Boomerang Illusion

The Boomerang Illusion (also known as the Jastrow Illusion) is an effect based on an optical illusion in which two boomerang shapes appear to shrink and grow.

The two figures are identical.

The optical illusion is often credited to Professor Joseph Jastrow, a pioneering psychologist, who used it to illustrate optional illusions in 1891. The same illusion was published by Felix Müller-Lyer in 1889 and by Wilhelm Wundt a year later. Psychologists often refter to it as the Jastrow-Wundt illusion of size.

This illusion has been used by psychologists to investigate how the we perceive size based on visual clues and has also been used as to test whether people with brain damage have a full field of vision. The psychological mechanism that causes this illusion is still a matter of discussion amongst researchers. The Jastrow Illusion has even been tested with chimpanzees to find out how our perception functions.

Performing the Jastrow Illusion as a magic trick requires a theatrical approach. Only showing the arcs to be swapping relative size will render it a mere puzzle. Many different variations on this theme have been published and marketed over the decades. The trick can be performed with boomerangs, fish, card fans, mud flaps or anything else that resembles an arch. Magic routines using this principle can use two, three or even four arches, adding layers of deception. When using small segments, there are also possibilities to deploy slight of hand for additional trickery.

It is often sold as part of many magic kits to children.

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References

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Peter Prevos, The Jastrow Illusion in Magic