers a little outwards, that is to the right. The result of this movement is, of course, to transfer the retinal picture to new nervous elements further to the right. And since, in this instance, the displacement is not produced in the ordinary way by the activity of the ocular muscle making itself known by certain feelings of movement, it is disregarded altogether, and the direction of the objects is judged as though the eye were stationary.

A somewhat similar illusion as to direction occurs in auditory perception. The sense of direction by the ear is known to be due in part to the action of the auricle, or projecting part of the ear. This collects the air-waves, and so adds to the intensity of the sounds, especially those coming from in front, and thus assists in the estimation of direction. This being so, if an artificial auricle is placed in front of the ears; if, for example, the two hands are each bent into a sort of auricle, and placed in front of the ears, the back of the hand being in front, the sense of direction (as well as of distance) is confused. Thus, sounds really travelling from a point in front of the head will appear to come from behind it.

Again, the perception of the unity of an object is liable to be falsified by the introduction of exceptional circumstances into the sense-organ. This is illustrated in the well-known experiment of crossing two fingers, say the third and fourth, and placing a marble or other small round object between them. Under ordinary circumstances, the two lateral surfaces (that is, the outer surfaces of the two fingers) now pressed by the marble, can only be acted on simultaneously by two objects having convex surfaces. Consequently, we cannot help feeling the presence of two objects in this exceptional instance. The illusion is analogous to that of the stereoscope, to be spoken of presently.

Exceptional External Arrangements.

Passing now to those cases where the exceptional circumstance is altogether exterior to the organ, we find a familiar example in the illusions connected with the action of well-known physical forces, as the refraction of light, and the reflection of light and sound. A stick half-immersed in water always looks broken, however well we may know that the appearance is due to the bending of the rays of light. Similarly, an echo always sounds as though it came from some object in the direction in which the air-waves finally travel to the ear, though we are perfectly sure that these undulations have taken a circuitous course. It is hardly necessary to remind the reader that the deeply organized tendency to mistake the direction of the visible or audible object in these cases has from remote ages been made use of as a means of popular delusion. Thus, we are told by Sir D. Brewster, in his entertaining Letters on Natural Magic (letter iv.), that the concave mirror was probably used as the instrument for bringing the gods before the people. The throwing of the images formed by such mirrors upon smoke or against fire, so as to make them more distinct, seems to have been a favourite device in the ancient art of necromancy.

Closely connected with these illusions of direction with respect to resting objects, are those into which we are apt to fall respecting the movements of objects. What looks like the movement of something across the field of vision is made known to us either by the feeling of the ocular muscles, if the eye follows the object, or through the sequence of locally distinct retinal impressions, if the eye is stationary. Now, either of these effects may result, not only from the actual movement of the object in a particular direction, but from our own movement in an opposite direction; or, again, from our both moving in the first direction, the object more rapidly than ourselves; or, finally, from our both moving in an opposite direction to this, ourselves more rapidly than the object. There is thus always a variety of conceivable explanations, and the action of past experience and association shows itself very plainly in the determination of the direction of interpretation. Thus, it is our instinctive tendency to take apparent movement for real movement, except when the fact of our own movement is clearly present to consciousness, as when we are walking, or when we are sitting behind a horse whose movement we see. And so when the sense of our own movement becomes indistinct, as in a railway carriage, we naturally drift into the illusion that objects, such as trees, telegraph posts, and so on, are moving, when they are perfectly still. Under the same circumstances, we are apt to suppose that a train which is just shooting ahead of us is moving slowly.

Similar uncertainties arise with respect to the relative movement of two objects, the eye being supposed to be fixed in space. When two objects seem to pass one another, it may be that they are both moving in contrary directions, or that one only is moving, or finally, that both are moving in the same direction, the one faster than the other. Experience and habit here again suggest the interpretation which is most easy, and not unfrequently produce illusion. Thus, when we watch clouds scudding over the face of the moon, the latter seems moving rather than the former, and the illusion only disappears when we fix the eye on the moon and recognize that it is really stationary. The probable reason of this is, as Wundt suggests, that experience has made it far easier for us to think of small objects like the moon moving rapidly, than of large masses like the clouds.[36]

The perception of distance, still more than that of direction, is liable to be illusory. Indeed, the visual recognition of distance, together with that of solidity, has been the great region for the study of "the deceptions of the senses." Without treating the subject fully here, I shall try to describe briefly the nature and source of these illusions.[37]

