Book T of C
Chap T of C
This is the 2007 version. Click here for the 2017 chapter 04 table of contents.
The visual system is studied more than the other perceptual systems in humans, because humans are dependent upon vision and have a larger portion of their brains devoted to sight than any other sense. People who are blind often become more sensitive to non-visual cues in the environment, not because they hear better than sighted people, but because they pay better attention to non-visual information. For most of us, however, vision is the dominant sense.
The sense organ for sight is the eyeball. It gathers light into a focused image on the back of the eye. Focusing is done by refracting (bending) light as it passes through the transparent tissues and fluids of the eyeball.
Why is it dangerous to look directly at the sun?
Refraction is familiar to anyone who has held a magnifying glass under the sun. Light from the sun is bent by the lens, concentrating the light on a small spot where it becomes intense enough to start a fire. The curvature of the eye performs a similar function. That is why it is dangerous to look directly at the sun: it can burn the back of your eye.
The back of the eye receives an inverted image
How did Descartes observe the inverted image on the back of the eye?
A magnifying glass creates a point of focus because its image is dominated by a single source of light, the sun. Light from the environment creates a focused image in our eyes because it comes from every direction. Each point in the environment is focused on a different point on the back of the eye. Notice on the above diagram that point B ends up above point A, on the back of the eye. The back of the eye receives an inverted (upside-down) image of the world. Descartes discovered the inverted image on the back of the eyeball in 1637. He scraped tissue off the back of an eye and inserted a small thin piece of paper in its place. Holding up the eye, he observed a small, sharp, upside-down image of the environment on the paper.
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Copyright © 2007-2011 Russ Dewey