safely without knowing or recognizing all conditions. Many are not easily
explained, especially when reported by others, who may not recognize
certain important conditions. For example, authentic observers have
reported that black letters on white paper appeared vivid red on a white
background, under certain conditions. Of the latter, the apparently
important one was "sun's rays falling aslant the forehead." When the eyes
were shaded with the hand the letters immediately appeared black as they
should.
The influence of the color of an object upon its apparent weight is
relatively slight, but there is evidence of a tendency to judge a red or
black object to be slightly heavier than a yellow or blue object of the
same weight. It appears that hue is a minor factor in influencing the
judgment and that there is no correlation between the affective quality of
a color and its influence upon apparent weight. Although the scanty
evidence available attributes but a slight influence to color in this
respect, it is of interest in passing as a reminder of the many subtle
factors which are at work modifying our judgments.
X
LIGHTING
It should be obvious by this time that the lighting of objects or of a
scene can alone produce an illusion, and that it can in still more cases
contribute toward an illusion. Furthermore, there are many cases of
illusions in lighting due to brightness and color. Many effects of
lighting have been described elsewhere with detailed analyses of the
underlying principles, but a condensed survey applying particularly to
illusions will be presented here.
The comparison of intaglio with low relief has been mentioned several
times in preceding chapters. Examples of these as related to lighting are
found in Figs. 70 to 73. Fig. 70 represents a bas-relief lighted from
above and Fig. 71 would ordinarily be taken to represent a bas-relief
lighted from below. However, the latter was made from a photograph of the
mold (intaglio) from which the bas-relief was made and Fig. 71 really
represents an intaglio lighted from above.
Similarly Fig. 72 represents the bas-relief lighted from the left and Fig.
73 ordinarily would be taken to be a bas-relief lighted from the right.
However, Fig. 73 was made from a photograph of an intaglio lighted from
the left. These amply demonstrate the effect of lighting as an influence
upon the appearance of objects and they indicate the importance of
correct assumptions in arriving at a correct judgment. In these cases the
concealment of the light-source and the commonness of bas-relief as
compared with intaglio are the causes for the illusion or the error in
judgment. Certainly in these cases the visual sense delivers its data
correctly.
[Illustration: Fig. 70.--A bas-relief lighted from above.]
[Illustration: Fig. 71.--An intaglio lighted from above.]
[Illustration: Fig. 72.--A bas-relief lighted from the left.]
[Illustration: Fig. 73.--An intaglio lighted from the left.]
[Illustration: Fig. 74.--_a._ A disk (above) and a sphere (below) lighted
from overhead. _b._ A disk and a sphere lighted by perfectly diffused
light.]
In Fig. 74 the upper object is a disk and the lower is a sphere. In _a_
Fig. 74 the lighting is due to a source of light of rather small
physical dimensions directly above the objects. The same objects
illuminated by means of highly diffused light (that is, light from many
directions and of uniform intensity) appear as in _b_. Both objects now
appear as disks. It is obvious that under appropriate lighting a disk
might be taken for a sphere and vice versa, depending upon which dominates
the judgment or upon the formulation of the attendant assumptions.
Incidentally an appearance quite similar to that of _a_, Fig. 74 is
obtained when the light-source is near the observer; that is, when it lies
near the line of sight.
[Illustration: Fig. 75.--A concave hemispherical cup on the left and a
convex hemisphere on the right lighted by a light-source of large angle
such as a window.]
Somewhat similar to the confusion of intaglio with bas-relief is the
confusion of the two hemispherical objects illustrated in Fig. 75. The one
on the left is concave toward the observer. In other words, both could be
hemispherical shells--one a mold for the other. Under the lighting which
existed when the original photographs were made they could both be taken
for hemispheres. The lighting was due to a large light-source at the left,
but if the object on the left is assumed (incorrectly) to be a hemisphere
convex toward the observer or a sphere, it must be considered to be
lighted from the right, which is also an incorrect assumption. Obviously,
if the direction of the dominant light is clear to the observer, he is not
likely to make the error in judgment. Incidentally the object on the right
might be assumed to be a sphere because a sphere is more commonly
encountered than a hemisphere.
[Illustration: Fig. 76.--The same as Fig. 75, but lighted by a very small
light-source.]
