The retina contains a layer of two kinds of light-sensitive
photoreceptors: cones and rods. Cones are used for day vision and color vision. They are most concentrated in the fovea,
where focused light produces the sharpest image. Rods are used for vision in dim light and for the detection of movement in
the visual field. Rods are concentrated in the periphery of the retina, hence, the tendency to focus away from the fovea (“look
to the side of the eye”) in darkness.