Confining ourselves first of all to near objects, we know that the smaller differences of distance in these cases are, if the eyes are at rest, perceived by means of the dissimilar pictures projected on the two retinas; or if they move, by this means, together with the muscular feelings that accompany different degrees of convergence of the two eyes. This was demonstrated by the famous experiments of Wheatstone. Thus, by means of the now familiar stereoscope, he was able to produce a perfect illusion of relief. The stereoscope may be said to introduce an exceptional state of things into the spectator's environment. It imitates, by means of two flat drawings, the dissimilar retinal pictures projected by a single solid receding object, and the lenses through which the eyes look are so constructed as to compel them to converge as though looking on a single object. And so powerful is the tendency to interpret this impression as one of solidity, that even though we are aware of the presence of the stereoscopic apparatus, we cannot help seeing the two drawings as a single solid object.

In the case of more remote objects, there is no dissimilarity of the retinal pictures or feelings of convergence to assist the eye in determining distance. Here its judgment, which now becomes more of a process of conscious inference, is determined by a number of circumstances which, through experience and association, have become the signs of differences of depth in space. Among these are the degree of indistinctness of the impression, the apparent or retinal magnitude (if the object is a familiar one), the relations of linear perspective, as the interruption of the outline of far objects by that of near objects, and so on. In a process so complicated there is clearly ample room for error, and wrong estimates of distance whenever unusual circumstances are present are familiar to all. Thus, the inexperienced English tourist, when in the clear atmosphere of Switzerland, where the impressions from distant objects are more distinct than at home, naturally falls into the illusion that the mountains are much nearer than they are, and so fails to realize their true altitude.

Illusions of Art.

The imitation of solidity and depth by art is a curious and interesting illustration of the mode of production of illusion. Here we are not, of course, concerned with the question how far illusion is desirable in art, but only with its capabilities of illusory presentment; which capabilities, it may be added, have been fully illustrated in the history of art. The full treatment of this subject would form a chapter in itself; here I can only touch on its main features.

Pictorial art working on a flat surface cannot, it is plain, imitate the stereoscope, and produce a perfect sense of solidity. Yet it manages to produce a pretty strong illusion. It illustrates in a striking manner the ease with which the eye conceives relations of depth or relief and solidity. If, for example, on a carpet, wall-paper, or dress, bright lines are laid on a dark colour as ground, we easily imagine that they are advancing. The reason of this seems to be that in our daily experience advancing surfaces catch and reflect the light, whereas retiring surfaces are in shadow.[38]

The same principle is illustrated in one of the means used by the artist to produce a strong sense of relief, namely, the cast shadow. A circle drawn with chalk with a powerful cast shadow on one side will, without any shading or modelling of the form, appear to stand out from the paper, thus:

The reason is that the presence of such a shadow so forcibly suggests to the mind that the object is a prominent one intervening between the light and the shaded surface.[39]

Even without differences of light and shade, by a mere arrangement of lines, we may produce a powerful sense of relief or solidity. A striking example of this is the way in which two intersecting lines sometimes appear to recede from the eye, as the lines a a', b b', in the next drawing, which seem to belong to a regular pattern on the ground, at which the eye is looking from above and obliquely.

Again, the correct delineation of the projection of a regular geometrical figure, as a cube, suffices to give the eye a sense of relief. This effect is found to be the more striking in proportion to the familiarity of the form. The following drawing of a long box-shaped solid at once seems to stand out to the eye.

This habitual interpretation of the flat in art as answering to objects in relief, or having depth, can only be understood when it is remembered that our daily experience gives us myriads of instances in which the effect of such flat representations answers to solid receding forms. That is to say, in the case of all distant objects, in the perception of which the dissimilarity of the retinal pictures and the feeling of convergence take no part, we have to interpret solidity, and relations of nearer and further, by such signs as linear perspective and cast shadow. On the other hand, it is only in the artificial life of indoors, on our picture-covered walls, that we experience such effects without discovering corresponding realities. Hence a deeply organized habit of taking these impressions as answering to the solid and not to the flat. If our experience had been quite different; if, for example, we had been brought up in an empty room, amid painted walls, and had been excluded from the sight of the world of receding objects outside, we might easily have formed an exactly opposite habit of taking the actual mountains, trees, etc., of the distant scene to be pictures laid on a flat surface.

