hung on brilliant green or rose walls. The changes in the appearance of
the object due to these highly colored environments are easily
demonstrated by viewing a piece of white paper pinned upon the wall. On
the green wall, the white paper appears pinkish; on the rose wall, it
appears bluish or greenish. A portrait or a picture in which there are
areas of white or delicate tints is subject to considerable distortions in
the appearance of its colors. Similarly, if a woman must have a colored
background, it is well to choose one which will induce the more desirable
tints in her appearance. The designer of gowns certainly must recognize
these illusions of color which may be desirable or undesirable.

The lighting of a picture has already been mentioned, but the discussion
was confined solely to distribution of light. The quality of the light
(its spectral character) may have an enormous influence upon the painting.
In fact with the same painting many illusions may be produced by lighting.
In general, paintings are painted in daylight and they are not the same in
appearance under ordinary artificial light. For this reason the artist is
usually entitled to the preservation of the illusion as he completed it.
By using artificial daylight which has been available for some years, the
painting appears as the artist gave it his last touch. Of course, it is
quite legitimate to vary the quality of light in case the owner desires to
do so, but the purpose here is to emphasize the fact that the quality of
light is a powerful influence upon the appearance of the painting. The
influence is not generally enough recognized and its magnitude is
appreciated by relatively few persons.

All other considerations aside, a painting is best hung upon a colorless
background and black velvet for this purpose yields remarkable results.
Gray velvet is better, when the appearance of the room is taken into
consideration, as it must be. However, the influence of dark surroundings
toward enhancing the illusion is well worth recognizing. In the case of a
special picture or a special occasion, a painting may be exhibited in a
booth--a huge shadow-box not unlike a show-window in which the
light-sources are concealed. Such experiments yield many interesting data
pertaining to the illusions which the painter strives to obtain.

[Illustration: Fig. 82.--Illustrating the apparent distortion of a picture
frame in which the grain of the wood is visible.]

Incidentally on viewing some picture frames in which the grain of the wood
was noticeable, the frames did not appear to be strictly rectangular. The
illusions were so strong that only by measuring the frames could one be
convinced that they were truly rectangular and possessed straight sides.
Two of these are represented in Figs. 82 and 83. In the former, the
horizontal sides appear bent upward in the middle and the two vertical
sides appear bowed toward the right. In Fig. 83, the frame appears
considerably narrower at the left end than at the right. Both these frames
were represented in the original drawings by true rectangles.

Many illusions are to be seen in furniture and in other woodwork in which
the grain is conspicuous. This appears to the author to be an objection in
general to this kind of finish. In Fig. 84 there is reproduced a
photograph of the end of a board which was plane or straight
notwithstanding its warped, or bowed, appearance. The original photographs
were placed so as to be related as shown in the figure. Various degrees of
the illusion are evident. The reader will perhaps find it necessary to
convince himself of the straightness of the horizontal edges by applying a
straight edge. These are examples of the same illusion as shown in Figs.
37 to 40.

[Illustration: Fig. 83.--Another example similar to Fig. 82.]

[Illustration: Fig. 84.--From actual photographs of the end-grain of a
board.]

Perhaps a brief statement regarding the modern _isms_ in art may be of
interest. In considering some of the extreme examples, we must revise our
idea that art is or should be always beautiful. The many definitions of
art would lead us too far afield to discuss them here but in its most
extended and popular sense, art may be considered to mean everything
which we distinguish from nature. Certainly art need not be beautiful,
although it does seem that the world would welcome the beautiful and would
get along contentedly without art that is ugly or repulsive. The modern
_isms_ must be viewed with consideration, for there are many impostors
concealing their inabilities by flocking to these less understood fields.
However, there are many sincere workers--research artists--in the modern
_isms_ and their works may best be described at present as experiments in
the psychology of light, shade, and color. They have cast aside or reduced
in importance some of the more familiar components such as realism and are
striving more deeply to utilize the psychology of light and color. Some of
them admit that they strive to paint through child's eyes and mind--free
from experience, prejudice, and imitation. These need all the scientific
knowledge which is available--and maybe more.

In closing this chapter, it appears necessary to remind the artist and
others that it is far from the author's intention to subordinate the
artist's sensibility to the scientific facts or tools. Art cannot be
manufactured by means of formulae. This would not be true if we knew a
great deal more than we do pertaining to the science of light, color, and
vision. The artist's fine sensibility will always be the dominating
necessity in the production of art. He must possess the ability to compose
exquisitely; he must be able to look at nature through a special
temperament; he must be gifted in eye and in hand; he must be master of
unusual visual and intellectual processes. But knowledge will aid him as
well as those in other activities. A superior acquaintance with scientific
facts lifted past masters above their fellows and what helped Leonardo da
Vinci, Rembrandt, Velasquez, Turner, Claude, Monet, and other masters will
help artists of today. What would not those past masters have accomplished
if they had available in their time the greater knowledge of the present!

XIII

ARCHITECTURE

Many illusions are found in architecture and, strangely enough, many of
these were recognized long before painting developed beyond its primitive
stages. The architecture of classic Greece displays a highly developed
knowledge of many geometrical illusions and the architects of those
far-off centuries carefully worked out details for counteracting them.
Drawings reveal many illusions to the architect, but many are not
predicted by them. The ever-changing relations of lines and forms in
architecture as we vary our viewpoint introduce many illusions which may
appear and disappear. No view of a group of buildings or of the components
of a single structure can be free from optical illusions. We never see in
the reality the same relations of lines, forms, colors, and brightnesses
as indicated by the drawings or blue-prints. Perhaps this is one of the
best reasons for justifying the construction of expensive models of our
more pretentious structures.

No detailed account of the many architectural illusions will be attempted,
for it is easy for the reader to see many of the possibilities suggested
by preceding chapters. However, a few will be touched upon to reveal the
magnitude of the illusory effect and to aid the observer in looking for
or recognizing them, or purely for historical interest. In architecture
the eye cannot be wholly satisfied by such tools as the level, the square,
and the plumb-line. The eye is satisfied only when the _appearance_ is
satisfactory. For the purpose of showing the extent of certain
architectural illusions, the compensatory measures applied by the Greeks
are excellent examples. These also reveal the remarkable application of
science to architecture as compared with the scanty application in
painting of the same period.

During the best period of Grecian art many refinements were applied in
order to correct optical illusions. It would be interesting to know to
what extent the magnitude of the illusions as they appeared to many
persons were actually studied. The Parthenon of Athens affords an
excellent example of the magnitude of the corrections which the designer
thought necessary in order to satisfy the eye. The long lines of the
architrave--the beam which surmounts the columns or extends from column to
column--would appear to sag if it were actually straight. This is also
true of the stylobate, or substructure of a colonnade, and of pediments
and other features. These lines were often convex instead of being
straight as the eye desires to see them.

In the Parthenon, the stylobate has an upward curvature of more than four
inches on the sides of the edifice and of more than two and a half inches
on the east and west fronts. Vertical features were made to incline inward
in order to correct the common appearance of leaning outward at the top.
In the Parthenon, the axes of the columns are not vertical, but they are
inclined inward nearly three inches. They are said also to be inclined
toward each other to such a degree that they would meet at an altitude of
one mile above the ground. The eleven-foot frieze and architrave is
inclined inward about one and one-half inches.

In Fig. 85, _a_ represents the front of a temple as it should appear; _b_
represents its appearance (exaggerated) if it were actually built like _a_
without compensations for optical illusions; _c_ represents it as built
and showing the physical corrections (exaggerated) in order that it may
appear to the eye as _a_ does.

Tall columns if they are actually straight are likely to appear somewhat
shrunken in the middle; therefore they are sometimes made slightly swollen
in order to appear straight. This outward curvature of the profile is
termed an entasis and in the Parthenon column, which is thirty-four feet
in height, amounted to about three-fourths of an inch. In some early
Grecian works, it is said that this correction was overdone but that its
omission entirely is quite unsatisfactory. Some authorities appear to
believe that an excellent compromise is found in the Parthenon columns.

[Illustration: Fig. 85.--Exaggerated illusions in architecture.]

One of the conditions which is responsible for certain illusions and has
been compensated for on occasions is represented in Fig. 86. On the left
are a series of squares of equal size placed in a vertical row. If these
are large so that they might represent stories in a building they will
appear to decrease in size from the bottom upward, because of the
decreasing projection at the eye. This is obvious if the eye is
considered to be at the point where the inclined lines meet. In order to
compensate for the variation in visual angle, there must be a series of
rectangles increasing considerably in height toward the top. The
correction is shown in the illustration. It is stated that an inscription
on an ancient temple was written in letters arranged vertically, and in
order to make them appear of equal size they were actually increased in
size toward the top according to the law represented in Fig. 86. Obviously
a given correction would be correct only for one distance in a given
plane.

