Open this image in full-size. Are those concentric circles, or one continuous spiral?

Let us revisit the Seemingly Bent category. We hadn’t had unique optical illusion that belongs to this section, in quite a while. To remind you once again, Seemingly Bent category consists of illusions that give you impression you are seeing slanted lines and shapes, while in fact they are perfectly straight (or mutually parallel). Most famous example to represent this category could be “Cafe Wall“, followed by both slanted square and slanted seesaw.

Now that we have repeated the matter, I can proceed explaining today’s two specimens. First one on the right is nothing new to be frank. It’s just a more recent version of “Circles or Spiral” illusion. You can open the image in full-size to see the full effect. In short: the image gives you impression you are seeing continuous spiral, while there are only 6 concentric (ones that share the same center) circles instead. The second image below is new work by Akiyoshi Kitaoka, and believe it or not – all of the lines in it (consisted of gray blocks) are placed perfectly horizontal. I’m not pretty sure why is that, and does the color combination help making the effect stronger.

Bourdon Optical Illusion

There’s a new optical illusion in town, and its name is “Bourdon Illusion“. The Bourdon  illusion is the apparent “bentness” of the straight edge of a figure consisting of two elongated triangles arranged apex to apex.

Like you see in the image on your left, the upper side of the left-lower triangle is actually aligned with the upper side of the right-upper triangle, the former appears to tilt counterclockwise while the latter clockwise. What we are seeing here is “seemingly bent” type of illusion. But why?

Further, I’ve added three rather subtle illusions, each showing bent lines. In Bourdon’s illusion, to the left, the straight left hand edge looks bent again. In Humphrey’s figure, (central object), the straight, loose line touching the corner of the cube looks bent. In the figure to the right, the straight line interrupted by the corner looks bent.

I don’t think we really understand any of these illusions, and since they are not very dramatic, you don’t notice them very often. There’s probably a different explanation for each. For example, both the left and middle figures show a bent line that is the backbone of two triangles meeting at a point, so you might think “Hello, we’re getting somewhere”. But then you notice that the lines bend in different directions relative to the triangles inside each illusion.

The Bourdon Illusion, Humphrey's Figure and Corner Illusion

A properly thrown curve ball spins in a way that makes the air on one side move faster than on the other. This causes the ball to move along a gradual curve. From the point of view of a batter standing on home plate, though, curve balls seem to “break,” or move suddenly in a new direction. This year’s winning illusion may explain this phenomena. Arturo’s animation shows a spinning ball that, when watched directly, moves in a straight line. When seen out of the corner of the eye, however, the spin of the ball fools the brain into thinking that the ball is falling towards the left. We had another VERY SIMILAR example posted few months ago.

So, as a baseball flies towards home plate, the moment when it passes from central to peripheral vision could exaggerate the movement of the ball, causing its gradual curve to be seen as a sudden jerk. Be sure to observe the falling ball directly, and indirectly (from aside) while reading the text embedded inside the animation. See the difference? Other winning illusions will be posted shortly. More can be found here.

Update 1 (02/22/2009): Since yesterday, I have received this enhanced animation from visitor that goes by the name Mr. Anonym. His version might help you in seeing the effect little better. You should be able to spot the moment after which the seesaw gets slanted.

Update 2 (02/22/2009): Alessandro aka SuperSandro sent another example where he modified this illusion. Now the seesaw is actually slanted, but appears perfectly balanced. Here is what Alessandro wrote – “Hi! Today I’m happy to send you this attached file. It’s the same as slanted Seesaw, but rotated for 4 degrees on the rectangle’s side. It seems to be horizontal, but of course it is not.”

P.S. I have updated Nike SB “Optical Illusion” Blazers with the crucial missing picture.