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Let's look at the human eye.
The eye is basically a sphere 2.4 cm in diameter, (a little less than one inch.)
Take a look at a one inch diameter plastic sphere to see a model of the eye.
We'll start with exhibits progressing from the Cornea to the retina.
Then we'll do some hands-on explorations including a web guided cow's eye dissection.
Cornea and lens
The eye forms an image on the retina by bending light, refracting light, through the cornea and the lens. Most of the bending comes from the cornea, however, the lens makes an important fine adjustment which sharpens the focus.
When you look at a small bright light often you will see lines radiating from it, painters often show these lines surrounding stars, see for example lines radiating from the Christmas star. Here is an investigation of the source of those lines.
After doing this activity you may begin to see why we started our exploration of science with a study of perception. The rays which you thought existed around a light bulb were not really there but were the result of your perception.
The cornea is made of a tough clear material. In humans it has 5 layers. The living bottom layer is nourished by the aqueous humor.
Iris and Pupil
The iris gives the eye its color. The iris expands and contracts forming the pupil which controls the amount of light entering the retina. Let's explore the pupil.
The vitreous humor is a gelatinous material that fills the eye behind the lens and presses the retina up against the back of the eye. Occasionally, the retina will hemorrhage so that red blood cells and white blood cells leak out into the vitreous humor. You can see these cells drifting about in your field of view when you look at a clear blue sky.
The retina is where it's at! Here rods and cones convert light into electrical signals. The human retina is built in a strange way with the blood supply and the neurons in front of the light sensitive cells. This allows us to look at our own retinal blood supply.
also Seeing Your Retina snack
When you come to the Exploratorium you must see an exhibit titled, "Corpuscles of the Eye." This exhibit is too expensive to build yourself. When you look into the eyepiece of what looks like a small telescope you see many bright points of light dancing across your field of view. They dance to a regular rhythm, your heartbeat. The particular narrow band of blue light wavelengths in this exhibit has been chosen to allow you to see the diffraction pattern of red blood cells as they pulse through the blood supply in front of your retina.
Another exhibit is named Macula, look at a disk of light that alternates between purple and blue. In the center of the blue light a dark shadowy shape may appear. This is the visual appearance of the macula. Not everyone can see their macula, but those who do report seeing disks, dandelions, and klingon bird of prey spacecraft shadows. Notice that the size of the macula changes as you approach and then go away from from the screen. The macula is a fixed portion of the retina, near the fovea, but no one knows exactly how it is made.
There are many more activities we could do but this is a good time to take a break.
Now let's get a hands-on experience with an eye. We'll do a cow's eye dissection. You can do this for real by calling up your local slaughterhouse and arranging to get a cow's eye. Or you can do this electronically at the Exploratorium's cow's eye dissection site. Nothing beats the real thing though, so go for it! Get a cow's eye and start exploring.
You can view a guide to the eye by downloading the Exploratorium's eye site!
Tomorrow we'll explore how the eye and brain work together.
Return to the Summer Institute
22 June 2009