[Illustration: Fig. 86.--Illustrating the influence of visual angle upon
apparent vertical height.]

In Chapter VIII the phenomenon of irradiation was discussed and various
examples were presented. It exerts its influence in the arts as elsewhere.
Columns viewed against a background of white sky appear of smaller
diameter than when they are viewed against a dark background. This is
illustrated in Fig. 87 where the white and the black columns are supposed
to be equal in diameter.

The careful observer will find numberless optical illusions and
occasionally he will recognize an attempt on the part of the architect to
apply an illusory effect to his advantage. In Fig. 88 some commonplace
illusions are presented, not for what they are worth, but to suggest how
prevalent they may be. Where the pole or column intersects the arches or
circle, there is an apparent change in the direction of the curved lines.
The different types of arches show different degrees of the illusion. It
may be of interest for the reader to refer to preceding chapters and to
ascertain what types of illusions are involved.

[Illustration: Fig. 87.--Irradiation in architecture.]

If a high wall ends in a series of long horizontal steps at a slightly
inclined sidewalk, the steps are not likely to appear horizontal.

Some remarkable illusions of depth or of solid form are given to flat
surfaces when snow is driven against them so as to adhere in decreasing
amounts similar to shading.

A suggestion of augmented height may be given to a low tower by
decreasing the size of its successive portions more rapidly than demanded
by perspective alone. The same principal can be applied in many ways. For
example, in Fig. 89 the roof appears quite extensive when viewed so that
the end-walls of the structure are not seen. Such illusions find
applications in the moving-picture studio where extensive interiors, great
fortresses, and even villages must be erected within small areas.
Incidentally the camera aids to create the illusion of magnitude in
photographs because it usually magnifies perspective, thereby causing
scenes to appear more extensive in the photographs than in the reality.

[Illustration: Fig. 88.--Some simple geometrical-optical illusions in
architecture.]

Balance in architecture is subject to illusions and might be considered an
illusion itself. For example, our judgment of balance is based largely
upon mechanical laws. A composition must appear to be stable; that is, a
large component such as a tower must not be situated too far from what we
take as a center of gravity, to appear capable of tipping the remainder
of the structure. In physics we would apply the term "moment." Each mass
may be multiplied by its distance from the center of gravity, thus
determining its moment. For a building or other composition to appear
stable the sum of these moments must be zero; that is, those tending to
turn the figure in one direction must be counterbalanced by those tending
to turn it in another direction. In appraising a composition, our
intellect summates the effects of different parts somewhat in this manner
and if satisfactory, balance is considered to have been attained. The
colors of the various components exert an influence in this respect, so it
is seen that illusions may have much to do with the satisfactoriness of
architectural compositions.

[Illustration: Fig. 89.--By decreasing the exposed length of shingles
toward the top a greater apparent expanse is obtained.]

Various illusions of height, of ceiling, composedness, etc., may be
obtained by the color of the ceiling. A dark cornice in an interior may
appear to be unsupported if the walls below are light in color, without
any apparent vertical supports for the cornice. We are then subjected to
the illusion of instability or incongruity. Dark beams of ceilings are not
so obtrusive because our intellect tells us that they are supports passing
over the top of the walls and are therefore able to support themselves.
Color and brightness in such cases are very important.

The architectural details on exteriors evolved under daylighting outdoors
so that their form has been determined by the shadows desired. The
architect leads his lights and shadows around the building modeling it as
he desires. An offset here and a depression there models the exterior in
light and shade. The forms must be powerful enough to resist the
obliterating effect of overcast skies but notwithstanding all precautions
the expression of an exterior varies considerably with nature's lighting.
Indoors the architect has a powerful controllable medium in artificial
light which he may draw upon for producing various expressions or moods in
rooms. The effect of shadows is interesting when viewing some structures
flood-lighted at night. In those cases where the light is directed upward
there is a reversal of shadows which is sometimes very unsatisfactory.

It is interesting to experiment with various ornamental objects lighted
from various directions. For example, a Corinthian capital lighted from
below may produce an unpleasant impression upon the observer. We do not
like to have the dominant light from below, perhaps because it is annoying
to the eyes. Possibly this is an instinct acquired by experience in
snow-fields or on the desert, or it may be a heritage of ancestral
experience gained under these glaring conditions. This dislike manifests
itself when we appraise shadow-effects and therefore our final impression
is tempered by it.

All sculptured objects depend for their appearance upon the lighting, and
they are greatly influenced by it. In sculpture, in a strict sense,
illusions play a lesser part than in other arts. Perhaps in those of very
large proportions various corrections have been applied. A minor detail of
interest is the small cavity in the eye, corresponding to a reversed
cornea. This depression catches a shadow which gives considerable
expression to the eye.

XIV

MIRROR MAGIC

Strictly speaking there are fewer illusions found in the practice of the
magician than is generally supposed; that is, the eye usually delivers
correctly to the intellect, but the judgment errs for various reasons. The
"illusion" is due to false assumptions, to the distracting words, to
unduly accented superfluous movements of the magician; or in general to
downright trickery. Much of the magician's success is due to glibness of
tongue and deftness of fingers, but many of the more notable "tricks" were
those involving the use of mirrors and the control of light. Black
curtains, blackened assistants, and controlled light have played prominent
parts in the older magic, but the principles of these are easily
understood. However, the mirror perhaps has done more to astound the
audience than any other device employed by the magician. For this reason,
and because its effects are commonly termed illusions, some representative
examples will be presented.

In a previous chapter attention was called to the simple but usually
overlooked fact that, for example, the image of a face in a mirror is
reversed as to right and left. When this fact is overlooked we may be
astonished at the changed expression of an intimate friend as we view the
face (reversed) in the mirror. Similarly our own features are reversed as
to right and left and we are acquainted with this reversed image rather
than the appearance of our face as it is. Inasmuch as faces are not
accurately symmetrical and many are quite unsymmetrical the effects of the
mirror are sometimes startling. It might be of interest for the reader to
study his face in the mirror and note that the right ear is the left ear
of the image which he sees. He will also find it of interest to compare
the face of a friend as viewed directly with the appearance of its image
in the mirror. If he desires to see himself as others see him, he can
arrange two mirrors vertically almost at a right-angle. By a little
research he will find an image of his own face, which is not reversed;
that is, an image whose right ear is really his right ear.

A famous "illusion" which astounded audiences was the sphinx illustrated
in Fig. 90. The box was placed upon a table and when opened there was
revealed a Sphinxian head, but why it was called a Sphinx is clothed in
mystery because upon some occasions it talked. As a matter of fact it
belonged to a body which extended downward from the table-top and this
kneeling human being was concealed from the audience by two very clean
plate-glass mirrors _M_ shown in the accompanying diagram. The table
actually appeared to have three legs but the audience if it noticed this
at all assumed the fourth leg was obscured by the foremost leg. The walls,
floor, and ceiling of the box-like recess in which the table was placed
were covered with the same material. It is seen by the diagram that the
mirrors _M_ reflected images of the side walls _W_ and these images were
taken by the audience to be portions of the rear wall _W_. Thus the table
appeared to be open underneath and the possibilities of the apparatus are
evident.

[Illustration: Fig. 90.--An example of a "mirror" illusion.]

The magician with a fine flow of language could dwell at length upon the
coming to life of the head of an ancient statue which he had in the box in
his hand. Walking to the table he could place the box over a trap-door and
by the time he had unlatched the door of the box, the assistant kneeling
under the table could have his head thrust upward through the trap-door of
the table-top into the box. After a few impressive words, supposed to be
Hindoo but in reality were Hoodoo, presto! and the Sphinx was revealed.
It conversed after a period of silence extending back to the days of
Rameses when a wrathful god condemned an unfortunate king to imprisonment
in the stone statue. The original trick awed audiences for many nights and
defied explanation until one night a keen observer noted finger-prints on
what proved to be a mirror. Doubtless a careless accomplice lost his job,
but the damage had been done, for the trick was revealed. This "illusion"
is so effective that it, or variations of it, are still in use.

[Illustration: Fig. 91.--Another example of "mirror magic."]

Another simple case is illustrated in Fig. 91. A large plate-glass mirror
_M_ was placed at an angle of approximately 45 degrees from the floor.
Through a hole in it an assistant's head and shoulders projected and the
edge of the opening was covered with a draped cloth. The audience saw the
image of the ceiling _C_ of the alcove reflected by the mirror but being
ignorant of the presence of the mirror, assumed this image to be the rear
wall. This trick was effective for many years. Obviously the mirrors must
be spotlessly clean and the illuminations of the walls, ceiling, and in
some cases, the floor must be very uniform. Furthermore, no large
conspicuous pattern could be used for lining the box-like recess.

