Text-book of ophthalmology . uite precise, yet serves to show how an astigmatic sees. Just as a myope sees likean emmetrope wearing a convex sphere and a hypermetrope like an emmetrope wearing a concavesphere (page 865, Note), so an astigmatic sees like an emmetrope wearing a cylinder with or without asphere.—D.] 889 890 TEXT-BOOK OF OPHTHALMOLOGY erect ellipse. At 2, where the rays of the horizontal meridian come exactlyto a focus, the image of the point is a vertical line. In the same way theshape of the cross section of the beam—i. e., the shape of the diffusionimage of the point—can be asc
Text-book of ophthalmology . uite precise, yet serves to show how an astigmatic sees. Just as a myope sees likean emmetrope wearing a convex sphere and a hypermetrope like an emmetrope wearing a concavesphere (page 865, Note), so an astigmatic sees like an emmetrope wearing a cylinder with or without asphere.—D.] 889 890 TEXT-BOOK OF OPHTHALMOLOGY erect ellipse. At 2, where the rays of the horizontal meridian come exactlyto a focus, the image of the point is a vertical line. In the same way theshape of the cross section of the beam—i. e., the shape of the diffusionimage of the point—can be ascertained for the more posteriorly situatedpoints from 3 to 7. This cross section is sometimes an erect or horizontalellipse, sometimes a vertical or horizontal line, according as it is more orless distant from the refracting surface. It is only at 4 that there is reallya diffusion circle, because here the rays passing through the horizontalmeridian diverge to the same extent that those of the vertical Fig. 389.—Refraction op the Rays in Regular Astigmatism. 785. Symptoms.—The vision of an astigmatic person is not simplyindistinct, like that of one who is near-sighted or far-sighted, but pre-sents special peculiarities on account of the elongated form of the diffu-sion images. Circular surfaces—e. g., the full moon—appear lines sometimes look distinct, sometimes indistinct, according tothe direction that they take. Let us assume that we have before us anastigmatic who sees the diffusion image of a point under the form of avertical line (2, Fig. 389). If this man looks at two lines standing per-pendicular to each other (Fig. 390 A) the horizontal line appears broad-ened and indistinct but the vertical line seems sharply defined. For, wemay imagine these two lines to be composed of an infinite number ofpoints. Each one of these points appears upon the retina of the astig-matic patienl under the form of a short vertical stroke, a
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