The refraction of the eye; including a complete treatise on opthalmometry; a clinical text-book for students and practitioners . 66. — Shmviiic focuses of the chief meridians in simple myopic astigmatism of2 I), fm- distant vision and for near vision at the age of forty-five years. The 1 I). of presbyopia neutralizes 1 D. of the myopicastigmatism in the vertical meridian (leaving 1 D. of it uncor-rected), and at the same time renders the horizontal meridian(which is emmetropic) presbyopic, or in effect hypermetropic1 D. Thus the eye for reading purposes is mixed astigmatic,since it focuss
The refraction of the eye; including a complete treatise on opthalmometry; a clinical text-book for students and practitioners . 66. — Shmviiic focuses of the chief meridians in simple myopic astigmatism of2 I), fm- distant vision and for near vision at the age of forty-five years. The 1 I). of presbyopia neutralizes 1 D. of the myopicastigmatism in the vertical meridian (leaving 1 D. of it uncor-rected), and at the same time renders the horizontal meridian(which is emmetropic) presbyopic, or in effect hypermetropic1 D. Thus the eye for reading purposes is mixed astigmatic,since it focusses rays of light in front of the retina in the verti-cal meridian, and back of the retina in the horizontal to correct this we can give either the cross-cylindricalglasses, + 1 1). cyl., 90° -ID. cyl., 180°; or we can givesphero-cylindrical glasses, + 1 D. — 2 D. cyl., 180°. In the 146 THE REFRACTION OF THE EYE first instance, with the cross-cylinders, the +1 D. cyl., 90°,corrects the 1 D. of presbyopia in the horizontal meridian, andthe —ID. cyl., 180°, corrects the one remaining diopter ofmyopic
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