Rod cells pick up movement out of the corner of the eye and also, in a normal eye it is the rods that operate in poor light or at night. There are about 120 million rods in each eye and they are more numerous towards the outer edge of the retina. The cone cells are used in colour vision and in close precision work like reading. There are not as many cones and they are more concentrated in the centre of the retina (the Macula).