In the standard version of the Poggendorff figure a transversal intersects two parallel verticals and the segment of the transversal between the two intersection points, A and B, is not shown. The two portions of the transversal outside the parallels then seem to be misaligned. Besides this illusion of direction, there is also an illusion of size, the distance AB being underestimated in the standard figure. The influence of configural components in determining this spatial distortion of the Poggendorff figure was examined by having subjects reproduce the inner oblique (at 45 degrees) extent AB in variations of the figure. This distance was found to be underestimated in Poggendorff variations which contained parallel (vertical) components that formed an acute angle with AB; and the underestimation increased as the number of these components present in the figure increased. The distance AB was found not be significantly distorted in figures which contained only those parallel (vertical) components that formed an obtuse angle with AB, yet their presence in the figure tended to counteract the underestimation. When the transversals were omitted, the underestimation was found to increase. The findings are interpreted in support of an explanation that reduces the Poggendorff effect to those factors which mediate the Müller-Lyer illusion.

On the basis of a version of the Zeigarnik (1927) demonstration, the effect of task interruption and closure on perceived duration was examined. Subjects estimated the time it took to solve a list of 10 three-letter anagrams; this group thus experienced closure when they completed the task. A second group of subjects was presented with a 20-item set of anagrams, the flrst 10 of which were identical to the items solved by the flrst group. These subjects were interrupted after solving the flrst 10 items, then they estimated the time. A signiflcant difference in the perceived duration of the task was found between the two groups: subjects who were interrupted signiflcantly overestimated the time it took to solve the flrst 10 anagrams. These flndings indicate that task interruption has a lengthening effect on perceived duration. Gestalt notions of closure and motivation are discussed.

Previous investigations have shown that the magnitude of the Müller-Lyer illusion is a function of the linear and angular dimensions of the figure. If the Müller-Lyer and Poggendorff illusions share a common basis, then the magnitude of the Poggendorff illusion should similarly be a function of the analogous configural dimensions. A study is reported in which changes were made in the dimensions of the Poggendorff figure that are analogous to the dimensions of the Müller-Lyer figure: the length of the parallel components (analogous to the wings of the Müller-Lyer figure); the length of the intertransversal extent (analogous to Müller-Lyer shaft length); and the angle formed between the parallel components and the intertransversal extent (analogous to the angle of wing attachment in the Müller-Lyer figure). The relationship between the magnitude of the illusion and the dimensions of the Poggendorff figure was found to be generally in line with previous findings relating to the Müller-Lyer illusion. Adaptation-level theory and the positive-context model accommodate the major findings of the present study.

In order to assess the role of the structural components of the Mueller-Lyer illusion, subjects reproduced the central extent of standard Mueller-Lyer figures and configural variations. Illusory magnitude of the underestimated wings-in and overestimated wings-out figures was examined with selective amputations of the oblique wings and central line segment (shaft). Varlations were presented at 0,45,90, and 135 deg from vertical. Orientation had no reliable effect on illusory magnitude. Elimination of the shaft effected a decrease in the apparent extent for all variations, presumably due to the addition of the filled-unfilled space illusory effect to the standard Mueller-Lyer effect. A second study corroborated this finding: The decrease in apparent extent consequent to shaft removal occurred independently of any response factor. Selective wing removal differentially decreased the illusory magnitude of the standard MuellerLyer figures; this was discussed with regard to a dual-illusion hypothesis. Finally, variations that contained no intersecting lines produced a significant illusion in the direction of the standard Mueller-Lyer figures, suggesting the involvement of higher level, nonperipheral distortion mechanisms.The familiar form of the Mueller-Lyer illusion pattern characteristically effects a measurable error in the apparent length of a central line segment when this segment is appropriately flanked by oblique angles (Figures lA and lB). Although a number of investigations have examined the contribution of various components (e.g., length of the Oblique angles, the angle of Oblique attachment, etc.; see Robinson, 1972, for a review of configural manipulations and a historical review of the Mueller-Lyer illusion), there has been little systematic study to determine the critical stimuli within the basic figure for effecting and affecting the magnitude of the illusion. The effect on