Fig. 13. Dissimilar targets viewed by each eye, either by allowing eyes to become exotropic or by inserting base-in prism before eyes so the intersection of lines of the X and the center of the horizontal line are superimposed. While fixating the center of the X, the eyes no longer perceive the center of the horizontal line. Conversely, while seeing the center of the horizontal line, they lose the center of the X. The portions of both the X and the horizontal line that are projected onto rod-populated retinas are simultaneously perceived. As the paper is tilted, the centers of the dissimilar targets move vertically apart. When both centers project onto the rod-populated retina, both entire dissimilar targets are seen. (Adler FH: Physiology of the Eye. St. Louis, CV Mosby, 1959)