Induced pupil hippus is an oscillation in an unstable pupil servomechanism. The Nyquist stability criterion is utilized to demonstrate how this instability is experimentally achieved by increasing loop gain. Quantitatively accurate predictions from servoanalytic theory have been put forward and confirmed experimentally. This validates the use of the linear servoanalytic approach in this analysis of the pupil reflex. The direct pupil reflex to light has been studied and compared with previous studies on the consensual pupil reflex. Certain autonomic drugs have been found useful in modifying system parameters.