Jastrow illusion

The Jastrow illusion is an optical illusion attributed to the Polish-American psychologist Joseph Jastrow. This optical illusion is known under different names: Ring-Segment illusion, Jastrow illusion, Wundt area illusion or Wundt-Jastrow illusion.

The illusion also occurs in the real world. In the illustration on the right, the two toy railway tracks are identical, although the lower one appears to be larger. There are three competing theories on how this illusion occurs.

This illusion is often included in magic kits and several versions are sold in magic shops and is commonly known under the name Boomerang Illusion.

The oldest reference to this illusion can be found in The World of Wonders, an 1873 book about curiosities of nature, science and art. The two arches are placed on top of each other. They are similar in size, but not the same. The inner radius of the upper arch is the same as the outer radius of the lower arch.

The first psychologist to describe this illusion was not German psychologist Franz Müller-Lyer in 1889. His article presents a collection of geometrical illusions of size, including what is now known as the Müller-Lyer illusion. His paper also includes the ring segments which we now know as the Jastrow Illusion.

Joseph Jastrow extensively researched optical illusions. One of his most famous creations is the Duck-Rabbit illusion, an image that can be interpreted as being both a rabbit or a duck. In 1892 he published a paper which introduced his version of what is now known as the Jastrow illusion. His version is different to the commonly used figure because the two arches taper to one end.

On the other side of the Atlantic, German scientist Wilhelm Wundt was also pioneering in psychology research. He wrote one of the first books about geometric optical illusions in which he copied the design previously published by Müller-Lyer.

There are several competing explanations of why the brain perceives the difference in size between the ring segments, none of which has been accepted as definitive.

One explanation relates to how the mind interprets the two-dimensional images on the retina as a three-dimensional world. Another explanation relates to the fact that the mind can only attend to a small field of vision, which is reconstructed by our consciousness. The most commonly used explanation is that the brain is confused by the difference in size between the large and the small radius. The short side makes the long side appear longer, and the long side makes the short side appear even shorter.

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