Various curves relating CFF to log I cross each other when these curves represent different PCF’s (pulse-to-cycle fractions). This was interpreted by Bartley to mean that PCF and conditions of flicker and fusion are not simply related, as has been generally taken for granted. He supposed that, whereas short pulses in a given repetitive cycle would produce flicker and longer pulses would produce fusion, still longer ones would reintroduce flicker, and finally the longest ones would produce fusion. An electronic fusion apparatus producing square waves and varying PCF from 0.02 to 0.98 was used to test this directly. PCF was varied over this range while CFF (and thus cycle length) and intensity were held constant. The supposition was readily confirmed. In addition, data were collected suggesting that the very shortest pulses would produce fusion rather than flicker at some intensity levels.
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