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index of 1.4, it is flexible, and it is the alteration of shape of this lens from fat to thin by muscles
within the eye which allows us to focus on nearby or distant objects. For distant objects the
front surface of the lens has a radius of curvature of about 10 mm, the back surface 6 mm. When
focusing on nearby objects, the front surface can bulge to less than 6 mm radius. Light is focused
through the vitreous humour onto the retina, which is composed of light sensitive cells. Nerves
go back from these cells into the eyeball, and emerge through the blind spot, from whence they
run to the brain. Curiously, light must pass through these nerves in order to reach the sensitive
cells. No one would design a television camera so that the light had to pass through the wires
to reach the sensitive surface, but the inside-out nature of our visual process arises as a feature
of the evolutionary process.
The opening of the lens is determined by the iris diaphragm, controlled by a set of involuntary
muscles. Look into a mirror, and suddenly shine a bright line into your eye. The iris diaphragm
closes within a fraction of a second, reducing the size of the pupil and the amount of light entering
the eye. It takes much longer for the iris to relax on entering a darkened room. With the iris
contracted, as on a bright sunny day, if we look at a white sheet of paper, we can see the shadows
cast on the retina of bits of muck, called ‘musca volentes’ floating in the vitreous humour.
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Figure 26. Molecular structure of the retinene group of the rhodopsin pigment. The structure is twisted as shown in the top
portion in the dark. Absorption of a light quantum causes the structure to straighten out.
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