The foregoing examples illustrate the principles involved in the
appearance of ghosts on the stage and of a skeleton or other gruesome
object in place of a human being. The possibilities of mirrors in such
fields are endless and they can be studied on a small scale by anyone
interested. The pseudoscope which produces effects opposite to those of
the stereoscope is an interesting device.

The foregoing is the faintest glimpse of the use of the mirror, but it
does not appear advisable to dwell further upon its use, for after all the
results are not visual illusions in the sense of the term as usually
employed throughout this book.

XV

CAMOUFLAGE

Illusions played many roles in the science and art of deception during the
World War, but they served most prominently in the later stages of the war
upon the sea. Inasmuch as the story of the science of camouflage is not
generally available, it appears worth while to present it briefly. Besides
being of interest, it will reveal to the reader the part that the science
of light, color, lighting, and vision played in deception. Furthermore,
the reader will sense the numberless illusions which are woven into
camouflage as developed in nature, and in human activities. The word
_camouflage_ by origin does not include all kinds of deception; however,
by extension it may and will here signify almost the entire art and
science of deception as found in nature and as practiced in the World War.

_Terrestrial Camouflage._--Camouflage is an art which is the natural
outgrowth of our instinct for concealment and deception when pitting our
wits against those of a crafty prey or enemy. It is an art older than the
human race, for its beginnings may be traced back to the obscurity of the
early ages of the evolution of animal life. The name was coined by the
French to apply to a definite art which developed during the Great War to
a high state, as many other arts developed by drawing deeply upon the
resources of scientific knowledge. With the introduction of this specific
word to cover a vast field of activity in scientifically concealing and
deceiving, many are led to believe that this is a new art, but such is not
the case. However, like many other arts, such as that of flying, the
exigencies of modern warfare have provided an impetus which has resulted
in a highly developed art.

Scientists have recognized for many years, and perhaps more or less
vaguely for centuries, that Nature exhibits wonderful examples of
concealment and deception. The survival of the fittest, as Darwin
expressed his doctrine, included those individuals of a species who were
best fitted by their markings and perhaps by peculiar habits to survive in
the environment in which they lived. Naturally, markings, habits, and
environment became more and more adapted to each other until the species
became in equilibrium with Nature sufficiently to insure its perpetuity.
If we look about us upon animal life we see on every hand examples of
concealing coloration and attitudes designed to deceive the prey or enemy.
The rabbit is mottled because Nature's infinite variety of highlights,
shadows, and hues demand variety in the markings of an animal if the
latter is to be securely hidden. Solid color does not exist in Nature's
landscapes in large areas. The rabbit is lighter underneath to compensate
for the lower intensity of illumination received on these portions. As
winter approaches, animals in rigorous climates need warmer coats, and the
hairs grow longer. In many cases the color of the hairs changes to gray or
white, providing a better coating for the winter environment.

Animals are known to mimic inanimate objects for the sake of safety. For
example, the bittern will stand rigid with its bill pointed skyward for
many minutes if it suspects an enemy. Non-poisonous snakes resemble
poisonous ones in general characteristics and get along in the world on
the reputation of their harmful relatives. The drone-bee has no sting, but
to the casual observer it is a bee and bees generally sting. Some animals
have very contrasting patterns which are conspicuous in shape, yet these
very features disguise the fact that they are animals. Close observation
of fishes in their natural environment provides striking examples of
concealing coloration. Vast works have been written on this subject by
scientists, so it will only be touched upon here.

There are many examples of "mobile" camouflage to be found in Nature.
Seasonal changes have been cited in a foregoing paragraph. The chameleon
changes its color from moment to moment. The flounder changes its color
and _pattern_ to suit its environment. It will even strive to imitate a
black and white checkerboard.

In looking at a bird, animal, insect, or other living thing it is
necessary to place it in its natural environment at least in the
imagination, before analyzing its coloration. For example, a male mallard
duck hanging in the market is a very gaudy object, but place it in the
pond among the weeds, the green leaves, the highlights, and the shadows,
and it is surprisingly inconspicuous. The zebra in the zoo appears to be
marked for the purpose of heralding its presence anywhere in the range of
vision, but in its reedy, bushy, grassy environment it is sufficiently
inconspicuous for the species to survive in Nature's continuous warfare.

Thus studies of Nature reveal the importance of general hue, the necessity
for broken color or pattern, the fact that black spots simulate shadows or
voids, the compensation for lower illumination by counter-shading, and
many other facts. The artist has aided in the development of camouflage,
but the definite and working basis of all branches of camouflage are the
laws and facts of light, color, and vision as the scientist knows them.

Just as lower animal life has unconsciously survived or evolved by being
fitted to do so, mankind has consciously, or at least instinctively,
applied camouflage of various kinds to fool his prey or his enemy. Many of
us in hunting ducks have concealed the bow of our sneak-boat with mud and
weeds, or in the season of floating ice, with a white cloth. In our quest
of water fowl we use decoys and grass suits. The Esquimau stalks his game
behind a piece of ice. In fact, on every hand we find evidences of this
natural instinct. The Indian painted his face and body in a variety of
colors and patterns. Did he do this merely to be hideous? It seems very
possible that the same instinct which made him the supreme master of
wood-craft caused him to reap some of the advantages of concealment due to
the painting of his face and body.

In past wars there is plenty of evidence that concealment and deception
were practiced to the full extent justifiable by the advantages or
necessity. In the World War the advent of the airplane placed the third
dimension in reconnaissance and called for the application of science in
the greatly extended necessity for concealment and deception. With the
advent of the airplane, aerial photography became a more important factor
than visual observation in much of the reconnaissance. This necessitated
that camouflage in order to be successful had to meet the requirements of
the photographic eye, as well as that of the human eye. In other words,
the special characteristics of the colors used had to be similar to those
of Nature's colors. For example, chlorophyl, the green coloring matter of
vegetation, is a peculiar green as compared with green pigments. When
examined with a spectroscope it is seen to reflect a band of deep red
light not reflected by ordinary pigments. In considering this aspect it is
well to bear in mind that the eye is a synthetic apparatus; that is does
not analyze color in a spectral sense. An artist who views color
subjectively and is rarely familiar with the spectral basis may match a
green leaf perfectly with a mixture of pigments. A photographic plate, a
visual filter, or a spectroscope will reveal a difference which the
unaided eye does not.

Some time before the Great War began, it occurred to the writer that
colored filters could be utilized in aiding vision by increasing the
contrast of the object to be viewed against its surroundings.[9] Studies
were made of various filters, made with the object of the experiment in
mind, in viewing the uniforms of various armies. Further developments were
made by applying the same principles to colored lights and painted
pictures. Many of these have been described elsewhere. With the
development of the science of camouflage, filters came into use for the
detection of camouflage. As a result of the demand for avoiding detection
by photographic plates and by various colored filters, some paints
provided for the camoufleur were developed according to the spectral
requirements. Many other applications of science were developed so that
camouflage can now be called an art based upon sound scientific
principles.

Natural lighting is so variable that it is often impossible to provide
camouflage which will remain satisfactory from day to day; therefore, a
broad knowledge of Nature's lighting is necessary in order to provide the
best compromise. There are two sources of light in the daytime, namely,
the sun and the sky. The relative amounts of light contributed by these
two sources is continually changing. The sky on cloudless days contributes
from one-tenth to one-third of the total light received by a horizontal
surface at noon. Light from the sky and light reflected from the
surroundings illuminate the shadows. These shadows are different in color
than highlights, although these finer distinctions may be ignored in most
camouflage because color becomes less conspicuous as the distance of
observation increases. In general, the distribution of brightness or light
and shade is the most important aspect to be considered.

The camoufleur worries over shadows more than any other aspect generally.
On overcast days camouflage is generally much more successful than on
sunny days. Obviously, counter-shading is resorted to in order to
eliminate shadows, and where this is unsuccessful confusion is resorted to
by making more shadows. The shape and orientation of a building is very
important to those charged with the problem of rendering it inconspicuous
to the enemy, but little attention has been paid to these aspects. For
example, a hangar painted a very satisfactory dull green will be
distinguishable by its shape as indicated by its shadow and shaded sides.
In this zone a hangar, for example, would be more readily concealed if its
length lay north and south. Its sides could be brought with a gradual
curve to the ground and its rear, which is during most of the day in
shadow, could be effectively treated to conceal the shadow. A little
thought will convince the reader of the importance of shape and
orientation.

