Emmert's law

Emmert's Law bears the name of Emil Emmert (1844–1911), since it was first described by him in 1881. Emmert noted that an afterimage appeared to increase in size when projected to a greater distance. It is unclear whether he intended this to mean physical distance or perceived distance, but most authors assume the latter. A modern version of the law states that objects that generate retinal images of the same size will look different in physical size (linear size) if they appear to be located at different distances. Specifically, the perceived linear size of an object increases as its perceived distance from the observer increases. This makes intuitive sense: an object of constant size will project progressively smaller retinal images as its distance from the observer increases. Similarly, if the retinal images of two different objects at different distances are the same, the physical size of the object that is farther away must be larger than the one that is closer.

The effect of viewing distance on perceived size can be observed by first obtaining an afterimage, which can be achieved by viewing a bright light for a short time, or staring at a figure for a longer time. It appears to grow in size when projected to a further distance. However, the increase in perceived size is much less than would be predicted by geometry, which casts some doubt on the geometrical interpretation given above.

Emmert's law is closely related to size constancy, and has been used to investigate the moon illusion (the apparent enlargement of the moon or sun near the horizon compared with higher in the sky). A recent neuroimaging study that examined brain activation when participants viewed afterimages on surfaces placed at different distances found evidence supporting Emmert's Law and thus size constancy played out in primary visual cortex (V1); i.e. the larger the perceived size of the afterimage, the larger the retinotopic activation in V1.

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