Fig. 2 Alternate cover test. Cover before right eye (A) is rapidly transferred to left eye (B) as examiner observes right eye for any shift it may make to establish fixation. Alternately transferring the cover from eye to eye while observing the uncovered eye discloses any heterodeviation that may be present, provided each eye can centrally fixate. C. Introducing a prism before one eye allows measurement of the heterodeviation (prism and alternate cover test).