Broken color or pattern is another fundamental of camouflage which, of
course, must be adapted to its environment. For our trucks, cannon, and
many other implements of war, dark green, yellow, dark blue, light gray,
and other colors have been used in a jumble of large patterns. A final
refinement is that of the blending of these colors at a distance, where
the eye no longer resolves the individual patches, to a color which
simulates the general hue of the surroundings. For example, red and green
patches at a distance blend to yellow; yellow and blue patches blend to a
neutral gray if suitably balanced, but if not, to a yellow-gray or a
blue-gray; red, green, and blue if properly balanced will blend to a gray;
black, white and green patches will blend to a green shade, and so on.
These facts are simple to those who are familiar with the science of
light and color, but the artist, whose knowledge is based upon the mixture
of pigments, sometimes errs in considering this aspect of color-blending
by distance. For example, it is not uncommon for him to state that at a
distance yellow and blue patches blend to make green, but the addition of
lights or of juxtaposed colors is quite different in result from the
addition of pigments by intimately mixing them.

In constructing such a pattern of various colors it is also desirable to
have the final mean brightness approximate that of the general
surroundings. This problem can be solved by means of the photometer and a
formula provided, which states, for example, that a certain percentage of
the total area be painted in gray, another percentage in green, and so on.
The photometer has played an important rôle in establishing the scientific
basis of camouflage. The size of the pattern must be governed by the
distance at which it is to be viewed, for obviously if too small the
effect is that of solid color, and if too large it will render the object
conspicuous, which is a disadvantage ranking next to recognizable.

Where the artist is concerned with a background which does not include the
sky, that is, where he deals only with _illuminated_ objects on the earth,
his trained eye is valuable provided the colors used meet the demands made
by photographic plates and visual color-filters. In other words, the sky
as a background gives trouble to all who are unfamiliar with scientific
measurements. The brightnesses of sky and clouds are outside the scale of
brightnesses ordinarily encountered in a landscape. Many interesting
instances of the artist's mistakes in dealing with these backgrounds could
be presented; however, the artist's trained eye has been a great aid in
constructing patterns and various other types of camouflage. One of the
most conspicuous aspects of the earth's surface is its texture. From great
heights it appears flat, that is, rolling land is ironed out and the
general contour of the ground is flattened. However, the element of
texture always remains. This is the chief reason for the extensive use of
netting on which dyed raffia, foliage, pieces of colored cloth, etc., are
tied. Such network has concealed many guns, headquarters, ammunition
dumps, communication trenches, roadways, etc. When this has been well done
the concealment is perfect.

One of the greatest annoyances to the camoufleur is the lack of dullness
or "flatness" of the paints, fabrics, and some of the other media used.
When viewed at some angles the glint of highlights due to specular
reflection renders the work very conspicuous. For this reason natural
foliage or such material as dyed raffia has been very successful.

Systems of network and vertical screens have been extensively employed on
roadways near the front, not for the purpose of concealing from the enemy
the fact that the roadways exist, but to make it necessary to shell the
entire roadway continually if it is hoped to prevent its use.

Although the camoufleur is provided with a vast amount of material for his
work, many of his requirements are met by the material at hand.
Obviously, the most convenient method of providing concealment for a
given environment is to use the materials of the environment. Hence,
rubbish from ruined buildings or villages supplies camouflage for guns,
huts, etc., in that environment. In woods the material to simulate the
woods is at hand. Many of these aspects are so obvious to the reader that
space will not be given to their consideration. The color of the soil is
important, for if it is conspicuous the camoufleur must provide screens of
natural turf.

In this great game of hocus-pocus many deceptions are resorted to.
Replicas of large guns and trenches are made; dummy soldiers are used to
foil the sniper and to make him reveal his location, and papier-maché
horses, trees, and other objects conceal snipers and observers and afford
listening posts. Gunners have been dressed in summer in green flowing
robes. In winter white robes have been utilized. How far away from modern
warriors are all the usual glitter and glamour of military impedimenta in
the past parades of peace time! The armies now dig in for concealment. The
artillery is no longer invisible behind yonder hill, for the eyes of the
aerial observer of the camera reveal its position unless camouflaged for
the third dimension.

In the foregoing only the highlights of a vast art have been viewed, but
the art is still vaster, for it extends into other fields. Sound must
sometimes be camouflaged and this can only be done by using the same
medium--sound. In these days of scientific warfare it is to be expected
that the positions of enemy guns would be detected by other means than
employed in the past. A notable method is the use of velocity of sound.
Records are made at various stations of the firing of a gun and the
explosion of the shell. By trigonometric laws the position of the gun is
ascertained. It is said that the Germans fired a number of guns
simultaneously with the "75-mile" gun in order to camouflage its location.
The airplane and submarine would gladly employ sound camouflage in order
to foil the sound detector if practicable solutions were proposed.

The foregoing is a brief statement of some of the fundamental principles
of land camouflage. Let us now briefly consider the eyes of the enemy. Of
course, much concealment and deception is devised to foil the observer who
is on the ground and fairly close. The procedure is obvious to the average
imagination; however, the reader may not be acquainted with the aerial
eyes from which concealment is very important. As one ascends in an
airplane to view a landscape he is impressed with the inadequacy of the
eyes to observe the vast number of details and of the mind to retain them.
Field glasses cannot be used as satisfactorily in an airplane as on solid
ground, owing to vibration and other movements. The difference is not as
great in the huge flying boats as it is in the ordinary airplane. The
camera can record many details with higher accuracy than the eye. At an
altitude of one mile the lens can be used at full aperture and thus very
short exposures are possible. This tends to avoid the difficulty due to
vibration. When the plates are developed for detail and enlargements are
made, many minute details are distinguishable. Furthermore, owing to the
fact that the spectral sensibilities of photographic emulsions differ from
that of the eye, contrasts are brought out which the eye would not see.
This applies also to camouflage which is devised merely to suit the eye.
Individual footprints have been distinguished on prints made from
negatives exposed at an altitude of 6000 feet. By means of photography,
daily records can be made if desired and these can be compared. A slight
change is readily noted by such comparison by skilled interpreters of
aerial photographs. The disappearance of a tree from a clump of trees may
arouse suspicion. Sometimes a wilted tree has been noted on a photograph
which naturally attracts attention to this position. It has been said that
the belligerents resorted to transplanting trees a short distance at a
time from day to day in order to provide clearance for newly placed guns.
By paths converging toward a certain point, it may be concluded from the
photographs that an ammunition dump or headquarters is located there even
though the position itself was well camouflaged. Continuous photographic
records may reveal disturbances of turf and lead to a more careful
inspection of the region for sapping operations, etc. By these few details
it is obvious that the airplane is responsible for much of the development
of camouflage on land, owing to the necessity which it created for a much
more extensive concealment. The entire story of land camouflage would
overflow the confines of a volume, but it is hoped that the foregoing will
aid the reader in visualizing the magnitude of the art and the scientific
basis upon which terrestial camouflage is founded.

_Marine Camouflage._--At the time of the Spanish-American war, our
battleships were painted white, apparently with little thought of
attaining low visibility. Later the so-called "battleship gray" was
adopted, but it has been apparent to close observers that this gray is in
general too dark. Apparently it is a mixture of black and white. The ships
of the British navy were at one time painted black, but preceding the
Great War their coats were of a warm dark gray. Germany adopted dark gray
before the close of the last century and Austria adopted the German gray
at the outbreak of the war. The French and Italian fleets were also
painted a warm gray. This development toward gray was the result of an aim
toward attaining low visibility. Other changes were necessitated by
submarine warfare which will be discussed later.

In the early days of unrestricted submarine warfare many schemes for
modifying the appearance of vessels were submitted. Many of these were
merely wild fancies with no established reasoning behind them. Here again
science came to the rescue and through research and consultation, finally
straightened out matters. The question of low visibility for vessels could
be thoroughly studied on a laboratory scale, because the seascape and
natural lighting conditions could be reproduced very closely. Even the
general weather conditions could be simulated, although, of course, the
experiments could be prosecuted outdoors with small models, as indeed
they were. Mr. L. A. Jones[10] carried out an investigation on the shore
of Lake Ontario, and laboratory experiments were conducted by others with
the result that much light was shed on the questions of marine camouflage.
This work confirmed the conclusion of the author and others that our
battleship gray was too dark. Of course, the color best adapted is that
which is the best compromise for the extreme variety in lighting and
weather conditions. These vary in different parts of the world, so
naturally those in the war zone were of primary importance. All camouflage
generally must aim to be a compromise best suited for average or
dominating conditions. For example, in foggy weather a certain paint may
render a ship of low visibility, but on a sunny day the ship might be
plainly visible. However, if ships are rendered of low visibility for even
a portion of the time it is obvious that an advantage has been gained.
Cloudiness increases generally from the equator northward, as indicated by
meteorological annals.

