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The pupil reflex to light has been considered as a servomechanism, a self-regulated error-actuated control device. This cybernetic approach, requiring the experimenter to make quantitative measurements in animals with a fully intact central nervous system, was made possible using a pupillometer designed for awake, cooperative human subjects. This instrument provided an electronically controlled light stimulus as well as continuous records of both pupil area and light intensity. Sinusoidal changes in light intensity, small enough for linearization assumptions, were injected in an open loop fashion to determine the transfer function for pupil system behavior. The pupil servo is quite stable and has a low gain with an attenuation slope of 18 db per octave beyond 1.5 cps. One line of investigation using pharmacological agents has suggested the triple lag to be contributed by the physical law representing the viscosity of the iris neuromuscular system. Another experiment used artificially increased gain to produce instability oscillations whose frequency was predictable from the low gain transfer function. Still another investigation has shown the pupil system to contain much noise. This noise is not a result of instability, nor generated by the smooth muscle of the iris, nor by other elements of the pupil servoloop, but is injected into the loop from another part of the brain. Further studies in progress are defining nonlinearities in the pupil and retinal system in order to set up an accurate analog model of the pupil system in the form of a program for a digital computer.