but of these only one will be specifically mentioned, for it will suffice.
Carr[3] has studied this type of illusion comparatively recently and
apparently quite generally, and his work will be drawn upon for examples
of this type. Apparently they may be divided into four classes: (1) Those
of pure distance; that is, an object may appear to be located at varying
distances from the observer, but no movement is perceived. For example, a
person might be seen first at the true distance; he might be seen next
very close in front of the eyes; then he might suddenly appear to be quite
remote; (2) illusions of pure motion; that is, objects are perceived as
moving in a certain direction without any apparent change in location. In
other words, they appear to move, but they do not appear to traverse
space; (3) illusions of movement which include a change in location. This
appears to be the most common illusion of depth; (4) those including a
combination of the first and third classes. For example, the object might
first appear to move away from its true location and is perceived at some
remote place. Shortly it may appear in its true original position, but
this change in location does not involve any sense of motion.
These peculiar illusions of depth are not as generally experienced as
those described in preceding chapters. A geometrical illusion, especially
if it is pronounced, is likely to be perceived quite universally, but
these illusions of depth are either more difficult to notice or more
dependent upon psychological peculiarities far from universal among
people. It is interesting to note the percentages computed from Carr's
statistics obtained upon interrogating 350 students. Of these, 17 per cent
had experienced depth-illusions and between one and two per cent had
voluntary control of the phenomenon. Of the 48 who had experienced
illusions of this type and were able to submit detailed descriptions, 25
per cent belonged to class (1) of those described in the preceding
paragraph; 4 per cent to class (2); 52 per cent to class (3); and 17 per
cent to class (4).
Usually the illusion involves all objects in the visual field but with
some subjects the field is contracted or the objects in the periphery of
the field are unaffected. For most persons these illusions involve normal
perceptual objects, although it appears that they are phases of
hallucinatory origin.
Inasmuch as these illusions cannot be illustrated diagrammatically we can
do no better than to condense some of the descriptions obtained and
reported by Carr.[3]
A case in which the peripheral objects remain visible and stationary at
their true positions while the central portion of the field participates
in the illusion is as follows:
The observer on a clear day was gazing down a street which ended a
block away, a row of houses forming the background at the end of the
street. The observer was talking to and looking directly at a
companion only a short distance away. Soon this person (apparently)
began to move down the street, until she reached the background of
houses at the end, and then slowly came back to her original position.
The movement in both directions was distinctly perceived. During the
illusory movement there was no vagueness of outline or contour, no
blurring or confusion of features; the person observed, seemed
distinct and substantial in character during the illusion. The
perceived object moved in relation to surrounding objects; there was
no movement of the visual field as a whole. The person decreased in
size during the backward movement and increased in size during the
forward return movement.
With many persons who experience illusions of depth, the objects appear to
move to, or appear at, some definite position and remain there until the
illusion is voluntarily overcome, or until it disappears without voluntary
action. A condensation of a typical description of this general type
presented by Carr is as follows:
All visual objects suddenly recede to the apparent distance of the
horizon and remain in that position several minutes, returning at the
end of this period to their original positions. This return movement
is very slow at the beginning, but the latter phase of the movement is
quite rapid. If the subject closes her eyes while the objects appear
at their distant position she cannot even _imagine_ those objects
located anywhere except at their apparent distant position.
In all cases (encountered by Carr) the motion in both directions is an
actual experience reality and the subject was helpless as to
initiating, stopping, or modifying the course of the illusion in any
way. Objects and even visual images (which are subject to the same
illusions) decrease in size in proportion to the amount of backward
movement and grow larger again on their return movement. The objects
are always clearly defined as if in good focus. In this particular
case the illusion occurred about twice a year, under a variety of
conditions of illumination, at various times of the day, but
apparently under conditions of a rather pronounced fatigue.
In regard to the variation in the size of objects, many who have
experienced these illusions of depth testify that the size seems to change
in proportion to the apparent distance, according to the law of
perspective. Some persons appear in doubt as to this change and a few have
experienced the peculiar anomaly of decreasing size as the objects
apparently approached.
