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Abstract: In order to facilitate the discrimination of two nearby flash light signals and to eliminate the resulting apparent movement, the effects of different intensity levels of dim phase, instead of dark phase, intervening the repetitive flash signals, on discrimination were tested under laboratory conditions. Results revealed that (1) the disjunctive reaction time of the flash signals increased with the intensity level of the dim phase; (2) there existed an optimum intensity level of the dim phase which yielded least discrimination errors; and (3) the occurence rate of apparent movement, caused by the two flash signals, decreased with the increasing intensity level of the dim phase.Field experiments were also conducted to compare the effects of dim phase and those of dark phase. Results indicated that (1) at day time, flash signals with dim phase gave longer reaction time and more errors than that with dark phase; (2) in the night, flashes with dim phase showed less unfavourable effects on discrimination, with a tendency of shortening the reaction time and diminishing errors, though the differences were not significant; (3) the dim phase, as compared with the dark one, had definite effects on the elimination of apparent movement of two simultaneous flash signals. This suggested the possibility of utilizing a dim phase in simultaneous flash signals under certain circumstances.

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