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-mache
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
debut 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 resume.
[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.