in brightness as the retinal process readjusts itself. An after-image of a
colored object may often appear of a color complementary to the color of
the object. This is generally accounted for by fatigue of the retinal
process. There are many conflicting theories of color-vision but they are
not as conflicting in respect to the aspect of fatigue as in some other
aspects. If the eye is directed toward a green surface for a time and then
turned toward a white surface, the fatigue to green light diminishes the
extent of response to the green rays in the light reflected by the white
surface. The result is the perception of a certain area of the white
surface (corresponding to the portion of the field fatigued by green
light) as of a color equal to white minus some green--the result of which
is pink or purple. This is easily understood by referring to the
principles of color-mixture. Red, green, and blue (or violet) mixed in
proper proportions will produce any color or tint and even white. Thus
these may be considered to be the components of white light. Hence if the
retina through fatigue is unable to respond fully to the green component,
the result may be expressed mathematically as red plus blue plus reduced
green, or synthetically a purplish white or pink. When fatigued to red
light the after-image on a white surface is blue-green. When fatigued to
blue light it is yellowish.
Further mixtures may be obtained by directing the after-image upon colored
surfaces. In this manner many of the interesting visual phenomena and
illusions associated with the viewing of colors are accounted for. The
influence of a colored environment upon a colored object is really very
great. This is known as simultaneous contrast. The influence of the
immediately previous history of the retina upon the perception of colored
surfaces is also very striking. This is called successive contrast. It is
interesting to note that an after-image produced by looking at a bright
light-source, for example, is projected into space even with the eyes
closed. It is instructive to study after-images and this may be done at
any moment. On gazing at the sun for an instant and then looking away, an
after-image is seen which passes in color from green, blue, purple, etc.,
and finally fades. For a time it is brighter than the background which may
conveniently be the sky. On closing the eyes and placing the hands over
them the background now is dark and the appearance of the after-image
changes markedly. There are many kinds, effects, and variations of
after-images, some of which are discussed in other chapters.
As the intensity of illumination of a landscape, for example, decreases
toward twilight, the retina diminishes in sensibility to the rays of
longer wave-lengths such as yellow, orange, and red. Therefore, it becomes
relatively more sensitive to the rays of shorter wave-length such as
green, blue, and violet. The effects of this Purkinje phenomenon (named
after the discoverer) may be added to the class of illusions treated in
this book. It is interesting to note in this connection that moonlight is
represented on some paintings and especially on the stage as greenish blue
in color, notwithstanding that physical measurements show it to be
approximately the color of sunlight. In fact, it is sunlight reflected by
dead, frigid, and practically colorless matter.
Some illusions may be directly traced to the structure of the eye under
unusual lighting conditions. For example, in a dark room hold a lamp
obliquely outward but near one eye (the other being closed and shielded)
and forward sufficiently for the retina to be strongly illuminated. Move
the lamp gently while gazing at a plain dark surface such as the wall.
Finally the visual field appears dark, due to the intense illumination of
the retina and there will appear, apparently projected upon the wall, an
image resembling a branching leafless tree. These are really shadows of
the blood vessels in the retina. The experiment is more successful if an
image of a bright light-source is focused on the sclerotic near the
cornea. If this image of the light-source is moved, the tree-like image
seen in the visual field will also move.
The rate of growth and decay of various color-sensations varies
considerably. By taking advantage of this fact many illusions can be
produced. In fact, the careful observer will encounter many illusions
which may be readily accounted for in this manner.
It may be said that in general the eyes are never at rest. Involuntary
eye-movements are taking place all the time, at least during
consciousness. Some have given this restlessness a major part in the
process of vision but aside from the correctness of theories involving
eye-movements, it is a fact that they are responsible for certain
illusions. On a star-lit night if one lies down and looks up at a star the
latter will be seen to appear to be swimming about more or less jerkily.
On viewing a rapidly revolving wheel of an automobile as it proceeds down
the street, occasionally it will be seen to cease revolving momentarily.
