Assimilation theory was employed to generate quantitative predictions relating the ingoing Müller—Lyer illusion to both angle of fin and gap between shaft and fin. Gap sizes were varied from zero to 18 mm in steps of 2 mm, and angles of fins were varied from 30° to 180° in steps of 30°. Five subjects adjusted the length of a comparison line to match the apparent length of the shaft at each combination of gap size and angle of fin. It was predicted that the gap necessary for a reversed illusion to occur, and the gap at which the maximal reversed effect occurred, would be inversely related to angle of fins. Empirical functions verified the predictions.
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