Modern war is often fought at night. This means that men must
learn to see in the dark--or at least to use their eyes in new
and unfamiliar ways.
This article is written to tell you how to make the
best use of your eyes at night. It will help you, whether
your job is in an airplane or a tank, on a ship, or
driving a truck, or just getting about on your own feet.
It will not give you the uncanny eyes of an owl or a
cat, but it may give you just the edge on the enemy you
need to get in the first shot--and to get home.
You already know that when you go into a dark room
from a bright one, it is hard to see until your eyes have
become used to the gloom. At a movie it takes a minute
or two to see the vacant seat. It may take another
minute or two to be able to recognize a friend. During
these minutes your eyes become more sensitive to
the faint light.
2. ADJUSTING FOR DARKNESS
Your eyes adjust in two ways for seeing in the dark. One
way is by opening up to let in more light or to
make maximum use of what little light there is. This
works in the same way as a camera diaphragm, which
can be opened up wide for taking pictures in dim light.
Your eye pupils open wide in dim light and close to a
pin-point opening when the light is very bright.
But this is not the most important change in the way
your eye works in dim lighting.
You actually have two kinds of sight. Your day eyes
use one kind of vision cells known as "cones." They
are principally located in the very center of the eye.
Your night eyes use an entirely different kind of
cells, the rod cells, which are mostly around the outside
edge of the eye.
The rod cells used by your night eyes are color blind. That
is why "all cats look gray at night." If you see a colored
light shining at night, and it looks red or
green or blue, it is only because it is bright enough so
that you can see it with your daylight eyes.
But your night vision is much more sensitive to light
of some colors than to others. Red is seen equally well
by night and day vision. Blue light, however, affects
your night eyes 1,000 times as much as it does your
day eyes. For this reason it is extremely dangerous to
use blue lights in a blackout because it affects
the enemy's eyes just as much as it does yours.
Night eyes lack the sharp vision for detail that your
day eyes have. If you want to see to read, if you want
to watch the dial of an instrument, if you must look at
a map, a road sign, or your watch, then you must use
your day vision. For this you must have good light--the more
the better. Especially if the print or other
forms are small, the light must be bright.
Night eyes are extraordinarily sensitive to faint light. This
is shown by calculations that an ordinary candle
flame could be seen at a distance of more than 100 miles
if the night were completely black and if haze, dust, and
the curvature of the earth did not interfere. A
lighted match is about as bright as a candle flame. Under
ordinary night conditions, a match can be seen
from a plane for many miles away.
Night vision is not in use as soon as you step into the
dark. It takes time--a half hour or more--before your
eyes are completely adapted to the dark. When you
leave a sunny street to go into a darkened theater, or step
from a brightly-lighted room into the dark outdoors, you
are completely blind at first.
Then several things happen. First the pupil of your
eye dilates, letting more light into your eyes. This is
a mechanical action.
Next the cones of your day vision adapt to the darkness. This
takes about 5 minutes, and after that you feel more comfortable
about moving around in the pitch dark.
After a much longer time, your rod vision adapts itself
to the darkness and you can begin to see shapes
and outlines in the gloom that were not even vague
bulking shadows when you first came in.
Just how this change-over from cone to rod cells is
accomplished is not completely understood, but it is at
least partly a chemical process.
The soldier who, at a command or an alert signal, leaves a lighted
room to run on duty without having
prepared his eyes is completely at the mercy of the
enemy insofar as his vision is concerned. By the time
he gains the use of his night eyes, the emergency may
be all over.
And even when your eyes are adapted to the dark, flashing
on a light, even for a very short time, may ruin
your night vision for another half hour. You can lose
in a few minutes all you gained by half an hour in the
dark. The brighter the light and the longer you look
at it, the more you lose.
3. GETTING YOUR EYES READY
Complete darkness is the best preparation for night
fighting. It pays to protect your eyes from light before
you start and while you are out. If you can't stay
in darkness, keep the lights around you as low as possible
and don't look straight at them. If it is necessary
to look at any lighted object, be as quick as you
can about it. Experiments have shown that looking
at an instrument dial lighted only by radium paint will
cut down the distance at which you can see a friendly
or an enemy plane by 50 percent. Don't look at the
dial any longer than you must or the loss will be greater.
Experienced gun pointers and spotters know that
they must not watch the flash of their guns as they
fire. The flash of a 6-inch gun may dull the eyes for
a minute or more. Under continuous fire at dawn or
dusk it is impossible to aim some rapid-fire guns accurately
at a target more than 7 times a minute if the
gunners watch the flash. At night the blinding effect
would be even greater. Looking away from the flash
gives almost complete protection. Luckily the flash
of rifles and small-caliber machine guns has much less
effect on the eyes.