These apparently are accounted for by involuntary eye-movements which take
place regardless of the effort made to fixate vision.
If the eyelids are almost closed, streamers appear to radiate in various
directions from a light-source. Movements of the eyelids when nearly
closed sometimes cause objects to appear to move. These may be accounted
for perhaps by the distortion of the moist film which covers the cornea.
The foregoing are only a few of the many visual phenomena due largely to
the structure of the eye. The effects of these and many others enter into
visual illusions, as will be seen here and there throughout the chapters
which follow.
III
VISION
A description of the eye by no means suffices to clarify the visual
process. Even the descriptions of various phenomena in the preceding
chapter accomplish little more than to acquaint the reader with the
operation of a mechanism, although they suggest the trend of the
explanations of many illusions. At best only monocular vision has been
treated, and it does not exist normally for human beings. A person capable
only of monocular vision would be like Cyclops Polyphemus. We might have
two eyes, or even, like Argus, possess a hundred eyes and still not
experience the wonderful advantages of binocular vision, for each eye
might see independently. The phenomena of binocular vision are far less
physical than those of monocular vision. They are much more obscure,
illusory, and perplexing because they are more complexly interwoven with
or allied to psychological phenomena.
The sense of sight differs considerably from the other senses. The sense
of touch requires solid contact (usually); taste involves liquid contact;
smell, gaseous contact; and hearing depends upon a relay of vibrations
from an object through another medium (usually air), resulting finally in
contact. However, we perceive things at a distance through vibration
(electromagnetic waves called light) conveyed by a subtle, intangible,
universal medium which is unrecognizable excepting as a hypothetically
necessary bearer of light-waves or, more generally, radiant energy.
It also is interesting to compare the subjectiveness and objectiveness of
sensations. The sensation of taste is subjective; it is in us, not in the
body tasted. In smell we perceive the sensation in the nose and by
experience refer it to an object at a distance. The sensation of hearing
is objective; that is, we refer the cause to an object so completely that
there is practically no consciousness of sensation in the ear. In sight
the impression is so completely projected outward into space and there is
so little consciousness of any occurrence in the eye that it is extremely
difficult to convince ourselves that it is essentially a subjective
sensation. The foregoing order represents the sense-organs in increasing
specialization and refinement. In the two higher senses--sight and
hearing--there is no direct contact with the object and an intricate
mechanism is placed in front of the specialized nerve to define and to
intensify the impression. In the case of vision this highly developed
instrument makes it possible to see not only _light_ but _objects_.
As we go up the scale of vertebrate animals we find that there is a
gradual change of the position of the eyes from the sides to the front of
the head and a change of the inclination of the optical axes of the two
eyes from 180 degrees to parallel. There is also evident a gradual
increase in the fineness of the bacillary layer of the retina from the
margins toward the center, and, therefore, an increasing accuracy in the
perception of form. This finally results in a highly organized central
spot or fovea which is possessed only by man and the higher monkeys.
Proceeding up the scale we also find an increasing ability to converge the
optic axes on a near point so that the images of the point may coincide
with the central spots of both retinas. These changes and others are
closely associated with each other and especially with the development of
the higher faculties of the mind.
Binocular vision in man and in the higher animals is the last result of
the gradual improvement of the most refined sense-organ, adapting it to
meet the requirements of highly complex organisms. It cannot exist in some
animals, such as birds and fishes, because they cannot converge their two
optical axes upon a near point. When a chicken wishes to look intently at
an object it turns its head and looks with one eye. Such an animal sees
with two eyes independently and possibly moves them independently. The
normal position of the axes of human eyes is convergent or parallel but it
is possible to diverge the axes. In fact, with practice it is possible to
diverge the axes sufficiently to look at a point near the back of the
head, although, of course, we do not see the point.
