any color of approximately the same brightness. The star which is really
white will now appear decidedly tinted and of a hue approximately
complementary to that of the surroundings. When the latter are of a green
color the white star will assume a purplish tinge; when red the white star
will appear of a blue-green tint; when yellow the white star will appear
bluish. This is an illusion in any sense of the term.
The strength of this illusion caused by simultaneous contrast is very
remarkable. For example, if a grayish purple star is viewed amid intense
green surroundings it will appear richly purple, but when the surroundings
are changed to a rich purple the grayish purple star will even appear
greenish. The apparent change of a color to its complementary by merely
altering its environment is really a remarkable illusion.
The importance of simultaneous contrast is easily demonstrated upon a
painting by isolating any colored object from its surroundings by means of
a hole in a gray card. For example, an orange flower-pot amid the green
foliage of its surroundings will appear decidedly different in color and
brightness than when viewed through a hole in a white, black, or gray
cardboard. By means of colored papers the same color may be placed in many
different environments and the various contrasts may be viewed
simultaneously. The extent of the illusion is very evident when revealed
in this simple manner. However, too much emphasis cannot be given to Figs.
62 and 66 as a powerful means for realizing the greatest effects.
_After-images._--After looking at bright objects we see after-images of
the same size and form which vary more or less in color. These
after-images are due to persistence or fatigue of the visual process,
depending upon conditions. After looking at the sun for a moment a very
bright after-image is seen. Undoubtedly this at first is due to a
persistence of the visual process, but as it decays it continuously
changes color and finally its presence is due to fatigue.
After-images may be seen after looking intently at any object and then
directing the eyes toward a blank surface such as a wall. A picture-frame
will be seen as a rectangular after-image; a checkered pattern will be
seen as a checkered after-image. When these after-images are projected
upon other objects it is obvious that the appearance of the latter is
apparently altered especially when the observer is not conscious of the
after-image. The effects are seen in paintings and many peculiar phenomena
in the various arts are directly traceable to after-images.
It appears unnecessary to detail the many effects for the explanations or
at least the general principles of after-images are so simple that the
reader should easily render an analysis of any given case.
Let us assume that vision is fixed upon a green square upon a gray or
white background. Despite the utmost effort on the part of the observer to
gaze fixedly upon this green square, the latter will begin to appear
fringed with a pinkish border. This is due to the after-image of the green
square and it is displaced slightly due to involuntary eye-movements.
After gazing as steadily as possible for a half minute, or even less, if
the point of sight is turned to the white paper a pink square is seen upon
it. Furthermore, this pink square moves over the field with the point of
sight. This is the type most generally noticed.
After-images have been classified as positive and negative. The former are
those in which the distribution of light and shade is the same as in the
original object. Those in which this distribution is reversed, as in the
photographic negative, are termed "negative." After-images undergo a
variety of changes in color but in general there are two important states.
In one the color is the same as in the original object and in the other it
is approximately complementary to the original color. In general the
negative after-image is approximately complementary in color to the color
of the original object.
After-images are best observed when the eyes are well rested, as in the
morning upon awakening. With a little practice in giving attention to
them, they can be seen floating in the air, in the indefinite field of the
closed eyes, upon a wall, or elsewhere, and the changes in the brightness
and color can be readily followed. Negative after-images are sometimes
very persistent and therefore are more commonly noticed than positive
ones. The positive after-image is due to retinal inertia, that is, to the
persistency of the visual process after the actual stimulus has been
removed. It is of relatively brief duration. If an after-image of a window
is projected on a white area it is likely to appear as a "negative" when
projected upon a white background, and as a "positive" upon a dark
background, such as is readily provided by closing the eyes. It may be of
interest for the reader to obtain an after-image of a bright surface of a
light-source and study its color changes with the eye closed. Upon
repeating the experiment the progression of colors will be found to be
always the same for the same conditions. The duration of the after-image
will be found to vary with the brightness and period of fixation of the
object.
It is interesting to note that an after-image is seen with difficulty when
the eyes are in motion, but it becomes quite conspicuous when the eyes are
brought to rest.
An after-image due to the stimulation of only one eye sometimes seems to
be seen by the other eye. Naturally this has given rise to the suggestion
that the seat of after-images is central rather than peripheral; that is,
in the brain rather than at the retina. However, this is not generally the
case and the experimental evidence weighs heavily against this conclusion.
If Fig. 52 is revolved about its center and fixated for some time striking
effects are obtained upon looking away suddenly upon any object. The
latter will appear to shrink if the spiral has seemed to run outward, or
to expand if the spiral has seemed to run inward. These are clearly
after-images of motion.
