Focus the center of the image for 20 seconds.

Explanation: Troxler’s fading.
Troxler, D. (I. P. V.) (1804). Himly, K.; Schmidt, J.A.. eds. “Über das Verschwinden gegebener Gegenstände innerhalb unseres Gesichtskreises. [On the disappearance of given objects from our visual field]” (in German). Ophthalmologische Bibliothek 2 (2): 1–53. OCLC 491712012.
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