AFTERIMAGES AND EMMERT’S LAW 
(Left)  Forming an afterimage.  
(Right)  Projecting the 
afterimage to differing 
distances.  The perceived size of 
the afterimage illustrates 
Emmert’s law.

An afterimage is a percept 
lasting only a few seconds that 
is caused by the fatiguing of 
those retinal cells that have 
been stimulated by the light 
from an object. To achieve this 
effect, briefly view, while 
keeping the eyes stationary, a 
highly contrasting region such 
as a small hole cut out of 
cardboard placed in front of a 
light. Then look at surfaces at 
varying distances from you. You 
should see a visual "thing" that 
is the same shape as the object 
that created it, located at the 
distance of whatever surface is 
now viewed, as shown above. 
Notice that, if the afterimage is 
viewed on a paper held at arm’s 
length, it will appear relatively 
small. If it is viewed farther 
away, however, as on a distant 
wall or billboard, it will appear 
much larger, even though the 
size and shape of the retinal 
image remain the same. The 
perceived size of the afterimage 
varies directly with the 
distance of the surface on 
which it is viewed. This 
relation is an instance of a 
more general perceptual 
relation known as Emmert’s 
law: The perceived size of an 
image of a particular visual 
angle is directly proportional to 
its perceived distance.