Our sense organs 45

The eye
– our window to the outside
We read in the Bible that “The eye never has
enough of seeing” (Eccl 1:8). The eye is actually
one of our most important sense organs, since
well over half of all the information we take in
about our surroundings comes to us by way of
our eyes. Through being able to perceive incident
light, we can read letters, newspapers, and books;
marvel at the colours of a blossom, the perspectives
of a landscape, the beauty of a dress, or
the artistic appeal of a painting. But especially,
we can see our loved ones and others whom we
encounter in our daily lives. The German word for
face is Gesicht which means the same as vision
or sight. The English (and French) word visage,
also meaning face, similarly refers to seeing
(from the Latin videre = to see).
Physiologically speaking, seventy percent of all
our sense receptors are located in the eyes. In
reality, we evaluate and understand our world
mostly from being able to see it. That’s why,
despite all their differences, all human languages
are rich in visual imagery. Figures of speech and
proverbs, though conveying abstract meanings,
are often easily visualised, like: “Up to one’s neck
in debt”; “Carrying your heart on your sleeve”;
“A rolling stone gathers no moss”.
In the Bible, the Creator commanded on the very
first day: “Let there be light!” Our visual sense
was thus provided for right from the beginning.
When He reviewed His creative works, we read
five times: “And God saw that it was good.” In
reviewing all He had made at the end of the six
creation days, we again find His evaluation based
on vision: “And God saw all that he had made,
and it was very good” (Gen 1:31).
Having established the importance of vision, we
now turn to the actual organ of sight.
General features of the eye: Visible light comprises
electromagnetic radiation with wavelengths
between 400 (violet) and 750 (red) nanometres
(1 nm = 10 -9 m = one millionth of a millimetre).
For the purpose of forming an image, the incident
light rays must be bent (refracted) and focused
sharply on the retina. The cornea handles most of
the refraction and the lens subsequently focuses
images at various distances by varying its curvature.
Through this ingeniously devised ability to
change its shape, the focal length of the lens can
vary between 69.9 mm and 40.4 mm. This is why,
unlike the best products of the optical industry,
we can manage with only one lens.
The iris acts like the diaphragm of a camera.
There are two opposing sets of muscles which
regulate the size of the aperture (the pupil)
according to the brightness of the light. The
shape of the eye is maintained by the vitreous
body, and the pressure in a fluid called the aqueous
humour which fills the anterior and posterior
chambers. This pressure depends on a balance
between the production of this fluid and its outflow.
The cornea is lubricated, and protected
against drying out, by the tear ducts and the
movements of the eyelids.
Of all our sense organs, our eyes have the greatest
range of detection sensitivity, as well as the
greatest adaptability. They have their own machinery
of movement, through special muscles
which enable vision to be directed towards a target.
The two-dimensional image on the retina
requires massive parallel processing in the subsequent
network of nerve fibres.
Structure of the eye: The eye can be divided
functionally into two parts, namely the physical
dioptric mechanism (Greek: dioptra = something
through which one looks) which handles incident
light, and the receptor area of the retina where
the light triggers processes in nerve cells. The
dioptric mechanism produces a miniaturised,
upside down image. To obtain a sharp image
requires an exact “fine tuning” between the refractory
(light-bending) properties of the optical
medium and the dimensions of the eye. A deviation
in the latter as small as 0.1 mm is enough
to cause faulty vision, requiring correction by
spectacles.
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