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7: Elliptical Trunk Illusion

Walk downhill through Memorial Glade, past the Bancroft Library (on your left) and the Moffitt Library (also on the left). On the left (south) side of the roadway, there is a large eucalyptus tree. Stand on the downhill side of the tree at a distance of about 15 feet.

You should be able to see a lamp post behind the tree on the left and a tall building (Evans Hall) behind the tree on the right. Look with one eye while holding the head still at the trunk of the tree. It might take 15 seconds or so of looking.

What is the apparent 3d shape of the trunk? Does it look cylindrical or does it look flatter than a cylinder (an elliptical cross section)?

Most people report that it appears elliptical. If you see that, open the closed eye and move your head and you’ll see that the shape becomes more cylindrical.

The explanation for this illusion is the same as #4. It works on this tree trunk because the texture of the bark doesn’t provide a good cue to its actual shape.

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