This is the condition in which both eyes look straight at the same object, whether both see it or not. There is not the slightest tendency of deviation. HETEROPHORIA This is the condition in which both eyes keep looking straight at the same object so long as both _see_ it; but as soon as one eye is excluded from vision (as by a screen) that eye deviates. This is then a tendency of deviation which is strong enough to become manifest when either eye is covered, but which is abolished or overcome by the compelling impulse of binocular fixation as soon as both eyes are used for seeing. A heterophoria thus produces a maximum deviation. The deviation is also said to be _latent_, since it is absent under ordinary conditions and is brought to light only under special conditions. A common though improper term for heterophoria is “insufficiency.” SQUINT Squint is the condition in which there is so great a tendency to deviation that even when both eyes are uncovered, one deviates and only one “fixes.” It differs, therefore, from heterophoria in that the deviation it produces is obvious under ordinary conditions. Squint is also called strabismus, or heterotropia. In other words, in orthophoria there is binocular fixation _all the time_ and under all conditions; in heterophoria it is present only when the two eyes are uncovered, so that both see the object looked at; while in squint it is not present at all. Or, in still plainer terms, in orthophoria both eyes are straight all the time; in heterophoria both are straight, but only so long as both are uncovered; and in squint only one eye is straight, no matter whether both eyes are uncovered or not. In squint, while binocular fixation is altogether absent, the ability to perform monocular fixation is almost always preserved; i.e., the squinting eye will “fix” at once if the other eye is covered. It is only when there is marked amblyopia, particularly as the result of a central scotoma (or spot on the cornea in the line of vision) that the squinting eye loses its power to fix at all, and wanders uncertainly about, receiving impressions now on one, now on another portion of the retina. The term imbalance is often used to denote the two conditions opposed to orthophoria; i.e., to denote collectively heterophoria and squint. VARIETIES OF HETEROPHORIA AND SQUINT 1. _Classification According to Direction of Deviating Eye_: Heterophoria and squint may be classified according to the direction assumed by the deviating eye. Thus we have the following varieties of heterophoria: HETEROPHORIA LATERAL DEVIATIONS Either eye deviates In, or toward the nose Esophoria Out, or toward the temple Exophoria VERTICAL DEVIATIONS The right eye goes up and the left down Right Hyperphoria The left eye goes up and the right down Left Hyperphoria