The anatomy of the muscles, ligaments, and fasciae of the orbit, including an account of the capsule of tenon, the check ligaments of the recti, and of the suspensory ligament of the eye / by C. B. Lockwood.

Lockwood, C. B.

Date:: [1885]

Credit: The anatomy of the muscles, ligaments, and fasciae of the orbit, including an account of the capsule of tenon, the check ligaments of the recti, and of the suspensory ligament of the eye / by C. B. Lockwood. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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$of accommodation on convergence negative glasses must be worn, and their prismatic effect ascertained. This necessitates a brief study of the subject. When each visual line passes through the optical centre of its corresponding glass, since it undergoes no practical deviation, the effect on convergence is nil. But if either traverses the glass away from this point, it undergoes a deflection—towards the optical centre in a convex lens, and away from it in a con- cave. The amount of deflection depends on—(1) the focal strength of the lens; and (2) the distance from the*optical centre that the glass is traversed by the visual line. In the accom- panying table I have worked out the angular bend or deflection experienced by the visual line in passing through lenses of different strength (left hand vertical column), and at different distances from the optical centre in millimetres (highest hori- zontal column).1 The deviation with glasses of low refraction is seen to be very trivial, but with cataract glasses of 20d each visual line would be turned more than 9° out of its course if each optica] centre lay one- third of an inch to the inner or outer side—an angular effect nearly equal to that obtained by wearing two clinical prisms, each marked 18°, or a single one of 36°. The table may be made of practical use in prescribing the exact distance between the optical centres required by different patients. It is often desired, in cases of so-called insufficiency of the converging function, to combine weak prisms with their bases towards the nose with the spectacles ordered, and thus more or less relieve convergence simultaneously with accom- modation. If the required prisms are weak enough, and the lenses strong- enough to obtain the effect by adjustment of the mutual dis- tances of the optical centres, all that is necessary is to run the eye along that horizontal column opposite the required strength of lens till the angle is found which is nearest to the decimating angle2 of each prism. The number of mm. marked at the head 1 The angle required is that whose tangent is obtained by dividing the distance of the visual line from the optical centre by the focal length of the lens. 2 The real decimating angle of each prism is about half the refracting angle by which it is marked. Since the only clinical use of prisms is to deflect the rays of light which traverse them, and since the index of refraction varies with the$