In order to study low visibility a scale of visibility must be
established, and it is essential to begin with the fundamentals of vision.
We distinguish objects by contrasts in brightness and in color and we
recognize objects by these contrasts which mold their forms. In researches
in vision it is customary to devise methods by which these contrasts can
be varied. This is done by increasing or decreasing a veil of luminosity
over the object and its surroundings and by other means. Much work has
been done in past years in studying the minimum perceptible contrast, and
it has been found to vary with hue, with the magnitude of brightness, and
with the size of the image, that is, with the distance of an object of
given size. In such problems as this one much scientific work can be drawn
upon. A simple, though rough, scale of visibility may be made by using a
series of photographic screens of different densities. A photographic
screen is slightly diffusing, still the object can be viewed through it
very well. Such methods have been employed by various investigators in the
study of visibility.

Owing to the curvature of the earth, the distance at which a vessel can be
seen on a clear day is limited by the height of the observer and of the
ship's superstructure. For an observer in a certain position the
visibility range varies as the square root of the distance of the object
from him. Such data are easily available, so they will not be given here.
So far we have considered the ship itself when, as a matter of fact, on
clear days the smoke cloud emitted by the ship is usually visible long
before a ship's superstructure appears on the horizon. This led to the
prevention of smoke by better combustion, by using smokeless fuels, etc.

The irregular skyline of a ship is perhaps one of the most influential
factors which tend to increase its visibility. Many suggestions pertaining
to the modification of the superstructure have been made, but these are
generally impracticable. False work suffers in heavy seas and high winds.

After adopting a suitable gray as, a "low-visibility" paint for ships,
perhaps the next refinement was counter shading; that is, shadows were
painted a lighter color, or even white. The superstructure was painted in
some cases a light blue, with the hope that it would fade into the distant
horizon. However, the effectiveness of the submarine demanded new
expedients because within its range of effectiveness no ingenuity could
render its intended prey invisible. The effective gun-fire from submarines
is several miles and torpedoes can be effective at these distances.
However, the submarine prefers to discharge the torpedo at ranges within a
mile. It is obvious that, in average weather, low visibility ceases to be
very effective against the submarine. The movement of a target is of much
less importance in the case of gun-fire than in the case of the torpedo
with its relatively low velocity. The submarine gunner must have the
range, speed, and course of the target in order to fire a torpedo with any
hope of a hit. Therefore, any uncertainties that could be introduced
pertaining to these factors would be to the advantage of the submarine's
prey. For example, low visibility gave way to confusibility in the
discussions of defence against the submarine and the slogan, "A miss is as
good as a mile" was adopted. The foregoing factors cannot be determined
ordinarily with high accuracy, so that it appeared possible to add
somewhat to the difficulties of the submarine commander.

Many optical illusions have been devised and studied by scientists. In
fact, some of these tricks are well known to the general reader. Straight
lines may appear broken, convergent, or divergent by providing certain
patterns or lines intermingled with them. Many of these were applied to
models in laboratory experiments and it has been shown that confusion
results as to the course of the vessel. The application of these on
vessels has resulted in the grotesque patterns to be seen on ships during
the latter stage of the war. It is well known that these illusions are
most effective when the greatest contrasts are used, hence black and white
patterns are common. Color has not been utilized as definitely as pattern
in confusibility, although there is a secondary aim of obtaining low
visibility at a great distance by properly balancing the black, white, and
other colors so that a blue-gray results at distances too great for the
individual patterns to be resolved by the eye. Color could be used for the
purpose of increasing the conclusion by apparently altering the
perspective. For example, blue and red patterns on the same surface do not
usually appear at the same distance, the red appearing closer than the
blue.

[Illustration: Fig. 92.--A primary stage in the evolution of the use of
geometrical-optical illusions on ships.]

Such apparently grotesque patterns aimed to distort the lines of the ship
and to warp the perspective by which the course is estimated. This was
the final type of marine camouflage at the close of the war. Besides
relying upon these illusions, ships zigzagged on being attacked and aimed
in other ways to confuse the enemy. No general attempt was made to
disguise the bow, because the bow-wave was generally visible. However,
attempts have been made to increase it apparently and even to provide one
at the stern. In fact, ingenuity was heavily drawn upon and many
expedients were tried.

After low-visibility was abandoned in favor of the optical illusion for
frustrating the torpedo-attack by the submarine, there was a period during
which merely a mottled pattern was used for vessels. Gradually this
evolved toward such patterns as shown in Fig. 92. In this illustration it
is seen that the optical-illusion idea has taken definite form. During the
period of uncertainty as to the course the pattern should take, a
regularity of pattern was tried, such as illustrated in Figs. 93 and 94.
Finally, when it dawned more or less simultaneously upon various
scientific men, who were studying the problems of protecting vessels upon
the seas, that the geometrical-optical illusion in its well-known forms
was directly adaptable, renewed impetus was given to investigation. The
scientific literature yielded many facts but the problems were also
studied directly by means of models. The latter study is illustrated by
Figs. 95 and 96, the originals having been furnished by Mr. E. L.
Warner,[11] who among others prosecuted a study of the application of
illusions to vessels. The final results were gratifying, as shown to some
extent in Figs. 97 and 98, also kindly furnished by Mr. Warner. It is
seen that these patterns are really deceiving as to the course of the
vessel.

[Illustration: Figs. 93 and 94.--Attempts at distortion of outline which
preceded the adoption of geometrical-optical illusions for ships.]

[Illustration: Figs. 95 and 96.--Illustrating the use of models by the
Navy Department in developing the geometrical-illusion for ships.]

The convoy system is well known to the reader. This saved many vessels
from destruction. Vessels of the same speed were grouped together and
steamed in flocks across the Atlantic. Anyone who has had the extreme
pleasure of looking down from an airplane upon these convoys led by
destroyers and attended by chasers is strongly impressed with the old
adage, "In unity there is strength."

Before the war began, a Brazilian battleship launched in this country was
provided with a system of blue lights for use when near the enemy at
night. Blue was adopted doubtless for its low range compared with light of
other colors. We know that the setting sun is red because the atmospheric
dust, smoke, and moisture have scattered and absorbed the blue and green
rays more than the red and yellow rays. In other words the penetrating
power of the red and yellow is greater than that of the blue rays. This
country made use of this expedient to some extent. Of course, all other
lights were extinguished and portholes were closed in ocean travel during
the submarine menace.

[Illustration: Figs. 97 and 98.--Examples of the geometrical-optical
illusion as finally applied.]

Naturally smoke-screens were adopted as a defensive measure on sea as well
as on land. Destroyers belch dense smoke from their stacks in order to
screen battleships. Many types of smoke-boxes have been devised or
suggested. The smoke from these is produced chemically and the apparatus
must be simple and safe. If a merchantman were attacked by a submarine
immediately smoke-boxes would be dumped overboard or some which were
installed on deck would be put into operation and the ship would be
steered in a zigzag course. These expedients were likely to render
shell-fire and observations inaccurate. This mode of defense is obviously
best suited to unarmed vessels. In the use of smoke-boxes the direction
and velocity of the wind must be considered. The writer is unacquainted
with any attempts made to camouflage submarines under water, but that this
can be done is evident from aerial observations. When looking over the
water from a point not far above it, as on a pier, we are unable to see
into the water except at points near us where our direction of vision is
not very oblique to the surface of the water. The brightness of the
surface of water is due to mirrored sky and clouds ordinarily. For a
perfectly smooth surface of water, the reflection factor is 2 per cent for
perpendicular incidence. This increases only slightly as the obliquity
increases to an angle of about 60 degrees. From this point the
reflection-factor of the surface rapidly increases, becoming 100 per cent
at 90 degrees incidence. This accounts for the ease with which we can see
into the water from a position directly overhead and hence the airplane
has been an effective hunter of submerged submarines. The depth at which
an object can be seen in water depends, of course, upon its clarity. It
may be surprising to many to learn that the brightness of water, such as
rivers, bays, and oceans, as viewed perpendicularly to its surface, is
largely due to light diffused within it. This point became strikingly
evident during the progress of work in aerial photometry.

A submerged submarine may be invisible for two reasons: (1) It may be deep
enough to be effectively veiled by the luminosity of the mass of water
above it (including the surface brightness) or, (2) It may be of the
proper brightness and color to simulate the brightness and color of the
water. It is obvious that if it were white it would have to attain
concealment by submerging deeply. If it were a fairly dark greenish-blue
it would be invisible at very small depths. In fact, it would be of very
low visibility just below the surface of the water. By the use of the
writer's data on hues and reflection-factors of earth and water areas it
would be easy to camouflage submarines effectively from enemies overhead.
The visibility of submarines is well exemplified by viewing large fish
such as sharks from airships at low altitudes. They appear as miniature
submarines dark gray or almost black amid greenish-blue surroundings.
Incidentally, the color of water varies considerably from the dirty
yellowish-green of shallow inland waters containing much suspended matter
to the greenish-blue of deep clear ocean waters. The latter as viewed
vertically are about one-half the brightness of the former under the same
conditions and are decidedly bluer.

