Emmert's law

Law stating that the perceived size of an afterimage is proportional to the distance of the surface on which it is projected. It is named after Emil Emmert (1844–1911) who observed it 1881. Emmert's law has been found to apply also to eidetic imagery (mental imagery that remains for some time after the stimulus has been removed).

In normal vision, size and distance are related: the further away an object is, the smaller is its image on the retina, but it is perceived as distant rather than smaller due to a compensatory mechanism known as size-constancy scaling. In the case of an afterimage, the size of the retinal image remains the same, but the same compensatory mechanism functions, making the afterimage appear larger when seen over a greater distance.