Liberal Arts Blog — Wednesday is the Joy of Science, Engineering, and Technology Day
Today’s Topic: The Miracle of the Human Eye — a few notes
The eye is a bundle of miracles. Too many for one post. Today, a few scribblings that I hope will trigger a cascade of expert insights and, perhaps, corrections. What do you find most mesmerizing about the eye?
THE CAMERA ANALOGY — the lens, the aperture, the film
1. Both the cornea (in front of the pupil) and the lens (behind it) jointly focus the light as does the lens of a camera.
2. The iris and the pupil function as the aperture.
3. The retina serves as the film.
THE FRONT LAYERS: Conjunctiva, Sclera, Cornea
1. Conjunctiva — the thin outermost layer, vascularized except for the part over the pupil. Can get blood shot. Also lines the inside of the eyelid. Together with lacrymal gland, helps lubricate the eye. Infection fo conjunctiva common among children — conjunctivitis.
2. Sclera — the white part except for the front of the eye where it is continuous with the cornea. Fibrous, supportive, and protective. Mostly collagen. Five-sixths of the connective tissue that surrounds the eye. Human sclera distinctive in that clearly visible when eye is open. The iris is smaller than in other mammals. Hypothesis — the visibility of the white imparts awareness of where the other person is looking which facilitates human communication.
3. Cornea — transparent front of eye covering the pupil and the iris. Refracts light. Accounts for two thirds of the optical power of the eye. Can be re-shaped by Lasik. Avascular. Most frequently transplanted human organ — 100,000 procedures per year. Lack of blood vessels means fewer antibodies, lower rejection rate — about 10%.
THE POWER OF THE HUMAN EYE: the sensitivity/resolution trade-off
1. Apparently the eye can distinguish between 10 million colors!
2. And detect a single photon of light!
3. Color cones (about 7 million) are high resolution, but low on sensitivity (eg. don’t do well at night). Rods (about 100 MM) are high on sensitivity (can detect shape and movement at night) but low on resolution.
NB: other mammals have a barely visible sclera which is often dark.