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The lower limit of visible motion has been determined by a number of investigators under different conditions. Porterfield 1 made the observation by fixating a moving object in an otherwise motionless but unrestricted field of vision. Some time later G. G. Schmidt, 2 Muncke, 8 and Valentin l made similar determinations upon the second hand of a watch. Aubert 5 brought the observation under more adequate experimental control by the use of lines on a rotating drum whose rate could be altered and measured accurately. He determined to some extent the effect of the surrounding field upon the limen. With an unrestricted field of stationary visible objects and with fixation on the moving object, Aubert's results agreed with those of the earlier investigators. When the visual field was restricted to the drum seen through an opening in an otherwise uniform black field, the limen was increased tenfold. The sides of the opening in the screen formed, of course, a stationary basis for spatial comparison with the moving stimulus. To eliminate all such bases of judgments, Aubert repeated his experiments with a point of light in a lightless field. Under these conditions the perception of motion became highly unstable. The observers perceived motion when the stimulus was stationary and contrariwise. The presence of a second stationary line in the dark field did not alter the results materially. Aubert concludes, therefore, that a stationary field of known objects is necessary as a background for the perception of motion. Bourdon •

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