![]() Réné Descartes proved this in the 17th century by setting a screen in place of the retina in a bull’s excised eyeball. That’s because the process of refraction through a convex lens causes the image to be flipped, so when the image hits your retina, it’s completely inverted. There’s an unlikely sounding quirk to this set-up, which is that mechanically speaking, our eyes see everything upside down. The retina detects photons of light and responds by firing neural impulses along the optic nerve to the brain. Light from an external source enters through the cornea and is refracted by the lens, forming an image on the retina-the light-sensitive membrane located in the back of the eye. The model of vision as we now know it first appeared in the 16th century, when Felix Platter proposed that the eye functions as an optic and the retina as a receptor. (Or at least some of us do: There’s evidence that a worryingly large proportion of American college students think we do actually shoot beams of light from our eyes, possibly as a side effect of reading too many Superman comics.) It gained so much credence that it dominated Western thought for the next thousand years. ![]() ![]() ![]() This "emission theory" of vision was endorsed by most of the great thinkers of the age including Plato, Euclid, and Ptolemy. In ancient Greece, for example, it was thought that beams of light emanate from our eyes and illuminate the objects we look at. Beliefs about the way visual perception works have undergone some fairly radical changes throughout history. ![]()
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