Many persons who have experienced these peculiar illusions report no
change in the distinctness of objects; almost as many are uncertain
regarding this point; and as many report a change in distinctness.
Apparently there are phases of hallucinatory origin so that there is a
wide variety of experiences among those subject to this type of illusion.
According to Carr's investigation internal conditions alone are
responsible for the illusion with more persons than those due to external
conditions alone. With some persons a combination of internal and external
conditions seem to be a necessity. Fixation of vision appears to be an
essential objective condition for many observers. That is, the illusion
appeared while fixating a speaker or singer in a church or a theater. With
others the illusion occurs while reading. Some reported that fixation upon
checkered or other regularly patterned objects was an essential condition.
Among the subjective conditions reported as essential are steady fixation,
concentration of attention, complete mental absorption, dreamy mental
abstraction, and fatigue.
Ocular defects do not appear to be essential, for the illusions have been
experienced by many whose eyes were known to be free from any
abnormalities.
Period of life does not appear to have any primary influence, for those
who are subject to these peculiar illusions often have experienced them
throughout many years. In some cases it is evident that the illusions
occur during a constrained eye position, while lying down, immediately
upon arising from bed in the morning, and upon opening the eyes after
having had them closed for some time. However, the necessity for these
conditions are exceptional.
The control of these illusions of depth, that is, the ability to create or
to destroy them, appears to be totally lacking for most of those who have
experienced them. Some can influence them, a few can destroy them, a few
can indirectly initiate them, but those who can both create and destroy
them appear to be rare.
It may seem to the reader that the latter part of this chapter departs
from the main trend of this book, for most of these illusions of depth are
to a degree of hallucinatory origin. Furthermore it has been the intention
to discuss only those types of illusions which are experienced quite
uniformly and universally. The digression of this chapter is excused on
the basis of affording a glimpse along the borderland of those groups of
illusions which are nearly universally experienced. Many other phases of
depth illusions have been recorded in scientific literature. The excellent
records presented by Carr could be drawn upon for further glimpses, but it
appears that no more space should be given to this exceptional type. The
reader should be sufficiently forewarned of this type and should be able
to take it into account if peculiarities in other types appear to be
explainable in this manner. However, in closing it is well to emphasize
the fact that the hallucinatory aspect of depth illusions is practically
absent in types of illusions to which attention is confined in other
chapters.
VIII
IRRADIATION AND BRIGHTNESS-CONTRAST
Many interesting and striking illusions owe their existence to contrasts
in brightness. The visual phenomenon of irradiation does not strictly
belong to this group, but it is so closely related to it and so dependent
upon brightness-contrast that it is included. A dark line or spot will
appear darker in general as the brightness of its environment is
increased; or conversely, a white spot surrounded by a dark environment
will appear brighter as the latter is darkened. In other words, black and
white, when juxtaposed, mutually reinforce each other. Black print on a
white page appears much darker than it really is. This may be proved by
punching a hole in a black velvet cloth and laying this hole over a
"black" portion of a large letter. The ink which appeared so black in the
print, when the latter was surrounded by the white paper, now appears only
a dark gray. Incidentally a hole in a box lined with black velvet is much
darker than a piece of the black velvet surrounding the hole.
The effects of brightness-contrast are particularly striking when
demonstrated by means of lighting, a simple apparatus being illustrated
diagrammatically in Fig. 62. For example, if a hole _H_ is cut in an
opaque white blotting paper and a large piece of the white blotting paper
is placed at _C_, the eye when placed before the opening at the right will
see the opening at _H_ filled with the background _C_. The hole _H_ may be
cut in thin metal, painted a dull white, and may be of the shape of a
star. This shape provides an intimacy between the hole and its environment
which tends to augment the effects of contrasts. _R_ and _F_ are
respectively the rear and front lamps. That is, the lamps _R_ illuminate
_C_, which "fills" the hole and apparently is the hole; and the lamps _F_
illuminate the diffusing white environment _E_. The two sets of lamps may
be controlled by separate rheostats, but if the latter are unavailable the
lamps (several in each set) may be arranged so that by turning each one
off or on, a range of contrasts in brightness between _E_ and _H_ (in
reality _C_) may be obtained. (By using colored lamps and colored papers
as discussed in Chapter IX the marvelous effects of color-contrast may be
superposed upon those of brightness-contrast.)
