Middle left stick

Calculator stick

In the diagram below the viewer is likely to perceive a vague bluish tint in the square at the centre of the four sets of concentric circles. The reality is that no square exists and that the colour of the background is uniform throughout. The implied square is merely suggested by the change in colour of the circles, and our visual system tends to spread colour within boundaries for which it feels it has a definition. The illusion is generally known as neon colour-spreading.


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