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After looking at the eucalyptus trunk, turn 180 deg (looking westward) and walk along the left side of the roadway. You approach a wooden fence on your left. The fence forms a right angle near the road. Stop when you are aligned with the extension of the first part of the fence (so you are standing near the right angle).

Now look down the fence as it extends westward (downhill). Compare the apparent width of the 1st segment of the fence (the distance between the posts) and the 10th segment. Do they appear equal in width or does the 10th look considerably narrower? Most people report that the 10th appears narrower.

This illusion occurs because we tend to under-estimate long distances. We estimate the distance from the 1st to the 2nd post accurately because they are at short range. We under-estimate the distance from the 9th to the 10th post because of the tendency to under-estimate relative distances at long distances.

This is a well-known phenomenon called “depth compression.” You can experience this quite strikingly when hiking outdoors. Large distant mountains appear much closer than they actually are.

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