EXPERIMENT 1 MethodThree subjects participated in the experiment, J.V.O., G.M.C., and R.R.H. G.M.C. and R.R.H. were emmetropic, and J.V.O. was corrected to 6/6 (20/20). All 3 subjects had normal stereothresholds and had no deviations of the alignment of the two eyes. Interpupillary distances for J.V.O., R.R.H., and G.M.C. were 6.6, 6.3, and 6.1 em, respectively.The stimuli were eight photographic prints of square-wave gratings that, at a viewing distance of35.6 em (14 in.), hadfundamental spatial frequencies of 0.3, 0.6, 0.9,1.1,1.5,3.2,4.2, and 5.7 cycles/degree (cpd) and a field size that subtended 43°x53°( II x 14 in.). The mean luminance of the 0.28-epd grating was 150 cd/rn"; its contrast was 75%. The highest frequency grating was also presented at a distance of 53 and 86 em (21 and 34 in.).These two stimulus conditions resulted in gratings of 8.2 and 13.3 cpd with field sizes of 29°x37°and 18°x23°, respectively. The 10 spatial frequencies were presented at seven orientations: 0°, 15°,30°,45°,60°, 75°, and 90°. The orientation values refer to the deviation from the vertical. Thus, 0°refers to the vertical, 90°to the horizontal, and 45°to an orientation midway between the vertical and the horizontal. All deviations from the vertical tilted to the right. The center of th~stimulus did not change as orientation was varied.The subjects viewed all stimuli at all seven orientations as they fixated the stimulus either with both eyes open or with the left eye closed. Although the subjects were not instructed to fixate any particular locus on the stimulus, they reported a tendency to fix below the center of the display because it improved the clarity of the illusion. The order of experimental conditions was counterbalanced across subjects and sessions. For each experimental condition, each subject made five ratings. The subjects were instructed to view the We report that the binocular presentation of gratings under appropriate conditions results in an illusion of depth. Multiple levels of depth are seen simultaneously, distinguishing the illusion from the wallpaper illusion. The levels are not overlapping but are 180°out of phase relative to one another. The conditions under which the illusion is visible and under which it disappears are presented. Probable causal mechanisms, the correspondence problem, the horopter, and differential image projections of large tangential stimuli are discussed.Recently, while examining binocularly a large photoBelow we present the results of three experiments that graphic print of square-wave gratings, we observed a sen-examined these depth sensations as a function of monocusation of several nonoverlapping, discrete levels of depth. lar or binocular viewing, fundamental spatial frequency The usual appearance of the illusion, if one's eyes fix-of the grating, orientation, field size, and viewing disated at eye level or slightly above, was that of a distant tance. The results indicate that the illusion depends on ellipse surrounded by more proximal ellipses. Figure 1 retinal disparity a...