A pseudoscope [soo-duh-skohp] (‘false view’) is a binocular optical instrument that reverses depth perception. It is used to study human stereoscopic 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. It typically uses sets of optical prisms, or periscopically arranged mirrors to swap the view of the left eye with that of the right eye.

 In the 1800s Victorian scientist Charles Wheatstone coined the name and used the device to explore his theory of stereo vision. Switching the two pictures in a standard stereoscope changes all the elevated parts into depressions, and vice versa.

The pseudoscope produces other inversions also, it changes convex into concave, and high-relief into low-relief. The pseudoscopic inversion of a complicated picture — a landscape, streets, etc., produces a bewildering impression. It seems as if all the objects — men, trees, etc., had been placed in a depression of the earth, and yet everything remains in its place. Therefore, nearer objects appear very large, because we imagine them to be at a great distance, and more distant objects smaller, because they seem to be nearer.

Before the pseudoscope itself was created intentionally, it existed in binocular instruments as an imperfection. The first binocular microscope was invented by the Capuchin monk Cherubin d’Orleans. Because his instrument consisted of two inverting systems, it produced a pseudoscopic impression of depth by accident, although not recognized by microscopists of the time. The instrument subsequently fell into complete neglect for nearly two centuries. It was revived in 1852 by Wheatstone, who published his ideas in his second great paper ‘On Binocular Vision.’ Wheatstone’s paper stimulated the investigation of binocular vision and many variations of pseudoscopes were created, chief types being the mirror or the prismatic.

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