Word: fovea
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...concentrates heat and light in a spot on the retina. As distances increase, the spot grows smaller but remains as bright. At four miles away, a nuclear fireball 90 feet across forms a brilliant spot on the retina 1/300th of an inch in diameter. This is larger than the fovea centralis, the part of the retina that is used in accurate vision...
Sharp vision and color vision have two other common denominators. One is a highly sensitive dimplelike spot on the retina (the fovea centralis-literally, "central pit"), which acts as a magnifying device to spread the image over a greater number of visual cells. In this dimple, common to the vertebrates with the highest acuity (some birds have two in each eye), there are no rod cells. The cones are slim and tight-packed. The other common denominator is the mechanism for accommodation-ability to focus the eye, maintain a sharp image of a moving object...
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