The front of the eye comprises, in simple terms, a lens to control the focusing point within the eye and an iris to control the light entering the eye. The back of the eye contains the retina which is made up of rod-and cone-shaped cells which are sensitive to light and are linked by optic nerves to the brain. The lens ensures that the image being viewed is focused on the retina and the iris controls the amount of light. Different cells in the retina are sensitive to different colours, and while the central part of the retina, known as the fovea, is sensitive to colours the peripheral areas are sensitive only to light intensity. A result is that colour vision disappears at low light levels.