Archives of ophthalmology and otology . shortening we may choose, as be-ing perpendicular to the visual line in the point of fixation, as inFig. 2, in which A represents the nodal point of the eye, B thepoint of fixation ; therefore A B the radius e of the semi-circleC B D. But it is evident, at the first glance, that equal arcs, pro-jected in the usual way upon the tangent line E F, do not giveequal rectilinear If we transfer the visual field from * Tliis instrument, when placed opposite a high, lightsome window, admits of asufficient intensity of illumination, and renders the ex
Archives of ophthalmology and otology . shortening we may choose, as be-ing perpendicular to the visual line in the point of fixation, as inFig. 2, in which A represents the nodal point of the eye, B thepoint of fixation ; therefore A B the radius e of the semi-circleC B D. But it is evident, at the first glance, that equal arcs, pro-jected in the usual way upon the tangent line E F, do not giveequal rectilinear If we transfer the visual field from * Tliis instrument, when placed opposite a high, lightsome window, admits of asufficient intensity of illumination, and renders the examination very convenient. •)• The arc B K gives B G, the arc L M = B K gives H I ; B G being considerably largerthan H I. Let r af be the projection of a small arc a b^ then we obtain c d ^^ a b. cos.(a b, c d) = fl b COS. a m «, or generalized (i) dp ^ d b cos. fl, if db denotes a small arc,/8, its angle of inclination toward the polar tangent, and dj> its rectangular projection. The Dctcrmi7iation of the Field of Vision. 369. 370 y. Hirschbcrg: the spherical surface to the plane of the paper by rectangularprojection, the radii of the parallels must be proportionate, notto the ujinibers of the degrees, but to their sinuses,* as is illus-trated in Fig. 3. I have here restricted myself to 12 meridians, respectively 12radii, as in Scherks perimeter, the end-points of which aremarked with Roman numbers, in the same manner and successionas the numbers of a clock. This is very convenient for keepingrecords. I note, for instance, of a patient suffering from retinitisdextra : December 23, 1874. F R 55, 80, 85, 85, 70, 62, 65, 60, 55, 60, 45, 50. In this way the two radii, which severally form a straight line,are placed the one above the other, so that by addition the totalextent of the visual field in each of the six directions may be as-certained. In the same easy way a circumscribed defect in thevisual field may be noted down, if we do not deviate from theonce chosen order
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