Eye to eye
Question: Why do some animals have a noncircular pupil? Cats and some snakes
have a vertical shape yet horses and goats have a horizontal one. What is the
reason behind the differences and, more importantly, how do the different shapes
affect how animals actually see things?
Not enough is known about what different animals see to provide a final
answer but we can guess that slit pupils do provide advantages in the range of
light adaptation and the depth of field they provide. The distribution and type
of receptors on the retina will be just as important in truly understanding what
an animal perceives鈥擡d
Answer: Slit pupils are often found among nocturnal animals, such as the cat.
These creatures need to be able to open their pupils very wide to let in as much
light as possible when they are operating in darkness. However, because of their
very sensitive retinas, they need to be able to make the pupil very small indeed
when they are out during the day.
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Such a wide range of adjustment is hard to achieve with a circular pupil
because of the amount of tissue stretching involved.
For example, a ring of tissue small enough to form a pinhole pupil of 0.5
millimetres across would have to stretch by a factor of 10 to form a pupil 5
millimetres across. A neat solution to this engineering problem is to use a slit
pupil. Simple geometry shows that a completely shut slit pupil 5 millimetres
long can turn into a circular aperture 5 millimetres across simply by stretching
its edges by a factor of only 1.6 (see Diagram).
Ben Craven
Menstrie, Clackmannanshire
This argument holds true only if circular pupils are ringed by muscle, a
point confirmed by the next author鈥擡d
Answer: The human pupil is closed by the 鈥渋ris sphincter鈥� muscle which is
ring-shaped and lines the circular edge of the pupil. When this contracts, the
pupil closes.
When the sphincter ceases to contract, the pupil opens, partly due to the
counteracting elastic network of blood vessels that radiate away from the pupil.
The pupil is also actively opened by another set of cells, comprising the
dilator.
Eric Warrant
Zoology Department
University of Lund, Sweden
Answer: A small iris opening lets in less light, but due to the pinhole
effect it also improves focus (resolution). With a circular iris, the focus is
maintained in both dimensions of the field of view. A slit iris will tend to
improve focus in the direction perpendicular to the slit.
Cats and snakes are carnivores designed to hunt small creatures moving across
a horizontal plane (the ground), so a vertical slit gives them the best view of
a running mouse, say.
Goats and similar animals are herbivorous and so are vulnerable to larger
carnivores while they are grazing with their heads close to the ground. When
their heads are bent down, the 鈥渉orizontal鈥� slit becomes vertical, giving them
the best possible view of any predators creeping up on them.
Fred Parkinson
Sandiacre, Nottinghamshire
Answer: Human pupils close while remaining circular, allowing the image to
fall upon the receptor-rich centre of the retina.
Most nocturnal grazing animals such as deer close their pupil as a horizontal
shutter. Their photoreceptors are not heavily concentrated in a central fovea
like ours, but are spread horizontally across the retina in a band called the
macula. This allows them to maintain the same panoramic peripheral vision over a
wide range of light levels. It is important that they can detect predators in
all levels of light.
Daniel Cutting
Orangeburg, South Carolina
Commuter conductor
Question: As I waited for a train yesterday, I noticed that my umbrella,
resting on a damp patch of the platform, tingled as I touched it. The tingling
stopped when I lifted the umbrella. Was this some kind of current induced by the
nearby overhead cables? Is it harmful?
Answer: The high voltage overhead lines carrying power to the engine have a
strong electric field around them, which falls off with the inverse of the
distance from the conductor. Any two isolated, conducting objects at the same
distance from the overhead line are at the same voltage, and if they touch no
current flows between them. If you, the traveller, a conducting object, touch
some other conducting object at a different distance from the power line, you
have different voltages, and a small, harmless current will flow between you and
the other object.
I have played with this effect many times while waiting for a train to
arrive. There is a metal signpost embedded in the platform at my station,
soundly earthed. If I stand between it and the overhead line, and touch the
post, I can feel a strong tingling as the current flows from me (wearing
insulating shoes), at a higher voltage than the post, into the post. If I reach
out an arm to bring part of my body closer to the power line, the tingling
increases. If I stand behind the post so that it is between me and the line,
there is no tingling sensation鈥攁ll the electric field heads for the
earthed post, and my body is shielded from it.
I was so fascinated to discover this that I didn鈥檛 mind the train being late.
Incidentally, the currents are small and harmless, because there is no
conducting link with the overhead line.
John Elliott
Bramhall, Stockport
This week鈥檚 question
Green eggs: I love hard-boiled eggs. My mum makes me them for breakfast at
the weekend. I asked my mum why the yolk goes a funny green colour around the
edge, but she doesn鈥檛 know. It doesn鈥檛 happen with my sister鈥檚 soft-boiled eggs.
Can you tell me why my eggs change colour and why my sister鈥檚 don鈥檛?
Louis Atkins (age 6)
Norwich, Norfolk