There is precicely the same proof that the eye was made for vision, as there is that the telescope was made for assisting it. They are made upon the same principles; both being adjusted to the laws by which the transmission and refraction of rays of light are regulated. - (Paley, 1857 p.14)
Eyes and light.
Basic anatomy - cornea, lens, iris, retina, optic nerve.
Lens focuses light on retina. Photoreceptor cells transduce light
Common designs of eyes.
Comparison of human and squid eyes. Anatomy.
Humans and squid have exactly the same design of eye. Eyeball with a lens to focus the light on a retina, with light sensing cells and nerves to carry the signal to the brain. An iris that opens and closes to regulate the amount of light entering the eye...
Comparison of human and squid eyes. Histology.
Cover your left eye. Look at the capital X below. Place your nose right in front of the capital X. Slowly move your head away from the screen, and back again keeping your right eye focused on the capital X. Somewhere around 8 inches (say 12 cm or so) away from the screen (depending on the settings of your browser), something should happen.
Did the small x go away? Yes? This is an optical illusion that tells us something about the anatomy of the eye, and something about the way our nerves process information from the eye. The x disapears when light coming from it is focused on the blind spot, the place on back of eye where optic nerve leaves the eye. The nerves from our eyes travel to the back of our brain, where they are mapped out into an area called the visual cortex. The visual cortex fills in the blind spot with whatever the nerves from directly around it are seeing, so the x gets replaced with the surrounding white background.
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