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For the en:Stereoscopy article.

To view the crossed-eye view shown here, the viewer should move slightly back from his or her normal viewing distance and place his viewpoint on a line perpendicular to the center of the image. A finger should be placed halfway between the eyes and the image, then the finger should be viewed. The three bright spots between the pictures should become four spots, and the two images become three. If the focus of the eyes is now allowed to drift to the surface of the screen without uncrossing the eyes, a three dimensional depth illusion will appear in the central image. The finger may now be removed from the view. A viewer may find that the extra side images become unimportant once in-depth view of the central image is stable.