. Refraction and motility of the eye, with chapters on color blindness and the field of vision . as simple hyperopic. If bothmeridians are hyperopic, but more so in one than the other,the astigmatism is compound hyperopic. If one meridianis emmetropic and the other myopic, the astigmatism issimple myopic, while, if both are myopic, but one more thanthe other, the astigmatism is compound myopic. Mixedastigmatism is present, when one meridian is myopic andthe other hyperopic. In all these cases the amount ofastigmatism simply measures the difference in curvature ofdifferent meridians. In any of
. Refraction and motility of the eye, with chapters on color blindness and the field of vision . as simple hyperopic. If bothmeridians are hyperopic, but more so in one than the other,the astigmatism is compound hyperopic. If one meridianis emmetropic and the other myopic, the astigmatism issimple myopic, while, if both are myopic, but one more thanthe other, the astigmatism is compound myopic. Mixedastigmatism is present, when one meridian is myopic andthe other hyperopic. In all these cases the amount ofastigmatism simply measures the difference in curvature ofdifferent meridians. In any of these we can make thehorizontal curve equal by adding a convex cylinder with itsaxis vertical (90°) or by subtracting from the verticalcurve by a concave cylinder axis 180°, whichever gives the best vision. Therefore, con-vex cylinders at the axis 90°,or concave at axis 180°, indi-cate an astigmatism with therule. Many writers considerthat cases in which the axis ofthe correcting glass is within45 degrees of the verticaland horizontal, should beincluded, as shown in theFig. 69. ASTIGMATISM. 171 Vision in Astigmatism of Various Types.—A lowdegree of astigmatism is not incompatible with normaldistant vision, but in the high degrees of whatever typethe visual acuity is diminished, and since astigmatism ismuch more marked at the periphery than the centre of thecornea, vision is often very much better when the pupil iscontracted than when it is dilated. This sometimes resultsin a patient under a mydriatic showing much greaterastigmatism than before its use. Experiments have provedthat the astigmatic eye sees best when the vertical focallines fall on the retina and the individual will, so far aspossible, endeavor to bring this about either by using hisaccommodation or by changing the position of his head orthat of the object of regard, so that while the horizontallines may be indistinct, the vertical ones shall be sharp andclear, which is a great help in reading. Thes
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