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   Also known as Oppel illusion, filled-space illusion, and filled/unfilled space illusion. The eponym Oppel-Kundt illusion refers to the German physicists Johann Joseph Oppel (1815-1894) and August Adolph Eduard Eberhard Kundt (1839-1894), who have both been credited with describing specific aspects of the concomitant " geometric-optical illusion described by Oppel in 1854. The Oppel-Kundt illusion involves the subjective impression that a distance divided by graduated lines is longer than a similar, yet undivided distance. This principle is also referred to as Kundt's rule. As early as the fourth century BC, this illusion was described by the Greek philosopher Aristotle (384-322 BC) in his book Problems. As Aristotle's text describes the divided line as appearing shorter, while it is known to appear longer, it has been suggested that it may not have been written by Aristotle but by one of his pupils. As is the case with other geometric-optical illusions, the Oppel-Kundt illusion is considered a physiological phenomenon that arises as a consequence of the inherent properties of the visual system - which prompts the brain to calculate a 'weighted mean value' that is spread out over a population of neurons, and leads the observer to overestimate the divided distance in comparison to the undivided one. The Oppel-Kundt illusion is generally classified as a geometric-optical illusion, which itself tends to be classified as a subtype of the "optical illusions.

   References

   Kundt, A. (1863). Untersuchungen über Augenmaß und optische Täuschungen. Poggen-dorffs Annalen der Physik und Chemie, 120, 118-158.

   Oppel, J.J. (1854/1855). Ueber geometrischoptische Täuschungen. (Zweite Nachlese.) In: Jahres-Bericht des physikalischen Vereins zu Frankfurt am Main, 37-47.