Our sense organs 45

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not mean that we can see colours – it only provides
the necessary preconditions.
The sensation of colour only arises in the brain
after a computational comparison of the excitation
of the three types of cones. There are about
100 million optic cells in the retina, but only one
million optic nerve fibres lead from them. This
means that many optic cells are interconnected in
a complex fashion. The optic nerve transmits
image information to various parts of the brain in
the form of electrical pulses. A small number of
fibres lead directly to the mid-brain, but most of
them converge on a switchboard which serves the
primary vision centre in the rear of the brain.
The images formed on both retinas are upside
down and also left-right inverted. But an
astounding fact is that the optic nerves from
both eyes split up and cross each other in
such a way that the left halves of the
images of both eyes are received by the
right half of the brain, and the right
halves end up in the left hemisphere of
the brain. Each half of the observer’s brain
receives information from only one half of the
image. In addition, these images are distorted,
because the region around the yellow spot (the
fovea – where we see best; the Latin word for a
hollow) forms an image which is ten times as
large as that of the peripheral area. The left side
of the brain only observes the left half of the
image ( = the right half of what we are looking
at) and this half is the right way up with the distortion
removed. At the same time the right side
of the brain deals with the other half of the field
of view.
Note that, although the brain processes the different
parts of the image in various remote locations,
the two halves of the field of vision
are seamlessly re-united, without any trace of
a joint – amazing! This process is still far from
being fully understood.
Hermann von Helmholtz (1821 – 1894), a famous
physicist and physiologist of the 19th century,
comparing the error count of eye imaging with
that of a lens, concluded as follows in 1863:
Structure of the retina
Rods
Horizontal cells
Bipolar cells
Amacrine cells
Ganglia
(singular = ganglion)
Direction of incident
light
Glial (supporting) cells
Cones
“If an optician sold me an instrument having the
errors exhibited by the eye, it would be in order
for me to express my dissatisfaction with the
quality of his work in the strongest terms, and
return his instrument forthwith.”
Helmholtz was wrong, since he only measured
the performance of the lens of the eye in comparison
with the light path in optical instruments.
But he forgot that no technologically produced
lens system can function faultlessly for the
length of a human lifespan. Neither is it protected
against heat and cold, dryness and humidity,
shocks and dust, nor can it repair itself in the
event of minor damage. Which optical instrument
available at that time could adjust itself auto-
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