[Illustration: Fig. 62.--Simple apparatus for demonstrating the remarkable
effects of contrasts in brightness and color.]
If, for example, _C_ is very feebly illuminated and _E_ is very bright,
_C_ will be pronounced black; but when the lamps _F_ are extinguished and
no light is permitted to reach _E_, the contrast is reversed, and _C_ may
actually appear "white." Of course, it is obvious that white and black are
relative terms as encountered in such a case. In fact in
brightness-contrasts relative and not absolute values of brightness are
usually the more important. In order to minimize the stray light which
emerges from _H_, it is well to paint the inside of both compartments
black with the exception of sufficiently large areas of _C_ and _E_. The
use of black velvet instead of black paint is sometimes advisable. It is
also well to screen the lamps as suggested in the diagram. This simple
apparatus will demonstrate some very striking effects of contrasts in
brightness and will serve, also, to demonstrate even more interesting
effects of contrasts in color.
Two opposite contrasts obtainable by means of a simple apparatus
illustrated in Fig. 62 may be shown simultaneously by means of white,
black, and gray papers arranged as in Fig. 63. In this figure the gray is
represented by the partially black _V_s, each of which contains equal
amounts of black and of white. When held at some distance this serves as a
gray and the same effect is apparent as is described for the case of
actually gray _V_s. An excellent demonstration may be made by the reader
by using two _V_s, cut from the same sheet of gray paper, and pasted
respectively upon white and black backgrounds, as in Fig. 63. It will be
apparent that the one amid the black environment appears much brighter
than the one (same gray) amid the white environment. This can be
demonstrated easily to an audience by means of a figure two feet long. It
is interesting to carry the experiment further and place a _V_ of much
darker gray on the black background than the _V_ on the white background.
The persistency of the illusion is found to be remarkable, for it will
exist even when the one _V_ is actually a much darker gray than the other.
To become convinced that the two grays are of the same brightness in Fig.
63, it is only necessary to punch two holes in a white or gray card at
such a distance apart that they will lie respectively over portions of the
two _V_s when the card is laid upon Fig. 63. The grays in the holes should
now appear alike because their environments are similar.
[Illustration: Fig. 63.--Illustrating brightness-contrast.]
The importance of contrasts in brightness and in color cannot be
overemphasized, and it appears certain that no one can fully realize
their effectiveness without witnessing it in a manner similar to that
suggested in Fig. 62.
[Illustration: Fig. 64.--An effect of brightness-contrast. Note the
darkening of the intersections of the white strips.]
Many illusions of brightness-contrast are visible on every hand. For
example, the point at which the mullions of a window cross will be seen to
appear brighter than the remaining portions of them when viewed against a
bright sky. Conversely, in Fig. 64, dark spots appear where the white bars
cross. This is purely an illusion and the same type may be witnessed by
the observant many times a day. In Fig. 64 it is of interest to note that
the illusion is weak for the crossing upon which the point of sight rests,
but by averted vision the illusion is prominent for the other crossings.
This is one of the effects which depends upon the location in the visual
field.
No brightness-contrasts are seen correctly and often the illusions are
very striking. If a series of gray papers is arranged from black to white,
with the successive pieces overlapped or otherwise juxtaposed, a series of
steps of uniform brightness is not seen. An instrument would determine the
brightness of each as uniform, but to the eye the series would appear
somewhat "fluted." That is, where a light gray joined a darker gray the
edge of the former would appear lighter than its actual brightness, and
the edge of the darker gray would appear darker than it should. This may
also be demonstrated by laying a dozen pieces of white tissue paper in a
pile in such a manner that a series of 1, 2, 3, 4, etc., thickness would
be produced. On viewing this by transmitted light a series of grays is
seen, and the effect of contrast is quite apparent. Such a pattern can be
made photographically by rotating before a photographic plate a disk with
openings arranged properly in steps.