The same objects are represented in Fig. 76 lighted from the left by means
of a light-source of relatively small dimensions; that is, a source
subtending a relatively small solid-angle at the objects. In this case the
sharp shadow due to the edge of the hemispherical cup (on the left) is
likely to cause the observer to inquire further before submitting his
judgment. The more gradual modulation of light and shade as in the case of
a sphere or a hemisphere convex toward the observer is not present in the
case of the cup. This should be sufficient information for the careful
observer to guide him, or at least to prevent him from arriving at the
definite conclusion that the left-hand object is a hemisphere with its
convex side toward him. Furthermore it should be noted that we often jump
at the conclusion that an object is a sphere even though we see with one
eye practically only a hemisphere and with two eyes hardly enough more to
justify such a conclusion. However, spheres are more commonly encountered
than hemispheres, so we take a chance without really admitting or even
recognizing that we do.
The foregoing figures illustrate several phases which influence our
judgments and the wonder is that we do not make more errors than we do. Of
course, experience plays a large part and fortunately experience can be
depended upon in most cases; however, in the other cases it leads us
astray to a greater extent than if we had less of it.
The photographer, perhaps, recognizes more than anyone else the pitfalls
of lighting but it is unfortunate that he is not better acquainted with
the fundamentals underlying the control of light. Improper lighting does
produce apparent incongruous effects but adequately controlled it is a
powerful medium whose potentiality has not been fully realized. The
photographer aims to illuminate and to pose the subject with respect to
the source or sources of light so that undesirable features are suppressed
and desirable results are obtained.
Finally his work must be accepted by others and the latter, being human,
possess (unadmittedly of course) a desire to be "good looking." Lighting
may be a powerful flatterer when well controlled and may be a base
revealer or even a creator of ugliness.
Incidentally, the photographer is always under the handicap of supplying a
"likeness" to an individual who perhaps never sees this same "likeness" in
a mirror. In other words, the image which a person sees of himself in a
mirror is not the same in general that the photographer supplies him in
the photographic portrait. The portrait can be a true likeness but the
mirrored image in general cannot be. In the mirror there is a reversal of
the parts from right to left. For example, a scar on the right cheek of
the actual face appears on the left cheek in the mirror. Faces are not
usually symmetrical and this reversal causes an individual to be familiar
with his own facial characteristics in this reversed form. This influence
is very marked in some cases. For example, suppose the left side of a
companion's face to be somewhat paralyzed on one side due to illness. We
have become more or less oblivious to the altered expression of the left
side by seeing it so often. However, if we catch a glimpse of this
companion's face in the mirror and the altered expression of the left side
now appears upon the right side of the face, the contrast makes the fact
very conspicuous. Perhaps this accounts for the difference which exists
between the opinions of the photographer (or friends) and of the subject
of the portrait.
All the illusions of brightness-contrast may be produced by lighting.
Surfaces and details may appear larger or smaller, harsh or almost
obliterated, heavy or light; in fact, lighting plays an important part in
influencing the mood or expression of a room. A ceiling may be "lifted" by
light or it may hang low and threatening when dark, due to relatively
little light reaching it. Columns may appear dark on a light background or
vice versa, and these illustrate the effects of irradiation. A given room
may be given a variety of moods or expressions by varying the lighting and
inasmuch as the room and its physical characteristics have not been
altered, the various moods may be considered to be illusions. It should be
obvious that lighting is a potent factor.
In connection with lighting it should be noted that contrasts play a
prominent role as they always do. These have been discussed in other
chapters, but it appears advantageous to recall some of the chief
features. The effect of contrast is always in the direction of still
greater contrast. That is, black tends to make its surroundings white; red
tends to make its surroundings blue-green (complementary), etc. The
contrast-effect is greatest when the two surfaces are juxtaposed and the
elimination of boundary lines of other colors (including black or white)
increases its magnitude. The contrast-effect of colors is most conspicuous
when there is no brightness-contrast, that is, when the two surfaces are
of equal brightness and therefore differ chiefly in hue. This effect is
also greatest for saturated colors. It has been stated that cold colors
produce stronger contrast-effects than warm colors, but experimental
evidence is not sufficiently plentiful and dependable to verify this
statement.
As the intensity of illumination increases, colors appear to become less
saturated. For example, a pure red object under the noonday sun is likely
to be painted an orange red by the artist because it does not appear as
saturated as it would under a much lower intensity of illumination. In
general, black and white are the final appearances of colors for
respectively very low and very high brightness. As the intensity of
illumination decreases, hue finally disappears and with continued decrease
the color approaches black. Conversely, as the intensity of illumination
increases, a color becomes apparently less and less saturated and tends
toward white. For example, on viewing the sun through a colored glass the
sun appears of a much less saturated color than the haze near the sun or a
white object illuminated by sunlight.