The movement of the eyes is rather complex. When they move together to one
side or the other or up and down in a vertical plane there is no rotation
of the optical axes; that is, no torsion. When the visual plane is
elevated and the eyes move to the right they rotate to the right; when
they move to the left they rotate to the left. When the visual plane is
depressed and the eyes move to the right they rotate to the left; when
they move to the left they rotate to the right. Through experience we
unconsciously evaluate the muscular stresses, efforts, and movements
accompanying the motion of the eyes and thereby interpret much through
visual perception in regard to such aspects of the external world as size,
shape, and distance of objects. Even this brief glimpse of the principal
movements of the eyes indicates a complexity which suggests the intricacy
of the explanations of certain visual phenomena.
At this point it appears advantageous to set down the principal modes by
which we perceive the third dimension of space and of objects and other
aspects of the external world. They are as follows: (1) extent; (2)
clearness of brightness and color as affected by distance; (3)
interference of near objects with those more distant; (4) elevation of
objects; (5) variation of light and shade on objects; (6) cast shadows;
(7) perspective; (8) variation of the visor angle in proportion to
distance; (9) muscular effort attending accommodation of the eye; (10)
stereoscopic vision; (11) muscular effort attending convergence of the
axes of the eyes. It will be recognized that only the last two are
necessarily concerned with binocular vision. These varieties of
experiences may be combined in almost an infinite variety of proportions.
Wundt in his attempt to explain visual perception considered chiefly three
factors: (1) the retinal image of the eye at rest; (2) the influence of
the movements of one eye; and, (3) the additional data furnished by the
two eyes functioning together. There are three fields of vision
corresponding to the foregoing. These are the retinal field of vision, the
monocular field, and the binocular field. The retinal field of vision is
that of an eye at rest as compared with the monocular field, which is all
that can be seen with one eye in its entire range of movement and
therefore of experience. The retinal field has no clearly defined
boundaries because it finally fades at its indefinite periphery into a
region where sensation ceases.
It might be tiresome to follow detailed analyses of the many modes by
which visual perception is attained, so only a few generalizations will be
presented. For every voluntary act of sight there are two adjustments of
the eyes, namely, focal and axial. In the former case the ciliary muscle
adjusts the lens in order to produce a defined image upon the retina. In
axial adjustments the two eyes are turned by certain muscles so that their
axes meet on the object looked at and the images of the object fall on the
central-spots of the retina. These take place together without distinct
volition for each but by the single voluntary act of _looking_. Through
experience the intellect has acquired a wonderful capacity to interpret
such factors as size, form, and distance in terms of the muscular
movements in general without the observer being conscious of such
interpretations.
Binocular vision is easily recognized by holding a finger before the eyes
and looking at a point beyond it. The result is two apparently
transparent fingers. An object is seen single when the two retinal images
fall on corresponding points. Direction is a primary datum of sense. The
property of corresponding points of the two retinas (binocular vision) and
consequently of identical spatial points in the two visual fields is not
so simple. It is still a question whether corresponding points (that is,
the existence of a corresponding point in one retina for each point in the
other retina) are innate, instinctive, and are antecedent of experience or
are "paired" as the result of experience. The one view results in the
_nativistic_, the other in the _empiristic_ theory. Inasmuch as some
scientists are arrayed on one side and some on the other, it appears
futile to dwell further upon this aspect. It must suffice to state that
binocular vision, which consists of two retinas and consequently two
fields of view absolutely cooerdinated in some manner in the brain, yields
extensive information concerning space and its contents.
After noting after-images, motes floating in the field of view (caused by
defects in the eye-media) and various other things, it is evident that
what we call the field of view is the external projection into space of
retinal states. All the variations of the latter, such as images and
shadows which are produced in the external field of one eye, are
faithfully reproduced in the external field of the other eye. This sense
of an external visual field is ineradicable. Even when the eyes are closed
the external field is still there; the imagination or intellect projects
it outward. Objects at different distances cannot be seen distinctly at
the same time but by interpreting the eye-movements as the point of sight
is run backward and forward (varying convergence of the axes) the
intellect practically automatically appraises the size, form, and distance
of each object. Obviously, experience is a prominent factor. The
perception of the third dimension, depth or relative distance, whether in
a single object or a group of objects, is the result of the successive
combination of the different parts of two dissimilar images of the object
or group.
