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Two of the most eminent authorities on muscle imbalance of the last half-century, Alexander Duane and Ernest E. Maddox, have enunciated the essential conditions of a satisfactory test for the muscle balance. Duane was the most strenuous advocate of the objective cover test with its subjective complement, the parallax test. Maddox, in referring to this cover test in an introductory address on heterophoria before the Ophthalmological Society of the United Kingdom, 1929, said that the most important thing was to keep the eye "covered long enough for the deviation to develop. " The prolonged occlusion test is a combination of these two conditions. It puts into practice the principles which these two authorities have preached. It seems doubtful, however, whether either of them put the combined principles into practice and carried out an actual prolonged test. Indeed, it is doubtful whether Maddox realized the full significance of his