_The Visibility of Airplanes._--In the Great War the airplane made its
début in warfare and in a short time made a wonderful record, yet when
hostilities ceased aerial camouflage had not been put on a scientific
basis. No nation had developed this general aspect of camouflage
systematically or to an extent comparable with the developments on land
and sea. One of the chief difficulties was that scientific data which were
applicable were lacking. During the author's activities as Chairman of the
Committee on Camouflage of the National Research Council he completed an
extensive investigation[12] of the fundamentals upon which the attainment
of low visibility for airplanes must be based. Solutions of the problems
encountered in rendering airplanes of low visibility resulted and various
recommendations were made, but the experiences and data will be drawn upon
here only in a general way. In this general review details would consume
too much space, for the intention has been to present a broad view of the
subject of camouflage.

The visibility of airplanes presents some of the most interesting problems
to be found in the development of the scientific basis for camouflage. The
general problem may be subdivided according to the type of airplane, its
field of operation, and its activity. For example, patrol craft which fly
low over our own lines would primarily be camouflaged for low visibility
as viewed by enemies above. (See Fig. 99.) High-flying craft would be
rendered of low visibility as viewed primarily by the enemy below.
Airplanes for night use present other problems and the visibility of
seaplanes is a distinct problem, owing to the fact that the important
background is the water, because seaplanes are not ordinarily high-flying
craft. In all these considerations it will be noted that the activity of
the airplanes is of primary importance, because it determines the lines
of procedure in rendering the craft of low visibility. This aspect is too
complicated to discuss thoroughly in a brief résumé.

[Illustration: Fig. 99.--Representative earth backgrounds for an airplane
(uncamouflaged) as viewed from above.]

The same fundamentals of light, color, and vision apply in this field as
in other fields of camouflage, but different data are required. When
viewing aircraft from above, the earth is the background of most
importance. Cumulus clouds on sunny days are generally at altitudes of
4000 to 7000 feet. Clouds are not always present and besides they are of
such a different order of brightness from that of the earth that they
cannot be considered in camouflage designed for low visibility from
above. In other words, the compromise in this case is to accept the earth
as a background and to work on this basis. We are confronted with seasonal
changes of landscape, but inasmuch as the summer landscape was of greatest
importance generally, it was the dominating factor in considering low
visibility from above.

On looking down upon the earth one is impressed with the definite types of
areas such as cultivated fields, woods, barren ground and water. Different
landscapes contain these areas in various proportions, which fact must be
considered. Many thousand determinations of reflection-factor and of
approximate hue were made for these types of areas, and upon the mean
values camouflage for low visibility as viewed from above was developed. A
few values are given in the accompanying table, but a more comprehensive
presentation will be found elsewhere.[12]

_Mean Reflection-Factors_

(From thousands of measurements made by viewing vertically downward during
summer and fall from various altitudes.)

                                            Per Cent

  Woods                                        4.3
  Barren ground                               13.0
  Fields (grazing land and growing crops)      6.8
  Inland water (rivers and bays)               6.8
  Deep ocean water                             3.5
  Dense clouds                                78.0

Wooded areas are the darkest general areas in a landscape and possess a
very low reflection-factor. From above one sees the deep shadows
interspersed among the highlights. These shadows and the trapping of
light are largely responsible for the low brightness or apparent
reflection-factor. This is best illustrated by means of black velvet. If a
piece of cardboard is dyed with the same black dye as that used to dye the
velvet, it will diffusely reflect 2 or 3 per cent of the incident light,
but the black velvet will reflect no more than 0.5 per cent. The velvet
fibers provide many light traps and cast many shadows which reduce the
relative brightness or reflection-factor far below that of the flat
cardboard. Cultivated fields on which there are growing crops are nearly
twice as bright as wooded areas, depending, of course, upon the denseness
of the vegetation. Barren sunbaked lands are generally the brightest large
areas in a landscape, the brightness depending upon the character of the
soil. Wet soil is darker than dry soil, owing to the fact that the pores
are filled with water, thus reducing the reflection-factor of the small
particles of soil. A dry white blotting paper which reflects 75 per cent
of the incident light will reflect only about 55 per cent when wet.

Inland waters which contain much suspended matter are about as bright as
grazing land and cultivated fields. Shallow water partakes somewhat of the
color and brightness of the bed, and deep ocean water is somewhat darker
than wooded areas. Quiet stagnant pools or small lakes are sometimes
exceedingly dark; in fact, they appear like pools of ink, owing to the
fact that their brightness as viewed vertically is almost entirely due to
surface reflection. If it is due entirely to reflection at the surface,
the brightness will be about 2 per cent of the brightness of the zenith
sky. That is, when viewing such a body of water vertically one sees an
image of the zenith sky reduced in brightness to about 2 per cent.

The earth patterns were extensively studied with the result that definite
conclusions were formulated pertaining to the best patterns to be used.
Although it is out of the question to present a detailed discussion of
this important phase in this résumé, attention will be called to the
manner in which the earth patterns diminish with increasing altitude. The
insert in Fig. 100 shows the actual size of an image of a 50-foot airplane
from 0 to 16,000 feet below the observer as compared with corresponding
images (to the same scale) of objects and areas on the earth's surface
10,000 feet below the observer.

For simplicity assume a camera lens to have a focal length equal to 10
inches, then the length _x_ of the image of an object 100 feet long will
be related to the altitude _h_ in this manner:

   _x_      100
  ----- =  -----  or _xh_ = 1000
    10      _h_

By substituting the values of altitude _h_ in the equation the values of
the length _x_ of the image are found. The following values illustrate the
change in size of the image with altitude:

  Altitude _h_ in feet            Size of image _x_ in inches

         1,000                                1.00
         2,000                                0.50
         3,000                                0.33
         4,000                                0.25
        10,000                                0.10
        20,000                                0.05

It is seen that the image diminishes less rapidly in size as the altitude
increases. For example, going from 1000 feet to 2000 feet the image is
reduced to one-half. The same reduction takes place in ascending from
10,000 to 20,000 feet. By taking a series of photographs and knowing the
reduction-factor of the lens it is a simple matter to study pattern. An
airplane of known dimensions can be placed in the imagination at any
altitude on a photograph taken at a known altitude and the futility of
certain patterns and the advantages of others are at once evident.

[Illustration: Fig. 100.--Illustrating the study of pattern for airplanes.
The photograph was taken from an altitude of 10,000 feet. The insert shows
the relative lengths (vertical scale) of an airplane of 50-foot spread at
various distances below the observer.]

It is impracticable to present colored illustrations in this résumé and
values expressed in numbers are meaningless to most persons, so a few
general remarks will be made in closing the discussion of low visibility
as viewed from above in spring, summer and fall. A black craft is of much
lower visibility than a white one. White should not be used. The paints
should be very dark shades. The hues are approximately the same for the
earth areas as seen at the earth's surface. Inland waters are a dirty
blue-green or bluish-green, and deep ocean water is a greenish-blue when
viewed vertically, or nearly so. Mean hues of these were determined
approximately.

Before considering other aspects of camouflage it is well to consider such
features as haze, clouds and sky. There appear to be two kinds of haze
which the writer will arbitrarily call earth and high haze, respectively.
The former consists chiefly of dust and smoke and usually extends to an
altitude of about one mile, although it occasionally extends much higher.
Its upper limit is very distinct, as seen by the "false" horizon. This
horizon is used more by the pilot when flying at certain altitudes than
the true horizon. At the top of this haze cumulus clouds are commonly seen
to be poking out like nearly submerged icebergs. The upper haze appears
somewhat whiter in color and appears to extend sometimes to altitudes of
several or even many miles. The fact that the "earth" haze may be seen to
end usually at about 5000 to 6000 feet and the upper haze to persist even
beyond 20,000 feet has led the author to apply different names for
convenience. The upper limit of the "earth" haze is determined by the
height of diurnal atmospheric convection. Haze aids in lowering the
visibility of airplanes by providing a luminous veil, but it also operates
at some altitudes to increase the brightness of the sky, which is the
background in this case.

The sky generally decreases considerably in brightness as the observer
ascends. The brightness of the sky is due to scattered light, that is, to
light being reflected by particles of dust, smoke, thinly diffused clouds,
etc. By making a series of measurements of the brightness of the zenith
sky for various altitudes, the altitude where the earth haze ends is
usually plainly distinguishable. Many observations of this character were
accumulated. In some extreme cases the sky was found to be only one-tenth
as bright when observed at high altitudes of 15,000 to 20,000 feet as seen
from the earth's surface. This accounts partly for the decrease in the
visibility of an airplane as it ascends. At 20,000 feet the sky was found
to contribute as little as 4 per cent of the total light on a horizontal
plane and the extreme harshness of the lighting is very noticeable when
the upper sky is cloudless and clear.

