The movements of the eye are designed primarily to effect fixation—that is, to bring upon the macula the image of the object that we wish to look at. When this has been accomplished, we know as a result of long experience, the direction of the object looked at and also direction of other neighboring objects. This knowledge is doubtless afforded us, in part, by our muscle sense. Thus we know that an object, A, is straight in front of us because we can see it sharply without moving either the head or the eyes from the position of rest or equilibrium; and we know that an object, B, is on the right of us because to see it sharply we have to move either the head or the eyes to the right, thus altering the muscular condition from one of rest to one of tension. But without moving either head or eye, we also know, while still looking at A, that B is to the right, for the image of B is then formed on a portion of the retina situated to the left of the macula. From long experience we also know that an image so situated means an object placed on our right. Moreover, the farther to the left of the macula the image B is, the farther to the right do we judge B itself to be.
Similarly, if B is so placed that its image falls below the macula, we then know B itself is really above A, which forms its image on the macula; and if the image of B is above the macula, we know that B itself is below A.
The [table on page 116] is suggested as a guide in cases of muscular imbalance:
Suppression of Image
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
| Name of diplopia | Image of right eye as compared with that of the left is | CAUSED BY | CORRECTED BY | ||||
|---|---|---|---|---|---|---|---|
| (1) By a natural deviation of | (2) Artificially by a prism placed, base | (1) Turning | (2) Prism placed with base | ||||
| Lateral |  | Homonymous | On the right | Either eye inward (esophoria, esotropia.) | In before either eye. | Both eyes outward (divergence.) | Out before either eye. |
| Heteronymous (or crossed) | On the left | Either eye outward (exophoria, exotropia.) | Out before either eye. | Both eyes inward (convergence.) | In before either eye. | ||
| Vertical |  | Right | Below | Right eye up or left eye down (right hyperphoria, right hypertropia, left hypotropia.) | Up before right eye, down before left eye. | Right eye down and left eye up (left supravergence.) | Down before right eye or up before left eye. |
| Left | Above | Right eye up or left eye down (right hyperphoria, right hypertropia, left hypotropia.) | Up before right eye, down before left eye. | Right eye up and left eye down (rightsupravergence.) | Up before right eye or down before left. | ||