that works perfectly. there are others, but they all do the same thing. The object stays in the center with one eye and not in the center with the other.not sure if this is it, but if you make a small triangle or circle (couple fo inches) with your two hands and focus on an object in the distance with both eyes and then close them one at a time and the one eye that still has it centered or close to center is your dominant eye...the other will not even see the image in teh circle or triangle