Hermann Grid

This model was built for the January 2010 Secretary’s Challenge which was to build a model demonstrating an optical illusion. It uses square cut 2½” x 2½” blue flexible plates on a sheet of white card, all in a frame of gold angles. You should see black (or grey) squares at the intersections. On the Meccano model it doesn’t always work, and for some people the illusion doesn’t work at all.

The illusion (in a slightly different version) is shown and described below:

Count the black dots in the second photo…

It’s impossible — as soon as you focus on the black dots, they disappear!

Your eye and brain have a very special mechanism for seeing edges clearly called ‘lateral inhibition’. This allows you to see a sharp boundary between an object (e.g. a person or a building) and the background.

It works by the light-sensitive receptors in your eye switching their neighbouring receptors off. This makes an edge look more pronounced.

Scientists do not understand exactly why you see the black dots, but they think it has something to do with lateral inhibition.

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