When foreshortening is 
isolated from other aspects of 
perspective, the equal vertical 
bars look different in length 
but the equal horizontal bars do 
not.

    If this conclusion is correct, 
it may explain a striking effect 
uncovered by Barbara Gillam. In 
the illustration to the left, 
foreshortening has been 
isolated from other usual 
concomitants of perspective. 
Equally spaced horizontal 
contours on a ground plane are 
represented as decreasingly 
separated vertically. One can 
achieve an impression of depth 
from it. Accordingly, one 
achieves a size illusion in 
viewing equal vertical extents 
on it.