ILLUSIONS OF PERCEPTION--_continued_.
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:
[Illustration: FIG. 1.]
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.
[Illustration: FIG. 2.]
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.
[Illustration: FIG. 3.]
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.
[Illustration: FIG. 4.]
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.