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Masson disk

American  
[mas-uhn] / ˈmæs ən /

noun

  1. a white disk on which a series of concentric gray circles appear to vanish intermittently when the disk is rotated, used for testing a person's fluctuation of attention and visual threshold.

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.