The ArgumentThis essay addresses the historiographical question of how to study scientific instruments and the connections between them without rigidly determining the boundaries of the object under historical scrutiny beforehand. To do this, I will explore an episode in the early history of the tachistoscope — defined, among other things, as an instrument for the brief exposure of visual stimuli in experimental psychology. After looking at the tachistoscope described by physiologist Volkmann in 1859, I will turn to the gravity chronometer, (...) constructed by Cattell at Wundt's Leipzig institute of psychology in the 1880s. Taking Wittgenstein's notion of family resemblances as a methodological suggestion to travel from one member to another to find out just how members relate to one another, I will investigate part of the family to which both the tachistoscope and the gravity chronometer turn out to belong. A detailed analysis of these instruments, using both historical sources and historical accounts of psychological instruments, may demonstrate that the instrument is not a standard package that, if well applied, will simply secure good results. Each package needs to be assembled again and again; the particular package that is assembled may differ on different occasions. Thus an alternative is developed to an understanding of instruments as univocally functioning material means. (shrink)