(a) Illusions teach us that we are not of our world.(b) A drawing that can be perceived in more than one way is said to be.
Answers: (a) passive observers; (b) ambiguous.Returning to the Moon illusion, why does the Moon appear larger on the
horizon than when it’s overhead? The illusion is a variation of the Ponzo illu-
sion,an illusion associated with linear perspective. Parallel lines, like those associ-
ated with railroad tracks or the sides of a roadway, appear to converge as they
approach the horizon. At the horizon itself they meet, and this is called the van-
ishing point.If in a drawing two objects of the same size are simultaneously
placed so that the first object is far from the horizon and the second one is near
the horizon, the second object will be perceived as being larger than the first one.
This is because, in a drawing, the retinal size of both objects is the same. However,
the second object seems to be larger than it is in terms of comparisons we auto-
matically make with other objects near the horizon.
Note that in everyday perception the Ponzo illusion does not occur. This is
because the retinal size of an object near the horizon is smaller than that of an
object closer to you. When the size of an image projected on the retina shrinks
with distance, the apparent size of the object remains the same. This is a percep-
tual phenomenon called size constancy.For example, an approaching friend first
seen when twenty feet away and then when closer to you appears to be the same
size. However, in the case of the Moon illusion, the size of the Moon’s image pro-64 PSYCHOLOGY
The vase-faces illusion.