o ask how much a person sees in a glance, you need to control the glance. The tachistoscope shows a picture or word for a brief, set time, then takes it away.

what it is

Not one instrument but a family. Benschop (1998) asked what a tachistoscope actually is and found no single answer: Volkmann's 1859 device, falling screens, gravity-driven shutters such as the one Cattell built at Wundt's Leipzig institute in the 1880s, and many later designs. He borrowed Wittgenstein's idea of family resemblances, and argued that the instrument is not a standard package handed from lab to lab but has to be reassembled, with its procedures, each time it is used [paraphrase, not a quotation]. See tachistoscope.

what it was used for

How many letters can be read at a glance; whether a word is seen faster than a letter; how much of a scene survives a brief look. The modern descendants are computer displays timed to the screen's refresh, used for masking, priming and change blindness.

Try one: spot the change shows two pictures alternating with a blank between them, which is the tachistoscope's trick turned into a puzzle.

Sources

1. Benschop (1998). What Is a Tachistoscope? Historical Explorations of an Instrument. Science in Context. doi:10.1017/s0269889700002908

Try it yourself. [playful] Small toys on the toy shelf that show this in action:   • spot the change — change blindness, with a blink

see also