It follows from this that, with respect to the distant parts of a scene, pictorial art possesses the means of perfect imitation; and here we see that a complete illusory effect is obtainable. I need but to refer to the well-known devices of linear and aerial perspective, by which this result is secured.[40] The value of these means of producing illusion at the command of the painter, may be illustrated by the following fact, which I borrow from Helmholtz. If you place two pieces of cardboard which correspond to portions of one form at the sides and in front of a third piece, in the way represented above, so as just to allow the eye to follow the contour of this last, and then look at this arrangement from a point at some little distance with one eye, you easily suppose that it stands in front of the side pieces. The explanation of the illusion is that this particular arrangement powerfully suggests that the outline of the whole figure, of which the two side pieces are parts, is broken by an intervening object. Owing to the force of these and other suggestions, it is easy for the spectator, when attending to the background of a landscape painting, to give himself up for a moment to the pleasant delusion that he is looking at an actual receding scene.

In connection with pictorial delusion, I may refer to the well-known fact, that the eye in a portrait seems to follow the spectator, or that a gun, with its muzzle pointing straight outwards, appears to turn as the spectator moves.[41] These tricks of art have puzzled many people, yet their effect is easily understood, and has been very clearly explained by Sir D. Brewster, in the work already referred to (letter v.). They depend on the fact that a painting, being a flat projection only and not a solid, continues to present the front view of an object which it represents wherever the spectator happens to stand. Were the eye in the portrait a real eye, a side movement of the spectator would, it is evident, cause him to see less of the pupil and more of the side of the eyeball, and he would only continue to see the full pupil when the eye followed him. We regard the eye in the picture as a real eye having relief, and judge accordingly.

We may fall into similar illusions respecting distance in auditory perception. A change of wind, an unusual stillness in the air, is quite sufficient to produce the sense that sounding objects are nearer than they actually are. The art of the ventriloquist manifestly aims at producing this kind of illusion. By imitating the dull effect of a distant voice, he is able to excite in the minds of his audience a powerful conviction that the sounds proceed from a distant point. There is little doubt that ventriloquism has played a conspicuous part in the arts of divination and magic.

Misconception of Local Arrangement.

Let us now pass to a class of illusions closely related to those having to do with distance, but involving some special kind of circumstance which powerfully suggests a particular arrangement in space. One of the most striking examples of these is the erroneous localization of a quality in space, that is to say, the reference of it to an object nearer, or further off than the right one. Thus, when we look through a piece of yellow glass at a dull, wintry landscape, we are disposed to imagine that we are looking at a sunny scene of preternatural warmth. A moment's reflection would tell us that the yellow tint, with which the objects appear to be suffused, comes from the presence of the glass; yet, in spite of this, the illusion persists with a curious force. The explanation is, of course, that the circumstances are exceptional, that in a vast majority of cases the impression of colour belongs to the object and not to an intervening medium,[42] and that consequently we tend to ignore the glass, and to refer the colour to the objects themselves.

When, however, the fact of the existence of a coloured medium is distinctly present to the mind, we easily learn to allow for this, and to recognize one coloured surface correctly through a recognized medium. Thus, we appear to ourselves to see the reflected images of the wall, etc., of a room, in a bright mahogany table, not suffused with a reddish yellow tint, as they actually are—and may be seen to be by the simple device of looking at a small bit of the image through a tube, but in their ordinary colour. We may be said to fall into illusion here in so far as we overlook the exact quality of the impression actually made on the eye. This point will be touched on presently. Here I am concerned to show that this habit of allowing for the coloured medium may, in its turn, occasionally lead to plain and palpable illusion.

The most striking example of this error is to be met with among the curious phenomena of colour-contrast already referred to. In many of these cases the appearance of the contrasting colour is, as I have observed, due to a temporary modification of the nervous substance. Yet it is found that this organic factor does not wholly account for the phenomena. For example, Meyer made the following experiment. He covered a piece of green paper by a sheet of thin transparent white paper. The colour of this double surface was, of course, a pale green. He then introduced a scrap of grey paper between the two sheets, and found that, instead of looking whitish as it really was, it looked rose-red. Whatever the colour of the under sheet the grey scrap took the complementary hue. If, however, the piece of grey paper is put outside the thin sheet, it looks grey; and what is most remarkable is that when a second piece is put outside, the scrap inside no longer wears the complementary hue.

There is here evidently something more than a change of organic conditions; there is an action of experience and suggestion. The reason of our seeing the scrap rose-red in one case and neutral grey in another, is that in the first instance we vividly represent to ourselves that we are looking at it through a greenish veil (which is, of course, a part of the illusion); for rose-red seen through a greenish medium would, as a matter of fact, be light grey, as this scrap is. Even if we allow that there always exists after an impression of colour a temporary organic disposition to see the complementary hue, this does not suffice as an explanation of these cases; we have to conclude further that imagination, led by the usual run of our experience, is here a co-operant factor, and helps to determine whether the complementary tint shall be seen or not.

