If a bright red beach ball comes whirling toward you, you see its color, shape, and motion all at once—but your brain deals with each of these characteristics separately.
The Molecular Basis of Visual Excitation find the receptor proteins in the retina that respond to color.
Rod cells function only in dim light and are blind to color. "Get up on a dark moonlit night and look around," suggests David Hubel of Harvard Medical School, a winner of the Nobel prize for his research on vision. "Although you can see shapes fairly well, colors are completely absent. It is remarkable how few people realize that they do without color vision in dim light."
But the human retina also contains another kind of photoreceptor cell: the cones, which operate in bright light and are responsible for high acuity vision, as well as color.
Rods and cones form an uneven mosaic within the retina, with rods generally outnumbering cones more than 10 to 1—except in the retina's center, or fovea. The cones are highly concentrated in the fovea, an area that Nathans calls "the most valuable square millimeter of tissue in the body."
Even though the fovea is essential for fine vision, it is less sensitive to light than the surrounding retina. Thus, if we wish to detect a faint star at night, we must gaze slightly to the side of the star in order to project its image onto the more sensitive rods, as the star casts insufficient light to trigger a cone into action.