Visual adaptation also plays a prominent part, and it may be stated that
all sensations of light tend toward a middle gray and all sensations of
color tend toward neutrality or a complete disappearance of hue. The
tendency of sensations of light toward a middle gray is not as easily
recognized as changes in color but various facts support this conclusion.
In lighting it is important to recognize the tendency of color toward
neutrality. For example, a warm yellow light soon disappears as a hue and
only its subtle influence is left; however, a yellow vase still appears
yellow because it is contrasted with objects of other colors. In the case
of colored light the light falls upon everything visible, and if there is
no other light-source of another color with which to contrast it, its
color appears gradually to fade. This is an excellent example of the
tremendous power and importance of contrast. It is the life of color and
it must be fully appreciated if the potentiality of lighting is to be
drawn upon as it should be.
Physical measurements are as essential in lighting as in other phases of
human endeavor for forming a solid foundation, but in all these activities
where visual perception plays an important part judgment is finally the
means for appraisal. Wherever the psychological aspect is prominent
physical measurements are likely to be misleading if they do not agree
with mental appraisals. Of course the physical measurements should be made
and accumulated but they should be considered not alone but in connection
with psychological effects.
The photometer may show a very adequate intensity of illumination;
nevertheless seeing may be unsatisfactory or even impossible. An
illumination of a few foot-candles under proper conditions at a given
surface is quite adequate for reading; however, this surface may appear
quite dark if the surroundings are bright enough. In such a case the
photometer yielded results quite likely to be misinterpreted as
satisfactory. It should be obvious that many illusions discussed in
preceding chapters are of interest in this connection.
An interesting example of the illusion of color may be easily demonstrated
by means of a yellow filter. For this purpose a canary glass is quite
satisfactory. When such a filter is placed before the eyes a daytime scene
outdoors, for example, is likely to appear to be illuminated to a greater
intensity than when the eyes are not looking through the filter. This is
true for a glass used by the author notwithstanding the fact that the
filter transmits only about one-half as much light as a perfectly clear
colorless glass. In other words, the brightnesses of objects in the scene
are reduced on the average about fifty per cent, still the subject is
impressed with an apparent _increase_ in the intensity of illumination
(and in brightness) when the filter is placed before the eyes. Of course,
the actual reduction in brightness depends upon the color of the object.
In such a case as the foregoing, true explanations are likely to involve
many factors. For this reason explanations are usually tedious if they are
to be sufficiently qualified to be reasonably near completeness. In this
case it appears that the yellow filter may cause one to appraise the
intensity of illumination as having increased, by associating such an
influence as the sun coming out from behind a cloud. If we look into the
depths where light and color accumulated their psychological powers, we
are confronted on every hand by associations many of which are more or
less obscure, and therefore are subtly influential.
The psychological powers of colors could have been discussed more
generally in the preceding chapter, but inasmuch as they can be
demonstrated more effectively by lighting (and after all the effect is one
of light in any case) they will be discussed briefly here. They have been
presented more at length elsewhere.
It is well known that the artist, decorator, and others speak of warm and
cold colors, and these effects have a firm psychological foundation. For
example, if a certain room be illuminated by means of blue light, it does
seem colder. A theater illuminated by means of bluish light seems
considerably cooler to the audience than is indicated by the thermometer.
If this lighting is resorted to in the summer time the theater will be
more inviting and, after all, in such a case it makes little difference
what the thermometer indicates. The "cold" light has produced an illusion
of coolness. Similarly "warm" light, such as yellow or orange, is
responsible for the opposite feeling and it is easily demonstrated that an
illusion of higher temperature may be produced by its use. As already
stated, color-schemes in the decorations and furnishings produce similar
effects but in general they are more powerful when the primary light is
colored. In the latter case no object is overlooked for even the hands and
faces of the beings in the room are colored by the light. In the case of
color-schemes not all objects are tinged with the desired "warm" or "cold"
color.
In the foregoing, associations play a prominent role. The sky has been
blue throughout the numberless centuries during which the human organism
evolved. The blue-sky during all these centuries has tinged the shadows
outdoors a bluish color. That shade is relatively cool we know by
experience and perhaps we associate coolness or cold with the aerial
realm. These are glimpses of influences which have cooeperated toward
creating the psychological effect of coldness in the case of bluish light.
By contrast with skylight, sunlight is yellowish, and a place in the sun
is relatively warm. South rooms are usually warmer than north rooms in
this hemisphere when artificial heat is absent and the psychological
effect of warmth has naturally grown out of these and similar influences.