As already stated, the perception of distance, size, and form is based
partly upon monocular and partly upon binocular vision, and the simple
elements upon which judgments of these are based are light, shade, color,
intensity, and direction. Although the interpretation of muscular
adjustments plays a prominent part in the formation of judgments, the
influences of mathematical perspective, light, shade, color, and intensity
are more direct. Judgments based upon focal adjustment (monocular) are
fairly accurate at distances from five inches to several yards. Those
founded upon axial adjustment (convergence of the two axes in binocular
vision) are less in error than the preceding ones. They are reliable to a
distance of about 1000 feet. Judgments involving mathematical perspective
are of relatively great accuracy without limits. Those arrived at by
interpreting aerial perspective (haziness of atmosphere, reduction in
color due to atmospheric absorption, etc.) are merely estimates liable to
large errors, the accuracy depending largely upon experience with local
conditions.
The measuring power of the eye is more liable to error when the distances
or the objects compared lie in different directions. A special case is the
comparison of a vertical distance with a horizontal one. It is not
uncommon to estimate a vertical distance as much as 25 per cent greater
than an actually equal horizontal distance. In general, estimates of
direction and distance are comparatively inaccurate when only one eye is
used although a one-eyed person acquires unusual ability through a keener
experience whetted by necessity. A vertical line drawn perpendicular to a
horizontal one is likely to appear bent when viewed with one eye. Its
apparent inclination is variable but has been found to vary from one to
three degrees. Monocular vision is likely to cause straight lines to
appear crooked, although the "crookedness" may seem to be more or less
unstable.
The error in the estimate of size is in reality an error in the estimation
of distance except in those cases where the estimate is based directly
upon a comparison with an object of supposedly known size. An amusing
incident is told of an old negro who was hunting for squirrels. He shot
several times at what he supposed to be a squirrel upon a tree-trunk and
his failure to make a kill was beginning to weaken his rather ample
opinion of his skill as a marksman. A complete shattering of his faith in
his skill was only escaped by the discovery that the "squirrel" was a
louse upon his eyebrow. Similarly, a gnat in the air might appear to be an
airplane under certain favorable circumstances. It is interesting to note
that the estimated size of the disk of the sun or moon varies from the
size of a saucer to that of the end of a barrel, although a pine tree at
the horizon-line may be estimated as 25 feet across despite the fact that
it may be entirely included in the disk of the sun setting behind it.
Double images play an important part in the comparison of distances of
objects. The "doubling" of objects is only equal to the interocular
distance. Suppose two horizontal wires or clotheslines about fifty feet
away and one a few feet beyond the other. On looking at these no double
images are visible and it is difficult or even impossible to see which is
the nearer when the points of attachment of the ends are screened from
view. However, if the head is turned to one side and downward (90 degrees)
so that the interocular line is now at right angles (vertical) to the
horizontal lines, the relative distances of the latter are brought out
distinctly. Double images become visible in the latter case.
According to Bruecke's theory the eyes are continuously in motion and the
observer by alternately increasing or decreasing the convergence of the
axes of the eyes, combines successively the different parts of the two
scenes as seen by the two eyes and by running the point of sight back and
forth by trial obtains a distinct perception of binocular perspective or
relief or depth of space. It may be assumed that experience has made the
observer proficient in this appraisal which he arrives at almost
unconsciously, although it may be just as easy to accept Wheatstone's
explanation. In fact, some experiences with the stereoscope appear to
support the latter theory.
