Have a piece of string? Test your 3-D vision

To perform the test, all you need is one piece of string that measures about the length of your arm. Tie a knot in the middle, and then a knot two to four inches from each end. Hold the string up to your nose with the closest knot about two inches away from your nose. Make sure your finger isn't obscuring your line of sight. Now you're ready to go. First, check to see how many knots you see as you look at the knot in the center. You should see only one. If you see two, you may have double vision, or diplopia, and should consult your optometrist. Next, check to see how many strings you see as you focus on the center knot. You should see two, crossing at the knot in the middle. If you don't see two strings, you don't see 3-D. I found that the strings kept popping in and out, so I definitely have issues with 3-D perception. If you see two strings, the next step is to determine where the strings cross. If they cross at the center knot, your eyes probably work well together and you can see 3-D. If they cross in front of the center knot, your eyes are pointing in too much and you have "convergence excess." And if the strings cross in back of the center knot, your eyes aren't pointing in enough and you have "convergence insufficiency."

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