Optical Illusions » Geometric illusions

Magic Ring

david — Tue, 27 Sep 2011 07:25:47 +0000

Here’s a movie of a brilliant, double spiral novelty illusion ring. It’s available to buy from Grand Illusions, and on that link you can also see another movie of the illusory effect. As the ring is rotated, it seems to expand when rotated one way, and contract when rotated the other [...]

Decor with Attitude

david — Mon, 05 Sep 2011 09:17:31 +0000

This is an installation called Zig-Zag Corridor by Czech artist Petr Kvicala, in the Dox art centre in Prague, Czech Republic. He’s an artist who produces dazzling patterned effects. In this one, beautiful diagonals meander through the patterns, although the linework is entirely made up of a continuous sequence of horizontal and vertical segments. However, [...]

The Thiery-Wundt and Muller-Lyer illusions

david — Fri, 03 Jun 2011 15:59:58 +0000

Top left is one of the simplest of all illusions. The yellow dot is just half way up the vertical height of the triangle, but looks decidedly nearer to the apex. The effect was reported over a century ago, and has been named for its original researchers the “Thiery-Wundt illusion” by recent experimenters Ross Day [...]

Competition and the Poggendorff and Muller-Lyer Illusions

david — Tue, 24 Aug 2010 17:00:38 +0000

I’ve not been posting much because I’ve been struggling with a mammoth revision of my technical site on the Poggendorff illusion. But now that’s done, here’s a post on another Poggendorff puzzle. In earlier posts I’ve shown examples of competition between illusions, and included a demo of a paradox when the Poggendorff and Muller-Lyer [...]

The Twisted Stairs (version 2)

david — Fri, 15 Jan 2010 15:28:39 +0000

I’ve been wanting to do a new version of my earlier post of The Twisted Stairs. That’s partly because the way I placed the figures in the original posting, they got in a bit the way of seeing the twist in the lateral flights of stairs. I reckon you can see the twist effect better [...]

Doggendorff and Moggendorff Illusions

david — Tue, 08 Dec 2009 17:15:35 +0000

Here are a couple more variants of the Poggendorff illusion (mog, or moggy, by the way, is a term of endearment for a cat in UK English, but I’m not sure it’ll be familiar if your background is in American English). The symmetry axes of the dog and cat heads are objectively aligned, but to [...]

Pinna’s Intertwining Illusion

david — Sat, 05 Dec 2009 12:55:37 +0000

This is a brilliant illusion discovered by Baingio Pinna of the University of Sassari in Italy. The circles appear to spiral and intersect, but are in fact an orderly set of concentric circles. The illusion is due to the way the orientation of the squares alternates from circle to circle, and that contrast alternates from [...]

Subtle misjudgments of horizontal and vertical

david — Sat, 28 Nov 2009 11:24:31 +0000

Back in 1987 James Walker and Matthew Shank in the university of Missouri were doing a study of the Bourdon illusion. In some figures they devised for comparisons in their study they noticed a new effect, quite unrelated to their study. The figure upper left is a version of their chance discovery. The centre line [...]

Shepard’s tables – What’s up? (post no. 3)

david — Fri, 23 Oct 2009 17:35:31 +0000

This is a third look at the Shepard’s tables illusion. If you didn’t see the earlier posts, you might like to get up to speed on the illusion by scrolling down two posts to an animated demo. The two pairs of table-tops in these views are absolutely identical, and within each pair the two lozenge [...]

Shepard’s Tables – what’s up? (post no. 2)

david — Sun, 18 Oct 2009 10:47:34 +0000

The previous post presented an animation of Shepard’s Tables. If you didn’t see that, you might want to check it out first (scroll down to the previous post) to get the basics of the illusion. This new version of the illusion, with nested tables, follows the pattern: all eight of the lozenge shaped table-tops are [...]