. The refraction of the eye; including a complete treatise on ophthalmometry; . ans if no corneal astig-matism was present; and C represents a composite of the two asthey actually are and the points where rays of light focus afterpassing through the two chief meridians of each. The horizontal meridians of both the cornea and the lens inthe above cases are emmetropic, as seen by the diagrams, andallow the rays of light to focus on the retina. The verticalmeridian of the cornea is more curved by .25 D. than its VARIATION IN AXIS OF THE ASTIGMATISM 65 horizontal meridian. This is demonstrated by
. The refraction of the eye; including a complete treatise on ophthalmometry; . ans if no corneal astig-matism was present; and C represents a composite of the two asthey actually are and the points where rays of light focus afterpassing through the two chief meridians of each. The horizontal meridians of both the cornea and the lens inthe above cases are emmetropic, as seen by the diagrams, andallow the rays of light to focus on the retina. The verticalmeridian of the cornea is more curved by .25 D. than its VARIATION IN AXIS OF THE ASTIGMATISM 65 horizontal meridian. This is demonstrated by the overlappingof the images of the mires to the extent of .25 D. when theyare turned from the horizontal to the vertical meridian afterhaving been approximated. But the vertical meridian of thelens is too little curved by .50 D. It therefore neutralizes D. of corneal astigmatism (which is with the rule) andleaves still .25 D. lenticular astigmatism against the rule in thevertical meridian to be corrected, which requires + .25 glass axis 180°. 180CORNEA. Fig. 29. — A shows the focuses of the two chief meridians of the cornea, assuming nolenticular astigmatism; B, the focuses of the two chief meridians of the lens,assuming no corneal astigmatism; C, the focuses of their combined action, D. corneal astigmatism with the rule and .50 D. of lenticular astigmatismagainst the rule. Oases showing the variation of axes of the chief meridians ofcurvature of the cornea from 90° a7id 1S0°.—The plurality ofcases of astigmatism have their chief meridians exactl}^ at 90°and 180°, and, as a rale, the meridian that has a shorter radiusof curvature is at 90°, while the meridian that has the longerradius of curvature is at 180°. Many times the two chiefmeridians are not exactly at 90°, but vary; and this variationin the majority of cases is symmetrical in character.^ That is, 1 Clairborne, JV. Y. Med. Jour. June 25, and July 2, 1892. See also refer-ence on
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