The degree of lateral squint may be measured along the lower lid in millime tres of change in the direction of the ere from the deviating to the fixing position. But it is more accurately measured by the angle of deviation. This may be ascertained by placing the deviating eye at the centre of the are of a perimeter, and directing the gaze toward a distant point in the axis of that arc. Then find ing the point of the arc toward which the deviating eye is turned, we read off the angle of deviation. The point toward which the deviating eye is turned is ascer tained by moving a candle-flame along the arc, until the surgeon's eye behind the flame sees its reflection in the cen tre of the pupil of the deviating eye. Priestley Smith's method is applicable without a perimeter. In it the surgeon reflects light on the deviating eye with a mirror held at his own eye one metre from the patient, and has the patient look at his finger, which is moved at a distance of one metre from the deviating eye until the corneal reflex from that eye appears at the centre of the pupil. The distance from the surgeon's eye to his finger is then the measure of the strabismus. It may be measured on a scale of tangents showing the degrees of squint, or each centimetre corresponds to about one 1 centrad or four-sevenths of a degree. When there is diplopia the amount of squint may also be measured by the dis tance of the false image from the true image, or the strength of the prism re quired to bring them together.
To discriminate between paralytic and comitant strabismus, we must note if the deviation of the squinting eye or the separation of the true and false images is confined to a part of the field of fixation, or is greater in some parts than in others.
To ascertain which muscle or muscles are paralyzed, note the direction in which the eyes must be turned in order to pro duce the greatest deviation, or widest separation of the two images, this being the direction in which the paralyzed mus cle is most needed to turn the eye. The false image, belonging to the eye which cannot be normally turned, always ap pears farthest in the direction the eyes are turned. Thus, on looking up, the false image appears higher than the true image; on looking to the right the false image appears the farther to the right. By alternately covering the eyes we can find to which eye the false image belongs, and so the exact muscle or muscles af fected.
Diplopia is the rule in paralytic stra bismus, unless one eye be blind or cov ered by a drooping lid; but it is the ex ception in comitant strabismus.
To recognize latent squint we must pre vent binocular vision. To secure binoc ular vision the strabismus is rendered latent, and when the effort necessary to prevent strabismus no longer secures bin ocular vision it is given up and the eyes are allowed to deviate. Binocular vision
is prevented by covering one eye. When this is done the covered eye deviates. But on removing the covering the eye quickly turns to the position of true fixation. The deviation of the eye under cover may be so slow as to be with dif ficulty noticeable; but the quick "recov ery" when the cover is removed is very apparent.
By shifting the cover quickly from one eye to the other the eyes may be made to deviate and "recover" alternately. By so shifting the cover back and forth while the patient gazes at a distant lamp flame, he will see the lamp-flame appear to jump back and forth from one position to another as the cover is shifted. The direction in which the eyes deviate and ‘'recover" and the direction in which the flame appears to jump will tell the variety of latent strabismus present.
Binocular vision may be prevented by making the image received in one eye so unlike the other that there will be little or no tendency to fuse them. This may be done by placing before one eye a dark blue or purple glass. On looking at a distant flame the patient then sees two: one of the natural color, the other blue or pink. In orthophoria these appear superimposed; but with heterophoria they appear separated. The direction in which they are removed from one an other indicates the kind and the distance the amount of latent strabismus.
Binocular vision may also be prevented by use of a prism which so displaces the image formed in one eye that it cannot be fused with the image formed in the other eye. Thus, in the "Graefe test" a prism of S or 10 centrads is held with its base up before one eye, and the gaze fixed upon a dot in the centre of a blank card. To the eye before which the prism is held the dot appears displaced downward. In orthophoria it appears directly below the true image. In esophoria the lower dot appears below and toward the side of the eye that sees through the prism; in exo phoria downward and toward the op posite side. The phorometers of Stevens and others are mostly based on this prin ciple.
The Maddox rod-test is really one in which one image is so distorted as to pre vent its fusion with that of the other eye. A very strong cylinder, either a piece of a small glass rod or a concave cylinder of similar strength, is placed be fore one eye. Seen through this a point of light appears as a long streak. The other eye being left uncovered, the streak appears in orthophoria to pass through the point of light. But in heterophoria the streak appears to pass to one side of the point of light. The side on which it appears to pass indicates the variety of latent squint, and the distance of the streak from the light, or the strength of prism required to cause it to pass through the light, shows the amount or degree of the tendency to deviation.