Many demonstrations of the chief illusion of brightness-contrast are
visible at night under glaring lighting conditions. It is difficult or
impossible to see objects beyond automobile headlights, and adjacent to
them, in the visual field. Objects similarly located in respect to any
surface sufficiently bright are more or less obscured. Characters written
upon a blackboard, placed between two windows, may be invisible if the
surfaces seen through the window are quite bright, unless a sufficient
quantity of light reaches the blackboard from other sources.
Stage-settings have been changed in perfect obscurity before an audience
by turning on a row of bright lights at the edge of the stage-opening. The
term "blinding light" owes its origin to this effect of
brightness-contrast.
The line of juncture between a bright and a dark surface may not be seen
as a sharp line, but as a narrow band of gray. When this is true it is
possible that an undue amount of area is credited to the white. In
preceding paragraphs we have seen the peculiar effect at the border-lines
of a series of grays. This may have something to do with the estimate;
however, irradiation may be due to excitation of retinal rods and cones
adjacent to, but not actually within the bright image.
A remarkable effect which may be partially attributable to irradiation can
be produced by crossing a grating of parallel black lines with an oblique
black line. At the actual crossings the black appears to run up the narrow
angle somewhat like ink would under the influence of surface tension. This
is particularly striking when two gratings or even two ordinary
fly-screens are superposed. The effect is visible when passing two
picket-fences, one beyond the other. If a dark object is held so that a
straight edge appears to cross a candle-flame or other light-source, at
this portion the straight edge will appear to have a notch in it.
Irradiation in general has been defined as the lateral diffusion of
nervous stimuli beyond the actual stimulus. It is not confined to the
visual sense but irradiation for this sense is a term applied to the
apparent enlargement of bright surfaces at the expense of adjacent darker
surfaces. The crescent of the new moon appears larger in radius than the
faint outline of the darker portion which is feebly illuminated chiefly by
light reflected from the earth's surface. A filament of a lamp appears to
grow in size as the current through it is slowly increased from a zero
value; that is, as it increases in brightness. In Fig. 65 the small inner
squares are of the same size but the white square appears larger than the
black one. It seems that this apparent increase is made at the expense of
the adjacent dark area. This phenomenon or illusion is strongest when the
brightness is most intense, and is said to be greatest when the
accommodation is imperfect. A very intense light-source may appear many
times larger than its actual physical size.
[Illustration: Fig. 65.--The phenomenon of irradiation.]
Doubtless a number of factors may play a part in this phenomenon. It
appears possible that there is a rapid spreading of the excitation over
the retina extending quite beyond the border of the more intensely
stimulated region, but this must be practically instantaneous in order to
satisfy results of experiments. Eye-movements may play some part for,
despite the most serious efforts to fixate the point of sight, a fringe
will appear on the borders of images which is certainly due to involuntary
eye-movements.
Irradiation has also been ascribed to spherical aberration in the eye-lens
and to diffraction of light at the pupil. Printed type appears
considerably reduced in size when the pupil is dilated with atropin and is
restored to normal appearance when a small artificial pupil is placed
before the dilated pupil. It has been suggested that chromatic aberration
in the eye-lens is a contributory cause, but this cannot be very
important, for the illusion is visible with monochromatic light which
eliminates chromatic aberration. The experimental evidence appears to
indicate that the phenomenon is of a physical nature.
There are variations in the effects attributable to radiation, and it is
difficult to reduce them to simple terms. Perhaps it may aid the reader to
have before him the classification presented by Boswell.[4] He describes
the varieties of irradiation as follows:
1. Very rapid spreading of the excitation over the retina extending
far beyond the border of the stimulated region and occurring
immediately upon impact of the stimulating light.
