Müller-Lyer illusion and perception of numerosity

Illusion of numerosity can be observed in many of the classical illusions of linear extent by replacing the uninterrupted lines with rows of dots. Using the method of constant stimuli both length and numerosity illusions move in the same direction, whereas using a magnitude-estimation method the two illusions move in opposite directions. Two experiments show that this inversion occurs also in the Müller-Lyer illusion.