Nightvision

                               a TêC production

                                  by Morpheus
                            With help from FM-21-75




You Can See at Night
--------------------
   You can see much more in the dark than you realize.  However, to take
maximum advantage of this ability, you must understand how your eyes are
constructed and how to use them to see best under conditions of poor
visibility.

Your Eyes Are Like a Camera
---------------------------

   Certain parts of your eyes compare to a simple camera.

a. The lens focuses light entering the eye just as does the lens of a camera.

b. The iris corresponds to the diaphragm of a camera, opening and closing to
regulate the amount of light entering the pupil.

c. The retina corresponds to the camera film.  Light rays strike the retina,
form an image, and cause an impression to be transmitted to the brain through
the optic nerve.  The brain tells us what we see.  In a camera, the image is
stored and fixed on film.

Day and Night Eyes
------------------

   The cone area is composed of cone cells and rod cells, so-called because of
their shapes.


a. Cone cells enable you to see color, shape, and sharp contrast.  A great
deal of light is required to activate them and they are blind during periods
of low illumination.  For this reason, they are your day eyes.  The cone cells
are concentrated in the cone region, directly behind the lens, and decrease in
number with distance from the center of the cone region.

b. Rod cells produce a chemical substance called visual purple which makes
them active in darkness or periods of low illumination.  They are your night
eyes.  Rod vision enables you to distinguish black, white, and shades of grey
and to distinguish general outlines.  Most of the rod cells are in the area
of the retina around the cone region.  A few are in the cone region.

Seeing at Night
---------------

   Using your eyes effectively at night requires the application of the
principles of night vision - dark adaptation, off-center vision, and scanning.

a. Dark Adaptation means allowing your eyes to become accustomed to low levels
of illumination.  It takes about 30 minutes for the rod cells to produce
enough visual purple to enable you to distinguish objects in dim light. This
may be accomplished by staying in a red-lighted area, or by wearing red
goggles for 20 minutes, followed by 10 minutes in darkness (to allow pupils to
open wide).

b. Off-center vision is the technique of keeping your attention focused on an
object without looking directly at it.  When you look directly at an object,
the image is focused on the cone region, which is not active at night.  When
you look slightly to the left, right, above, or below an object, the image is
formed in the area of the retina containing rod cells, which are sensitive in
the darkness.  The most sensitive area in individuals varies, but usually is
found by looking 6ø to 10ø away from an object.  In effect, you look out of
the corner of your eye.

c. Scanning is using off-center vision to observe an area or an object.  When
you use rod vision, the visual purple in the rod cells bleaches or blacks out
in 4 to 10 seconds, and the object observed disappears.  As the visual purple
in one area bleaches out, you must shift your eyes slightly so fresh rod cells
are being used.  Move your eyes in short, abrupt, irregular movements over and
around your target, but do not look directly at it.  Pause a few seconds at
each point of observation because your eyes cannot see while in motion.

Factors Affecting Night Vision
------------------------------

   Visual purple is chemically related to vitamin A, and a serious lack of
vitamin A impairs your night vision.  However, excessive amounts of vitamin A
will not necessarily improve your night vision.  Colds, headaches, fatigue,
narcotics, heavy smoking, and excessive use of alcohol reduce your ability to
see at night.  Exposure to bright light for extended periods impairs both day
and night vision.

Preserving Night Vision
-----------------------

   Night vision is quickly destroyed if bright light is allowed to enter the
eye.  If this cannot be avoided, such as when you must enter a lighted area
or observe in a temporarily lighted area, close and cover one eye to preserve
the night vision in that eye.  When the light goes away, the night vision
retained in your protected eye enables you to see until the other eye becomes
adapted to the darkness.

Confidence
----------

   Confidence is very important. You usually use your eyes where there is
plenty of light, so you are used to sharp outlines and bright colors.  In
darkness, objects are faint, have no sharp outlines, and have little or no
color.  You must believe what your eyes tell you.  Gain confidence by faithful
practice in using the principles of night vision.

Movement
--------

   Usually you must move more quietly in the night than in the day. Here are
some general rules to help you --

a. Move around thick undergrowth, dense woods, and ravines. Your field of
observation is reduced and it is difficult to move quietly.
b. Move as quickly as circumstances allow, but avoid running if possible. You
may fall or make unnecessary noise.


Have Phun..


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