Geometrical llusions, of which Muller-Lyer is an illustration, is an example of how our discriminal processes do not faithfully represent the objective world. We do not always see things as they exist in physically measured reality. In the Muller-Lyer illusion, we tend to underestimate the length of the arrowhead line. The constant error gives the quantitative measure of the extent of illusion. The movement error is the bias which the subject may
have for moving the variable outward (ascending series) as compared to moving it inward (descending series), or vice-versa.
