On the Time Constants of Tachistoscopes

Abstract

The time constants of commercially available, electronic tachistoscopes are shown to be much poorer than is widely assumed. When a band near the centre of the visible spectrum is isolated, the output of a typical tachistoscope lamp takes approximately 18 ms to reach 90% of its asymptotic value; the time to decay to 10% is also typically 18 ms and we demonstrate conditions under which the decaying trace remains visually effective against a dark field for more than 40 ms after the nominal offset of the lamps. Many reports of tachistoscopic experiments published in the last two decades must be quantitatively in error; and we provide examples of experiments where qualitative conclusions may have been drawn invalidly.