As stated elsewhere, we may have illusions of after-images as well as of
the original images. For example, if a clearly defined plane geometrical
figure such as a cross or square is bright enough to produce a strong
after-image, the latter when projected upon a perspective drawing will
appear distorted; that is, it is likely to appear in perspective.
A simple way of demonstrating after-images and their duration is to move
the object producing them. For example, extinguish a match and move the
glowing end. If observed carefully without moving the eye a bluish
after-image will be seen to follow the glowing end of the match. In this
case the eyes should be directed straight ahead while the stimulus is
moving and the observation must be made by averted or indirect vision.
_Growth and Decay of Sensation._--Although many after-images may not be
considered to be illusions in the sense in which the term is used here,
there are many illusions in which they at least play a part. Furthermore,
it is the intention throughout these chapters to adhere to a discussion of
"static" illusions, it is difficult to avoid touching occasionally upon
motion. The eyes are in motion most of the time, hence, certain effects of
an illusory nature may be superposed upon stationary objects.
The persistence of vision has been demonstrated by every small boy as he
waved a glowing stick seized from a bonfire. Fireworks owe much of their
beauty to this phenomenon. A rapidly revolving spoked wheel may appear to
be a more or less transparent disk, but occasionally when a rapid
eye-movement moves the point of sight with sufficient speed in the
direction of motion, the spokes reappear momentarily. Motion-pictures owe
their success to this visual property--the persistence of vision. If a
lantern-slide picture be focused upon black velvet or upon a dark doorway,
the projected image will not be seen. However, if a white rod be moved
rapidly enough in the plane of the image, the latter may be seen in its
entirety. The mixture of colors, by rotating them on disks, owes its
possibility to the persistence of the color-sensations beyond the period
of actual stimulation. The fact that it takes time for sensations of light
to grow and decay is not as important here as the fact that the rates of
growth, and also of decay, vary for different colors. In general, the
growth and the decay are not of similar or uniform rates. Furthermore, the
sensation often initially "overshoots" its final steady value, the amount
of "overshooting" depending upon the intensity and color of the stimulus.
These effects may be witnessed in their extensive variety by rotating
disks so constructed that black and various colors stimulate the retina in
definite orders.
An interesting case of this kind may be demonstrated by rotating the disk
shown in Fig. 67. Notwithstanding the fact that these are only black and
white stimuli, a series of colored rings is seen varying from a reddish
chocolate to a blue-green. Experiment will determine the best speed,
which is rather slow under a moderate intensity of illumination. The
reddish rings will be outermost and the blue-green rings innermost when
the disk is rotated in one direction. Upon reversing the direction of
rotation the positions of these colored rings will be reversed. By using
various colors, such as red and green for the white and black
respectively, other colors will be produced, some of which are very
striking. The complete explanation of the phenomenon is not clear, owing
to the doubt which exists concerning many of the phenomena of
color-vision, but it appears certain that the difference in the rates of
growth and decay of the various color-sensations (the white stimulus
includes all the spectral hues of the illuminant) is at least partially,
if not wholly, responsible.
[Illustration: Fig. 67.--By rotating this Mason (black and white) disk
color-sensations are produced.]
An interesting effect, perhaps due wholly or in part to the differences in
the rates of growth and decay of color-sensations, may be observed when a
colored pattern is moved under a low intensity of illumination, the eyes
remaining focused upon a point in space at about the same distance as the
object. A square of red paper pasted in the center of a larger piece of
blue-green paper is a satisfactory object. On moving this object gently,
keeping the point of sight fixed in its plane of movement, the central red
square will appear to shake like jelly and a decided trail of color will
appear to cling to the lagging edge of the central square. Perhaps
chromatic aberration plays some part in making this effect so conspicuous.
A similar case will be noted in a photographic dark-room illuminated by
red light upon observing the self-luminous dial of a watch or clock. When
the latter is moved in the plane of the dial, the greenish luminous
figures appear separated from the red dial and seem to lag behind during
the movement. For such demonstrations it is well to experiment somewhat by
varying the intensity of the illumination and the speed of movement.
Relatively low values of each appear to be best.
Although the various color-sensations grow and decay at different rates,
the latter depend upon conditions. It appears that blue-sensation rises
very rapidly and greatly overshoots its final steady value for a given
stimulus. Red ranks next and green third in this respect. The overshooting
appears to be greater for the greater intensity of the stimulus. The time
required for the sensation to reach a steady value depends both upon the
spectral character and the brightness of the color but is usually less
than a second.
_Chromatic Aberration._--It is well known that the eye focuses different
spectral colors at different points. This is true of any simple lens and
the defect is overcome in the manufacture of optical instruments by
combining two lenses consisting respectively of glasses differing
considerably in refractive index. If a white object is viewed by the eye,
it should appear with a purplish fringe; however, the effect is observed
more readily by viewing a light-source through a purple filter which
transmits only violet and red light. The light-source will have a red or a
violet fringe, depending upon the accommodation or focus of the eye.
