The illusion of afterimages 
appearing to vary in size despite 
a constant retinal image is 
precisely what we must predict 
if perceived size is governed not 
only by visual angle but also by 
distance. The two seemingly 
different facts, that images of 
the same size lead to 
perceptions of different size 
(Emmert’s law), and that images 
of different size lead to 
perceptions of the same size 
(constancy), in fact illustrate 
the same principle: Distance is 
taken into account in 
computing object size from 
image size. This leads to 
illusions such as in Emmert’s 
law when the image remains 
constant and to veridicality (or 
constancy) when the image 
diminishes with distance. This 
principle also helps explain a 
common illusion of size in 
viewing the moon.