Misinterpretation of Form.

More complex and circumscribed associations take part in those errors which we occasionally commit respecting the particular form of objects. This has already been touched on in dealing with artistic illusion. The disposition of the eye to attribute solidity to a flat drawing is the more powerful in proportion to the familiarity of the form. Thus, an outline drawing of a building is apt to stand out with special force.

Another curious illustration of this is the phenomenon known as the conversion of the concave mould or matrix of a medal into the corresponding convex relief. If, says Helmholtz, the mould of a medal be illuminated by a light falling obliquely so as to produce strong shadows, and if we regard this with one eye, we easily fall into the illusion that it is the original raised design, illuminated from the opposite side. As a matter of fact, the visual impression produced by a concave form with the light falling on one side, very closely resembles that produced by a corresponding convex form with the light falling on the other side. At the same time, it is found that the opposite mode of conversion, that is to say, the transformation of the raised into the depressed form, though occurring occasionally, is much less frequent. Now, it may be asked, why should we tend to transform the concave into the convex, rather than the convex into the concave? The reader may easily anticipate the answer from what has been said about the deeply fixed tendency of the eye to solidify a plane surface. We are rendered much more familiar, both by nature and by art, with raised (cameo) design than with depressed design (intaglio), and we instinctively interpret the less familiar form by the more familiar. This explanation appears to be borne out by the fact emphasized by Schroeder that the illusion is much more powerful if the design is that of some well-known object, as the human head or figure, or an animal form, or leaves.[43]

Another illustration of this kind of illusion recently occurred in my own experience. Nearly opposite to my window came a narrow space between two detached houses. This was, of course, darker than the front of the houses, and the receding parallel lines of the bricks appeared to cross this marrow vertical shaft obliquely. I could never look at this without seeing it as a convex column, round which the parallel lines wound obliquely. Others saw it as I did, though not always with the same overpowering effect. I can only account for this illusion by help of the general tendency of the eye to solidify impressions drawn from the flat, together with the effect of special types of experience, more particularly the perception of cylindrical forms in trees, columns, etc.

It may be added that a somewhat similar illustration of the action of special types of experience on the perception of individual form may be found in the region of hearing. The powerful disposition to take the finely graduated cadences of sound produced by the wind for the utterances of a Iranian voice, is due to the fact that this particular form and arrangement of sound has deeply impressed itself on our minds, in connection with numberless utterances of human feeling.

Illusions of Recognition.

As a last illustration of comparatively passive illusions, I may refer to the errors which we occasionally commit in recognizing objects. As I have already observed, the process of full and clear recognition, specific and individual, involves a classing of a number of distinct aspects of the object, such as colour, form, etc. Accordingly, when in a perfectly calm state of mind we fall into illusion with respect to any object plainly visible, it must be through some accidental resemblance between the object and the other object or class of objects with which we identify it. In the case of individual identification such illusions are, of course, comparatively rare, since here there are involved so many characteristic differences. On the other hand, in the case of specific recognition there is ample room for error, especially in those kinds of more subtle recognition to which I have already referred. To "recognize" a person as a Frenchman or a military man, for example, is often an erroneous process. Logicians have included this kind of error under what they call "fallacies of observation."

Errors of recognition, both specific and individual, are, of course, more easy in the case of distant objects or objects otherwise indistinctly seen. It is noticeable in these cases that, even when perfectly cool and free from emotional excitement, we tend to interpret such indistinct impressions according to certain favourite types of experience, as the human face and figure. Our interpretative imagination easily sees traces of the human form in cloud, rock, or tree-stump.

Again, even when there is no error of recognition, in the sense of confusing one object with other objects, there may be partial illusion. I have remarked that the process of recognizing an object commonly involves an overlooking of points of diversity in the object, or aspect of the object, now present. And sometimes this inattention to what is actually present includes an error as to the actual visual sensation of the moment. Thus, for example, when I look at a sheet of white paper in a feebly lit room, I seem to see its whiteness. If, however, I bring it near the window, and let the sun fall on a part of it, I at once recognize that what I have been seeing is not white, but a decided grey. Similarly, when I look at a brick viaduct a mile or two off, I appear to myself to recognize its redness. In fact, however, the impression of colour which I receive from the object is not that of brick-red at all, but a much less decided tint; which I may easily prove by bending my head downwards and letting the scene image itself on the retina in an unusual way, in which case the recognition of the object as a viaduct being less distinct, I am better able to attend to the exact shade of the colour.

