How to perform this exercise:

Hold your hands 5-7 inch away from your eyes, and point up with your fingers so you can see them.
Place your hands farther apart from each other on your sides, keeping them right in front of you at the same level as your eyes.
Slowly move your fingers closer towards each other while continuing to look at the center between the two fingers.
It is very important to keep your focus in the center between the fingers and not get distracted by anything else.
Slowly bring the fingers even closer until you see the double fingers overlap and become one, creating an illusion of a third finger.
Continue concentrating at the third finger until it becomes less blurry.
If the third finger disappears or you cannot see it anymore, try this exercise from the beginning.