3: The Sinking Tower Illusion
Now walk slowly toward the archway keeping the tower centered in the arch. You’ll notice that the tower seems to sink as you walk forward. The most likely explanation is the following.
The brain estimates the distance from you to the arch and from you to the tower. The tower is quite large and quite distant, so the brain actually under-estimates its distance (and its size, but that’s not important to the explanation).
Suppose, for example, that the ratio of actual distances (tower/arch) is 20 (a pretty good approximation), but that the ratio of perceived distances is 5 (another pretty good approximation).
Given the assumed ratio of 5, when you step forward, the top of the tower should rise in your visual field at 1/5 the rate as the top of the arch (from the geometry). It doesn’t rise that fast, so the brain concludes (erroneously) that the tower is sinking.
This illusion is related to the well-known size-distance illusion. It works better during earthquakes. There is an interesting variant of this illusion, which we will call the Growing Tower Illusion.
After positioning yourself to see the Sinking Tower Illusion, walk slowly backward in a straight line away from the archway. Keep the archway centered in the arch as you did when walking forward.
You will notice that the tower seems to grow as you move farther away. Also notice the difference on how the tower grows and shrinks.
Upon moving forward, the tower shrank in increments and upon moving backward, the tower grows at a more constant rate. The explanation for the Growing Tower Illusion is the basically the same as explanation for the Sinking Tower Illusion.
When you move backward, the image of the tower goes higher in the archway. The distance to the tower is great, so we don’t take its distance into account properly. As a result we see the tower appear to rise.
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