Measuring eye deviation using the alternating cover test and prism bar. One or other eye is always covered during the test. To elicit the presence or absence of a heterophoria along with type of deviation (eso-, exo-, hyper- and cyclophoria), effect of refractive correction, accommodation and CHP. The alternating cover test cannot differentiate between heterophoria and heterotropia but can be used to detect an intermittent heterotropia i.e. a heterophoria that changes (breaks down) to heterotropia on repeated cover testing. If the heterophoria breaks down it is likely to recover quickly during remainder of eye examination. The alternating cover test is fully dissociative and reveals the total deviation i.e. the habitual heterotropia or heterophoria experienced day-to-day by the patient. A prism bar can be used with increasing prism value and appropriate base direction until eye movement during alternating cover ceases.