Pseudoscope

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A pseudoscope is an optical instrument that reverses depth perception. Objects viewed through it appear inside out, for example: a box on a floor, would appear as a box shaped hole in the floor.

[edit] How it works

"Whereas stereoscopic depth perception is the result of fusing the different images received by the two eyes, pseudoscopic perception is the result of switching the inputs to the eyes before they are fused, so that the right eye receives information normally received by the left eye, and the left eye receives information normally received by the right eye.

The pseudoscope does to front and back what a mirror does to left and right. This means that foreground becomes background and visible background becomes foreground, or more simply, background advances, foreground recedes. So when you view objects with this scope, the convex becomes concave. So a brick lying on the ground appears to be a brick-shaped hole sunk into the ground. A tree turns inside out as it were - its front branches appear at the back, whilst the back branches come out in front, hanging or suspended in mid-air as their support is eclipsed by branches in front.

Early pseudoscopes were prismatic (they used two prisms to reverse the images). M. C. Escher used one, and it became one of the most important inputs in his work. However prismatic pseudoscopes have three limitations; these are a small field of view, lateral inversion, and a weak pseudoscopic effect. A mirror pseudoscope has a substantially larger field of view, and no lateral inversion."^[1]