In the field of astronomy one often hears about
telescopes. It is, however, perhaps rather less frequent in this
context to hear about the eye itself
The eye is the optical instrument that most of us are endowed with
all our lives, from birth to the grave. Only a minority of people
own, or could even afford, a large astronomical telescope; but the majority
of us, in whatever society we live, possess eyes. Optical telescopes
have been around for only a few hundred years, yet the field of astronomy
and observations was known to ancient people in many parts of the world.
The Babylonians could predict eclipses; the five naked-eye planets
were well known to ancient peoples, and there are ancient records of such
things as novae, to name just a few examples. Copernicus never possessed
a telescope. The Supernova of 1054 which gave birth to the Crab Nebula
is well recorded, as are the one in Cassiopeia in 1572, known as Tycho's
star, and the one in Ophiucus in 1604, known as Kepler's star - believed
to have been observed only a short time before Galileo turned his telescopes
to the sky and made his great discoveries.
I shall here attempt to give a brief look at the workings of the eye.
Figure 1 is a rough section of the human eye. Optically, it works
very like a camera or telescope.. The cornea and the lens combine to
focus rays of light on the retina, which is the sensitive layer, converting
light photons into electrical nerve signals, which are sent via the optic
nerve to the visual parts of the brain. The iris regulates the amount
of light getting in, contracting the pupil in response to bright light and
expanding it when the light is dim. An iris which is heavily pigmented
looks brown, whereas one which is less pigmented generally looks blue or
In the normal human eye the cornea at the front possesses most of the
focusing power, leaving only a relatively small part for the inner lens
to play. The latter, however, is believed to perform most of the change
in focussing power necessary for focussing objects at differing distances
onto the retina (by a process known as accommodation, in which the curvature
of the anerior surces of the lens changes in response to muscular action).
In other creatures the structure of the eye often differs from the
human eye, depending on the creature's habitat. The cat, for example,
has an eye specially developed to enable it to see in dim light. In
aquatic animals, e.g. fish, the cornea, being of almost the same refractive
index as the surrounding water, does not contribute much towards the focussing
of light onto the retina; and thus may be shaped so as to conform well
with the general streamlining of the animal. In these creatures the
inner lens must perform virtually all the focussing; and the lens is often
thick. Some animals which do not rely too heavily on sight have eyes
which are small and often flattened. This way the eyes do not take
up too much space in the head. The cameleon is unusual in that it
is believed to have an inner lens of negative power.
This may enable the animal to have a large retinal image and thus
good visual acuity, without the very-long eyeball which would normally have
to go with it. This negative-powered inner lens acts in a similar
manner to the telephoto lens of some cameras, or the negative-powered Barlow
lens in some telescopes, so as effectively to increase the focal length
of the objective without requiring an exceptionally long telescope tube
to go with it.
The retina itself is shown schematically in Figure 2. There are basically
two types of receptor cell, rods and cones. The former react to low
light levels and thus give sensitive night vision, whereas the latter give
good visual accuity and colour vision where light conditions are more favourable.
The receptor cells respond to light by sending signals to bipolar
cells, which in turn activate ganglion cells, signals then being sent along
the optic nerve to the brain. Horizontal cells and amacrine cells modify
the signals in various ways, depending on the activity of adjacent receptors.
In good light, the signals sent along the optic nerve to the brain are
believed in large measure to represent contrast rather than the state of illumination
of each receptor cell taken in isolation. This way, form vision is
enhanced, by strong awareness of outlines. Another thing that one
generally tends to notice easily is changes in illumination, as well as
movement of an image across the retina.
The rods and cones are by no means uniformly distributed on the human
retina. In and around the fovea (Figure 1) there are few or no rods
(which is why one can sometimes see a very dim star more easily if one looks
slightly away from it rather than directly at it). The fovea is where
cones are most densly populated, and where the photopic or daytime vision
is at its most acute. Away from the fovea cones become more scarse,
and visual acuity more degraded; but rods become more numerous. The
total number of recepters (rods and cones) generally exceeds the number of
fibres in the optic nerve which take the signal to the brain.
As with the optical adaptions of the eye, so the retina is not the
same in all species. Animals which live chiefly at night have retinas
which consist chiefly of rods, whereas diurnal animals tend to have retinas
with more cones. Some creatures do not have notably good visual acuity,
but never the less are able to detect motion easily. Some birds seem
to have one or more involutions or projections (called pectens) into the
vitreous chamber of their eye. Whilst it is not certain exactly what
functions these serve, they may assist vision under certain circumstances
by casting (moving) shadows on the retina. Some animals, e.g. the
cat, have reflective layers in their retinas, in the pigmented epithelium.
This is why a cat's eyes sometimes appear to glow in the dark. It
should be noted that, unlike the retinas of octopods (whose eyes are similar
in many respects to ours), our retinas are inverted. This means that
the receptive layer of the retina is the layer nearest the outer coat of
the eye, and light has to travel through the other retinal cells to get to
it. This, however, possibly assists in the nutrition of the recepters
cells, which in warm-blooded creatures such as ourselves, are very energy-hungry.
One or two minor modification to the article 13 September 2018.
HTML Validator added 27 March 2005.