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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
20-30 minutes and should
always be allowed for when
night flying (actually, the cones
take 7 minutes, and the retina
can take up to 45 minutes).
The greatest visual acuity (that
is, the ability to see small detail)
is obtained by cones in the fovea,
in the central part of the retina,
so you must look directly at an
object to see it best. At night,
look slightly to one side,
because the rods that are
sensitive to lower levels are
outside the fovea, at the
peripheral of the retina.
With distance, objects on the
retina become smaller.
Human Factors 405
Refraction
The transparent part of the
sclerotic is known as the cornea,
behind which is the lens, whose
purpose is to bend light rays
inwards, so they focus on the
retina. If this happens in front
of it, short sightedness results. You
get long sightedness with the
point of focus behind the retina.
Both conditions cause blurred
vision, correctable by glasses,
that vary the refraction of the
light waves until they focus in
the proper place.
Blurred vision can also be
caused by stress, causing
nervous tension, and excessive
eye muscle activity leading to
eyestrain. Just relaxing often
helps this condition.
70% of light is refracted by the
cornea, and 35% by the lens.
Optical illusions
Searching for an object in a
swimming pool is difficult,
because the light rays bend as
they pass the surface and the
object appears to be displaced.
Similarly, rain on a windscreen
at night gives the impression
that objects are further away. A
good fixed wing example of an
optical illusion is a wider
runway tending to make you
think the ground is nearer than
it actually is; a narrow runway
delays your reactions, possibly
leading to a late flare and early
touchdown. Any object (like a
runway) that you think is
smaller than it actually is, will
also appear to be nearer, and
vice versa. If the object is
brighter than its surroundings (a
well-lit runway), you will think
you are higher than you are, so
on an approach, you might start
early and be lower than you
should. In haze, objects appear
to be further away.
An approach to a downsloping
runway should be started
higher, with a steeper angle, and
one to an upsloping runway
should be started lower, at a
shallower angle.
When mountain flying, it's often
difficult to fly straight and level
because the sloping ground
around affects your judgment.
Similarly, you can't judge your
height when landing on a peak.
Even going to the cinema is an
optical illusion; still frames are
shown so quickly it looks as if
movement is taking place—the
switching is done in the brain.
Vectional illusions are caused by
movement, as when sitting in a
railway carriage and wondering
whether it's the train next to you
or the one you're in that is
moving. The autokinetic effect is
the illusion that an object is
moving, where it is actually your
eye that is moving. Distant
objects become less colourful
and less distinct.
A high speed aircraft
approaching head on will grow
the most in size very rapidly in
the last moments, so it's
possible for an approaching
aircraft to be hidden by a bug
on the windscreen for a high
proportion of its approach time
(you might only see it in the last
few seconds). Lack of relative
406 Canadian Professional Pilot Studies
movement makes an object
more difficult to detect.
Ears
These are important because an
auditory stimulus is the one most
often attended to. How many times
do you answer the phone when
you're busy, even though you've
ignored everything else for hours?
Sound waves make the eardrum
　
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