Wheatstone discovered that the dissimilar pictures of an object or scene,
when united by means of optical systems, produce a visual effect similar
to that produced by the actual solid object or scene provided the
dissimilarity is the same as that between two retinal images of the solid
object or scene. This is the principle upon which the familiar stereoscope
is founded. Wheatstone formulated a theory which may be briefly stated as
follows: In viewing a solid object or a scene two slightly dissimilar
retinal images are formed in the two eyes respectively, but the mind
completely fuses them into one "mental" image. When this mental fusion of
the two really dissimilar retinal images is complete in this way, it is
obvious that there cannot exist a mathematical coincidence. The result is
a perception of depth of space, of solidity, of relief. In fact the third
dimension is perceived. A stereoscope accomplishes this in essentially the
same manner, for two pictures, taken from two different positions
respectively corresponding to the positions of the eyes, are combined by
means of optical systems into one image.
Lack of correct size and position of the individual elements of
stereoscopic pictures are easily detected on combining them. That is,
their dissimilarity must exactly correspond to that between two views of
an object or scene from the positions of the two eyes respectively (Fig.
2). This fact has been made use of in detecting counterfeit notes. If two
notes made from the same plate are viewed in a stereoscope and the
identical figures are combined, the combination is perfect and the plane
of the combined images is perfectly flat. If the notes are not made from
the same plate but one of them is counterfeit, slight variations in the
latter are unavoidable. Such variations will show themselves in a wavy
surface.
The unwillingness of the visual sense to combine the two retinal images,
if they are dissimilar to the extent of belonging to two different
objects, is emphasized by means of colors. For example, if a green glass
is placed over one eye and a red glass over the other, the colors are not
mixed by the visual sense. The addition of these two colors results
normally in yellow, with little or no suggestion of the components--red
and green. But in the foregoing case the visual field does not appear of a
uniform yellow. It appears alternately red and green, as though the colors
were rivaling each other for complete mastery. In fact, this phenomenon
has been termed "retinal rivalry."
The lenses of the stereoscope supplement eye-lenses and project on the
retina two perfect images of a near object, although the eyes are looking
at a distant object and are therefore not accommodated for the near one
(the photographs). The lenses enlarge the images similar to the action of
a perspective glass. This completes the illusion of an object or of a
scene. There is a remarkable distinctness of the perception of depth of
space and therefore a wonderful resemblance to the actual object or scene.
It is interesting to note the effect of taking the two original
photographs from distances separated by several feet. The effect is
apparently to magnify depth. It is noteworthy that two pictures taken from
an airplane at points fifty feet or so apart, when combined in the
stereoscope, so magnify the depth that certain enemy-works can be more
advantageously detected than from ordinary photographs.
Stereoscopic images such as represented in Fig. 2 may be combined without
the aid of the stereoscope if the optical axes of the eye can be
sufficiently converged or diverged. Such images or pictures are usually
upon a card and are intended to be combined beyond the plane of the card,
for it is in this position that the object or scene can be perceived in
natural perspective, of natural size, of natural form, and at natural
distance. But in combining them the eyes are looking at a distant object
and the axes are parallel or nearly so. Therefore, the eyes are focally
adjusted for a distant object but the light comes from a very near
object--the pictures on the card. Myopic eyes do not experience this
difficulty and it appears that normal vision may be trained to overcome
it. Normal eyes are aided by using slightly convex lenses. Such glasses
supplement the lenses of the eye, making possible a clear vision of a near
object while the eyes are really looking far away or, in other words,
making possible a clear image of a near object upon the retina of the
unadjusted eye. Stereoscopic pictures are usually so mounted that
"identical points" on the two pictures are farther apart than the
interocular distance and therefore the two images cannot be combined when
the optical axes of the eyes are parallel or nearly so, which is the
condition when looking at a distant object. In such a case the two
pictures must be brought closer together.
[Illustration: Fig. 2.--Stereoscopic pictures for combining by converging
or diverging the optical axes.]
[Illustration: Fig. 3.--Stereoscopic pictures.]