Visual Illusion Cards by Y&B Associates Inc.. $9.95. Prepared by professors of psychology in the Gestalt tradition, Dr. J. Block and Dr. H. Yuker of Hofstra University. Contains 52 different visual illusions and instructions. Examples: illusions in color, color reversal, ambiguous figures, size and figure distortion. Card size 3-1/2" L x 2-1/2" W. Two different sets available.