This effect is perhaps best witnessed on viewing a line spectrum such as
that of the mercury arc, focused upon a ground glass. The violet and blue
lines are not seen in good focus when the eyes are focused upon the green
and yellow lines. Furthermore, the former can be seen in excellent focus
at a distance too short for accommodating the eyes to the green and the
yellow lines. This experiment shows that the focal length of the optical
system of the eye is considerably shorter for the spectral hues of shorter
wave-length (violet, blue) than for those of longer wave-length (such as
yellow). Narrow slits covered with diffusing glass and illuminated
respectively by fairly pure blue, green, yellow, and red lights may be
substituted.
The effect may be demonstrated by trying to focus fine detail such as
print when two adjacent areas are illuminated by blue and red lights
respectively. It is also observed when fine detail such as black lines are
held close to the eye for colored fringes are seen. This optical defect is
responsible for certain visual illusions.
An excellent demonstration of chromatic aberration in the eye is found by
viewing fine detail through a purple filter. Now if a red filter be
superposed on the purple one only the red light is transmitted.
Notwithstanding the decrease in illumination or rather of light reaching
the eye, measurement shows that finer detail can be discriminated than in
the first case. A similar result is found on superposing a blue filter
upon the purple one.
_Retiring and Advancing Colors._--For years the artist and the decorator
have felt that certain colors seem to advance nearer than others or that
the latter seem to retire more than the former. The author[5] obtained
actual measurements of this phenomenon, but the evidence also indicated
that the effects were not the same for all persons. The phenomenon is very
noticeable in the case of the image of a colored lantern-slide projected
upon a screen and is readily observed when the image consists of letters
of various colors. In the case of red and green letters, for example, the
former appear (to most persons) to be considerably nearer the observer
than the green letters. It has appeared to the writer that the illusion is
apparent even for white letters upon a dark background. In general, the
colors whose dominant hues are of the shorter wave-lengths (violet, blue,
blue-green, green) are retiring and those whose dominant hues are of the
longer wave-lengths (yellow, orange, red) are advancing.
[Illustration: Fig. 68.--For demonstrating retiring and advancing colors.]
In order to obtain experimental measurements two light-tight boxes, each
containing a light-source, were arranged to run independently upon tracks.
Over the front end of each a diaphragm was placed so that the observer saw
two characters as in Fig. 68. A saturated red filter was placed over one
and a saturated blue filter over the other. In a dark room the observer
saw a blue _E_ and a red _H_ standing out in the darkness. One of these
boxes was fastened so as to be immovable and the observer moved the other
to and fro by means of a cord over pulleys until the two characters
appeared equi-distant from him. This was done for a series of distances of
the stationary box from the observer's eye. Nearly all the observers
(without being acquainted with the positions) were obliged to set the red
_H_ further behind the blue _E_ in order that both appeared at the same
distance. This added distance for the red _H_ was as much as 2.4 feet when
the blue _E_ was at a distance of 24 feet. In other words the difference
in the positions of the two was as much as 10 per cent of the total
distance in this case.
Many other interesting data were obtained but most of these are not
particularly of interest here. Some of the experiments tended to show the
effect of certain optical defects in the eye and the variations and even
reversal of the effect for some persons were accounted for by differences
in the curvatures, etc., of certain eye-media for the observers. These
details are not of interest here but it may be of interest to know that
the phenomenon may be accounted for by the chromatic aberration in the
eye. This may not be the true explanation, or it may be only partially
correct. Perhaps some of the illusion is purely psychological in origin.
Certainly the illusion is very apparent to most careful observers.
_Color-sensibility of the Retina._--This aspect was touched upon in
Chapter III, but the differences in the sensibility of various areas of
the retina to various colors are of sufficient importance to be discussed
further. The ability to distinguish light and color gradually fades or
decreases at the periphery of the visual field, but the actual areas of
the fields of perception vary considerably, depending upon the hue or
spectral character of the light reaching the retina. The extreme
peripheral region of the visual field is "color-blind"; that is, color
ceases to be perceived before brightness-perception vanishes in the
outskirts of the visual field. These fields for various colors depend in
size and contour not only upon the hue or spectral character of the
light-stimuli but also upon the intensity and perhaps upon the size of
the stimuli. There is some disagreement as to the relative sizes of these
fields but it appears that they increase in size in the following order:
green, red, blue, white (colorless). The performances of after-images, and
the rates of growth and decay of sensation vary for different colors and
for different areas of the retina, but it would be tedious to peruse the
many details of these aspects of vision. They are mentioned in order that
the reader may take them into account in any specific case.
