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I'm trying to understand Manishearth's experiment in the answer here,

To try this out, close one eye. Now hold your arms straight out, nearly stretched, but not completely stretched. Now extend your index fingers (there should be about one inch between them). Now try touching them together. Try this a few times, then repeat with both eyes open. You'll realise how necessary the second eye is for judging depth.

I tried the experiment a few times but I am really not sure what I am supposed to see/ how the experiment works.

The one part I do understand is the reason they have said not to completely stretch the arms. If one does that, then by the sensation of arm being stretched will give a sense of depth, so it is necessary to not extend till to total arm length.

P.S: I completely understand the mathematics and fact we need two rays, but I think I am not getting the correct result for the experiment. Ideally an answer with pictures would be best.

Clemens Bartholdy
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2 Answers2

78

The problem with the two finger experiment is that your body’s sense of proprioception is so accurate and so instinctive that you don’t need binocular vision to touch your finger tips together. In fact, you don’t need vision at all. Try the experiment with your eyes closed. You will find that you can still touch your finger tips together quite accurately without even seeing them.

To get a better sense of the power of binocular vision, use a pen or pencil held in each hand instead of finger tips, to reduce the effect of proprioception. Wave the pens/pencils around to randomise the starting positions, and then try to make the ends of the pens/pencils meet. With both eyes open this task is very easy. With only one eye open you will find it is surprisingly difficult.

gandalf61
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You do not need two eyes to determine depth, although having two eyes certainly helps.

There are many ways to retain depth perception with one eye. See here: https://en.wikipedia.org/wiki/Depth_perception#Monocular_cues

Two rays is necessary, but you can attain two rays with one eye given special circumstances, by moving your one eye to two places, for example.

It is not uncommon for people to try this experiment and find that they have a fine time with depth perception with one eye closed.

In fact, you might even have a fine time doing this with both eyes closed, using your sense of proprioception. Better to try identifying a depth to an object which is not your own body. https://en.wikipedia.org/wiki/Proprioception

Alwin
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