Pupillary reactions have been studied for centuries. Their use in physiological research as indicators of autonomic nervous activity dates back to the 18th century (du Petit, 17271), and the interest of physicians, to a much earlier period (Galen). This interest was intensified during the 19th century. Horner's description of the effects of peripheral sympathetic lesions in man (18693), Argyll Robertson's discovery of the syndrome which bears his name (18694), and numerous other reports on pupillary behavior under normal and pathological conditions stimulated and sustained it. Unfortunately, accurate observation of pupillary reactions is difficult. The smallness of the pupils, the swiftness of their motions, the impossibility of observing the patient's two eyes simultaneously, and the changes in pupillary behavior under different experimental conditions are some of the factors contributing to this difficulty. For these reasons, many methods of pupillary measurement have been developed in the course of time.