. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . + D and — D. 47. Examination of the cornea with the keratoscopic disc. ThePlacidos disc is a common form of keratoscopic disc; it consists essen-tially of a circular plate on which is painted alternate rings of blackand white. At the center there is a circular aperture or a convex lens 65 PHYSIOLOGICAL OPTICS which acts as a simple microscope. When the patient looks towardthe center of the disc the images of the circles seen apparently justback of the cornea are, in the normal eye, circl
. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . + D and — D. 47. Examination of the cornea with the keratoscopic disc. ThePlacidos disc is a common form of keratoscopic disc; it consists essen-tially of a circular plate on which is painted alternate rings of blackand white. At the center there is a circular aperture or a convex lens 65 PHYSIOLOGICAL OPTICS which acts as a simple microscope. When the patient looks towardthe center of the disc the images of the circles seen apparently justback of the cornea are, in the normal eye, circles; in an astigmaticcornea they are elongated along the meridian of least refraction. Byhaving the patient look toward the border of the disc it is easy to seeand establish the peripheral flattening of the cornea. Fig. 24 presentssome keratoscopic figures copied from Javal. C represents directfixation; H, upward; B, downward; D, to the right and G, to the figures show that the central part of the cornea (Fig. 24, C)was affected with a pronounced astigmia while the middle zones are. Fig. 25.—Keratoscopic Figures of a Case of Keratoconus. (After Javal.) scarcely affected at all; in fact the central ring of figure C, whichcorresponds to the middle of the cornea, is much lengthened while themore peripheral rings are almost circular. The aberration periphery are noticed in H, G, B and D. In cases of irregularastigmatism the circles assume irregular forms and one may obtainimportant information from them. Fig. 25 is a reproduction of thekeratoscopic figures in a case of keratoconus after Javal. In such casesthe image of the disc is small at the summit but a slight deviation ofthe look causes a change of form by lengthening the image Fig. 26 are reproduced the forms of the horizontal section of thecornea according to Cordiale. Curve I is for a normal cornea; Curves 66 PHYSIOLOGICAL OPTICS II, III and IV for a case of keratoconus. Curve I, for e
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