Two experiments are reported. The first experiment studies the intensity-duration function for peripheral threshold stimulation of the human eye, over a range of durations from 0.02 second to 0.24 second. The form of the flashes presented involves a rapid onset and cessation (i.e., a nearly rectangular wave). The second experiment determines the total energy required for threshold excitation for flashes of various wave forms (i.e., with different onset and cessation times), the maximum duration always being below the critical duration obtained in the first experiment.
The results of the first experiment agree with previous studies in showing that below a certain critical duration the product of light intensity and duration is constant. Above this critical duration the intensity remains constant. The transition from the relationship It=C to I=Const. is abrupt and occurs at 0.10 second.
The results of the second experiment indicate that changes in the wave form, within the limits of variation introduced, do not affect the total amount of energy required for threshold below the limit of a critical duration. These findings give additional generality to the Bunsen-Roscoe law and to the view that the essential element in producing a constant effect in response to flashes of light is the total energy of the stimulus.
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