Failure of Constancy

In many instances in daily life, 
objects located at very great 
distances appear extremely 
small. Consider, for example, 
the striking smallness of 
houses viewed from a plane 
traveling at an altitude of 
20,000 feet or the size of people 
on the street when viewed from 
a skyscraper. Students of 
perception have used such 
expressions as "the falling off 
of constancy" and 
"underconstancy" in 
describing these instances.

    The logic behind this 
terminology is straightforward. 
Constancy can be no better 
than the sensory information 
on which it is based. Because 
our sensory capacities are 
limited, we are not always able 
to achieve full constancy. With 
respect to size constancy, for 
example, one might say that 
the maximum distance that can 
be detected perceptually is on 
the order of hundreds or, at 
most, thousands of yards. We 
can thus hardly expect that the 
distance to, say, the moon, 
which is 250,000 miles away, 
will be perceived accurately. 
Instead we should expect the 
moon over the horizon to 
appear to be about the size of a 
house or other terrestrial 
object of the same visual angle 
on the horizon. Since the moon 
is roughly 2000 miles in 
diameter, one might say that we 
have here a considerable 
departure from constancy. But 
it is important to note that the 
failure does not lie with the 
perceptual apparatus 
responsible for constancy but 
with the information available 
to it.