MAGNITUDE OF THE ZOLLNER ILLUSION AS A FUNCTION OF THE PERCEIVED LENGTH OF THE OBLIQUES

Summary.-The average magnitude of the Zollner illusion was increased by increasing the perceived length of the obliques, even though the physical length of the obliques was held constant. Subjects were 18 male and 42 female undergraduates. The ZijlIner illusion consists of s perceived convergence or divergence of physically parallel lines, as a consequence of viewing the parallel lines against a background of obliques. In previous studies of this illusion, Wallace (1969) and Oyama (1975) reported that the magnitude of the illusion increased with an increase in the physical length of the obliques which intersect the parallel lines. In the current investigation, the purpose was to determine whether the magnitude of the ZlIner illusion would increase with an increase in the perceived length of the obliques, while the physical length of the obliques was held constant. Differences in perceived length were effected by making the obliques crossing the parallels either a fins-out (the overestimated shaft) version of the Miiller-Lyer or a fins-in version of the Miiller-Lyer. The Ztillner illusion was drawn on a stencil and mimeographed. Two parallel lines were drawn 4 cm. apart, were each 9.5 cm. long, and were horizontal. Eighteen obliques were drawn through each horizontal. Each oblique was 2 cm. long and intersected the horizontal at an angle of 58". On one stencil, the ends of all obliques had arrow tips pointing our, i.e., the fins-in version of the Miiller-Lyer. On a second stencil, the ends of all obliques had arrow tips pointing in, i.e., the fins-out version of the Miiller-Lyer. The angle between the lines forming the arrow tip, in every instance, was 32" and the length of each line forming the arrow was .5 cm. To the right of the parallel horizontals and 7 un. from their ends, a vertical, 12.5-cm. long line appeared, and upon this line, each observer was to indicate the apparent projection of each horizontal. Given the current configuration, the usual illusion would be that the parallels diverge, hence, the observers' indicated apparent projections would be expected to exceed the physical separation of 4 un. between the parallel lines. A copy of the Zollner illusion with the obliques having "fins-in" was combined in a booklet with a copy of the Zollner illusion whose obliques were "fins-out." The Zllner with "fins-in" obliques was counterbalanced with the Zollner having "fins-out" obliques so that each appeared first in half of the booklets. In each booklet, opaque yellow paper separated and concealed the Zollner configurations so that the observer could see a configuration only as instructed and only one configuration at a time. Observers were 18 male and 42 female undergraduate studeno. Their average age was 18.6 yr. with a standard deviation of 1.3. Observers were tested in a single group.' They were seated, and the booklet containing the illusion patterns was before 'Boone, NC 28608. 'This method of obtaining observers' responses is based on a procedure used by MacKay and Newbigging (1977) in their investigation of the Poggendorff illusion. An individual testing procedure using few observers was considered, but the procedure described here was selected in that it permitted data collection from a large number of observers, uiz., N = 60 vs Oyama's (1975) N = 4 and Wallace's (1969) N = 22.