Transmission electron micrograph of iris showing a series of large radial vessels gives rise to a complex network of intermediate and small vessels throughout the length of the iris. The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. Eye color is defined by that of the iris. In optical terms, pupil is the eye's aperture, while iris is the diaphragm that serves as the aperture stop. Magnification unknown.