2. Irradiation within the stimulated portion of the retina after the
form of a figure becomes distinctly perceptible.
3. Emanations of decreasing intensity extend themselves outward and
backward from a moving image until lost in the darkness of the
background.
4. A well known form of irradiation which occurs when a surface of
greater intensity enlarges itself at the expense of one of less
intensity.
5. A form having many of the characteristics of the first type, but
occurring only after long periods of stimulation, of the magnitude of
30 to 60 seconds or more.
IX
COLOR
In order to simplify the presentation of the general subject, discussions
of color have been omitted in so far as possible from the preceding
chapters. There are almost numberless phenomena involving color, many of
which are illusions, or seemingly so. It will be obvious that many are
errors of sense; some are errors of judgment; others are errors due to
defects of the optical system of the eye; and many may be ascribed to
certain characteristics of the visual process. It is not the intention to
cover the entire field in detail; indeed, this could not be done within
the confines of a large volume. However, substantial glimpses of the more
important phases of color as related to illusions are presented in this
chapter. In the early chapters pertaining to the eye and to vision some of
the following points were necessarily touched upon, but the repetition in
the paragraphs which follow is avoided as much as possible.
_Simultaneous Contrast._--That the life of color is due to contrast is
demonstrable in many ways. If a room is illuminated by deep red light, at
first this color is very vivid in consciousness; however, gradually it
becomes less saturated. After a half hour the color is apparently a much
faded red but upon emerging from the room into one normally lighted, the
latter appears very markedly greenish in tint. The reason that the pure
red light does not appear as strongly colored as it really is, is due to
the lack of contrast. In a similar manner at night we see white objects as
white even under the yellowish artificial light. The latter appears very
yellow in color when it is first turned on as daylight wanes but as
darkness falls and time elapses it gradually assumes a colorless
appearance.
An apparatus constructed after the plan of Fig. 62 is very effective for
demonstrating the remarkable effects of color-contrast but some additions
will add considerably to its convenience. If the lamps _F_ are divided
into three circuits, each emitting, respectively, red, green, and blue
primary colors, it is possible by means of controlling rheostats to
illuminate _E_, the environment, with light of any hue (including purple),
of any saturation, and of a wide range of intensities or resulting
brightnesses. Thus we have a very simple apparatus for quickly providing
almost numberless environments for _H_. The same scheme can be applied to
lamps _R_, with the result that a vast array of colors may be seen through
the hole _H_. If the hole is the shape of the star in Fig. 66 it will be
found very effective. The observer will actually see a star of any desired
color amid an environment of any desired color. Care should be taken to
have the star cut in very thin material in order to eliminate conspicuous
boundary lines. It is quite satisfactory to use a series of colored papers
on a slide at _C_ and ordinary clear lamps at _R_. By means of this
apparatus both contrasts--hue and brightness--may be demonstrated. Of
course, for black and white only brightness-contrast is present; but in
general where there is color-contrast there is also brightness-contrast.
The latter may be reduced or even eliminated if the brightness of the star
and of its surroundings are made equal, but it is difficult to make a
satisfactory balance in this respect. Assuming, however, that
brightness-contrast is eliminated, we have left only hue and saturation
contrast, or what will be termed (rather loosely, it is admitted)
color-contrast.
[Illustration: Fig. 66.--An excellent pattern for demonstrating
color-contrast.]
If the surroundings are dark and, for example, an orange star is seen
alone, it does not appear very colorful. However, if the surroundings are
now made bright with white light, the star appears quite saturated. With
blue or green light the orange star appears even more intensely orange,
but when the color-contrast is reduced, as in the case of yellow or red
surroundings, the vividness of the orange star again decreases. This may
be summarized by stating that two widely different colors viewed in this
manner will mutually affect each other so that they appear still more
different in hue. If their hues are close together spectrally this effect
is not as apparent. For example, if orange and green are contrasted, the
orange will appear reddish in hue and the green will appear bluish.