the iris!!! iris is the coloured part of your eye,and its a muscle that controls your pupil(the black dot) to let how much light you're eye needs. The iris regulates the amount of light by controlling the size of the pupil.
Iris regulates the amount of light that enter your eye ball. It constricts when there is light of high intensity. It dilates when there is less light. So it protects the retina from light of high intensity and enables you to see in dim light.