The momentary blindness experienced on going from bright to dim light, or from dim to bright, is the pause needed for the other set of nerve cells to take over. The rods are spread toward the edges of the retina, so that vision in dim light is general but not very sharp or clear. The cones are clustered thickly in the center of the retina, in the fovea centralis. When the eyes are turned and focused on the object to be seen the image is brought to the central area of the retina. In very dim light, on the other hand, an object is seen more clearly if it is not looked at directly, because then its image falls on an area where the rods are thicker.
Color vision is a function of the cones.
The most widely accepted theory of color vision is that there are three types of cones, each type containing chemicals that respond to one of the three primary colors-red, green, and violet. The brain can then interpret the impulses from these cones as various colors. The great majority of people with some degree of color blindness lack either red or green cones, and cannot distinguish between the two colors. True color blindness, in which none of the sets of color cones works, is very rare. Most color blindness is inherited, mostly by male children through their mothers from a color-blind grandfather.