ABSTRACT
The retina incorporates two kinds of nerve endings: cones and rods. A normal eye has some 6.5 million cones (in and around the fovea), which are sensitive to both quantity and quality (colour) of light, but operate only in good lighting ( photopic vision). The retina also has some 125 million rods , which are more sensitive than the cones, but perceive only quantity of light, do not distinguish colour ( scotopic vision ).
