Seeing things

01FinsMeasure

Make the lower line as long as the upper one. Judge the lines, not the fins.

The lines were the same length. Fins splayed outward make a line look longer, and arrowheads make it look shorter. Franz Müller-Lyer published the figure in 1889, and why it works is still argued over.

02GridWatch

Let your eyes wander over the grid. Dark dots flicker in the white discs you aren't looking at, and vanish from any disc you look at straight on.

There are no dark dots. Bend the bars and they go away, which is hard to square with the old textbook account of cells in the retina sharpening edges.

03TablesMeasure

Stretch the right tabletop until it is the same shape as the left one.

The two tops were the same shape, one turned a quarter turn. The legs make you see surfaces lying in depth, and the eye lengthens the one that seems to point away from you. After Roger Shepard's tables.

04ChaserWatch

Look at the cross and keep looking for half a minute. The gap becomes a disc of the opposite colour running round, and the others fade away.

Nothing on the screen is that colour. Each gap leaves an afterimage in the opposite colour, and blurred discs you hold still in your gaze fade from view. Sharpen them and they stop fading. Jeremy Hinton made the first one.

05MortarMeasure

The grey lines between the rows look tilted. Tilt them back until they all look level.

The lines were level at 0°. The tilt needs mortar of a shade between the dark and light tiles, and goes if the contrast does. It is named after the tiled front of a café in Bristol.

06BusesWatch

Watch the two bars cross the stripes. They seem to take turns stepping forward, like feet.

They move together at one steady speed. A bar looks faster while its ends stand out against the stripes and slower while they blend in, and the yellow and blue bars blend in at opposite moments. Fade the stripes and they glide. Stuart Anstis's stepping feet.

07HaloMeasure

Size the right centre disc until it matches the left one.

They were the same size. Among large discs a disc looks smaller, and among small ones it looks larger. Hermann Ebbinghaus's figure, made famous by Edward Titchener.

08SpiralWatch

Press start and keep your eyes on the red dot while the spiral turns. When it stops, keep looking.

Once it stops, the still spiral seems to flow the other way, and so does whatever you look at next. Cells that answer to one direction of motion tire while you stare, and those tuned the other way win for a while. Robert Addams described it in 1834, after watching a waterfall.

09StripesMeasure

Set the grey of the right bars to match the left bars.

The two greys were the same. Which way the stripes push a bar depends on whether it lies in a black stripe or a white one, and why is still argued over. Michael White described it in 1979.

10DriftWatch

Let your eyes roam over the wheels. They seem to turn, though the picture is still.

Each ring repeats black, dark, white, light. Out of the corner of the eye the pattern seems to creep from black toward dark and from white toward light, so reversing the order reverses the turn. Alan Fraser and Kimberley Wilcox found the effect in 1979; Akiyoshi Kitaoka made it famous in colour.