The illusion shown here and 
before in realistic settings, is 
named after the Italian 
psychologist Mario Ponzo. 

The Ponzo illusion in its 
original form.

The illusion, of course, 
depends on the presence of the 
two oblique lines within each 
illustration. To distinguish 
among the elements of an 
illusion figure, psychologists 
term the features that undergo 
distortion the test components, 
and the remaining features, 
which create the illusory 
effect, the inducing 
components.

    How can we explain the 
effect the two oblique lines 
have on our perception of the 
relative lengths of the 
horizontal lines? According to 
the currently favored theory, 
the Ponzo illusion results from 
the same perceptual process 
that leads to constancy of size. 
Because the inducing lines 
appear to converge, they create 
the impression, on a two-
dimensional surface, of 
parallel lines receding in 
depth, such as the sides of a 
road or railroad tracks. Thus we 
perceive the figure as if it had 
depth. Because the top line 
appears to be farther away than 
the bottom line, it must look 
longer. Like the moon illusion 
discussed in Chapter 2, the 
Ponzo illusion can thus be 
regarded as an example of 
Emmert’s law, according to 
which objects yielding retinal 
images of the same size will 
look different in size if they 
appear to be located at different 
distances.