On a bright day with a clear sky, a table like the one you see in a physician's office is placed in a large football field. A person, the subject of the experiment, is laid on his back on the table, facing directly upwards. He closes his eyes until asked to open them again.
Two poles, as high as the ceiling are planted on the left and right of the table, a few meters apart from each other, so that the person has to turn his head left and right to see that there are poles in the field.
The experimenter brings a new and shiny steel wire of small thickness, about 1-2mm, but nevertheless clearly visible to the human eyes, and ties it straight, flat and horizontally between the poles, directly above the subject's eyes. The only condition for this experiment is that the subject, when asked to open his eyes, must see only the wire and the cloudless sky in front of him.
Now, the subject is asked to open his eyes. He is not allowed to turn his head and see the poles or the ground, or the test is considered invalid.
The test is this: Will the subject, regardless of his birth and origin, be able to tell the distance between his eyes and the wire with a reasonable accuracy, say with a +/-10% error in his estimate?
Based on my experience, humans usually lose their focus no matter how hard they try in this experiment. Even if they are able to focus on the wire, they lose it again after a few seconds. So, what is it that the subject should do to estimate the distance of the wire from his eyes, given that he has not seconds, but hours to keep looking at the wire? Do you know the answer?
