When watching an object, the images of the object on the retinas of both eyes overlap with each other in the central pivot of brain and become a complete and sole image with three dimension, which is called the binocular single vision. But crossed eyes can not both focus on the same object, i.e., when one eye watches an object, the other cannot look at the same object because it inclines to the inner side. People with crossed eyes can only see things without three dimension, and they cannot distinguish whether the object is near or far away, which will of course many troubles in life, for example, they cannot thread a needle when doing sewing work, they cannot put things into a certain hole, they cannot fetch a small object nearby, and so on. Crossed eyes can influence people's daily life and study and work, so people with crossed eyes should go to doctor's as soon as possible for treatment, and it takes patience and perseverment.
