We habitually use the eyes together, fixating with both at once; that is, we direct the eyes in such a way that the image of the object to which the attention is directed falls on the fovea of each eye.
Where both eyes are accurately directed to an object at which one or both are looking, the condition is known as “binocular fixation,” which is commonly understood to mean that both eyes are straight.
The ability to produce and maintain binocular fixation—to keep both eyes directly straight—is acquired early in life. The impulse to maintain it grows with exercise, and soon becomes so strong that after the age of infancy binocular fixation is present in the great majority of persons, and in most of them is present all the time.
Binocular fixation must be distinguished through three conditions—orthophoria, heterophoria and squint.
Orthophoria
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.