There are several ways by which one can become
dark-adapted or maintain dark-adaptation, even
though working in a fairly bright light. Each method
is suitable for certain types of jobs, and each has its
limitations and dangers. A patch worn over one eye
will keep this eye ready for night duty at any time, but
vision from one eye alone is not as accurate as
binocular (two-eye) vision, especially in judging distances
of nearby objects. An individual may work in
red light, or wear close-fitting red goggles, either of
which are effective since red light has little effect on
the rod cells and leaves one ready for nearly instant
action in the dark. It is wisest to consult a medical
officer concerning the necessity for such preparation, and
the methods best suited for the task at hand.
4. USING YOUR EYES PROPERLY
Always remember that you must look a little to one
side in order to see best on a very dark night. Learn
to pay attention to things which are just a little off to
the side. Learn to keep from looking directly at any
object. As you feel your eyes drawn irresistibly toward
what you want to see, just let them slide on over it to the
other side and look again with the tail of your eye. It
takes practice to learn to do this without fail, but it
is worth the trouble to learn the trick.
And don't keep looking steadily to the same side of
an object. This will make it disappear, too.
Try it out yourself and see how your eyes at night
can play "parlor magic" tricks on you.
When in your darkened room or outdoors, hold up
your finger and look steadily at it. It will disappear. Look a little
to one side and it will appear again. But if you
keep staring at this side it will soon be gone again. Move
your eyes to the other side and back and it will reappear.
This means that in searching the sea or the sky for a dark
object, you must look at first one area and then
another. When you think you have spotted something, keep
looking first on one side of the object and then
at the other, or above and below it.
But don't try to sweep your eyes over the sky or the
horizon--you can't see well while the eyes are moving. "Scan" the
sky, don't sweep over it. Night eyes are
slow in responding. At night a faint object may not
be recognizable until after you have looked near it a
number of times. If you have ever hunted quail in
the morning or watched deer in the dusk, you know that
you can look right at such a camouflaged object for a
while before you notice it. In darkness such an object
is even harder to pick out because you won't see it at
all if you stare. You have to look again and again at
points near it.
5. CONTRAST HELPS NIGHT VISION
Another thing that affects our vision at night is the
contrast between an object and its background. If the
thing observed is very different from its background, it
is much more easily seen. An airplane may be clear if
you look up at it against the night sky; but invisible if
you look down on it against the dark ground. A ship
may show up clearly against a star-lit sky, but fade
into the background if you are looking at it against a
background of dark water.
If light from the moon is reflected onto the under
side of an airplane from white clouds below, the plane
may become almost invisible from any angle.
To notice small differences in contrast, it is essential
to have clear vision. It is for this reason that windshields
must be kept clean and free of scratches or fog. These
tend to scatter light in all directions and reduce
contrast. Careless night fighters have been known to
tolerate enough dirt on their windshields to double the
time it takes to see a plane moving along near by. And
sailors on ships sometimes let the salt from spray pile
up in blotches on the glass. This is courting death.
For the same reason it is important to keep down the
lights on your side of a windshield. Any light on your
side reduces the contrast because stray light spreads
over the whole glass and reflects in your eyes. That is
why you push up close to a window when you try to look
out at night. By coming up close, you shade part of the
glass and increase the contrast of the objects seen
through this part. If it is necessary to have any light
on your side, keep it as dim as you can and screen it
from the glass. This also helps your adaptation to
There has been a good deal of talk about the effect of
shortages of vitamins A and C on ability to see at night. These
are the vitamins in fresh vegetables, cheese, and
fruit. People who don't get enough of these vitamins
do become poor in night vision, but regular Army and
Navy rations supply plenty of these vitamins. Occasionally
when boats are on long trips or when fighting
lasts until fresh foods are all gone, a shortage of vitamins
may occur. In these cases medical officers will supply
men who are likely to be on night duty with
vitamin capsules. Extra vitamins don't improve night
vision if your diet or your night vision is already normal.
Night vision is affected by fatigue. Anything that
reduces your physical well-being has a greater effect on
night vision than on day vision. Experiments have
shown that hangovers, slight illnesses, or excessive
fatigue may double or even triple the amount of light
needed to see an object/ The night fighter must train
for his job as a boxer trains for a big match. The boxer
who is not at the peak of training is likely to be knocked
out. The night fighter whose eyes are not at the peak of
efficiency is likely to be killed.
7. REMEMBER THESE THINGS
a. Protect your eyes from light before you go on night duty and while you are out.
b. Don't look directly at any light or illuminated object. If you must, be quick about it.
c. Use the corners of your eyes. Night targets are more clearly seen
when you don't look directly at them.
d. Keep your eyes moving. Quick, jerky movements and short pauses are better
than long, sweeping movements and long pauses.
e. Keep your windshield spotless and free of scratches and fog.
f. Keep yourself wide awake and on the alert. Don't break training. Use
good sense about eating, drinking, and smoking.
g. Practice what you know about seeing at night until
it becomes second nature to use your eyes to the best
advantage. Use every possible device to aid you in
learning to recognize ships, planes, and other important
objects from slight cues.