Doubtless, it has been commonly noted that airplanes are generally very
dark objects as viewed from below against the sky. Even when painted white
they are usually much darker than the sky. As they ascend the sky above
them becomes darker, although to the observer on the ground the sky
remains constant in brightness. However, in ascending, the airplane is
leaving below it more and more luminous haze which acts as a veil in
aiding to screen it until, when it reaches a high altitude, the
combination of dark sky behind it and luminous haze between it and the
observer on the ground, it becomes of much lower visibility. Another
factor which contributes somewhat is its diminishing size as viewed from a
fixed position at the earth. The minimum perceptible contrast becomes
larger as the size of the contrasting patch diminishes.

Inasmuch as there is not enough light reflected upward from the earth to
illuminate the lower side of an opaque surface sufficiently to make it as
bright as the sky ordinarily, excepting at very high altitudes for very
clear skies, it is necessary, in order to attain low visibility for
airplanes as viewed from below, to supply some additional illumination to
the lower surfaces. Computations have shown that artificial lighting is
impracticable, but measurements on undoped airplane fabrics indicate that
on sunny days a sufficient brightness can be obtained from direct sunlight
diffused by the fabric to increase the brightness to the order of
magnitude of the brightness of the sky. On overcast days an airplane will
nearly always appear very much darker than the sky. That is, the
brightness of the lower sides can in no other manner be made equal to that
of the sky. However, low visibility can be obtained on sunny days which is
an advantage over high visibility at all times, as is the case with
airplanes now in use. Many observations and computations of these and
other factors have been made, so that it is possible to predict results.
Transparent media have obvious advantages, but no satisfactory ones are
available at present.

Having considered low visibility of aircraft as viewed from above and from
below, respectively, it is of interest to discuss briefly the possibility
of attaining both of these simultaneously with a given airplane. Frankly,
it is not practicable to do this. An airplane to be of low visibility
against the earth background must be painted or dyed very dark shades of
appropriate color and pattern. This renders it almost opaque and it will
be a very dark object when viewed against the sky. If the lower surfaces
of the airplane are painted as white as possible the airplane still
remains a dark object against the blue sky and a very dark object against
an overcast sky, except at high altitudes. In the latter cases the
contrast is not as great as already explained. A practicable method of
decreasing the visibility of airplanes at present as viewed from below is
to increase the brightness by the diffuse transmission of direct sun-light
on clear days. On overcast days clouds and haze must be depended upon to
screen the craft.

In considering these aspects it is well to recall that the two sources of
light are the sun and the sky. Assuming the sun to contribute 80 per cent
of the total light which reaches the upper side of an opaque horizontal
diffusing surface at midday at the earth and assuming the sky to be
cloudless and uniform in brightness, then the brightness of the horizontal
upper surface will equal 5 _RB_, where _R_ is the reflection-factor of the
surface and _B_ is the brightness (different in the two cases) of the sky.
On a uniformly overcast day the brightness of the surface would be equal
to _RB_. Now assuming _R{e}_ to be the mean reflection-factor of the
earth, then the lower side of a horizontal opaque surface suspended in the
air would receive light in proportion to _R{e}B_. If this lower surface
were a perfect mirror or a perfectly reflecting and diffusing surface its
brightness would equal 5 _R{e}B_ on the sunny day and _R{e}B_ on the
overcast day where _B_ is the value (different in the two cases) of the
brightness of the uniform sky. The surface can never be a perfect
reflector, so on an overcast day its brightness will be a fraction
(_RR{e}_) of the brightness _B_ of the uniform sky. Inasmuch as _R{e}_
is a very small value it is seen that low visibility of airplanes as
viewed from below generally cannot be attained on an overcast day. It can
be approached on a sunny day and even realized by adopting the expedient
already mentioned. Further computations are to be found elsewhere.[12]

Seasonable changes present no difficulties, for from a practical
standpoint only summer and winter need be generally considered. If the
earth is covered with snow an airplane covered completely with white or
gray paint would be fairly satisfactory as viewed from above, and if a
certain shade of a blue tint be applied to the lower surfaces, low
visibility as viewed from below would result. The white paint would
possess a reflection-factor about equal to that of snow, thus providing
low visibility from above. Inasmuch as the reflection-factor of snow is
very high, the white lower sides of an airplane would receive a great deal
more light in winter than they would in summer. Obviously, a blue tint is
necessary for low visibility against the sky, but color has not been
primarily considered in the preceding paragraphs because the chief
difficulty in achieving low visibility from below lies in obtaining
brightness of the proper order of magnitude. In winter the barren ground
would be approximately of the same color and reflection-factor as in
summer, so it would not be difficult to take this into consideration.

Seaplanes whose backgrounds generally consist of water would be painted of
the color and brightness of water with perhaps a slight mottling. The
color would generally be a very dark shade, approximating blue-green in
hue.

Aircraft for night use would be treated in the same manner as aircraft for
day use, if the moonlight is to be considered a dominant factor. This is
one of the cases where the judgment must be based on actual experience. It
appears that the great enemy of night raiders is the searchlight. If this
is true the obvious expedient is to paint the craft a dull jet black.
Experiments indicate that it is more difficult to pick up a black craft
than a gray or white one and also it is more difficult to hold it in the
beam of the searchlight. This can be readily proved by the use of black,
gray, and white cards in the beam of an automobile head-light. The white
card can be seen in the outskirts of the beam where the gray or black
cannot be seen, and the gray can be picked up where the black one is
invisible. The science of vision accounts for this as it does for many
other questions which arise in the consideration of camouflage or low
visibility.

Some attempts have been made to apply the principle of confusibility to
airplanes as finally developed for vessels to circumvent the submarine,
but the folly of this appears to be evident. Air battles are conducted at
terrific speeds and with skillful maneuvering. Triggers are pulled without
computations and the whole activity is almost lightning-like. To expect to
confuse an opponent as to the course and position of the airplane is
folly.

The camouflage of observation balloons has not been developed, though
experiments were being considered in this direction as the war closed.
Inasmuch as they are low-altitude crafts it appears that they would be
best camouflaged for the earth as a background. Their enemies pounce down
upon them from the sky so that low visibility from above seems to be the
better choice.

In the foregoing it has been aimed to give the reader the general
underlying principles of camouflage and low visibility, but at best this
is only a résumé. In the following references will be found more extensive
discussions of various phases of the subject.

REFERENCES

1. A Study of Zöllner's Figures and Other Related Figures, J. Jastrow,
Amer. Jour. of Psych. 1891, 4, p. 381.

2. A Study of Geometrical Illusions, C. H. Judd, Psych. Rev. 1899, 6, p.
241.

3. Visual Illusions of Depth, H. A. Carr, Psych. Rev. 1909, 16, p. 219.

4. Irradiation of Light, F. P. Boswell, Psych. Bul. 1905, 2, p. 200.

5. Retiring and Advancing Colors, M. Luckiesh, Amer. Jour. Psych. 1918,
29, p. 182.

6. The Language of Color, 1918, M. Luckiesh.

7. Apparent Form of the Dome of the Sky, Ann. d. Physik, 1918, 55, p. 387;
Sci. Abs. 1918, No. 1147.

8. Course on Optics, 1738, Robert Smith.

9. Color and Its Applications, 1915 and 1921; Light and Shade and Their
Applications, 1916, M. Luckiesh.

10. Report of The Submarine Defense Association, L. T. Bates and L. A.
Jones.

11. Marine Camouflage Design, E. L. Warner, Trans. I. E. S. 1919, 14, p.
215.

12. The Visibility of Airplanes, M. Luckiesh, Jour. Frank. Inst. March and
April, 1919; also Aerial Photometry, Astrophys. Jour. 1919, 49, p. 108.

13. Jour. Amer. Opt. Soc., E. Karrer, 1921.

The foregoing are only a few references indicated in the text. Hundreds of
references are available and obviously it is impracticable to include such
a list. The most fruitful sources of references are the general works on
psychology. E. B. Titchener's Experimental Psychology (vol. 1) contains an
excellent list. A chapter on Space in William James' Principles of
Psychology (vol. II) will be found of interest to those who wish to delve
deeper into visual perception. Other general references are Elements of
Physiological Psychology by Ladd and Woodworth; the works of Helmholtz; a
contribution by Hering in Hermann's Handb. d. Phys. Bk. III, part 1;
Physiological Psychology by Wundt; E. B. Delabarre, Amer. Jour. Psych.
1898, 9, p. 573; W. Wundt, Täuschungen, p. 157 and Philos. Stud. 1898, 14,
p. 1; T. Lipps, Raumaesthetik and Zeit. f. Psych. 1896, 12, 39.

