National Geographic : 2001 Oct
we feel when we look at the sun is evidence that the light is entering the eye and not the other way around. Centuries later Leonardo da Vinci realized that the eye is akin to the camera obscura, pioneered by Alhazen, in which light passes through a pinhole into a darkened room and casts an inverted image of the exterior world onto a wall. Descartes later did a rather dramatic examination of the eyeball of an ox, scraping away the back of the eye and peering through it. He saw that the eye cap tures an inverted, upside-down image of the world. Why doesn't the world look upside down? Because our minds correct the image. Sight has both a physical and psychological element. Light soon passed through the laboratory of Isaac Newton and never looked the same again. In the 1660s Newton demonstrated that white light is composed of all the colors of the spectrum. Using a prism, he broke sunlight into a rainbow, then later used a second prism to cohere the colors back into white light. "Whatever light be," he told the Royal Society in 1675, "I would sup pose, it consists of Successive Rays differing from one another in contin gent circumstances, as bigness, forms, or Vigour, like as the Sands on the Visually impaired children swarm around a light table to learn shapes and colors at Marquette School in Detroit, Michigan. "The kids crave light so much," says Carol Walker, who works with 30 students in the Detroit public school system. "The light levels in most homes are inadequate for them. They need more light to stimu late their vision." NATIONAL GEOGRAPHIC, OCTOBER 2001 II) i?