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well, giving a difference between
seeing and perceiving, discussed below.
This is because the eye's optical
quality is actually very poor (in fact,
you would get better results from a
pinhole camera), hence the need for
the brain, which can actually modify
what you see, based on experience,
and so is reliant on expectations. If
the brain fills in the gaps wrongly,
you get visual illusions.
The eye can, however react quickly
to changes in light conditions,
although it is slower to adapt from
light to dark because a chemical
(visual purple) needs to be created.
The optic nerve carries signals from
the eye to the brain, the lens focuses
light on to the retina, the iris controls
the diameter of the pupil (the black
bit in the middle where light gets
through), and the cornea refracts light
onto the lens. The area of sharp
vision is actually very small (at 4 feet
the size of a small coin), because of
the relatively small size of the fovea.
5°away from the foveal axis, it
reduces by a quarter, and onetwentieth
at 20° away. You should
be able to see another aircraft
directly at 7 miles, or 2.5 miles if it
was 45° off – at 60° it's down to half
a mile! The reason why you are
taught to scan is because the eye
needs to latch on to something,
difficult to do with a clear blue sky.
With an empty field of vision, your
eyes will focus at short distances,
about 1-2 metres ahead, and miss
objects further away.
Peripheral vision also has a different
neurological route to the brain. To
prove this, stand up, cover one eye
and make a fist with the other, to
cover up the good eye's central
vision. Now stand on one foot. Now
do it again, with your forefinger and
thumb making a very small circle
where the fist was – in other words,
you are using your central vision to
look through the circle. Standing on
one foot is now more difficult – and
this is what we look at our
instruments with!
Accommodation is the change in
refraction of light as it enters the eye,
240 JAR Private Pilot Studies
caused by altering the curvature of
the lens.
The major causes of defective vision
are:
·  Hypermetropia – where the
eyeball is too short, and images
focus behind the retina (log
sight). Requires a convex lens.
·  Myopia – where the eyeball is
too long, and images focus in
front of the retina (short sight).
Needs a concave lens.
·  Presbyopia – the lens hardens,
leading to hypermetropia
(comes with old age).
·  Cataracts – the lens becomes
opaque.
·  Glaucoma – pressure inside the
eyeball.
·  Astigmatism – unequal curvature
of the cornea or lens.
Laser surgery, by the way, only
corrects one aspect of vision, that is,
long or short sight, but not both.
Rods and Cones
The retina consists of light
sensors (actually, neurons)
which are called rods and cones,
because of the way they are
shaped. Each is more efficient
than the other in different kinds
of light. The point where the
optic nerve joins the retina is
mostly populated with cones,
which work best in daylight and
become less effective at night,
or where oxygen levels are
reduced (which is significant for
smokers, whose blood has less
oxygen carrying capacity), so
you get a blind spot in the direct
field of vision, which is why you
see things more clearly at night
if you look slightly to the side of
what you want to look at. Rods
cannot distinguish colours,
either, which is why things at
night seem to be in varying
shades of grey (you see colours
simply because the vibrations
they give out are strong enough
to wake the cones up).
The more your iris is open, the
less depth of field you have, so in
darkness it is difficult to see
beyond or before a certain
distance, and you may require
glasses to help (the depth of
field in photography is an area
between the camera and the
subject in which everything is in
focus. The wider the aperture,
or iris, the shorter this distance
is, and vice versa).
The retina contains enormous
amounts of vitamin A, which is
necessary for adapting between
light and darkness. Too little
vitamin A could therefore result
in night blindness. The changeover
from light to dark takes about
　
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