Slope and the Zollner illusion

Informal observation suggests that the magnitude of the Zollner illusion is reduced when the figure is viewed on a sloping plane. The hypothesis that this effect derives from the enlargement of the acute angle of intersection between the obliques and the verticals in the figure when it is viewed on a sloping plane is here investigated. The magnitude of the Zollner illusion was measured with the use of a visual analogue scale. The results show tha t the change in the magnitude of the Zollner effect as a function of the sl ope of the figure is different from that for corresponding figures, with en larged angles of intersection between the obliques and the verticals, prese nted vertically. It is concluded that the enlargement of the angles of inte rsection can only partly account for the reduction of the Zollner effect wh en the figure is viewed under slope, and that some other factor must be inv olved. An alternative hypothesis is evaluated whereby the effects result fr om the diminution in the contrast of the obliques when the figure is viewed under slope. Data are also presented to show that observers are able to pe rceive the enlarged or foreshortened angles of intersection veridically.