Figure 3.

Stereogram of a Tuscan landscape (photograph © 1999 by Ansen Seale), modified, with permission, to include identical moons in each half-field. Fusing the two half-fields creates the impression that the left-hand moon is closer and also substantially smaller than the right-hand moon, as reported by subjects of Experiment (i) described in text. One should view the scene by staring at a point some distance behind the page. An animation simulating the experiment is available in the supplementary material on the PNAS web site, www.pnas.org.