This room is locked. The plan of the building does not show a west wing. The front desk keeps the keys.

e said the east wing was not finished, and it isn't. Nobody mentioned a west wing. It turns out to be finished, swept, and lit, and it has been for a while. (A playful room; the exhibits are real.)

the hall of thresholds

The question in this hall is the oldest one in experimental psychology: how much has to change before you notice?

	the grey card — the middle of the tones, and why it sits at about 18 percent
	Weber, 1834 — the just-noticeable difference grows with the size of what you start from
	Fechner, 1860 — Elemente der Psychophysik: measuring the mind by its thresholds
	the staircase, 1962 — step the stimulus down until they miss, up until they see
	test patterns — the screen version of a faint mark on grey

Weber. Ernst Heinrich Weber, in De pulsu, resorptione, auditu et tactu (1834), reported that the smallest difference in weight a person can feel is a roughly constant fraction of the weight they start with, not a fixed amount. (Paraphrase.)

Fechner. Gustav Theodor Fechner's Elemente der Psychophysik (1860) turned that into a program: build a scale of sensation out of just-noticeable differences, and the mind becomes something you can measure. (Paraphrase.)

The staircase. Tom Cornsweet's “The staircase-method in psychophysics” (The American Journal of Psychology, 75(3), 485, 1962; doi:10.2307/1419876) describes the procedure: lower the stimulus after each detection, raise it after each miss, and the steps settle around the threshold. (Paraphrase.)

In the tour, there is a staircase in the dark room too, and somebody stops it. That is the tour's story.

At the end of the hall, a door marked Fellows. And, under a dust sheet, an oracle somebody forgot to put back.