The Poggendorff Illusion and estimates of transverse extent.

The underestimation of transverse extent relative to longitudinal extent, in the Poggendorff Illusion, was tested by varying oblique line orientation, interparallel line distance, and presence or absence of obliques. 20 subjects made estimates of the transverse extent on both a longitudinal and transverse extent. The results indicated that, although underestimation was found for some stimulus conditions, overestimation was found fot others. It was argued that even though presence of obliques affected judgmental error the longitudinal-transverse illusion could not form a basis for the Poggendorff Illusion.