All deviations should be and probably are primarily associated with diplopia. Yet in the great majority of cases of established squint, especially convergent squint, there is no double vision. This is due to the mental suppression of the image by the squinting eye. In such cases all attempts to evoke diplopia by our tests may be futile, the patient not appreciating the presence of double images even when they are widely separated by prisms. Moreover, this suppression usually persists after the squint is cured, so that even though there are two retinal images of the same object, the mind perceives but _one_ and ignores the _other_, just as though it were not present. In this case there is no true stereoscopic, or solid, vision. MONOCULAR DIPLOPIA Binocular diplopia, due to deviation of the eyes or to prisms, must be distinguished from monocular diplopia, or the condition in which the patient sees double with one eye alone. This occurs as the result of astigmatism, plus spherical aberration and other conditions found occasionally in squint. It can readily be differentiated by the fact that binocular diplopia disappears when the patient shuts either eye; while monocular diplopia, of course, does not. TABLE OF DIPLOPIA ==================+===========+=====================================+ | Image of | | | right eye | CAUSED BY | |as compared+-------------------+-----------------+ Name of |with that | |(2) Artificially | diplopia | of the | (1) By a natural | by a prism | | left is | deviation of | placed, base | ------------------+-----------+-------------------+-----------------+ {Homonymous | On the |Either eye inward |In before either | { | right | (esophoria, | eye. | Lat-{ | | esotropia.) | | eral{ | | | | {Heteronymous| On the |Either eye outward |Out before either| {(or crossed)| left | (exophoria, | eye. | | | exotropia.) | | ------------------+-----------+-------------------+-----------------+ {Right | Below |Right eye up |Up before right | { | | or left eye down | eye, down before| Vert-{ | |(right hyperphoria,| left eye. | ical{ | | right hypertropia,| | { | | left hypotropia.) | | { | | | | {Left | Above |Right eye down |Down before right|