As already stated, the central part of the visual field--the fovea upon
which we depend for acute vision--contains a yellowish pigmentation, which
is responsible for the term "yellow spot." This operates as a yellow
filter for this central area and modifies the appearance of visual fields
quite the same as if a similar yellow filter was placed in the central
position of the field of vision. The effect of the selectivity of the
"yellow spot" is noticeable in viewing certain colors.
_Purkinje Effect._--The relative sensibility of the retina varies for
different colors with a change in brightness; or it may be better to state
that the relative sensations for various colors alters as the brightness
values are reduced to a low intensity. For example, if a reddish purple
(consisting of red and blue or violet rays) be illuminated in such a
manner that the intensity of illumination, and consequently its
brightness, may be reduced from normal to a low value (approximating
moonlight conditions), it will be seen to vary from reddish purple to
violet. In doing this its appearance changes through the range of purples
from reddish to violet. This can be accomplished by orientation of the
purple surface throughout various angles with respect to the direction of
light or by reducing the illumination by means of screens.
In general the Purkinje effect may be described as an increasing
sensibility of the retina for light of shorter wave-lengths (violet, blue,
green) as the brightness decreases, or a corresponding decreasing
sensibility for light of longer wave-lengths (yellow, orange, red). The
effect may be seen on any colored surfaces at twilight illumination. A
blue and a red flower, which appear of the same brightness before sunset
will begin to appear unequal in this respect as twilight deepens. The red
will become darker more rapidly than the blue if there are no appreciable
changes in the color of the daylight. Finally all color disappears. It is
better to perform this experiment under artificial light, in order that
the spectral character of the illuminant may be certain to remain
constant. In this case rheostats must not be used for dimming the light
because of the attendant changes in color or quality of the light.
The Purkinje effect may be noticed by the careful observer and it is
responsible for certain illusions. Apparently it cannot operate over one
portion of the retina, while the remainder is stimulated by normal
intensities of light.
_Retinal Rivalry._--Many curious effects may be obtained by stimulating
the two retinas with lights, respectively different in color. For example,
it is interesting to place a blue glass before one eye and a yellow or
red one before the other. The two independent monocular fields strive for
supremacy and this rivalry is quite impressive. For a moment the whole
field may appear of one color and then suddenly it will appear of the
other color. Apparently the fluctuation of attention is a factor. Usually
it does not seem to be possible to reach a quiescent state or a perfect
mixture of the two colors in this manner. The dependence of one monocular
field upon the other, and also their independence, are emphasized by this
experiment. It is of interest to consider the illusions of reversible
perspective and others in Chapter V in this connection.
[Illustration: Fig. 69.--By combining these stereoscopically the effect of
metallic lustre (similar to graphite in this case) is obtained.]
One of the interesting results of retinal rivalry is found in combining
two stereoscopic pictures in black and white with the black and white
reversed in one of them. The apparently solid object will appear to
possess lustre. The experiment may be tried with Fig. 69 by combining the
two stereoscopic pictures by converging or diverging the axes of the eyes
as described in connection with Figs. 2 and 3.
It will be noted that in order for two stereoscopic pictures, when
combined, to produce a perfect effect of three dimensions their
dissimilarity must be no more than that existing between the two views
from the two eyes respectively. The dissimilarity in Fig. 69 is correct as
to perspective, but the reversal of white and black in one of them
produces an effect beyond that of true third dimension. When the colors
are so arranged in such pictures as to be quite different in the two the
effects are striking. There is, in such cases, an effect beyond that of
perfect binocular combination.
By means of the stereoscope it is possible to attain binocular mixture of
colors but this is usually difficult to accomplish. The difficulty
decreases as the brightness and saturation of the colors decrease and is
less for colors which do not differ much in hue and in brightness. These
effects may be studied at any moment, for it is only necessary to throw
the eyes out of focus for any object and to note the results. Many simple
experiments may be arranged for a stereoscope, using black and white, and
various combinations of colors. For example, Fig. 65 may be combined by
means of double images (produced by converging or diverging the optical
axes) so that the two inner squares are coincident. Actual observation is
much more satisfactory than a detailed description.
_Miscellaneous._--There are many interesting effects due to diffraction of
light by edges of objects, by meshes such as a wire screen or a
handkerchief, by the eye-media, etc. On looking at a very bright small
light-source it may be seen to be surrounded by many colors.
Streamers of light appear to radiate from brilliant sources and all
bright areas colored or colorless, when viewed amid dark surroundings,
appear to be surrounded by diffuse brushes of light. These brushes are
likely to be of a bluish tint.