rods and cones

The rods are more sensitive than the cones. However, they are not sensitive to color. They are responsible for our dark-adapted vision…The optimum dark-adapted vision is obtained only after a considerable period of darkness, say 30 minutes or longer, because the rod adaption process is much slower than that of the cones…If you see a dim star in your peripheral vision, it may disappear when you look at it directly since you are then moving the image onto the cone-rich fovea region which is less light sensitive.

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html