Mate in five moves?! December 31, 2005
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Puzzle 7

"I saw another incredible game at the club last night"

"Here we go again. What happened?"

"I can only remember that the game ended with 5...Rh1#."

"Did that take the white rook?"

"No, it wasn’t a capture at all."

What was the game?

John Nunn

Visual Illusions

Remember the following optical puzzle we gave you a few years ago?

Our questions at the time were: (a) which of the two knights is the white knight (i.e. which is the lighter of the two)? Is it the one in the middle of the chessboard, or the one on the top left? (b) Which square is lighter, the knight square in the middle or the knight square at the top?

For those who have not seen it before the correct solution is, incredibly: both knights, and both squares on which they stand, are exactly the same shade of grey (RGB = 107, 107, 107), with identical light shading of the pieces.

The solution is hard to believe, and we expended a great deal of effort convincing our readers that both elements are exactly the same shade of grey, and explaining why they look so different to the human eye. When confronted with all of this my son Martin announced that he was returning his brain to the manufacturer as defective, since there were obviously serious bugs in its design.

Prepare for a shock, Martin! The web site eChalk has produced some excellent Flash pages to demonstrate and explain the above illusion, and some that are even more difficult to believe. Take a look at the following (our favourites):

• Chessboard and similar illusions – There are four, one more startling than the other.

• Colour perception – More of the same. Number three will really cause you to lose all faith in your visual senses. The effects were created by R. Beau Lotto, whose book "Why we see what we do" is definitely worth reading.

  
  Would you believe that the "brown" square in the middle of the top surface is exactly the same colour as the "yellow" square in the middle of the front surface? Yikes!

• Motion induced blindness – Follow the instructions carefully, the dots actually disappear. The illusion was created by Yoram Bonneh, Alexander Cooperman and Dov Sagi.
• Pseudo motion images – Patterns that trick the brain into seeing a rotating motion that just isn't there. Quite unnerving.

There are a host of other more or less well-known illusions at eChalk. Try "Dimple or pimple", "Coloured text mind masher" or "Jumbled words". Then call the hotline of the creator to find out if they have a firmware update for the current version of our brains.

Frederic Friedel