We could go further into the psychology of light and color and conjecture
regarding effects directly attributable to color, such as excitement,
depression, and tranquillity. In so doing we would be led far astray from
illusions in the sense of the term as used here. Although this term as
used here is still somewhat restricted, it is broader in scope than in its
usual applications. However, it is not broad enough to lead far into the
many devious highways and byways of light and color. If we did make these
excursions we would find associations almost universally answering the
questions. The question would arise as to innate powers of colors and we
would find ourselves wondering if all these powers were acquired (through
associations) and whether or not some were innate. And after many
interesting views of the intricate subject we would likely conclude that
the question of the innateness of some of the powers of color must be left
unanswered.
As an example let us take the case of the restfulness or depression due to
blue. We note that the blue sky is quite serene or tranquil and we find
that the delicate sensibilities of poets verify this impression. This
association could account for the impression or feeling of tranquillity
associated with blue. On proceeding further, we would find nature's
solitudes often tinged with the blue skylight, for these solitudes are
usually in the shade. Thus their restfulness or even depressiveness may be
accounted for--partially at least. These brief glimpses are presented in
order that they may suggest to the reader another trend of thought when
certain illusions of light and color are held up for analysis. Besides
these our individual experiences which have molded our likes and dislikes
must be taken into account. This phase of light and color has been treated
elsewhere.[6]
A very unusual kind of optical illusion is illustrated by the phenomenon
of the apparent ending of a searchlight beam which has attracted much
attention in connection with the powerful searchlights used for locating
aeroplanes (Fig. 77). For years the apparent ending has more or less
carelessly been attributed to the diminution of the density of atmospheric
fog or haze, but recently Karrer[13] has suggested what appears to be the
correct explanation.
When the beam of light from a powerful searchlight is directed into space,
its path is visible owing to the scattering of some of the light by dust
and moisture particles and the molecules of the air itself. While
obviously the beam itself must go on indefinitely, its luminous path
appears to end abruptly at no very great distance from the source. This is
true whether the beam is photographed or viewed with the naked eye.
[Illustration: Fig. 77.--Apparent ending of a searchlight beam.]
The fact that the appearance of the beam is no different when it is
directed horizontally than when directed vertically proved that the common
assumption pertaining to the ending of the haze or fog is untenable.
Furthermore, photometric measurements on the different portions of the
beam as seen from a position near the searchlight show that the beam is
actually brighter at its outer termination than near its origin. Again,
the apparent length of the beam varies with the position of the observer,
and bears a direct ratio to his distance from the searchlight.
The fact is, that the luminous path of the beam has no definite ending,
and extends to a very great distance--practically to infinity. It appears
to be sharply cut off for the same reason that the boundary between earth
and sky in a flat landscape is a sharp line. Just as the horizon recedes
when the landscape is viewed from an elevation, so the beam appears longer
when one's distance from it is increased. The outer portion appears
brighter, because here the line of sight pierces it to great depth.
That the ending of the beam appears _close at hand_ is no doubt partly due
to the brightness distribution, but is also a matter of perspective
arising from the manner in which the beam is adjusted. Searchlight
operators in the army were instructed to adjust the light to throw a
parallel beam. Accordingly, the adjustments were so made that the beam
appeared the same width at its outer extremity as at its base. The result
seems to be a short parallel shaft of light, but is really a divergent
cone of infinite extent, its angle of divergence being such as exactly to
offset the effects of perspective.
If the beam were a truly parallel one it would seem to come to a point,
just as the edges of a long straight stretch of country road seem to meet
at the horizon. If the sides of the road were not parallel, but diverged
from the observer's eye at exactly the rate at which they ordinarily would
appear to converge, then the road would seem to be as wide where it passed
out at the horizon as at the observer's feet. If there were no other
means in the landscape of judging the distance of the horizon than by the
perspective afforded by the road, it would likely be inferred that the
road only extended a short distance on the level, and then went down a
hill, that is, passed abruptly from the observer's view.
These conditions obtain ideally in the case of the searchlight beam. There
is no other means of judging the position in space of the "end" of an
unobstructed searchlight beam than by the perspective of the beam itself,
and the operator in adjusting it to appear parallel eliminates the
perspective.
The angle at which the beam must diverge to appear parallel to an observer
depends upon the distance of the observer from the searchlight. A beam
which seems parallel to a person close to it will not appear so at a
distance. This fact probably accounts for the difficulties encountered
during "searchlight drill" in the army in getting a beam which satisfied
both the private operating the lamp and the officer down the field as to
its parallelity.
To summarize, the apparent abrupt ending of a searchlight beam is purely
an optical illusion. It really has no ending; it extends to infinity.
XI
NATURE