Circles or Ovals?
You have two eyes, yet you see only one image of your environment. If your eyes receive conflicting information, what does your brain do?
Hold the round tube up to one eye and hold the flattened tube up to your other eye. Look through both tubes at the white screen, wall, or paper.
Angle the ends of the tube so the two spots you see overlap. Do you see the circle or the oval?
Switch the tubes and repeat. If you saw only the circle before, you may see the oval now.
Your eyes and brain have trouble merging the different shapes. Most people have a dominant eye; the brain will choose to see the image that is coming from the dominant eye. Some people do not have a dominant eye and therefore see the two shapes overlapped. The best baseball hitters don't have a dominant eye.
You probably know your dominant hand (are you right- or left-handed?), but do you know which eye is your dominant eye?
To find out, cut or tear a coin-sized hole in a piece of paper, pick a distant object, and look through the hole at that object. Slowly bring the piece of paper toward your face, keeping the object in view through the hole. At some point, the piece of paper will be right up against your face—and you will have found your dominant eye.
You've probably been using your dominant eye without realizing it when you look through a single lens view finder on a telescope or camera.