Nowhere is this inattention to the sensation of the moment exhibited in so striking a manner as in pictorial art. A picture of Meissonier may give the eye a representation of a scene in which the objects, as the human figures and horses, have a distinctness that belongs to near objects, but an apparent magnitude that belongs to distant objects. So again, it is found that the degree of luminosity or brightness of a pictorial representation differs in general enormously from that of the actual objects. Thus, according to the calculations of Helmholtz,[44] a picture representing a Bedouin's white raiment in blinding sunshine, will, when seen in a fairly lit gallery, have a degree of luminosity reaching only to about one-thirtieth of that of the actual object. On the other hand, a painting representing marble ruins illuminated by moonlight, will, under the same conditions of illumination, have a luminosity amounting to as much as from ten to twenty thousand times that of the object. Yet the spectator does not notice these stupendous discrepancies. The representation, in spite of its vast difference, at once carries the mind on to the actuality, and the spectator may even appear to himself, in moments of complete absorption, to be looking at the actual scene.

The truly startling part of these illusions is, that the direct result of sensory stimulation appears to be actually displaced by a mental image. Thus, in the case of Meyer's experiment, of looking at the distant viaduct, and of recognizing an artistic representation, imagination seems in a measure to take the place of sensation, or to blind the mind to what is actually before it.

The mystery of the process, however, greatly disappears when it is remembered that what we call a conscious "sensation" is really compounded of a result of sensory stimulation and a result of central reaction, of a purely passive impression and the mental activity involved in attending to this and classing it.[45] This being so, a sensation may be modified by anything exceptional in the mode of central reaction of the moment. Now, in all the cases just considered, we have one common feature, a powerful suggestion of the presence of a particular object or local arrangement. This suggestion, taking the form of a vivid mental image, dominates and overpowers the passive impression. Thus, in Meyer's experiment, the mind is possessed by the supposition that we are looking at the grey spot through a greenish medium. So in the case of the distant viaduct, we are under the mastery of the idea that what we see in the distance is a red brick structure. Once more, in the instance of looking at the picture, the spectator's imagination is enchained by the vivid representation of the object for which the picture stands, as the marble ruins in the moonlight or the Bedouin in the desert.

It may be well to add that this mental uncertainty as to the exact nature of a present impression is necessitated by the very conditions of accurate perception. If, as I have said, all recognition takes place by overlooking points of diversity, the mind must, in course of time, acquire a habit of not attending to the exact quality of sense-impressions in all cases where the interpretation seems plain and obvious. Or, to use Helmholtz's words, our sensations are, in a general way, of interest to us only as signs of things, and if we are sure of the thing, we readily overlook the precise nature of the impression. In short, we get into the way of attending only to what is essential, constant, and characteristic in objects, and disregarding what is variable and accidental.[46] Thus, we attend, in the first place, to the form of objects, the most constant and characteristic element of all, being comparatively inattentive to colour, which varies with distance, atmospheric changes, and mode of illumination. So we attend to the relative magnitude of objects rather than to the absolute, and to the relative intensities of light and shade rather than to the absolute; for in so doing we are noting what is constant for all distances and modes of illumination, and overlooking what is variable. And the success of pictorial art depends on the observance of this law of perception.

These remarks at once point out the limits of these illusions. In normal circumstances, an act of imagination, however vivid, cannot create the semblance of a sensation which is altogether absent; it can only slightly modify the actual impression by interfering with that process of comparison and classification which enters into all definite determination of sensational quality.

Another great fact that has come to light in the investigation of these illusions is that oft-recurring and familiar types of experience leave permanent dispositions in the mind. As I said when describing the process of perception, what has been frequently perceived is perceived more and more readily. It follows from this that the mind will be habitually disposed to form the corresponding mental images, and to interpret impressions by help of these. The range of artistic suggestion depends on this. A clever draughtsman can indicate a face by a few rough touches, and this is due to the fact that the spectator's mind is so familiarized, through recurring experience and special interest, with the object, that it is ready to construct the requisite mental image at the slightest external suggestion. And hence the risk of hasty and illusory interpretation.

These observations naturally conduct us to the consideration of the second great group of sense-illusions, which I have marked off as active illusions, where the action of a pre-existing intellectual disposition becomes much more clearly marked, and assumes the form of a free imaginative transformation of reality.