Thursday, October 13, 2011

Human Eyes

Marilyn vos Savant recently had an interesting column in Parade magazine. She addressed the fact that human eyes have two kinds of photosensitive cells, cones and rods.

"Cones are used for seeing when it's relatively dark. No animals, not even owls, can see in total darkness. The center of your eye contains only cones; the perimeter is dense with rods.

When we look directly at a star, for instance, it disappears because the cones cannot "see" it. But when you look nearby, the star reappears because the rods can see it.

This is the reason observers so often feel that they're never looking in quite the right place during a meteor shower. They are correct! Mostly we see the shooting stars only in our peripheral vision.

Chickens have nearly all cones, so they can barely see at night. Rats are the opposite. Their eyes have nearly all rods, which is why they dislike daylight so much."

The American Heritage Dictionary has these definitions:

Cone=A photoreceptor in the retina of the eye.
Rod=Rod-shaped cells in the retina of the eye that respond to dim light.

You may be wondering what this has to do with color, the subject of this blog. It is this: these same rods and cones which help us see stars and comets also help us see color and shades of colors.

I am grateful we have eyes able to distinguish nuances of color in our surroundings. It is difficult to imagine seeing only in light and dark.

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