Perceived size of an afterimage grows with the distance of the surface it is projected onto, showing perception computes size from inferred distance.
Emmert's 1881 observation. It demonstrates that the visual system computes size from assumed distance rather than reading it off the retina, which is the mechanism behind most size illusions.
In practice
- A projected slide read at three metres seems the same size as one at ten, until you notice the second is four times the sheet.
- A photograph of the moon looks wrong because the camera did not share the eye's assumption about how far away it was.
- Further reading
- Wikipedia
- Subject
- Psychology & Behavior
- Essay
- A century and a half of measuring the mind