n.
Drawing or painting technique that gives a distorted image of the subject when seen from the usual viewpoint, but when viewed from a particular angle or reflected in a curved mirror shows it in true proportion.
Its purpose is to amuse or mystify. It was a curious by-product of the discovery of perspective in the 14th15th century and was regarded as a display of technical virtuosity. The first examples appear in Leonardo da Vinci 's notebooks.