Published: January 08, 2009 11:40 pm      

Earth's creatures use many methods to see

Wonders of vision

Despite many years of research, human vision is still not completely understood. For a broad look at the intriguing history of eye research, the British writer Simon Ings has written “A Natural History of Seeing,” published by W.W. Norton in 2007.

In relating some of the fascinating topics covered by Ings, let’s consider the early ideas of seeing. In 1500 BCE, Egyptians living in the New Kingdom believed that all seeing was due to Ra, the sun god. If Ra looked at an object, it could be seen.

The early Greek philosophers felt that an object became visible by casting out a thin film that would be caught in our eyes. To the Greek thinkers Empedocles and Plato, humans saw because their eyes emitted emanations or visual darts that bounced off the objects, returning to their eyes; in the same way that we use flashlights at night.

Aristotle rejected this view, asking how could we see stars that were great distances away. To Aristotle, light was due to fire that could bounce off objects, allowing us to see. Modern science tells us that light comes in bundles of tiny energy called photons, which can behave either as particles or as waves, depending on the circumstances.

What creatures were the first to see? The first creatures with eyes go back to the Cambrian Explosion, the remarkable first appearance of hard shelled fossils about 540 million years ago. One of the smallest creature to see is the Protozoa, Euglena gracilis, an algal flagellate.

In the simplest kind of seeing, light hits rods, special nerves whose pigments lose their colors. This change in the rods makes them grow longer and excites nearby nerves, whose “chattering” sends a signal from nerve cell to nerve cell, eventually reaching the brain. While most life forms lack eyes, many are light-sensitive, such as flowers that open during the day and unfold their leaves so as to face the sun.

The smallest creatures have compound eyes, with hundreds of images, allowing them to sense life all around them, particularly things in motion. Single chambered eyes developed about 85 million years after the Cambrian Explosion in fishes. Once the first amphibians came ashore, their eyes flattened out to allow sharp focus for objects far away. The Icthyosaurs were aquatic dinosaurs who could reach 15 meters in length and had eyes 12 inches across.

What makes our seeing better than most animals? Most humans are trichromatic, seeing in three colors with special cones concentrated in our macula (part of retina, near the back of our eyes). Most animals see in monochrome (like a black and white television).

In our eyes, there are cones sensitive to red, green and blue light. But at night, we see by our rods, nerve cells that only sense light but not color. The rods are much more sensitive than the cones, allowing sharp eyed observers to see the light of a candle miles away! In each eye there are 100 million rods, 24 million cones for red or green light and only 2 million cones for blue light.

Light enters through our cornea (outer surface of eye), through the pupil and then into the lens (made of dead protein cells) to which muscles are attached for focusing. The light then goes into a gel called the vitreous humor, then through a thin layer of blood vessels, the transparent retina whose rods and cones face backward and then to the back of the eyeball where an upside down image is formed.

All the visual information is compressed into a million fibers of each optic nerve which carries the signals into the brain.

At night, most stars look white because we are seeing them just with our rods. There is not sufficient light to activate the cones, except for the brighter stars that show tints. Look at the golden Capella, a bright star nearly overhead in January evening’s sky. If you find Orion with his three-star belt, look above and to the left of the belt for Betelgeuse (say “Beetle Juice”), which has a pink tint.

Polar skies and club meeting

Our new public planetarium program for January is “Polar Skies” with free presentations each Sunday at 4 p.m. and 7 p.m. through the end of the month. What kind of observations can you make in the polar regions, not possible in most parts of the world?

Also, there will be an informal tour of our current evening sky using our Planetarium projector. The Planetarium is in Tawes 302, just inside the front lobby that faces Frampton Hall and the Compton Science Center.

The Cumberland Astronomy Club will meet Friday at 7:30 p.m. in the LaVale Public Library just off Route 40 (about a mile to the East of the State Police Barrack). All interested sky gazers are invited at attend.

Readers are invited to email Bob Doyle at rdoyle@frostburg.edu or leave a message at (301) 687-7799.