by an ink dot upon the edge of the lower eyelid, m, and s. Next, cover the left eye with a screen, S (Fig. 83, B), and, on the patient again directing his gaze to the object, the right eye will be used for fixation, and then the position of the external border of the cornea is marked on the lower lid, m, with an ink dot. The space between the two dots will give the linear deviation of the squinting eye-the primary deviation. When the left eye is screened and moved inwards, FIG. 83.--Measurement of strabismic deviation.-FUCHS. A, Primary position. B, Secondary position. the right has turned into the position of fixation (Fig. 83, B). The left, L, is then in the secondary deviation position. A dot is made on the lower eyelid corresponding to the position of the outer edge of the left cornea, s, and the space between these indicates the linear amount of the secondary deviation, s, m. The two deviations, primary and secondary, in concomitant strabismus are equal, a fact which constitutes a difference between it and paralytic squint. With an eye of normal size, I mm. of linear deviation corresponds approximately to an angle of 5°. Mr. Priestley Smith uses a tape measure. The position of the patient's deviating eye is estimated by the position, centric or eccentric, of the corneal reflex FIG. 84.-Priestley Smith's tape measure for strabismus. of a flame. An ophthalmoscope held either before or below the observer's eye is the immediate source of the light. A double tape is employed, the use of which is explained by the accompanying figures (Fig. 84): "Fig. 1.—The ring O is placed by the observer on his forefinger, or on the handle of his ophthalmoscope. To it are attached two tapes, each I metre in length, one black, the other coloured. The black tape ends in a ring, P, which the patient places on his forefinger, and holds against his cheek below either eye, or against his chin; it determines the distance between observer and patient, and has no other purpose. The coloured tape is divided by lines into twelve parts, and figured 5, 10, 15, and so on up to 60; it ends in a small weight, which keeps it stretched when the hand of the observer passes along it in either direction, the tape sliding between the fingers. "Fig. 2 illustrates the measurement of a convergent strabismus of the right eye. The patient, seated below the ophthalmoscope lamp, and holding the tape as above described, is told to look at the mirror. The observer, holding the ring, O, and the mirror in the right hand, throws the light on the left eye, L. He sees the corneal reflection in the centre of the pupil, and knows thereby that this eye is fixing properly. He then throws the light on the right eye, R, and sees the reflex situated eccentrically outwards, and knows that this eye deviates inwards. Taking the graduated tape between the fingers of his left hand, and telling the patient to watch this hand, he moves it outwards along the tape (see Fig. 1), and meanwhile watches the corneal reflex in the right eye. When the reflex reaches the middle of the pupil, he reads the position of the hand upon the tape. The axis of the deviating eye, R, has moved from R' to O, through the angles RR O. The axis of the non-deviating eye, L, has moved through an equal angle OL L'. The angular movement of L, as measured by the tape, equals the angular deviation of R. Fig. 3 illustrates the measurement of a divergent strabismus of the right eye. In this case the observer sees the reflex of the deviating eye, R, situated eccentrically inwards. Taking the mirror and ring O in his left hand, he takes the graduated tape in his right, tells the patient to look at the hand, and moves it outwards along the tape until the reflex of R lies in the centre of the pupil. Then, as before, the position of the hand upon the tape indicates the angular deviation. "The hand should in all cases be held edgeways towards the patient; in this position, and at the distance of 1 metre, it forms a sufficiently small fixation object. "It is obvious that since the position of the measuring tape does not exactly correspond with the arc of a circle round the non-deviating eye, the angular measurements will not be absolutely precise; but if the observer remembers to keep both his hands as nearly as may be at the same distance from the patient's face, no important error will arise." The angular method (Landolt).—A perimeter is necessary. Direct the patient to fix a distant point in the median line with both eyes (Fig. 85). Then move a taper or candle along the arc of the perimeter, until its reflection is seen in the centre of the pupil of the squinting eye. The degree on the arc at this point will indicate the size of the strabismus angle. Tangent strabismometer.-Maddox has devised this method, and thus briefly describes it :— The scale in Fig. 78, p. 147, provides its lower series of smaller figures for this purpose. The only FIG. 85.-Measurement of the angle of a squint with the addition required is a piece of string, I metre long, suspended from the candle. "Stand the patient I metre away, as measured by the string held up to his eye, place your own head between him and the flame, but a little lower down, so as not to intercept the light, bidding him look at the candle, |