INDEX

  Aberration, 19
    spherical, 122
    chromatic, 135

  Aerial perspective, 165, 183

  After-images, 24, 25, 59, 128, 186
    positive and negative, 129

  Airplanes, visibility of, 233
    camouflage for different types, 234
    size of image at various altitudes, 238
    camouflage for various conditions, 240

  Alhazen, 8

  Angles, influence of, 76
    various effects of, 81
    contours and, 87
    apparent effect on length, 91
    theories, 98

  Animals, protective coloration, 211

  Architecture, 195
    balance in, 201

  Arcs, circular, illusions due to, 86

  Areas, juxtaposed, illusions of, 96

  Artist, 179

  Attention, fluctuation of, 65, 106, 141, 164

  Aubert, 49

  Auerbach's indirect vision theory, 100

  Aureole, 178

  Balance in architecture, 201

  Bas-relief, 143

  Battleships, 222

  Binocular disparity, 105

  Binocular vision, 29, 31

  Blending of colors in camouflage, 216

  Blind spot, 21

  Blue light on war-vessels, 230

  Boswell, varieties of irradiation, 122

  Brightness,
    illusions due to
      variations in, 107
      and color contrasts, 114
      apparatus, 115
      and hue, 125
      sky, 241

  Brucke's theory, 37

  Brunot's mean distance theory, 101

  Camouflage, 210
    terrestrial, 210
    detection of, 215
    marine, 222
    airplane, 234

  Carr, observations on distance illusions, 108

  Chromatic aberration, 19, 135

  Chlorophyl, 214

  Circle, 11
    arcs of, illusion, 86
    contracting and expanding illusion, 98

  Clouds, 235

  Color, 124
    after-images, 128
    blending in camouflage, 216
    contrasts and brightness, 114
    growth and decay of sensation, 131
    illusions of, 156
    retiring and advancing, 138
    saturation, 154
    sensibility of retina, 138
    warm and cold, 158

  Confusability, 226

  Confusion theory of angular illusions, 100

  Contour, illusions of, 52
    and angles, illusions, 87

  Contracting and expanding circles, illusion of, 98

  Contrasts, illusions of, 53
    simultaneous, 124, 154
    apparatus for, 115, 125
    color, 114, 154, 188
    brightness, 114

  Convergence, illusions of, 108, 191

  Cord, twisted, illusion, 88

  Daylight, artificial, 189

  Decoration, painting and, 179

  Decorator, 188

  Dember, 167

  Depth and distance, illusions, 102

  Direction, illusions of
    Zöllner's, 76
    Wundt's, 79
    Hering's, 80

  Disk, Mason, 132

  Distance and depth, illusions, 102
    and size, 35, 104, 166

  Distance illusions, Carr's observations, 108

  Double images, 37

  Dynamic theory of angular illusions, 99

  Enlargement of sun and moon at horizon, apparent, 169

  Equivocal figures, 64

  Euclid, 8

  Extent, interrupted, illusions of, 48

  External image, 15, 17, 34

  Eye, physiology, 13
    position, 30
    adjustments, 33
    defects, 19

  Fatigue, 128

  Field, visual, effect of location in, 44

  Figures, equivocal, 64

  Filters, color, 214

  Fluctuation of attention, 65, 106, 141, 164

  Focusing, 14

  Form of sky, apparent, 166

  Fovea centralis, 22, 23, 139

  Frames, picture, effect of wood grain, 190

  Geometrical illusions, 44

  Glare, 119

  Grain of wood, apparent distortions due to, 190

  Grecian art, 196

  Growth and decay of color sensation, 131

  Guttman, 175

  Hallucination, 4, 72

  Halo, 178

  Haze, illusions, etc., 103, 166, 183
    earth and high, 240

  Helmholtz, 13, 74

  Hering, 74
    illusion of direction, 80

  Hue and brightness, 125

  Illusions, geometrical, 44
    equivocal figures, 64
    influence of angles, 76
    of depth and distance, 102
    irradiation and brightness contrast, 114
    color, 124
    light and shadows, 144
    in nature, 164
    in painting and decoration, 179
    mirror, 205
    camouflage, 210

  Image
    after-, 24, 25, 59, 128, 129, 186
    double, 37
    external, 15, 17, 34
    retinal, inversion of, 16
    of airplane, size at various altitudes, 238

  Indirect vision theory, Auerbach's, 100

  Intaglio, 143

  Interrupted extent, illusions of, 48

  Iris, 15

  Irradiation, 120
    and brightness contrast, 114
    varieties of (Boswell), 122
    in architecture, 199

  James, 170

  Jastrow, 80

  Jones, L. A., 223

  Judd, 86, 93

  Judgment, 1

  Karrer, 160

  Kepler, 8

  Light, effect of spectral character, 189

  Lighting, illusions of depth and distance due to, 102
    contrasts, 154
    diffusion, effect of, 145
    direction, effect of, 144, 151
    ending of searchlight beam, 160
    warm and cold colors, 158

  Lipps, 10, 11
    dynamic theory of, 99

  Location in visual field, effect, 44

  Mean distance theory, Brunot's, 101

  Mechanical, esthetic unity, 11

  Magician, 3

  Magic, mirror, 205

  Marine camouflage, 222

  Mason disk, 132

  Mirage, 3, 176

  Mirror magic, 205

  Miscellaneous color effects, 140

  Moon, apparent size at horizon, 8, 36, 169
    theories of, 173
    apparent radius of crescent, 121

  Müller-Lyer illusion, 53, 93

  National Research Council, Committee on Camouflage, 234

  Nature, 164

  Necker, 74

  Oppel, 9

  Painting and decoration, 179

  Painter, 2, 179, 186

  Parallax, 105

  Parthenon of Athens, 196

  Persistence of vision, 131

  Perspective, 58
    in architecture, 198
    aerial 165, 183
    theory, 98

  Photographer, 152

  Photography, use in detection of camouflage, 220

  Photometer, 156, 217

  Pigments, 184

  Poggendorff illusion, 85

  Protective coloration, animals, 211

  Psychology, 2, 6, 157
    of light, 193

  Purkinje phenomenon, 26, 139

  Reflection-factors, 236

  Retina, 14, 20
    inertia, 130
    color sensibility, 138

  Retinal rivalry, 140

  Retiring and advancing colors, 136

  Reversal of mirror image, 205

  Rods and cones, 21

  Schröder's staircase, 70

  Sculpture, 204

  Searchlight beam, ending of, 160

  Sensation, color, growth and decay, 131

  Sense, 1

  Shading, counter, for vessels, 224

  Shadows, importance in camouflage, 215

  Size and distance, 35, 36
    illusions of, 104, 166

  Sky
    apparent form of, 166
    brightness, 241

  Skylight and sunlight, relative proportions of, 215, 243

  Smith, Robert, 173

  Smoke-screens, 230

  Spectral character of light, 189

  Sphere, illusions, 145, 150, 151

  Spherical aberration, 19

  Sphinx illusion, 206

  Spiral illusions, 90

  Spraying, paint, 187

  Stereoscope, 39, 142

  Stereoscopic vision, 38, 41, 141

  Submarines, 225
    camouflage for, 232

  Sun, apparent enlargement at horizon, 169

  Sunlight and skylight, relative proportions in nature, 215, 243

  Terrestrial camouflage, 210

  Theory of influence of angles, 98
    perspective, 98
    dynamic, 99
    confusion, 100
    indirect vision, 100
    mean distance, 101

  Thiéry's figure, 71

  Thiéry's perspective theory, 99

  Transparencies, 185

  Twisted cord illusions, 88

  Uibe, 167

  Vertical vs. horizontal distances, 11, 36, 46

  Visibility, low, for vessels, 222
    of airplanes, 233

  Vision, 29
    persistence of, 131
    stereoscopic, 38

  Visual perception, 32, 33

  Warm and cold colors, 158

  Warner, E. L., 227

  Wheatstone, 37

  Wood grain, illusions caused by, 191

  World War, 213

  Wundt, 10, 11, 32, 74
    illusion of direction, 79

  Yellow spot, 139

  Zöllner's illusion, 67, 76

  Zoth, 175

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Transcriber's Notes:

Passages in italics are indicated by _italics_.

Subscripted characters are indicated by {subscript}.

The following misprints have been corrected:
  "imgaes" corrected to "images" (page 128)
  "bove" corrected to "above" (page 177)
  "verticle" corrected to "vertical" (page 239)
  "colo" corrected to "color" (Index)

Other than the corrections listed above, inconsistencies in spelling and
hyphenation have been retained from the original.

The inverted "8" and "S" characters at the top of page 45 cannot be
properly represented in this text version.