Abstract
From beneath a glass table (Fig. 1), you peer up at a smooth stack of razor-thin disks: You see a silhouette. But of what? Each disk has the same apparent diameter. Replacing any of these opaque disks with a glass disk would not alter the appearance. So you cannot be viewing the bottom disk. It is causally idle, completely enveloped in a shadow cast from above. This shadow is not cast by any intermediate disk because none of them block light. Therefore, you see the silhouette of the top disk. The disks are so flat that they are held together by a vacuum effect. The top disk is the back surface of this cohesive stack. Now you understand why I believe that we see the backs of silhouetted objects.
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