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Since the discovery of photoperiodism by Garner and Allard (2), scientists have been concerned with controlling the length of day to which experimental plants or animals are exposed. If plants or animals are grown in rooms with artificial illumination it is possible to control day length precisely by operating lamps on an ordinary time clock. The experiment becomes more complicated, however, if one wishes to expose a series of experimental organisms to different day lengths in order to compare the effects of different treatments. One may operate a series of small chambers each containing an experimental group and control illumination of each chamber by a time clock. This becomes expensive; problenms of temperature control often become serious. Another procedure is to have an experimentally controlled illuminated room with an adjacent dark room so the organisms can be moved from the illuminated room to dark room and back again at desired intervals. This requires manual manipulation at specified times, twice a day for each treatmenit, and care must be exercised that the treatments are shifted at exactly the correct time each day. Most of the literature on photoperiodism has dealt with the influence of duration of light and dark periods in cycles of 24 hours. Recently, however, considerable interest has developed in the effects of cycle lengths other than 24 hours duration (1, 3). Such experiments have presented extreme (lifficulties in manipulation. If the organisms are to be transferred manually from liglht to (lark room or vice versa. it is necessary for someone to be available night and day throughout the experimental period. Failure to shift one group of organisms at the correct tinme may lead to failure of the entire experiment. We have devised a simple apparatlus for automatically shifting plants from light to dlarkness and vice versa with cycle lengths of any desired duration. While the apparatus was designed for work with -plants, it should work equally well with most small animals. We have called this apparatus a photocyclerr

Automatic Device for Controlling Lengths of Light and Dark Periods in Cycles of any Desired Duration