Jastrow Illusion
What to see
On the right you may observe a red and a green arc. Which one is bigger? [For me
it looks like a large bowl resting on a smaller bowl.]
While you are pondering this question, they exchange their places (whenever the
little progress wheel’s indicator is on top). Unexpectedly, when stacked in inverse
order, their size also appears to be exchanged.
Of course, as you guessed, their size, in fact, is identical.
What to do
You can click on the arc shapes and move them around. So you can check for yourself
that they are identical.
Comments
This is a classical “geometric illusion”, a variation on the version first described
by
Joseph Jastrow in 1891 (a lively biography here). A number of studies have examined
it, still this illusion is not well understood. Jastrow himself wrote (I edited
his words so they make sense without his context, and they apply to his figure depicted
below on the right): “The lower figure seems distinctly the larger, because its long
side is brought into contrast with the shorter side of the other figure. … In judging
areas we cannot avoid taking into account the lengths of the lines by which the
areas are limited, and a contrast in the lengths of these is carried over to the
comparision oft the areas. We judge relatively even when we most desire to judge
absolutely.”
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Jastrow’s original (1892, p 398, Fig. 28)
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What does “understanding an illusion” mean anyway? For me any explanation would
follow naturally from a general understanding of the mechanisms underlying vision.
For instance: the “stepping feet” illusion
follows directly from the property of motion receptors being (nearly) color blind.
For the Jastrow illusion it is easy to come up with hand-waving explanations invoking
local size comparisons etc., but these would be ad-hoc explanations, specific only
for the phenomenon at hand and thus not very satisfying.
Jastrow, by the way, also described the famous “duck-rabbit” bistable image.
Sources
Jastrow J (1892) Studies from the laboratory of experimental psychology of the University
of Wisconsin – II. Am J Psychol 4(3):381–428
Created: 2012-05-07
Last update: 2013-10-04