The optical-geometrical illusion "corner Poggendorff" re-examined.

In 1988 Greene noted, if a straight pair of obliques are drawn outside of two orthogonal lines, the segments appears to be angled slightly one relative to the other. This illusion, designated as "corner Poggendorff" is different from the Poggendorff effect (the two obliques seem to lie on a parallel path). The results of the present experiments (N = 76 students) suggest that the general conditions for the corner Poggendorff are similar to those for the Judd illusion.

Obliquity integration: variations on a theme of Delboeuf.

An optical-geometrical illusion, described by Delboeuf and not familiar to specialists, is investigated. The results of two experiments show that the divergence between a bar filled with parallel slanting lines and a line drawn above it is clearly related to this angle of the lines which fill the bar. The illusion is already present when this angle is 10 degrees, reaches its maximum at 20 degrees, decreases at 30 degrees, and almost disappears at 40 degrees. These results are similar to those found for the tilt illusion, are slightly different from those found for the rod-and-frame illusion, and differ greatly from those found for the Zöllner illusion. The other variables considered-the distance between the slanting lines filling up the bar, the distance between the upper line and the bar, and the width of the bar--do not influence the illusion as much. Since either the line appears as diverging from the bar, or the bar seems inclined in relation to the line, the illusion should be considered a complex one. The small oblique lines inside the bar induce obliquity in the opposite sense in the display, but which of the elements is seen as diverging from the other depends on which of the two is established as the frame of reference.

Wheels: a new illusion in the perception of rolling objects.

A new illusory effect is described which consists of a conspicuous perceptual overestimation of the speed at which a wheel rolling across an observer's visual field appears to be rotating. Wheels appear to revolve much faster than is compatible with their linear displacement. Experimental verification of the genuineness and magnitude of the effect is reported, along with a discussion of some of the variables upon which it may depend.

On misoriented letters.

An experiment is reported in which subjects were presented with misoriented letters and asked to identify the transformations to which normal letters had been subjected to produce the misorientation. When two or more transformations were equally justifiable from the geometrical point of view, they did not have necessarily the same relevance from the perceptual standpoint. For instance, it was easier for the subjects to see an upside-down letter R as the outcome of two reflections (one about the horizontal axis and one about the vertical axis) rather than of a 180 degrees rotation in the picture plane. Contrary to expectation, for the same final result a reflection--a movement in three dimensions--was preferred to a rotation--a movement in two dimensions--and two reflections were preferred to a single rotation. To explain the distribution of the responses, a hypothesis on the 'mode of appearance' of equivocally oriented objects is presented. This postulates that the directional axes assigned to the objects tend to remain in correspondence with those of the environment. Moreover, the polarization of the up-down axis tends to be preserved, while that of the left-right axis does not. Implications of this simple model for the perception of misorientations are discussed.

Orientation sensitivity in the peripheral visual field.

Orientation sensitivity in the peripheral visual field has been tested in two tasks: (a) setting horizontal the orientation of a grating at various retinal eccentricities, and (b) matching the orientation of a peripherally viewed grating as close as possible to an oblique reference viewed foveally. Both performances fall off with increasing retinal eccentricity. Magnification of the stimulus optimizes peripheral performance. Peripheral performance, optimized by magnification, varies with retinal eccentricity. It approaches, but does not reach, the foveal value (tested by the same method) at 10 deg of eccentricity, and is much lower at 20 and 30 deg of eccentricity.

Another optical-geometrical illusion.

A new optical-geometrical illusion is described. The parallelism of short rows of dots is affected by some unknown factor, so that the rows appear as pivoting on their middle point. Some explanations of the illusion are considered, but with no success.