SIZE of the pupil.The pupil is a hole through which light enters into our eye.Iris is a muscle in our eye that controls the size of the pupil. It DILATES and contracts because of which size of the pupil varies.So we can say Iris controls the amount of light entering into our eye.Iris is the ring of pigmented tissue SURROUNDING the pupil that varies in colour from person to person.Iris helps regulate the amount of light entering the eye.In humans and most mammals and birds, the iris is a thin, annular structure in the eye that is responsible for controlling the pupil's diameter and size and thus the amount of light reaching the retina.The colour of the eye is defined as that of the iris. Visually, the pupil is the opening of the eye, while the iris is the diaphragm.The pupil is a dark void in the middle of the iris of the eye that enables the retina to be penetrated by the sun.The retina is the innermost, light-sensitive layer of the eye tissue of most vertebrates and some molluscs.The cornea is the translucent portion of the eye that protects the anterior HALF of the body.